P-Delta

[Anonymous]

Hello,

I am a new user of Robot and maybe this is a stupid question but I hope you will be kind enough to reply.

I am performing a seismic LSA on an existing building and I need to include P-Delta effects.

Would it be enough to include the P-Delta on the Load Cases? will that affect the combinations I create from the load cases? Or should I define the P-Delta non-linearity directly to the load combination?

When I do the latter, the combination including the seismic load case from the Linear Static analysis does not converge and crashes with Error 5000.

Thank you in advance and I would really appreciate your help with this!

[Rafacascudo]

you have to create it directly for the combinations.

Search the forum for error 5000

[mustafahesenow]

Hi @Anonymous

Send us the model please.

[Anonymous]

Hi @mustafahesenow

please see the model attached

[Anonymous]

Could someone please explain why it does not converge when P-Delta is considered in the seismic combination? It seems to be working fine for the vertical load combinations.

Thank you again!

[Artur.Kosakowski]

Hi @Anonymous

This is the deformation for linear static analysis.

It cannot be that big (as it would be even larger for the 2nd order analysis and this is why IMHO the nonlinear analysts diverges). I'd say that either the seismic loads are too big or the model is wrong (lack of horizontal restrains / diaphragms).

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi @Artur.Kosakowski

Thank you for your reply. I am certain about the seismic loads because I have checked it and compared with the load derived from hand calculations.

As for the model, as already mentioned it is an existing structure relying solely on moment frames to resist the seismic forces. The extensive bracing of the roof should allow it to perform as a semi-rigid diaphragm and similarly the concrete slab for the mezzanine level.

The purpose of the this analysis is to evaluate the building and therefore I would appreciate any assistance I can get for the assumptions already made or those that I might have missed by creating the model.

Thank you again for your help so far.

[mustafahesenow · Accepted solution]

1

[Anonymous]

Hi @mustafahesenow

Thank you for your reply.

The code parameters you specify are correct when it is regarding new buildings where ASCE 7-10 applies.

For evaluation of existing buildings ASCE 41 is to be followed which specifies much larger forces to reach the real inelastic displacement.

Because Robot does not have in-built ASCE 41 parameters, I had to adjust the parameters specified on ASCE 7 to reach the load applied according to ASCE 41 (which really is 81% of total load).