Double curvature in excess camber

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Double curvature in excess camber

The excess camber in my exterior girders is showing double curvature (the max. excess camber occurs near the 1/3 point of the girder, than falls again toward the center). This only occurs in the exterior girders, and I have never seen it before. Is it possible that this is real, or is it an error? The analysis file and a screenshot of the camber details are attached.

Rick Brice
double curvature in excess camber

I can't spend too much time of this because of WSDOT's support policy. However, it is an interesting question so I did look into what is going on.

The exterior girders are more stiff than the interior girders and thus have less initial camber. The deflections due to the traffic barrier, overlay, and user defined loads over come the initial plus creep deflections at mid-span of the exterior girders. This results in the mid-span deflection that is less than that at the 1/3 points. If you look at deflections in the analysis results view, you'll see the typical parabola which begins to flatten out and then "sag" in the middle as dead load is added.

Is this real? Maybe, maybe not. Remember that PGSuper is doing a simple 2D line girder analysis. There may be load sharing between adjacent girders that is not accounted for in your model. For example, you have selected "Sufficient to make girders act as a unit" for the girder connectivity, which yields a better distribution of the live load. However, you are only distributing the traffic barrier load to the two exterior girders. Perhaps the traffic barrier load distributes to more than two girders, reducing the deflection, and resulting in an excess camber that are more familiar.

I've spot checked the computations and they appear to be correct. I'll leave it to you to do a complete review of the analysis results.

Mike, I agree with Rick B's

I agree with Rick B's assessment here. The program is computing results correctly base in your input. Basically what is going on is the rate of change in the final-stage upward creep3 deflection is larger in the mid span area than that for dead loads. Hence, causing the double curvature. Here are some factors contributing to this:
- The model has a very heavy barrier
- The barrier weight is distributed to only two girders causing the late stage creep to be large.
- The final summed excess camber at mid-span is relatively small. This means that small differences in component deflections seem amplified in the final summation.

If the inflection seems errant to you. Here are some things you can try to alleviate it:
- Change to project criteria to distribute the barrier load to 3 or more girders.
- Try changing the creep duration to a smaller value


Richard Pickings, P.E.
BridgeSight Inc.

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