Equivalent Seismic Lateral Forces for Inclined Steel Roof

[JMC85]

I have a simple rectangular (single story) steel structure with a lightweight roof supported by trusses. After modeling this in ROBOT, and creating a single Story level to include the roof steelwork members, I then set-up the Seismic analysis (Equivalent Lateral Forces) and can run this without creating a diaphragm for the roof level. The programme generates all the equivalent forces for the nodes in the roof structure successfully (at the top story).

However, in reality the roof is on an incline. When adjusting the roof geometry to account for the incline, the programme will no longer generate the equivalent lateral forces in the nodes of the roof structure (it only does this for the nodes corresponding to the set story height).

Im guessing the Seismic (ELF) analysis works to the set Storey height, and will only produce equivalent lateral forces for that set height?

What is the correct procedure for generating seismic equalivalent later forces for an inclined steel frame? Is a diaphragm required to do this correctly? If so, how is one applied on an incline?

[Artur.Kosakowski]

In fact this method works for flat stories only. The solutions I can think of are:

- run a model with flat roof and note the values of forces in its nodes > change model geometry and run calculations again > create a new load cases with forces from the roof nodes of the original model altered with the values obtained for the inclined roof model > create combination with both loads.

- use the dynamic spectra seismic analysis instead.

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