Highway Bridge Collapse In Minneapolis • Nursing Area

Highway Bridge Minneapolis, Minnesota

ACCIDENT REPORT NTSB/HAR-08/03 PB2008-916203National

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Transportation Safety Board

National Transportation Safety Board 490 LEnfant Plaza, S.W. Washington, D.C. 20594

Highway Accident Report Collapse of I-35W Highway Bridge

Minneapolis, Minnesota August 1, 2007

490 LEnfant Plaza, S.W. Washington, D.C. 20594

NTSB/HAR-08/03 PB2008-916203 Notation 7975C

November 14, 2008

The National Transportation Safety Board is an independent Federal agency dedicated to promoting aviation, railroad, highway, marine, pipeline, and hazardous materials safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board Act of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The Safety Board makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and statistical reviews.

Recent publications are available in their entirety on the Web at <http://www.ntsb.gov>. Other information about available publications also may be obtained from the Web site or by contacting:

National Transportation Safety Board Records Management Division, CIO-40 490 LEnfant Plaza, S.W. Washington, D.C. 20594 (800) 877-6799 or (202) 314-6551

Safety Board publications may be purchased, by individual copy or by subscription, from the National Technical Information Service. To purchase this publication, order report number PB2008-916203 from:

National Technical Information Service 5285 Port Royal Road Springfield, Virginia 22161 (800) 553-6847 or (703) 605-6000

The Independent Safety Board Act, as codified at 49 U.S.C. Section 1154(b), precludes the admission into evidence or use of Board reports related to an incident or accident in a civil action for damages resulting from a matter mentioned in the report.

National Transportation Safety Board. 2008. Collapse of I-35W Highway Bridge, Minneapolis, Minnesota, August 1, 2007. Highway Accident Report NTSB/HAR-08/03. Washington, DC.

Abstract: About 6:05 p.m. central daylight time on Wednesday, August 1, 2007, the eight-lane, 1,907-foot-long I-35W highway bridge over the Mississippi River in Minneapolis, Minnesota, experienced a catastrophic failure in the main span of the deck truss. As a result, 1,000 feet of the deck truss collapsed, with about 456 feet of the main span falling 108 feet into the 15-foot-deep river. A total of 111 vehicles were on the portion of the bridge that collapsed. Of these, 17 were recovered from the water. As a result of the bridge collapse, 13 people died, and 145 people were injured. On the day of the collapse, roadway work was underway on the I-35W bridge, and four of the eight travel lanes (two outside lanes northbound and two inside lanes southbound) were closed to traffic. In the early afternoon, construction equipment and construction aggregates (sand and gravel for making concrete) were delivered and positioned in the two closed inside southbound lanes. The equipment and aggregates, which were being staged for a concrete pour of the southbound lanes that was to begin about 7:00 p.m., were positioned toward the south end of the center section of the deck truss portion of the bridge and were in place by about 2:30 p.m. About 6:05 p.m., a motion-activated surveillance video camera at the Lower St. Anthony Falls Lock and Dam, just west of the I-35W bridge, recorded a portion of the collapse sequence. The video showed the bridge center span separating from the rest of the bridge and falling into the river.

Major safety issues identified in this investigation include insufficient bridge design firm quality control procedures for designing bridges, and insufficient Federal and State procedures for reviewing and approving bridge design plans and calculations; lack of guidance for bridge owners with regard to the placement of construction loads on bridges during repair or maintenance activities; exclusion of gusset plates in bridge load rating guidance; lack of inspection guidance for conditions of gusset plate distortion; and inadequate use of technologies for accurately assessing the condition of gusset plates on deck truss bridges. As a result of this accident investigation, the Safety Board makes recommendations to the Federal Highway Administration (FHWA) and the American Association of State Highway and Transportation Officials. One safety recommendation resulting from this investigation was issued to the FHWA in January 2008.

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Contents

Glossary of Bridge-Related Terms as Used in This Report …………………………….. vi

Executive Summary ………………………………………………………………………………………..xiii

Factual Information ……………………………………………………………………………………………1 Accident Synopsis …………………………………………………………………………………………………….1 The Accident …………………………………………………………………………………………………………….1 Emergency Response ………………………………………………………………………………………………..3

Initial Response …………………………………………………………………………………………………….3 Incident Command ……………………………………………………………………………………………….4

Injuries ………………………………………………………………………………………………………………………5 Accident Location ……………………………………………………………………………………………………..5 Bridge Description …………………………………………………………………………………………………….5

General …………………………………………………………………………………………………………………5 Deck Truss Portion ……………………………………………………………………………………………….9 Construction of Deck Truss …………………………………………………………………………………12

Study of Collapse Video ………………………………………………………………………………………….14 Damages …………………………………………………………………………………………………………………16

Approach Spans ………………………………………………………………………………………………….16 Deck Truss Spans ………………………………………………………………………………………………..16 Piers and Bearings ………………………………………………………………………………………………21

Recovery of Structure ……………………………………………………………………………………………..21 Bridge Renovations and Modifications ……………………………………………………………………22

1977 Renovation: Increased Deck Thickness ……………………………………………………….23 1998 Renovation: Median Barrier, Traffic Railings, and Anti-Icing System ……………………………………………………………………………………………….23 2007 Repair and Renovation: Repaving ………………………………………………………………24

Staging of Construction Materials on Bridge Deck ………………………………………………….24 Bridge Traffic and Construction Loading at Time of Collapse …………………………………28 Mn/DOT Policies Regarding Construction Loads on Bridges …………………………………28 Bridge Load Rating and Posting ……………………………………………………………………………..33

Mn/DOT Draft Bridge Rating Manual ……………………………………………………………….36 Load Ratings for I-35W Bridge ……………………………………………………………………………38 Bridge Posting …………………………………………………………………………………………………….39 Bridge Rating and Load Posting Report, 1979 …………………………………………………….39 Bridge Rating and Load Posting Report, 1995 …………………………………………………….40 FHWA Assessment of Load Rating Records for I-35W Bridge ……………………………42

Federal Bridge Inspection Requirements …………………………………………………………………44 State Inspections and Inspection Reporting …………………………………………………………….45 Inspection Results and Condition Ratings for I-35W Bridge ……………………………………47

Rust and Corrosion …………………………………………………………………………………………….52 Mn/DOT Underwater Bridge Inspection ……………………………………………………………53

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Washout Hole Underneath Bridge ………………………………………………………………………53 Other Repairs ……………………………………………………………………………………………………..53

Fatigue Cracking in Minnesota Bridges …………………………………………………………………..54 Lafayette Bridge ………………………………………………………………………………………………….54 Dresbach Bridge ………………………………………………………………………………………………….54 Dartmouth Bridge ……………………………………………………………………………………………….55 Lexington Bridge ………………………………………………………………………………………………..55 Hastings Bridge …………………………………………………………………………………………………..55 Oar Dock Bridge …………………………………………………………………………………………………55

Preexisting Fatigue Cracking in I-35W Bridge …………………………………………………………56 University of Minnesota Fatigue Assessment of I-35W Bridge ……………………………57 URS Corporation Inspections and Reports ………………………………………………………….58

Bowed Gusset Plates on I-35W Bridge …………………………………………………………………….61 Examination of Deck Truss Fracture Areas ……………………………………………………………..64

South Fracture Area, Main Trusses ……………………………………………………………………..64 South Fracture Area, Deck and Stringers …………………………………………………………….69 South Fracture Area, Floor Trusses and Braces …………………………………………………..70 North Fracture Area, Main Trusses …………………………………………………………………….71 North Fracture Area, Deck and Stringers ……………………………………………………………74

Tests and Research ………………………………………………………………………………………………….74 Preexisting Features of Structural Components ………………………………………………….74 I-35W Bridge Gusset Plate Adequacy Analysis …………………………………………………..75 Analysis of Other Aspects of I-35W Bridge Design ……………………………………………..78 Materials Testing ……………………………………………………………………………………………….79 Finite Element Modeling of I-35W Bridge …………………………………………………………..80

Design History of I-35W Bridge ………………………………………………………………………………87 Evolution of Design …………………………………………………………………………………………….88 Initial Design and Preliminary Engineering Report, October 1962April 1963 …………………………………………………………………………………….89 Preliminary Design, April 1963March 1964 ……………………………………………………….90 Final Design, March 1964March 1965 ………………………………………………………………..91

Other Information …………………………………………………………………………………………………..92 Mn/DOT Postaccident Actions …………………………………………………………………………..92 Deficient Gusset Plates on Other Minnesota Bridges …………………………………………..93 Deficient Gusset Plates on Ohio Bridges ……………………………………………………………..95 Guidance and Training for Inspecting Gusset Plates …………………………………………..97 FHWAAASHTO Joint Study of Gusset Plates …………………………………………………101 Bridge Design Firm Process for Design and Design Review ……………………………..102 Mn/DOT Bridge Design Review Process ………………………………………………………….108 FHWA Bridge Design Review Process ………………………………………………………………110 Design Review Processes in Other States ………………………………………………………….112 Examples of Bridge Design Errors …………………………………………………………………….114 Mn/DOT Certification of Bridge Inspectors ……………………………………………………..116 Minnesota Emergency Preparedness and Lessons Learned ………………………………117

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Analysis …………………………………………………………………………………………………………..119 Collapse Sequence …………………………………………………………………………………………………119 Other Possible Collapse Scenarios Considered ………………………………………………………123

Corrosion Damage in Gusset Plates at L11 Nodes …………………………………………….123 Fracture of Floor Truss ………………………………………………………………………………………125 Preexisting Cracking …………………………………………………………………………………………125 Temperature Effects ………………………………………………………………………………………….126 Pier Movement ………………………………………………………………………………………………….126

Emergency Response …………………………………………………………………………………………….127 Design of the Main Truss Gusset Plates …………………………………………………………………127 Origin of the Inadequate Gusset Plates ………………………………………………………………….129

Materials Substitution ……………………………………………………………………………………….129 Design Calculations …………………………………………………………………………………………..129 Design Quality Control ……………………………………………………………………………………..131

FHWA, Mn/DOT, and Other State DOTs ……………………………………………………………..132 Bridge Load Rating ……………………………………………………………………………………………….133 Effect of Added Loads Over Time …………………………………………………………………………136 Guidance for Allowing Construction Loads on Bridges ………………………………………..138 Inspections of I-35W Bridge …………………………………………………………………………………..140

Condition Rating ………………………………………………………………………………………………141 Gusset Plate Corrosion ……………………………………………………………………………………..142 Gusset Plate Bowing………………………………………………………………………………………….143

Inspecting U.S. Bridges for Gusset Plate Adequacy ……………………………………………….145 Corrosion on Gusset Plates ……………………………………………………………………………….145 Distortion of Gusset Plates ………………………………………………………………………………..146 Guidance for Inspecting Gusset Plates ………………………………………………………………147 Training for Bridge Inspectors …………………………………………………………………………..147

Conclusions …………………………………………………………………………………………………….149 Findings ………………………………………………………………………………………………………………..149 Probable Cause ……………………………………………………………………………………………………..152

Recommendations …………………………………………………………………………………………..153 New Recommendations ………………………………………………………………………………………..153 Previously Issued Recommendation Resulting From This Accident Investigation …………………………………………………………………………………..155

Appendixes A: Investigation …………………………………………………………………………………………………….157 B: Selected Bridge Information …………………………………………………………………………….158 C: Previous Safety Board Actions Regarding Bridge Inspections ……………………….161

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Glossary of BridGe-related terms as Used in this report

This glossary also defines the underlined terms within definitions.

Abutment: A retaining wall that supports the ends of a bridge.

American Association of State Highway and Transportation Officials (AASHTO): A nonprofit association whose voting membership consists of representatives of the highway and transportation departments of every State, Puerto Rico, and the District of Columbia. AASHTO guides and specifications are used to describe loading requirements for highway bridges. The organization was formed in 1914 and, until 1973, was known as the American Association of State Highway Officials, or AASHO.

Approach span: The portion of a bridge that carries traffic from the land to the main parts of the bridge.

Bearing: A device located between the bridge structure and a supporting pier or abutment.

Box member: A hollow, rectangular, four-sided structural member built up from pieces of steel joined to form a box. On the I-35W bridge, the upper and

lower surfaces of these members were referred to as cover plates; the east and west sides were referred to as side plates. Transverse metal plates (diaphragms) were welded at intervals inside the box members to add rigidity.

Bridge load rating: The live-load-carrying capacity of a bridge, determined by review of bridge plans and field inspection data. This rating is used to determine whether specific legal or overweight

vehicles can safely cross the structure, whether the structure needs to be load posted, and the level of load posting required.

Box member

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Cantilever: A structural member that projects beyond a supporting column or wall and is supported at only one end.

Compression: A force that pushes or presses toward the center of an object or from the ends toward the middle of a structural member.

Compression member: A truss member that is subjected to compressive (compression) forces. In the I-35W bridge, some structural members were always under compression; some were always under tension; and some, depending on the live load, reversed, changing from tension to compression or vice versa.

Condition ratings: According to the National Bridge Inspection Standards, condition ratings are used to describe an existing bridge compared with its condition when new. The ratings are based on materials, physical condition of the deck, superstructure, and substructure. General condition ratings range from 0 (failed condition) to 9 (excellent). Based on the bridges condition, a status is assigned. The status is used to determine eligibility for Federal bridge replacement and rehabilitation funding. Current Federal Highway Administration status ratings are Not Deficient, Structurally Deficient, and Functionally Obsolete.

Culvert: A drain, pipe, or channel that allows water to pass under a road, railroad, or embankment.

Dead load: The static load imposed by the weight of materials that make up the bridge structure itself.

Deck: The roadway portion of a bridge, including shoulders. Most bridge decks are constructed as reinforced concrete slabs.

Deck truss bridge: A truss bridge with the truss underneath the roadway, supporting traffic traveling along the top of the main structure. In a through truss, traffic travels through the superstructure, which is cross-braced above and below traffic.

Deck trussThrough truss

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Diagonal: A structural member connecting the upper and lower chords on the diagonal (as opposed to the vertical). See gusset plate.

Diaphragm: Bracing that spans between the main beams or girders of a bridge and assists in the distribution of loads. On the I-35W bridge, the box members also contained internal diaphragms.

Expansion joint: A meeting point between two parts of a structure that is designed to allow for independent movement of the parts due to thermal expansion while protecting the parts from damage. Expansion joints are commonly

visible on a bridge deck as a hinged or movable connection perpendicular to the roadway.

Fatigue: In metal, a brittle cracking mechanism caused by repetitive loading over time.

Finite element analysis: An analysis of a structure based on a computer model of its material or design. A finite element model describes a virtual assembly of simplified structural elements used to approximate a complex structure.

The behavior of the complex structure is then calculated by combining the actions of the interconnected simpler elements.

Floor truss: A welded truss perpendicular to the main trusses, used to support the deck.

Fracture-critical member: A steel member within a non-load-path-redundant structure, the failure of which would cause a partial or total collapse of the structure.

Functionally obsolete: A bridge status assigned by the Federal Highway Administration under the National Bridge Inspection Standards. A Functionally Obsolete bridge is one that was built to standards that are not used today. These bridges are not considered inherently unsafe, but they may have lane widths, shoulder widths, or vertical clearances that are inadequate for current traffic.

Girder: A horizontal structural member supporting vertical loads by bending. Larger girders are typically made of multiple metal plates that are welded or riveted together.

Typical expansion joint

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Gusset plate: A metal plate used to unite multiple structural members of a truss.

H member: A structural steel member with two flat flanges separated by a horizontal steel plate (web) to form an H, made as either a rolled or welded shape.

Legal load: The maximum load for each vehicle configuration permitted by law by the State in which a State highway bridge is located. In the United States, the current weight limit for the Interstate system as set by the Federal Highway Administration is 80,000 pounds gross vehicle weight and 20,000 pounds for an axle.

Live load: Operational or temporary loads such as vehicular traffic, impact, wind, water, or earthquake.

Load: Force applied either from the weight of the structure itself (dead load) or from traffic, temporary loads, wind, or earthquake (live load).

Load posted: Any bridge or structure restricted to carrying loads less than the legal load limit. The National Bridge Inspection Standards require the load posting of any bridge that is not capable of safely carrying a legal load.

Lower chord: The bottom horizontal, or almost horizontal, member of a truss. The lower chord extends the length of the deck truss but consists of shorter chord members spliced together at nodes.

Member: An individual angle, beam, plate, or built-up piece intended to become an integral part of an assembled frame or structure. The major structural elements of the truss (chords, diagonals, and verticals) are called members.

Gusset plate

I-35W bridge main truss node

H member H member

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National Bridge Inspection Standards: Federal standards first established in 1971 to set national requirements for bridge inspection frequency, inspector qualifications, report formats, and inspection and rating procedures. The legislative authority for the standards is found at 23 Code of Federal Regulations Part 650.

Node: On the I-35W bridge, a connecting point where the upper or lower chords were joined to vertical and diagonal members with gusset plates. The bridge had 56 nodes on each of its two main trusses, for a total of 112 nodes. See the node illustration at gusset plate.

Nondestructive evaluation: Also referred to as nondestructive testing or nondestructive inspection, this evaluation does not damage the test object. Technologies for nondestructive evaluation include x-ray and ultrasound, which may be used to detect such defects as cracking and corrosion.

Non-load-path-redundant: The condition where fracture of an individual structural element (a fracture-critical element) could lead to a partial or total collapse of the entire bridge. A bridge that is non-load-path-redundant is not inherently unsafe, but it does lack redundancy in the design of its support structure. Such bridges are sometimes referred to as fracture critical. The I-35W bridge was of a non-load-path-redundant design.

Pier: A vertical structure that supports the ends of a multispan superstructure at a location between abutments.

Post-tensioning: A method of stressing concrete using steel rods or cables that are stretched after the concrete has hardened. This stretching of the rods or cables puts the concrete in compression, with the compressive stresses designed to counteract the tensile (tension) forces on the concrete once it is under load.

Rivet: A metal fastener, used in construction primarily before 1970, made with a rounded preformed head at one end and installed hot into a predrilled or punched hole. The other end was hammered (upset) into a similarly shaped head, thereby clamping the adjoining parts together.

Rocker bearing: A bridge support bearing that accommodates thermal expansion and contraction of the superstructure through a rocking action.

Roller bearing: A bridge bearing comprising a single roller or a group of rollers housed so as to permit longitudinal thermal expansion or contraction of a structure.

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Section loss: A loss of metal, usually resulting from corrosion, that reduces the thickness of a steel bridge component.

Shear: A force that causes parts of a material to slide past one another in opposite directions.

Snooper: An inspection bucket or platform at the end of a long articulating boom (usually mounted to a truck) that provides access to the undersides of bridges.

Span: The horizontal space between two supports of a structure. A simple span rests on two supports, one at each end, the stresses on which do not affect the stresses in the adjoining spans. A continuous span comprises a series of consecutive spans (over three or more supports) that are continuously or rigidly connected (without joints) so that bending moment may be transmitted from one span to the adjacent ones.

Specifications: A document that explains material and construction requirements of the bridge structure.

Splice plate: A plate that joins two chord members of a truss or that is used to extend the length of a member. On the I-35W bridge, the chord member joints used four splice platestop, bottom, east, and west.

Stiffener: A structural steel shape, such as an angle, that is attached to a flat plate such as a gusset plate or the web of a member to add compression strength.

Stringer: A beam aligned with the length of a span that supports the deck.

Structurally deficient: A bridge status assigned by the Federal Highway Administration under the National Bridge Inspection Standards. A bridge is classified Structurally Deficient if it has a general condition rating for the deck, superstructure, substructure, or culvert of 4 (poor condition) or less.

A snooper being used to inspect the I-35W bridge

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A rating of Structurally Deficient does not indicate that the bridge is unsafe but that it typically requires significant maintenance and repair to remain in service and eventual rehabilitation or replacement to address deficiencies.

Substructure: The bridge structure that supports the superstructure and transfers loads from it to the ground or bedrock. The main components are abutments, piers, footings, and pilings.

Superstructure: The bridge structure that receives and supports traffic loads and, in turn, transfers those loads to the substructure. It includes the bridge deck, structural members, parapets, handrails, sidewalk, lighting, and drainage features.

Tension: A force that stretches or pulls on a material.

Tension member: Any member of a truss that is subjected to tensile (tension) forces. In the I-35W truss bridge, some structural members were always under tension; some were always under compression; and some, depending on the live load, reversed, changing from tension to compression or vice versa.

Truss bridge: A bridge typically composed of straight structural elements connected to form triangles. In large structures, the ends of the members are connected with gusset plates. Geometry ensures that the members are primarily loaded in direct tension or compression. The I-35W bridge structure was in the form of a Warren truss with verticals, which has alternating tension and compression diagonals.

Upper chord: The top horizontal, or almost horizontal, member of a truss. The upper chord extends the length of the deck truss, but it is made up of shorter chord members joined at nodes.

Vertical: The vertical member connecting the upper and lower chords at like-numbered nodes.

Wearing surface: The topmost layer of material applied on a roadway to receive traffic loads and to resist the resulting disintegrating action, also known as wearing course.

Web: The vertical portion of an I-beam or girder.

Yield stress: The stress above which permanent (plastic) deformation occurs.

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exeCUtive sUmmary

About 6:05 p.m. central daylight time on Wednesday, August 1, 2007, the eight-lane, 1,907-foot-long I-35W highway bridge over the Mississippi River in Minneapolis, Minnesota, experienced a catastrophic failure in the main span of the deck truss. As a result, 1,000 feet of the deck truss collapsed, with about 456 feet of the main span falling 108 feet into the 15-foot-deep river. A total of 111 vehicles were on the portion of the bridge that collapsed. Of these, 17 were recovered from the water. As a result of the bridge collapse, 13 people died, and 145 people were injured.

On the day of the collapse, roadway work was underway on the I-35W bridge, and four of the eight travel lanes (two outside lanes northbound and two inside lanes southbound) were closed to traffic. In the early afternoon, construction equipment and construction aggregates (sand and gravel for making concrete) were delivered and positioned in the two closed inside southbound lanes. The equipment and aggregates, which were being staged for a concrete pour of the southbound lanes that was to begin about 7:00 p.m., were positioned toward the south end of the center section of the deck truss portion of the bridge and were in place by about 2:30 p.m.

About 6:05 p.m., a motion-activated surveillance video camera at the Lower St. Anthony Falls Lock and Dam, just west of the I-35W bridge, recorded a portion of the collapse sequence. The video showed the bridge center span separating from the rest of the bridge and falling into the river.

The National Transportation Safety Board determines that the probable cause of the collapse of the I-35W bridge in Minneapolis, Minnesota, was the inadequate load capacity, due to a design error by Sverdrup & Parcel and Associates, Inc., of the gusset plates at the U10 nodes, which failed under a combination of (1) substantial increases in the weight of the bridge, which resulted from previous bridge modifications, and (2) the traffic and concentrated construction loads on the bridge on the day of the collapse. Contributing to the design error was the failure of Sverdrup & Parcels quality control procedures to ensure that the appropriate main truss gusset plate calculations were performed for the I-35W bridge and the inadequate design review by Federal and State transportation officials. Contributing to the accident was the generally accepted practice among Federal and State transportation officials of giving inadequate attention to gusset plates during inspections for conditions of distortion, such as bowing, and of excluding gusset plates in load rating analyses.

Executive Summary

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Before determining that the collapse of the I-35W bridge initiated with failure of the gusset plates at the U10 nodes, the Safety Board considered a number of potential explanations. The following factors were considered, but excluded, as being causal to the collapse: corrosion damage in gusset plates at the L11 nodes, fracture of a floor truss, preexisting cracking, temperature effects, and pier movement.

The following safety issues were identified in this investigation: Insufficient bridge design firm quality control procedures for designing bridges, and insufficient Federal and State procedures for reviewing and approving bridge design plans and calculations. Lack of guidance for bridge owners with regard to the placement of construction loads on bridges during repair or maintenance activities. Exclusion of gusset plates in bridge load rating guidance. Lack of inspection guidance for conditions of gusset plate distortion. Inadequate use of technologies for accurately assessing the condition of gusset plates on deck truss bridges.

As a result of this accident investigation, the Safety Board makes recommendations to the Federal Highway Administration and the American Association of State Highway and Transportation Officials. One safety recommendation resulting from this investigation was issued to the Federal Highway Administration in January 2008.

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faCtUal information

Accident Synopsis

About 6:05 p.m. central daylight time on Wednesday, August 1, 2007, the eight-lane, 1,907-foot-long I-35W highway bridge over the Mississippi River in Minneapolis, Minnesota, experienced a catastrophic failure in the main span of the deck truss. As a result, 1,000 feet of the deck truss collapsed, with about 456 feet of the main span falling 108 feet into the 15-foot-deep river. (See figure 1.) A total of 111 vehicles were on the portion of the bridge that collapsed. Of these, 17 were recovered from the water. As a result of the bridge collapse, 13 people died, and 145 people were injured.

The Accident

A total of 111 vehicles were documented as being on the bridge when it collapsed.1 Of these, 25 were either construction vehicles or vehicles belonging to construction workers. One of the nonconstruction vehicles was a school bus carrying 63 students and the driver. After the collapse, 17 vehicles were found in the river or on a submerged portion of the bridge deck.

1 Trailers and nonmotorized construction-related equipment are not included in this total.

Factual Information

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North approach spans

Deck truss

South approach spans

North

(Top) Aerial view (looking northeast) of I-35W bridge (arrow) about 2 hours 15 minutes Figure 1. before collapse. This photograph was taken by a passenger in a commercial airliner departing Minneapolis/St. Paul International Airport. (Bottom) I-35W bridge after collapse.

Factual Information

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Emergency Response

Initial Response About 6:05 p.m., Minnesota State Patrol dispatchers were notified of

the accident by cell phone through the 911 system. After verifying the collapse using the Minnesota Department of Transportation (Mn/DOT) freeway camera system, dispatchers contacted the Minneapolis 911 dispatch, which is a combined emergency dispatch center for the Minneapolis fire and police departments. The first call from Minneapolis 911 dispatch went out at 6:07 p.m. At 6:08 p.m., Minneapolis 911 dispatch made a distress call over the interstate radio system requesting that all available emergency assistance providers respond to the I-35W bridge.

Some of the first to become involved in the rescue effort were citizens who were in or near the area when the collapse occurred. The Minneapolis police captain responsible for the on-scene investigation estimated that 100 citizens assisted in the total rescue effort. These people included construction workers who had just left or arrived for shift change, passersby, a group of medical personnel who were in training at the nearby Red Cross building, and a number of University of Minnesota students and staff. He said that 3040 of these individuals went into the river to pull drivers and construction workers to safety.

Initial reports stated that the entire span of I-35W over the Mississippi River had collapsed while and a full construction crew. About 6:10 p.m., the first Minneapolis Police Department unit arrived on scene. At 6:11 p.m., the first of 19 engine units from the Minneapolis Fire Department arrived. The first Hennepin County Sheriffs Office personnel arrived on the river at 6:14 p.m. to begin conducting the search and rescue of numerous people reportedly trapped in their vehicles in the river. The Hennepin County Medical Center also initiated its disaster plan, which involved calling in additional medical personnel, notifying other local hospitals, and dispatching all available ambulances to the scene.

About 6:10 p.m., a unified command post was established in the parking lot of the Red Cross building, located on the south side of the river just west of the area of the collapse.

The sheriffs office established its river incident command at 6:25 p.m. near the University of Minnesota River Flats area along the north bank of the river. This site had historically been used as a base for water rescue operations and was the nearest facility that had an adequate number of boat ramps for rescue operations. Within the first hour of the collapse, 12 other public safety agencies responded with 28 watercraft to assist with river rescue operations.

Factual Information

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About 7:27 p.m., the fire department incident commander and the sheriffs office river operations incident commander decided to change the water operations from rescue mode to recovery mode.

Incident Command The city of Minneapolis and Hennepin County use the unified command

system in which the type of response required for an incident determines who will serve as the incident commander. In this accident, the assistant fire chief of the Minneapolis Fire Department was the incident commander, and the fire department was the lead agency responsible for overall operations as well as for issues related to the structural collapse of the bridge. The Minneapolis Police Department was responsible for the on-scene investigation (landside operations) and scene security, the Hennepin County Sheriffs Office was responsible for river rescue and recovery2 (waterside operations), and the Hennepin County Medical Center ambulance service was in charge of emergency medical services operations.

On August 2, 2007, the U.S. Coast Guard established a temporary security zone on the Mississippi River from the Upper St. Anthony Falls Lock and Dam to the Lower St. Anthony Falls Lock and Dam. Access through this portion of the river was granted to emergency vessels only. About 7:00 p.m. on August 2, the Minneapolis Fire Department transferred incident command to the Minneapolis Police Department because the area had been declared a crime scene (because of the possibility that the bridge had been the target of a terrorist attack). The Hennepin County Sheriffs Office continued to be responsible for coordinating public safety dive teams searching the area around the bridge collapse and for using side-scanning sonar to attempt to locate vehicles and victims reported missing through Monday, August 6.

On Saturday, August 4, 2007, the sheriffs office requested and received the help of the Federal Bureau of Investigation (FBI) underwater search and evidence response team (USERT) and the U.S. Naval Sea Systems Command (NAVSEA) mobile diving and salvage teams. On August 5, USERT arrived and began river recovery operations. On August 6, Navy teams arrived and, along with USERT, assisted sheriffs office personnel, who continued to coordinate all water recovery operations until the last victim was recovered.

On September 6, 2007, river access was increased to allow limited commercial barge traffic. The Mississippi River was completely reopened for all river traffic on October 6.

2 By Minnesota State statute (MSS 86B801), county sheriffs are responsible for recovering bodies from any waterway within their counties.

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Injuries

A total of 190 people (vehicle occupants, construction workers, and Mn/DOT personnel) were confirmed to have been on or near the bridge when the collapse occurred. Medical records obtained as part of this accident investigation indicated that 145 people were transported to or treated at 12 area hospitals, medical centers, and clinics. The information in table 1 is based on data provided by the Hennepin County medical examiner and the treating hospitals.

Injuries.Table 1.

Injuriesa Total Fatal 13 Serious 34 documented Minor 111 (70 documented) None or unknown 32 Total 190 ATitle 49 Code of Federal Regulations (CFR) 830.2 defines a fatal injury as any injury that results in death within 30 days of the accident. A serious injury is defined as any injury that requires hospitalization for more than 48 hours, commencing within 7 days of the date the injury was received; results in a fracture of any bone (except simple fractures of the fingers, toes, or nose); causes severe hemorrhages or nerve, muscle, or tendon damage; involves any internal organ; or involves second- or third-degree burns, or any burns affecting more than 5 percent of the body surface.

Accident Location

The accident occurred in the city of Minneapolis in Hennepin County, Minnesota. The I-35W bridge was located about 1 mile northeast of the junction of I-35W with Interstate 94. (See figure 2.) In addition to spanning the Mississippi River, the bridge also extended across Minnesota Commercial Railway railroad tracks and three roadways: West River Parkway, 2nd Street, and the access road to the lock and dam.

Bridge Description

General The I-35W bridge (National Bridge Inventory structure no. 9340) was

designed by the engineering consulting firm of Sverdrup & Parcel and Associates, Inc., of St. Louis, Missouri, a predecessor company of Sverdrup Corporation, which was acquired in 1999 by Jacobs Engineering Group, Inc. The bridge design was developed over several years, with plans for the foundation approved in 1964 and final design plans certified by the Sverdrup & Parcel project manager (a registered professional engineer) on March 4, 1965. The bridge design plans were approved by Mn/DOT on June 18, 1965.3

3 For more information about revisions to the initial bridge design, see Design History of I-35W Bridge later in this report.

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Construction of some piers started in 1964, and the bridge was opened to traffic in 1967.4 The design was based on the 1961 American Association of State Highway Officials (AASHO)5 Standard Specifications for Highway Bridges and 1961 and 1962 Interim Specifications, and on the 1964 Minnesota Highway Department Standard Specifications for Highway Construction. The bridge was built by Hurcon, Inc., with erection of the structure engineered and staged by the Industrial Construction Division of Allied Structural Steel Company, which was also the steel fabricator for the project. The structure used welded built-up steel beams for girders and truss members, with riveted and bolted connections. The bridge was 1,907 feet long and carried eight lanes of traffic, four northbound and four southbound. The structure inventory report for the bridge indicated average daily traffic in 2004 (the most recent available figures) as 141,000 vehicles. Average daily traffic of heavy commercial vehicles was 5,640. The earliest average daily traffic figures available for the bridge were from 1976 and indicated an average daily traffic at that time of 60,600 vehicles.

The bridge had 13 reinforced concrete piers and 14 spans, numbered south to north. (See figure 3.) Eleven of the 14 spans were approach spans to the deck truss portion. The bridge deck in the approach spans either was supported by continuous welded steel plate girders or by continuous voided slab concrete spans. The south approach spans were supported by the south abutment; by

4 The bridge was not opened to I-35W traffic until 1971, when construction of the I-35W highway was completed. Between 1967 and 1971, the I-35W bridge was used for traffic detoured from another bridge that was being renovated.

5 AASHO became AASHTO (American Association of State Highway and Transportation Officials) in 1973.

Accident location.Figure 2.

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piers 1, 2, 3, and 4; and by the south end of the deck truss portion. The north approach spans were supported by the north abutment; by piers 9, 10, 11, 12, and 13; and by the north end of the deck truss portion. The 1,064-foot-long deck truss portion of the bridge encompassed a portion of span 5; all of spans 6, 7, and 8; and a portion of span 9. The deck truss6 was supported by four piers (piers 5, 6, 7, and 8). (See figure 4.)

The original bridge design accounted for thermal expansion using a combination of fixed and expansion bearings for the bridge/pier interfaces. The fixed bearing assemblies were located at piers 1, 3, 7, 9, 12, and 13. Expansion (sliding) bearings were used at the south and north abutments and at piers 2, 4, 10, and 11. Expansion roller bearings were used at piers 5, 6, and 8. The roller bearings at pier 6 contained four large-diameter rollers; those at piers 5 and 8 were similar in design but contained only three large-diameter rollers. (See figure 5.)

6 A truss bridge is typically composed of straight structural elements connected to form triangles. In a classical pin-jointed truss, the ends of the members are connected by pins that allow free rotation so the members only carry direct tension or compression with no bending. In large structures, pin joints are impractical, and members are joined with gusset plates. Geometry ensures that the members are still primarily loaded in direct tension or compression, but the gusset plate joints allow for transfer of some secondary bending stresses. Depending on the live load, the stress in some members may reverse, changing from tension to compression or vice versa. The I-35W bridge structure was in the form of a Warren truss with verticals. A Warren truss has alternating tension and compression diagonals and has the advantage that chord members are continuous across two panels, with the same force carried across both panels. This design simplifies, somewhat, the design calculations. In a deck truss bridge, the roadbed is along the top of the truss structure.

Center span of I-35W bridge, looking northeast. The center span is supported by pier 6 Figure 4. on the near (south) riverbank and pier 7 on the far (north) riverbank.

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The deck of the I-35W bridge consisted of two reinforced concrete deck slabs separated by about 6 inches. Each deck slab accommodated four 12-foot-wide traffic lanes and two 2-foot-wide shoulders. The deck slabs widened at the north end of the approach to accommodate on and off ramps and curved slightly at the south approach spans to match the roadway alignment. The bridge deck had 11 expansion joints.

The total width of the deck slabs was about 113 feet 4 inches. The northbound and southbound traffic lanes (four in each direction) were each about 52 feet wide. The northbound lanes were separated from the southbound lanes by a 4-foot-wide median. The railing along the outside edges of the bridge was about 2 feet 8 inches wide. When the bridge was opened to traffic in 1967, the cast-in-place concrete deck slab had a minimum thickness of 6.5 inches. As discussed later in this report, bridge renovation projects eventually increased the average thickness of the concrete deck by about 2 inches.

Deck Truss Portion The deck truss portion of the bridge comprised two parallel main

Warren-type trusses (east and west) with verticals. The upper and lower chords of the main trusses extended the length of the deck truss portion of the bridge and were connected by straight vertical and (except at each end of the deck truss) diagonal members that made up the truss structure. The upper and lower chords

Expansion roller bearings at pier 5, looking east. (Source: URS Corporation)Figure 5.

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were welded box members, as were the diagonals and verticals, designed primarily for compression. The east and west sides of the box members are referred to as the side plates, and the top and bottom as the cover plates. The vertical and diagonal members designed primarily for tension were H members consisting of flanges welded to a web plate.

Riveted steel gusset plates at each of the 112 nodes (connection points) of the two main trusses tied the ends of the truss members to one another and to the rest of the structure.7 The gusset plates were riveted to the side plates of the box members and to the flanges of the H members. All nodes had at least two gusset plates, one on either side of the connection point.8 A typical I-35W main truss node, with gusset plates, is shown in figure 6.

7 At some gusset locations, bolts were used in place of some rivets, apparently to facilitate installation. 8 At some locations, the configuration of the connection required two overlapping gusset plates on each

side of the node.

Gusset plates

Upper chord

Diagonals

Vertical

Typical five-member node (two upper chord members, one vertical member, and two Figure 6. diagonal members) on I-35W bridge. (Source: URS Corporation)

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The east and west main trusses were spaced 72 feet 4 inches apart and were connected by 27 transverse welded floor trusses spaced on 38-foot centers along the truss and by two floor beams at the north and south ends. The floor trusses were cantilevered out about 16 feet past the east and west main trusses. The concrete deck for the roadway rested on 27-inch-deep wide-flange longitudinal stringers attached to the transverse floor trusses and spaced on 8-foot-1-inch centers. (See figure 7.) The 14 stringers were continuous for the length of the deck truss except for five expansion joints. Diaphragms (generally C sections except at expansion joints, where they were wide-flange sections) connected the webs of adjacent stringers to transfer lateral loads and maintain structural rigidity and geometry. The ends of these diaphragms were riveted or bolted to vertical plates, called stiffeners, which were welded to and projected from the stringer webs.

Bracing members

Floor trusses

East main trussWest main truss

Stringers

Interior structure of north portion of deck truss, looking north. (Source: URS Figure 7. Corporation)

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The bridge design documents and inspection records identified the 112 main truss nodes (29 upper and 27 lower nodes9 on each of the two main trusses) by number. Because the bridge was longitudinally symmetrical (the north and south halves were almost mirror images), the nodes were numbered from each end starting at 0 on the south end and at 0 (0 prime) on the north end. The node numbers increased from each end until reaching node 14, which was at the center of the bridge. Thus, n and n identified the corresponding nodes on the bridges south and north halves, respectively. The letters U (for upper chord) and L (for lower chord) further identified the nodes, along with E or W to specify the east or west main truss. For example, the U5E node refers to the sixth node from the south end of the bridge (because the node number begins at 0) on the upper chord of the east main truss. The U5E node refers to the corresponding node on the north half of the bridge. (See figure 8.)

These node numbers were also used to identify the connecting main truss structural members. For example, the upper chord member that connected the U7 node to the U8 node was designated U7/U8. The vertical member that connected the U8 node to the L8 node was designated U8/L8. The diagonal members were similarly designated. Thus, the two diagonal members that extended from the U8 node to the two nodes on the lower chord were L7/U8 and U8/L9. Again, E and W were used to designate the east or west main truss.

The upper chord of each transverse floor truss was supported on and connected to the upper chords of the main trusses at the like-numbered nodes. For example, the floor truss that connected the main truss U10E and U10W nodes was designated floor truss 10.

The I-35W bridge was designed and built before metal fatigue cracking in bridges was a well-understood phenomenon. In the late 1970s, when a better understanding of metal fatigue cracking was established within the industry, deck truss bridges such as the I-35W bridge were recognized as being non-load-path-redundantthat is, if certain main truss members (termed fracture-critical) failed, the bridge would collapse. According to Federal Highway Administration (FHWA) 2007 data, of the 600,000 bridges in the National Bridge Inventory, 19,273 are considered non-load-path-redundant. About 465 bridges within the inventory have a main span that is a steel deck truss.

Construction of Deck Truss The method and sequence of construction of the deck truss portion of

the bridge were specified in a series of engineering drawings. The span lengths required falsework (temporary support) below the trusses between piers 5 and 6 and between piers 7 and 8. Following erection of the main trusses between their south ends and pier 6 and their north ends and pier 7, the trusses were erected

9 The 0 and 0 nodes were upper nodes only.

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toward the midpoint of the center span, with each half cantilevered from piers 6 and 7. The main trusses were then joined in the center, and final adjustments were made in the vertical positioning at piers 5 and 8.

Study of Collapse Video

As noted earlier, a motion-activated video surveillance camera at the Lower St. Anthony Falls Lock and Dam, just southwest of the bridge center span, activated as the collapse began. The camera captured a total of about 10 seconds (23 separate images) of the collapse sequence and saved it to a digital video recorder. FBI agents retrieved the recorder and forwarded it to the Safety Boards vehicle recorder laboratory in Washington, D.C., for analysis.

The primary structure visible in the video is the west truss of span 7, the bridge center span that crossed the river. Portions of the east truss are also visible behind the west truss. The visible section of the lower chord of the west truss extends from the L11W node, which is partially visible, northward through the L9W node. The visible section of the upper chord extends from U11W northward through U8W. Some upper nodes north of U8W can also be seen, but they are generally too far away to be resolved clearly. Most of the west truss members are visible within this region. Most of pier 7 (on the north riverbank) can be seen, though a fence in the foreground obscures the west pier column. (See figure 9.)

L11W

U8 ‘W

Pier 7

L9 ‘W

U12W

View of a portion of the west side of I-35W bridge center span about 8 minutes before Figure 9. collapse. (Image extracted from surveillance camera video.)

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The video does not show any element of the west truss south of vertical member U11/L11W or any element of the east truss south of the L13E node. None of the lower nodes on the west truss north of L9W can be seen. Most of the east truss upper nodes that are visible to the camera are either too dark or blurred to be identified or are obscured by the west truss.

The video system activated at 18:04:57 central daylight time,10 just after initiation of the collapse. The first video image of the collapse (see figure 10) shows that the southern end of the visible structure of span 7 has dropped downward as compared to its position in precollapse recorded images. In the north portion of the span, a bend appears in lower chord members L9/L10W and L10/L11W.11 In the south portion, deck stringer 14, the westernmost stringer, has begun to separate from the deck at the U11W node. Over the next 3 seconds, the entire center span separates from the rest of the bridge structure and falls into the river. The video shows that during the collapse of this span, the bridge deck remained generally level east to west, but the north end of the span remained higher than the south end as the bridge section fell.

10 All times in this section reflect the U.S. Naval Observatory clock, offset to central daylight time. 11 This chord section was actually one continuous member between the L9W and L11W nodes that

passed through the L10W node.

Close-up of first image of collapse sequence, captured by surveillance camera at Figure 10. 18:04:57. The south (near) end of the center span can be seen to have dropped from its position as shown in precollapse images (compare with figure 9).

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Pier 7, on the northern bank of the river, appears to have remained vertical and stationary until after the collapse began. Following the collapse, survey measurements indicated that pier 7 was tilted approximately 9 to the south.

Damages

Approach Spans Although the primary damage occurred in the deck truss portion of the

I-35W bridge, the approach spans also sustained damage in areas where the cantilevered ends of the spans had been supported by the ends of the deck truss. The damage sustained was consistent with a loss of this support, which caused the ends of the approach spans to drop. Although the steel girders in the approach spans contained previously documented fatigue cracks, no evidence was found that these cracks affected the damage patterns to these spans.

Deck Truss Spans During the collapse sequence, the deck truss portion of the bridge separated

into three large sections. Most of the center span, between piers 6 and 7 (referred to here as the center section), separated from the remainder of the truss and fell into the river. The section south of pier 6 (the south section) fell onto land on the south side of the river, and the section north of pier 7 (the north section) fell onto land on the north side of the river. (See figure 11.) General damage to each of these sections is detailed below. The fractures in the members between these sections are discussed later in this report in the Examination of Deck Truss Fracture Areas.

Center section North section

South section

North fracture area

South fracture area

Collapsed deck truss sections of I-35W bridge.Figure 11.

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South Section. Figure 12 shows the collapsed south section of the deck truss. The deck and floor stringers north of the U8 node (directly over pier 6) separated at this node and remained attached to the center section of the structure, which fell into the river. The deck and stringers south of the U8 node remained approximately in their original positions relative to the upper chords of the truss. The U8 and L8 nodes, east and west (at pier 6), remained with the collapsed south section of the truss.

The L1 nodes were found north of pier 5, and the L8 nodes were found north of pier 6, indicating that, in general, the entire south section of the truss had displaced some amount north toward the river. Associated with this movement, the lower surface of lower chord members L7/L8 east and west contained impact and scraping damage from contact with the expansion rollers and upper surface of pier 6. Loading associated with this damage was severe enough that lower chord member L7/L8E was partially fractured at approximately mid-length. Additionally, member L7/L8E had broken away from the L7E node (through shearing of the rivets between the member side plates and the gusset plates) and

Pier 6 westPier 6 east

L8W

L9W

U8W

L9/U10E L9/U10W

U9W

U8E

Looking south at south fracture area and a portion of collapsed south section.Figure 12.

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the L8E node (through fracture of the member and shearing of the remaining rivets between the member side plates and the gusset plates).

The upper chords of the south section were intact between nodes 1 and 8. (The upper chord on both trusses was fractured between nodes 0 and 1.) The lower chord of the west truss was fractured between the L3W and L4W nodes, and the lower chord of the east truss was fractured adjacent to the L1 node. Also, as previously noted, each end of lower chord member L7/L8E separated from the L7 and L8E nodes, primarily through shearing of the gusset plate rivets.

In the postcollapse position, the portion of the south section between nodes 4 and 8 was toppled (laid over) to the east. Lower chord member L7/L8W remained on top of pier 6 west, while the L7E and L8E nodes struck the ground. Most of the main truss members and nodes in the south section contained compression or bending damage consistent with ground impact. Mating fracture areas in the lower chords were displaced, indicating continued translation to the north after the fractures were created in the lower chords.

The floor trusses from node 8 southward remained at least partially attached to the nodes on the east and west main trusses. The sway braces and lateral braces from nodes 08 showed no evidence of primary failure.

Center Section. See figure 13 for a photograph of the center span before collapse. In the postcollapse position, the center section of the truss was relatively flat and almost directly below its original location. (See figure 14.) As noted previously, the video of a portion of the collapse sequence captured by a nearby surveillance camera showed that the south end of the center section (just north of pier 6) dropped before the north end (just south of pier 7) and that the center section remained relatively level east to west as it fell. Many of the vehicles in the inner northbound lanes of the bridge remained in their lanes as the collapse occurred, indicating that the east and west main trusses at the south end of the center section fractured at about the same time and that the center section dropped into the river with a minimum of lateral roll.

The main truss upper chord members from nodes 1212 remained intact and above the water, with the floor trusses at these nodes at least partially attached and supporting the stringers and deck. The video recording showed no failure or deformation occurring in the lower chords between the L12E and L12E nodes and between the L11W and L12W nodes. The L11W node appeared to remain intact during the collapse. Three of the five truss members that meet at the L11W node are visible in the video; and these members appeared to maintain their orientation relative to one another, within the resolution of measurement, as span 7 collapsed.

The lower chord members, lower lateral braces, and sway braces from the center section were generally under water. The fractured L9 node ends of the lower chord members L9/L10 east and west were resting against the lock guide wall near pier 6.

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U10W

L8 W’

L13 W’ L13W L11W L9W

L8W L8E

West – Pier 6 – East

Pier 7

Center span of I-35W bridge (before collapse), looking northeast, with certain nodes Figure 13. labeled.

Collapsed bridge center section, looking southeast.Figure 14.

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North Section. An overall view of the north portion of the deck truss postcollapse is shown in figure 15. A large portion of the deck truss above and north of pier 7 had rotated to the north as a rigid unit. This portion included the deck and upper chords from the U8 to U6 nodes, the lower chords from the L9 to L7 nodes, and the diagonals and verticals between these nodes. In the postcollapse position, lower chord members L7/L8 east and west were contacting the north side of the upper end of the pier 7 columns. The video shows that the rigid portion of the deck truss initially remained in position and did not rotate as the center span dropped into the river. Later in the recording, after the water splash clears and the debris settles, this portion of the structure can be seen to have rotated to the north.

Between this rigid body portion and pier 8, the deck truss had collapsed almost straight downward, resulting in severe vertical compression damage. In this area, the upper chords and upper nodes generally were displaced 1825 feet to the north relative to the lower chord and lower nodes. The upper truss chords at nodes 2 were separated as a result of fractures of the gusset plates at these nodes.

No rth

Collapsed north section of bridge.Figure 15.

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The trusses, stringers, and deck at the U1 node (above pier 8) were bent down over the pier. The deck had fractured at this bend location, but the stringers were still mostly intact and had severe bending and lateral buckling. The L1 nodes were on the ground, resting against the north face of pier 8. The upper chord of floor truss 1 was on top of or on the north side of the pier, and the lower chord of the floor truss was on the south side of the pier. Nodes 0 east and west were on the north side of the pier. Based on the postcollapse position of floor truss 1 and the deck truss above pier 8, the truss collapsed across the pier without significant movement relative to the top of the pier. As the portion of the truss south of pier 8 dropped, the pier would have been pulled to the south, consistent with its cracking and tilt to the south.

Piers and Bearings Piers 5 and 6 were minimally damaged during the collapse, and postcollapse

survey measurements indicated that these piers exhibited no settlement or displacement. Piers 7 and 8 were both tilted about 9 south, toward the river, in their postcollapse positions. The video recording showed no evidence that pier 7 shifted before the initiation of the collapse, indicating that its movement occurred during the collapse. Excavation at these damaged piers showed that their tilted postcollapse positions occurred because of separations above the bases of the piers. Pier 7 (the fixed bearing location) hinged about the top of the pier footing, and the pier 8 columns hinged about a section approximately 3.5 feet above the top of the footings, at the termination of the footing dowel reinforcement. There was no evidence that the footings of these piers shifted.

The bearing rollers at piers 5 and 6 had come off the north side of the piers (with the exceptions of one roller that remained on pier 5 west and one roller that was found on the south side of pier 5 east). The wear patterns on the bearing sole plates indicated that the rollers had been moving annually by as much as 5 inches on pier 5 and 2.5 inches on pier 6, which was consistent with the design.

The wear patterns on the bearings at pier 8 showed evidence of normal movement over a distance of 2.5 inches, similar to the amount of movement for the rollers in pier 6. The bearing rollers at pier 8 had come off the south side of the pier (with the exception of one roller that was found on the north side of pier 8 west). The roller wear marks on the bearing plates at piers 5, 6, and 8 were approximately in the center of the plates, indicating that there was no significant longitudinal movement of the piers before the collapse.

Recovery of Structure

Initial removal of the collapsed truss structure from the accident site took place under the direction of the Hennepin County Sheriffs Office as part of the search for accident victims. Because finding and identifying victims had a higher

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priority than preserving evidence, some postaccident damage was done to the bridge structural components as they were removed from the scene. For example, during the initial phase of removal and before documentation of the preaccident locations of the deck stringers, the reinforced concrete deck and the stringers were cut or pulled apart with a shear-equipped backhoe, which caused additional damage to the stringers. The longitudinal locations of some of these stringers could be determined by the locations of shear studs along the top flange of the stringer ends. Also during this phase of the recovery, some larger members of the main truss were torch cut to permit barge access for victim recovery; these cuts were made under the guidance of Safety Board investigators.

Following this initial removal phase and in situ inspections and documentation, the removal of bridge components was directed by Mn/DOT, with the concurrence of Safety Board investigators. Individual pieces were assigned a salvage number and noted with the identification of the structural member, postaccident location, and date of removal.

Because much of the bridge center span collapsed into the river, its structural components were inaccessible for detailed inspection until after their removal from the site. During the earliest stages of recovery, some of these members were cut with a shear and pulled and twisted in an attempt to remove them. Later, less aggressive methods were employed, but some structural members were lifted out of the water while still connected to other members, and additional deformation may have occurred.

The recovered truss portions of the bridge were moved to Bohemian Flats Park, just downriver and east of the accident site, where the components were laid out in their relative original positions in the structure and subjected to detailed Safety Board examination. The results of those examinations are discussed later in this report.

Bridge Renovations and Modifications

Between the time of its opening in 1967 and the accident, the I-35W bridge underwent three major renovation/modification projects, two of which increased the dead load12 on the structure.13 The following sections provide details of those projects.

12 Dead load refers to the static load imposed by the weight of materials that make up the bridge structure itself.

13 As part of another project in 1999, portions of the deck truss were painted, and coverings (bird screens) were installed over the openings in the box members.

Factual Information

National Transportation Safety Board

H I G H W A Y Accident Report

1977 Renovation: Increased Deck Thickness The 1977 construction plan for renovation of the I-35W bridge involved

milling the bridge deck surface to a depth of 1/4 inch and adding a wearing course of 2 inches of low-slump concrete.14

According to Mn/DOT representatives, the bridge was initially constructed with 1.5 inches of concrete cover over the uncoated top reinforcing bars (rebar) in the bridge deck. By the early 1970s, States with a similar thickness of covering over rebar and with severe operating environments, like Minnesota, were experiencing rebar corrosion. To combat this problem, Mn/DOT adopted a policy of using an additional layer of concrete overlay to increase to 3 inches the cover over the upper layer of deck rebar. According to Mn/DOT officials, this additional cover reduced the likelihood that harsh road chemicals could reach and react with the steel rebar, thus substantially extending the life of bridge decks. Based on measurements after the accident, this project increased the average deck thickness to approximately 8.7 inches. The additional concrete applied as part of this project increased the dead load of the bridge by more than 3 million pounds, or 13.4 percent.

1998 Renovation: Median Barrier, Traffic Railings, and Anti-Icing System Work done on the I-35W bridge in 1998 involved replacing the median

barrier, upgrading outside concrete traffic railings, improving drainage, repairing the concrete slab and piers, retrofitting cross girders, replacing bolts, and installing an anti-icing system.

Mn/DOT representatives stated that several features of the bridge, including the median barrier and outside traffic railings, did not meet then-current (1998) safety standards. Also, the original median barrier and traffic railings were deteriorating from corrosion and traffic impact. The permanent changes to the median barrier and railings increased the dead load on the bridge by about 1.13 million pounds, or 6.1 percent. During the 1998 construction project, temporary barriers were erected in both the northbound and southbound lanes to protect workers in the median and to protect the new median barrier while it was curing. The temporary barriers were then moved to outside lanes to protect workers while the outside traffic railings were upgraded. The two rows of temporary barriers weighed 1.60 million pounds (840 pounds per foot), which was evenly distributed along the 1,907-foot length of the bridge.

The I-35W bridge was a candidate for installation of an anti-icing system because of the high incidence of winter traffic accidents on the bridge. The anti-icing system worked with a combination of sensors, a computerized control system, and a series of 38 valve units and 76 spray nozzles that applied potassium acetate to the roadway.

14 Slump refers to the consistency of the concrete and is measured in inches. Concrete with a lower ratio of water in the mix will have less slump than concrete with a high water content. According to Mn/DOT, the acceptable slump for a (low-slump) concrete overlay is 3/4 inch versus about 4 inches for other parts of a bridge deck.

Factual Information

National Transportation Safety Board

H I G H W A Y Accident Report

2007 Repair and Renovation: Repaving Roadway work was underway on the I-35W bridge at the time of the

collapse. This workwhich was being performed by Progressive Contractors, Inc. (PCI), of St. Michael, Minnesotainvolved removing the concrete wearing course to a depth of 2 inches and adding a new 2-inch-thick concrete overlay. The construction plan also called for removing unsound concrete from the curb and patching it with concrete, reconstructing the expansion joints, and removing and replacing the anti-icing system spray disks and sensors in the deck. The project began in June 2007 and had a scheduled substantial completion date of September 21, 2007, with final completion expected by October 26, 2007. PCI had completed seven pavement section overlays and was preparing for the eighth section overlay pour when the bridge collapsed.

At the time of the bridge collapse, four of the eight travel lanes (the two outside lanes northbound and the two inside lanes southbound) were closed to traffic. The preexisting wearing surface was still in place on the two inside lanes northbound, where the average deck thickness was 8.7 inches. The new overlay was already in place on the two outside northbound lanes (average deck thickness of 8.8 inches) and the two outside southbound lanes (average deck thickness of 8.9 inches). The surface of the two inside southbound lanes had been milled for the entire length of the bridge, removing about 2 inches of material. The weight of the concrete that had been milled from the two inside southbound lanes in preparation for the overlay was calculated by the FHWA to be 585,000 pounds, of which about 250,000 pounds would have come from the bridge center span.

Staging of Construction Materials on Bridge Deck

Of the seven overlays that PCI had completed on the I-35W bridge before the collapse, five involved staging construction materials, primarily aggregates, on the bridge ramps; one involved placing materials on the bridge deck; and one involved staging materials on both the ramps and the deck.

The staging of aggregates and other construction materials on a bridge is sometimes considered necessary to allow timely delivery of concrete to the job site. For example, because the low-slump concrete used for the overlay has a lower water and higher cement content than typical concrete, it sets up very quickly after mixing. Minnesota specifications require that all concrete overlays be mixed at the job site, with a 1-hour window from initial mixing to final concrete screeding, or setting of the final grade, and a 15-minute window from initial deposit of concrete on the deck to final screeding. Because of the requirements for quick placement of concrete and the relatively low volumes of concrete needed for overlay pours, the use of ready-mix concrete trucks is not practical; and, according to Mn/DOT, ready-mix concrete has not been used on State bridges. In the case of the I-35W bridge, storing materials off the bridge deck and thus farther from the job site would

Factual Information

National Transportation Safety Board

H I G H W A Y Accident Report

have required that small motorized buggies be used to transport the mixed concrete several hundred feet and might have required closing additional traffic lanes.15

The first time PCI staged construction materials on the I-35W bridge deck (though not on the deck truss portion of the bridge) was on the night of July 6, 2007. A portion of the materials for a projected 750-foot overlay on the outside southbound lanes from pier 6 northward toward pier 8 was staged on the bridge deck, centered approximately over the midpoint between north approach spans 10 and 11. This staging of equipment and aggregates was similar to the arrangement on the day of the accident except for two additional loads of sand and two additional loads of gravel that were off the bridge.

According to PCI, vehicles and aggregates were staged on the deck truss of the bridge for a pour that began on July 23, 2007. The overlay was a planned 589-foot pour on the northbound lanes extending from near the midpoint of span 8 just south of pier 8 (approximately at node 4) to the expansion joint at the end of span 7 (approximately at node 8). The staging area for this pour was about 183 feet long and extended from node 4 to the north end of the deck truss. The staging area had three 24-ton loads of gravel and three 24-ton loads of sand. PCI representatives told the Safety Board that a Mn/DOT bridge construction inspector was on scene for each pour.

According to Mn/DOT, bridge construction inspectors are assigned to a project to ensure that the contractor fulfills its responsibilities and that all materials used meet required standards. On the I-35W bridge project, this would include checking to see that the concrete met Mn/DOT specifications. Bridge construction inspectors are not engineers, nor are they trained in bridge inspections. In a postaccident interview, the PCI job foreman (who had worked for the company for 21 years and had been on the I-35W bridge project for 3 weeks) told the Safety Board that he had asked a Mn/DOT bridge construction inspector if materials could be staged on the bridge for the July 23 pour in the northbound lanes. He said the inspector evidenced no concern about the staging, which the job foreman interpreted as permission. The foreman said the reason he asked was because of the time and labor that would be required to move materials and clean the area after delivery. He did not indicate that he considered the weight of the materials to be an issue.

On the afternoon of August 1, 2007, PCI was preparing to pour a 530-foot overlay in the southbound inside lanes. The pour, which was not to begin until 7:00 p.m. because of the days high temperature, would extend between node 14 near the center of the deck truss northward to node 0. The PCI job foreman calculated and ordered the materials that would be needed for the job. The order included four end-dumps of sand, four end-dumps of gravel, and three cement tankers. Two of the cement tankers were fully loaded 80,000-pound vehicles

15 Mn/DOT representatives told the Safety Board that during the repaving of three other State bridges similar in length to the I-35W bridge, the concrete overlay material was mixed off the bridge in the and transported to the job site in buggies.

Factual Information

National Transportation Safety Board

H I G H W A Y Accident Report

that would be positioned off the deck. One tanker was of legal weight (less than 80,000 pounds) and would be staged on the deck.

The aggregates and associated construction vehicles would come to occupy an estimated 228-foot-long section of the two closed southbound lanes over the main river span (span 7). The area where the equipment and materials were stored was established by interviewing witnesses, by examining the limits of the work area, and by examining photographs, including one (figure 1 top) taken about 3:45 p.m. by a passenger aboard a commercial airliner. After this photograph was taken (see figure 16), PCI workers moved the materials closer to the median barrier to allow more room for the movement of construction vehicles and traffic.

The construction aggregates were distributed in eight adjacent piles (four sand and four gravel) placed along the median in the leftmost southbound lane just north of pier 6. The combined aggregates occupied a space about 115 feet long and 1216 feet wide, with its southern boundary about 10 feet north of pier 6. (See figure 17.) This staging placed the aggregate piles generally centered longitudinally over the deck truss U10 nodes. Along with the aggregates in this area were a water tanker truck with 3,000 gallons of water, a cement tanker, a concrete mixer, one small loader/excavator, and four self-propelled walk-behind or ride-along buggies for moving smaller amounts of materials.

Pier 6

Construction equipment and aggregates (indicated by white box) stockpiled on Figure 16. southbound lanes of bridge about 2 hours 15 minutes before collapse. (This figure is an enlargement of a section of a photograph taken by a passenger in a commercial airliner departing Minneapolis/ St. Paul International Airport.)

Factual Information

National Transportation Safety Board

H I G H W A Y Accident Report

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Factual Information

National Transportation Safety Board

H I G H W A Y Accident Report

Bridge Traffic and Construction Loading at Time of Collapse

The load on the I-35W bridge at the time of the collapse included vehicular traffic as well as temporary loads associated with the construction work on the bridge. The documented delivered weights for the aggregates staged on the bridge were 184,380 pounds of gravel and 198,820 pounds of sand, for a total weight of 383,200 pounds. The estimated weight of the parked construction vehicles, equipment, and personnel in this area was 195,535 pounds, for a total estimated weight of 578,735 pounds positioned over the inner west side of the bridge center span just north of pier 6.

Based on postaccident vehicle positions, photographs, and witness statements, investigators were able to determine the types and general positions of the 111 vehicles (including construction vehicles) on the bridge at the time of the collapse. The positions of the 17 construction workers on the bridge were also documented.

The vehicles that were on the bridge were weighed either during removal from the accident site or later at an impound lot. These weights were adjusted for occupants and cargo and, in some cases, for water and debris. Weights were also estimated for the various pieces of construction and related equipment and for construction personnel. These weights were totaled to arrive at the approximate traffic and construction loads shown in table 2 for various points along the bridge at the time of the collapse.

Summary of approximate loads along I-35W bridge at time of collapse.Table 2.

Lanes

South of pier 6 (pounds)

Center span between piers 6 and 7 (pounds)

North of pier 7 (pounds)

Total (pounds)

Open southbound 112,200 64,650 98,050 274,900

Closed southbound 41,900 578,735 91,691 712,326

Open northbound 66,300 57,100 44,950 168,350

Closed northbound 104,750 0 0 104,750

Total 325,150 700,485 234,691 1,260,326

Mn/DOT Policies Regarding Construction Loads on Bridges

Mn/DOT representatives told the Safety Board that any questions regarding the stockpiling of materials on the bridge should have been formally directed to the project engineer (in writing) rather than orally directed to a construction inspector. Mn/DOTs Standard Specifications for Construction 1509 and 1510, which are part of the contract documents for every project, address the respective authorities and duties of the project engineer (1509) and construction inspectors (1510). Specification 1509 stated:

The Project Engineer is the engineer with: (1) Immediate charge of the engineering details of the construction Project. (2) Responsibility for the

Factual Information

National Transportation Safety Board

H I G H W A Y Accident Report

administration and satisfactory completion of the Project. (3) Authority commensurate with the duties delegated to the Engineer. (4) Authority to reject defective material and to suspend any work that is being improperly performed.

Specification 1510 stated:

Inspectors employed by the Department [Mn/DOT] will be authorized to inspect all work done and materials furnished. The inspectors will not be authorized to alter or waive the provisions of the Contract, to issue instructions contrary to the Contract, or to act for the Contractor.

As a representative of the Engineer, the inspector will report progress and acceptability of the work being performed, and will call to the attention of the Contractor any failures and infringements on the part of the Contractor. Should any dispute arise as to the materials or work performance, the inspector may reject materials and suspend operations until the question at issue can be referred to and be decided by the Engineer.

The Mn/DOT project engineer for the ongoing work on the I-35W bridge stated that he typically made three or four trips per week to the bridge and relied heavily on the experience of the Mn/DOT project construction supervisor to bring potential problems to his attention.

The project construction supervisor had worked for Mn/DOT for more than 20 years and had been the inspector for one previous large bridge overlay job. He said that on one occasion during the I-35W bridge project, the contractor inquired about using the pavement planing/milling machine to go below the standard 2-inch depth to remove larger sections of unsound concrete. The construction supervisor referred the question to the Mn/DOT Bridge Office, whichafter conducting an experiment at another locationdetermined that the milling should not exceed 2 inches because of concerns that the vibration might damage the structure.

The construction supervisor told Safety Board investigators that he had had many contractors approach him about placing heavy machinery on bridges over the years but that he had never been asked about placing heavy piles of aggregates. He said, however, that most of the more than 60 projects he had worked on involved shorter bridges where there would have been no need to store materials on the bridge itself. He said that he was not at work on August 1 and was not aware of the stockpiling of materials. Asked whether he would have objected to the stockpiling had he known about it, he said:

Im not sure. I would have had to look at the loads. My best guess is it would have been a 50-50 chance that I might have done something, and that is only because of my close working relationship with the engineers in the bridge office and my many years of experience.

Factual Information

National Transportation Safety Board

H I G H W A Y Accident Report

A Mn/DOT bridge construction inspector was on scene when the materials were delivered and placed on the bridge and was standing near the stockpiled materials when the bridge collapsed.

According to Mn/DOT, the agencys policies at the time of the accident did not specifically address the placement of construction aggregates on bridges. Mn/DOT told the Safety Board:

The contractor can request to place larger than legal loads on a new or remodeled bridge with Mn/DOT Construction Project Engineers approval. Although not a written policy, when a contractor proposes a load that exceeds legal loads, it is a practice for the Mn/DOT Construction Project Engineer to consult with the Regional Construction Engineer in the Bridge Office. The construction loading information is provided to the Load Rating Unit or Design Unit for evaluation to determine if the loading is acceptable or if any special procedures such as use of the load distribution mats are required. Some examples of loads that exceed legal loads are mobile cranes or heavy earth moving equipment.

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