Thoughts on AASHTO Fatigue Loading

[opko]

I'm designing a transverse floorbeam for a 6 lane typical section bridge. The floorbeam supports longitudinal stringers which in turn support the concrete deck. The floorbeam is supported at it's 1/3 points thus creating cantilevered ends. The maximum positve stress at mid-floorbeam is caused by the two interior lanes loaded and the maximum negative stress at mid-floorbeam is caused by the two exterior lanes loaded. AASHTO allows a 25% lane load eduction when 4 or more lanes produce maximum stresses. Does this apply here even though the stress range is caused by 4 consecutive, rather than simultaneous lanes loaded? Bottom line, due to the odd geometry, the beam is about 100% overdesigned for strength, but exceeds the allowble fatigue range.

 

[mike80]

Opko,

I don't believe you can reduce the loadings by 25% unless you are loading four lanes simultaneously to produce the maximum stress. AASHTO 3.12.1 indicates the allowable reduction is due to the fact it is unlikely to have loads in position to produce maximum stresses in all four lanes simultaneously. In your case, the maximum load in the cantilever is actually produced by loading only one or two lanes, which does not have a reduction factor.

Some states do allow for a smaller distribution factor when considering fatigue compared to stress calculation.

mike