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URS Fatigue Evaluation and Redundancy Analysis 6/21/06

Bridge 9040 I35 over Mississippi River

Summary of Draft Report Executive Summary

Objectives:

Identify critical superstructure members most susceptible to cracking

Evaluate Structural consequences if one of the critical members should sever in terms of loadredistribution and load capacities of remaining members

Develop contingency repairs to selected members

Establish measures for improving structural redundancy and minimizing tensile stresses in the

deck and develop a preferred deck replacement staging plan.

Load Capacity.

Under factored design loads several members of the truss are can be overstressed by as much as 77%.

But, under actual in service legal loads, no member stressed beyond its ultimate capacity.

And, no sign of over stress that has been reported in the bridges 40 year history.

The bridge doesnt have the margin of safety a new bridge would have, but we are not overstressing thebridge under current loadings.

Fatigue Analysis

Bridge was built in 1967 prior to AASHTO Fatigue Design requirements.

Also, it was built prior to Charpy material toughness requirements which help control how fast cracks

grow, particularly in cold temperatures.

Analysis shows truss members have an infinite fatigue life. The probability of fatigue crack development

for category D details is very remote.

However, fatigue concerns cannot be completely discounted :

Access to fatigue susceptible details inside the truss sections is very limited for inspection at the

weld toes where cracks would typically initiate. Therefore a timely discovery of a crack is

unlikely should one happen to occur as a result of some unusual unanalyzed cause. There may be some fabrication errors or changes that may be more critical than those analyzed

according to plan dimensions.

The fatigue truck used to predict fatigue life may not adequately represent the heavy truck traffic

on this bridge.

Redundancy Analysis

Only eight members were analyzed to determine if total failure would cause collapse of the bridge.

Selection of the eight was based on:

- Subject to tension under dead and live loads- Fatigue susceptible welded details at interior diaphragms

- Among members susceptible to the highest fatigue load stress range

5 of the 8 members analyzed are fracture critical. They represent 20 different locations on the bridge.

There is a chance there may be other members not identified

Recommendations:

Retrofit the 20 fracture critical members

Before the retrofit, the fatigue susceptible details at the internal diaphragms inside the identified

fracture critical truss tension cords should be inspected with the access hole cover plates removed

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Replace the deck throughout the main truss span with a continuous, composite and light weight

deck

Discussion Item:

If there is a delay in retrofit of the most critical members, we should develop a critical response plan for the

bridge should cracks be discovered in critical truss members. The plan should include consideration of

how to detour traffic completely off the bridge, and under more favorable circumstances, onto one side ofthe bridge. It should also identify temporary repair methods and materials, and estimate material

availability and repair time.