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LRFD vs USD

LRFD vs USD

LRFD vs USD

(OP)
Greetings:

We are currently working with a contractor on the construction of a new wastewater treatment plant facility where we provide consultation, review and certification of shop drawings and provide professional engineering services for the project. One of the current shop drawings we are preparing is for a grated walkway above the influent channel and some other open channels.

The beams are ASTM 992 and most are W4x13. The CM has asked for computations to show the flexural capacity of these beams and herein lies the question.

The contractor has asked us to base the computations on USD to achieve the highest load combinations possible, while the CM requires the computations to be based on AISC 13 (LRFD/ASD).

The fact that this was already designed but that the contractor now has to "confirm" the design with separate computations is a story for another day.

My question is: Since the beams are there simply to support the fiberglass grate walkway and they are not structural, should not we be allowed to select the analytical and design method which we feel best addresses the situation? The unfactored loads are 124#/ft DL and 800 #/ft LL. The beams rest on the edges of the channels and are not structurally connected or secured.

Our chief designer says the contractor is correct since the walkway beams are non-structural pieces, but the CM says LRFD is a code requirement even for non-structural components.

No one in the office is familiar with anything in IBC or ASCE-SEI to confirm either position.

Have any of you ever encountered a similar issue, or can direct us to a reference?

Thanks.

RE: LRFD vs USD

What was spec'd? Either work. Unless spec'd or required by code, it should not be an issue.

Dik

RE: LRFD vs USD

Your post says "provide consultation, review and certification of shop drawings" and then says "current shop drawings we are preparing". These are not compatible.

What is your role, Contractor, Engineer, or equipment supplier?

Have you read the Contract? The Contractor typically spells out the responsibilities of the parties and the design requirements. If you are the Engineer, you don't want to assume responsibility for the equipment supplier's design unless you are being paid to do it.

Without knowing what is in the Contract, it is hard to propose a solution.

RE: LRFD vs USD

First, if the beams support the walkway, how are they not structural?

Second, why is the contractor dictating the method of analysis? If you are in the US, please cite the code requirement that requires LRFD be used.

Third, in all but the rare case, if the beam is ok by ASD, then it should work if designed by LRFD.

Just for clarification, LRFD is synonymous with USD (ultimate strength design). LRFD and ASD are the methods that differ.

" the highest load combinations possible "........what does that mean?

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