Design Section for Negative Moment

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Design Section for Negative Moment

I have a question regarding the critical design section for negative moment at interior piers of simple made continuous structures.

The first location that PGSuper will report negative moment capacity & demand for is at CL bearing. I understand that some engineers have traditionally taken the face of diaphragm as the design section for negative moment.

I have not seen guidance in AASHTO as to where the design section should be taken. Can you explain why PGSuper currently reports values at the sections that it does, and maybe some of the thought process and assumptions that were involved?

Thanks in advance.

Rick Brice
PGSuper evaluates flexural

PGSuper evaluates flexural strength requirements for negative moment at the same sections it uses for positive moment. These sections include the 10th point between CL-Bearings and at any location where either demand and capacity change. These points include locations of externally applied loads, harping points, diaphragm locations, section changes, reinforcement changes, and development length locations.

As described in the PGSuper Theoretical Manual in the Structural Analysis Models section, the length of each span is taken to be the CL-Bearing to CL-Bearing length. At intermediate piers, the CL-Bearing on both sides of the pier are taken to be at the same location. The resulting analysis model is therefore just a bit shorter than the CL-Pier to CL-Pier length of the span.

All sections are taken to be design sections and the controlling section is the one that has the minimum capacity to demand (C/D) ratio. C/D ratios are listed in the specification checks. The negative moment with greatest magnitude will be at the CL-Bearing location given the way PGSuper performs the structural analysis.

Sorry for taking so long to get back to you. I’ve been swamped with other duties lately. Just as a friendly reminder to everyone on the peer-to-peer support list, open source software works best when everyone contributes. If you know the answer to a question, please post it. If immediate support is needed, our gracious host BridgeSight offers per-incident and ongoing support options.

Best regards,

Richard Brice, PE
Software Applications Engineer
WSDOT Bridge and Structures Office

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