Page 139 - Bridge and Highway Structure Rehabilitation and Repair

114 SECTION 1 ADMINISTRATIVE ISSUES

Roadway construction was being conducted on the deck truss portion of the bridge when
the bridge collapsed. Machinery and paving materials were being parked and stockpiled on
the center span.
5. The Safety Board is concerned that, for at least the I-35W bridge:
• The bridge was designed with gusset plates that were undersized.
• The design error was not detected when plans were created. Because of this design er-
ror, the riveted gusset plates became the weakest member of this fracture-critical bridge.
Normally due to low cost, gusset plates are expected to be stronger than the beams they
connect.
• The methods used in calculating load ratings and the inspections conducted through the
National Bridge Inspection Standards (NBIS) program are not expected to uncover original
mistakes in gusset plate designs or calculations.
6. Because of this accident, the Safety Board cannot dismiss the possibility that other steel
truss bridges with non-redundant load paths may have similar undetected design errors. Con-
sequently, before any future major modifications or operational changes are contemplated,

owners should ensure that the original design calculations for this type of bridge have been
made correctly.
7. The National Transportation Safety Board makes the following recommendation to the
Federal Highway Administration: All non-load-path-redundant steel truss bridges within the
national bridge inventory require that owners conduct load capacity calculations to verify that
the stress levels in all structural elements, including gusset plates, remain within applicable
requirements whenever planned modifications or operational changes may signifi cantly

increase stresses.

3.18 A POSTMORTEM OF FAILURES
3.18.1 The Need for Applying Safety Engineering
1. Developments in safety engineering require that engineers should be trained in the basic
concepts of identifying and controlling hazards. It involves recognition, diagnosis, and
implementation of a control selected from one or more options.
2. Murphy’s Law seems to apply to bridges as well, especially when the number of bridges
runs into the millions. For some bridges, whatever can possibly go wrong will.
An engineer’s goal is to prevent hazards and prevent the fulfillment of Murphy’s Law.

In the planning, design, construction, and maintenance of bridges there are many single
and chains of activities that can contribute to a disaster. In the numerous decision making
tasks required in planning and design, engineers may inadvertently create hazards at sites,
in equipment, in operation, or by neglecting the environment.
Such activities are related to quality of materials, products, equipment, processes and the
environment. Any error in calculation will be in the direction of maximum harm. Where
hindsight is missing, the risk of failure is increased.
3. A hazard may be defi ned as a condition or circumstance that can lead to adverse or harmful
consequences. Hazards are seldom created deliberately, but are usually created unintention-
ally or inadvertently. Sources of hazards are:
• Human limitations
• Errors in judgment
• Poor assumptions
• Pressure to meet commitments or unrealistic schedules

• Over ambitious incentives and penalties offered to the contractor to finish the job
• Poor communication