topic Re: Seismic shear forces differences in ASCE7-10 ELFM and IBC 2009 in Robot Structural Analysis Forum

Seismic shear forces differences in ASCE7-10 ELFM and IBC 2009

Anonymous — Fri, 27 Feb 2015 17:15:16 GMT

Hi ,

I'm have started recently with some dynamic according ASCE7. I've spotted quite big a differnces in base shear using ELF comparing to modal IBC.

Shoud the base shear be simmilar?

Model attached.

Thanks,

Frank

Re: Seismic shear forces differences in ASCE7-10 ELFM and IBC 2009

Rafal.Gaweda — Tue, 03 Mar 2015 11:06:42 GMT

Results for my test model http://we.tl/RUsiPkZna0 :

Re: Seismic shear forces differences in ASCE7-10 ELFM and IBC 2009

Anonymous — Tue, 03 Mar 2015 16:02:00 GMT

Thank you Rafal,

Looks the shear is the same for both cases for dynamic and static ELF method as we define the perionds manually for each direction X and Y.

However, there are still some discrepancies, which are not clear to me. I have ammended the model you have attached this morning by removing the user defined perion in dynamic and ELF static method.

The summ of reaction for ELF is much bigger than for Dynamic. Not sure why. Some other questions (screenshoot).

Thanks and Regards,

Frank

Re: Seismic shear forces differences in ASCE7-10 ELFM and IBC 2009

Rafal.Gaweda — Wed, 04 Mar 2015 11:45:23 GMT

It seems change of S1 parameter is the cause.
It must be deeper checked by developers.
Thank You for reporting

Re: Seismic shear forces differences in ASCE7-10 ELFM and IBC 2009

Rafal.Gaweda — Mon, 16 Mar 2015 11:04:35 GMT

Frank,

As far as we checked period T is calculated as T <= Cu * Ta, according to 12.9.4
Could you please check, verify, clarify if our assumptions are wrong?

Etabs does it in the same way : point 2.9.1 here http://www.ottegroup.com/manuals/ETABSLateral%20Loads%20Manual.pdf

Re: Seismic shear forces differences in ASCE7-10 ELFM and IBC 2009

Anonymous — Mon, 16 Mar 2015 15:13:33 GMT

Rafal,

Looks that assupmtions are correct. It is not only "Precise(modal analysis)" but combination of modal and empirical one and its comparation. What I'm missing is the imput paramenter Ct as RSA is taking Ct=0.02 (0.0488) as default (Table 12.8-2 of ASCE), in which some cases it might be too conservative. ETABS seems have this option as stated in 2.9.1 (Program calculated).

Thanks,

Frank

Re: Seismic shear forces differences in ASCE7-10 ELFM and IBC 2009

Rafal.Gaweda — Tue, 17 Mar 2015 09:58:21 GMT

OK, registered on wishlist.
Thx