Summary of Released MnDOT I-35W Bridge Documents: An Ugly Read

by Staff
Published: August 9,2007
Time posted: 1:00 am
Tags: Bridge Collapse, Document, Download, I-35W Bridge, Minneapolis, MnDOT, PDF, Transportation, University of Minnesota, URS

MnDOT has released five documents on its website relating specifically to the I-35W bridge (bridge number 9340) which we have
packaged
into a zip file (35 MB). The
files
include an outside consultant review, a
University of Minnesota Civil
Engineering field report, two brief status
summary documents, and, most
troubling, a MnDOT “Fracture Critical”
engineering summary which reveals in
candid descriptions and shocking photographs
the deterioration of many critical
bridge elements. Here’s my
summary of the relevant
parts of the documents, which we posted in our special Bridge edition of the Weekly Report last Friday:

A 299-page draft report prepared for MnDOT by
the URS Corporation of Minneapolis
entitled, “Fatigue Evaluation and Redundancy
Analysis” for Bridge No. 9340,
released July 2006.

  • It recommended “steel plating of all 52
    fracture critical truss members” via
    “additional plates bolted to the existing
    webs” as the best retrofit option.
  • The bridge was designed in accordance
    with the 1961 AASHO
    Standard
    Specifications for Highway
    Bridges     - a completely different
    fatigue design method that was
    revamped in the 1974 interim edition.
    “The poor fatigue details on the truss
    spans, particularly those inside the main
    truss tension chords that are
    difficult to inspect, have raised
    concerns on the the consequence of a
    possible main truss member failure
    triggered by a fatigue crack.” The 1961
    AASHO specs don’t account for “bending
    moments” on truss members; only
    “axial force” is accounted for.
  • URS developed a 3D model to simulate
    stress on the bridge. Pages after 169
    contain bridge failure simulations.
  • Page 171-174 have eerie wireframe
    simulations of bridge collapse:
    computer-generated ghosts of future
    events. Page 174 shows middle span
    failure resembling the collapse video
    released yesterday.
  • Page 176 indicated a single upper chord
    main truss member collapse would
    cause more than a dozen floor trusses to
    fail under eight different
    simulated load conditions
  • Page 231 has a drawing of major internal
    collapse of center simulated from a
    single truss chord failure
  • Page 236 explores retrofit schemes: “Due
    to the nearly double symmetry of
    the structure, these eight members
    actually represent thirty-two truss
    members on the bridge.” Carbon-fiber
    reinforced polymer plating (CFRP),
    which “would be expected to take over the
    member forces” but would be
    difficult to apply and “determined
    inappropriate” as a potential patch.
    Pre-tensioned bars are deemed
    inappropriate due to non-satisfactory
    performance
  • Page 265 considers high-performance steel
    plating as the most suitable.
    Design loads were determined from the 3D
    computer model, and new plates
    would have to be installed extremely
    carefully to avoid increasing stresses.
  • Page 267 says replacing the deck would
    provide better strength by more
    widely distributing the load.
  • Page 299: in the conclusion someone
    underlined the passage: “it is more
    desirable to keep this symmetrical
    loading condition during deck replacement
    as much as possible,” meaning that load
    symmetry across both sides should be
    maintained while work is underway, since
    removing either the northbound or
    southbound decks alone would introduce
    even more stress to the structure.

The brief MnDOT Structure Inventory Report (3
pages) for the last inspection on
May 15, 2006, (which was generated and posted
by MnDOT on August 2, 2007)
indicates the deck was 6% unsound. Condition
codes were: Deck: 5;
Superstructure: 4; Substructure: 6; Channel:
7. Appraisal ratings were:
structure evaluation: 4; Deck Geometry: 4;
Underclearances: 7; Waterway
Adequacy: 9; Approach Alignment: 8. The
in-depth inspection for “fracture
critical” status was on a 48-month cycle
apparently last performed May 2003.
Underwater inspections were on a 60-month
cycle last performed December 2004.
Bonus: 15% of the paint was unsound, it was
painted in 1968, the primer type was
“LEAD,” and the finish was “OTHER (UNKNOWN),”
though the more detailed Fracture
Critical report has more information about
repainting.

The Bridge Inspection Report (4 pages) has
all the noted inspection issues going
back to the 1970s. Current highlights include
Element 412: “Relief joints need
re-sealing”; Element 131: “Main truss members
have numerous poor weld details
(some cracked tack welds)”; Element 984:
Drainage: “Pier 6: horizontal drain
trough has inadequate slope (usually
clogged).”

The March 2001 University of Minnesota
Fatigue Evaluation of the Deck Truss of
Bridge 9340 by Heather M.
O’Connell,
Robert J.
Dexter, P.E., and
Paul M.
Bergson, P.E., conducted by the
Department of Civil Engineering, used strain
gages to measure the effects of
load stress upon the bridge members.

  • Researchers installed “on main trusses
    and floor truss to measure the live
    load stress ranges.” While trucks of
    known weight crossed the bridge,
    researchers measured the stress gages
    then developed computer models to
    calculate stress ranges throughout the
    deck truss.
  • The Abstract notes “the bridge’s deck has
    not experienced fatigue cracking
    but it has many poor fatigue details on
    the main truss and floor truss
    system,” and they concluded that “fatigue
    cracking of the deck truss is not
    likely.” The project also “verified that
    the use of strain gages at key
    locations combined with detailed analysis
    help predict the bridge’s
    behavior.”
  • Pages 54-57 have interesting charts of
    stress measurements
  • After page 68, simulation data renders
    results impressively similar to their
    field stress tests. That’s some quality
    computer modeling.

Finally, the most grim
50-page report
came from MnDOT engineers in June 2006.
Entitled “Fracture Critical Bridge
Inspection: In Depth Report” for “Bridge
#9340 (Squirt Bridge)”. MnDOT engineers
reported a profusion of “pack rust,” which is
essentially the
formation
of rusting cavities between steel
members. The executive summary is grim:

If bridge replacement is
significantly delayed, the bridge should be
re-decked. The design of the main river spans
do not allow for deck widening.
Any re-decking contract should also include a
complete re-painting of the
superstructure, elimination of the hinge
joint in span #2, and reconfiguration
of the deck drainage system.

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