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Component loads obtained from a model that includes friction

Component loads obtained from a model that includes friction

Component loads obtained from a model that includes friction

(OP)
Hi all,
Hope somebody can help me out here.
We normally model large diameter mains using CAESAR, mainly as a flexibility analysis, support movement check, expansion joint movements etc, as well as providing component loads for structural designers. The Structural engineers need the component loads W, P, T, W1, U1 etc in order to apply their own' factors' to design the support structure beneath. In order to extract the correct component loads, the analysis has to be linear. So we run operating cases and see where supports want to lift and we remove those supports from the analysis and re-run, until all supports are sitting down during operation. Hence we have a linear analysis with a consistent set of supports.
However, and I think I am answering my own question here, if friction is introduced, this then becomes a non-linear analysis, and the component loads are no longer valid? I noticed that combining the component loads manually gave a different answer to the same operating loads i.e. Adding individual loads from W, P1 and T1, did not result in the same value as the loads from the load case W+P1+T1 (OPE).
Is there any way to extract the correct component loads from a model with friction?
My guess is that it isn't but thought I'd ask the experts first, before I go into print.

RE: Component loads obtained from a model that includes friction

Indeed your conclusion is correct. Friction does make a flexibility analysis non-linear, and as a consequence superposition does not hold. This is why a load case with W+P1+T1+U1+D1 does not yield the same result as running those five load primitives independently and summing the results. If you have a copy of (Roarks 5th Edition) "Formulas for Stress and Strain", read section 3.2 on page 43 - especially the 2nd paragraph.

You could ask your Structural Engineers what loads they want, and you should be able to setup a corresponding load case in CAESAR II for them. For example, you could do this: (1.3W + T1 + P1 + D1 + 1.7U1)

I hope this helps.

Richard Ay
Hexagon PPM (CAS)

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