Comparison of two analyses with different seismic factor q

[fsapostolou]

Hello Seismosoft team,

I ran two RSAs in SeismoBuild: Analysis A: q = 2 & Analysis B: q = 1.
Theoretically, forces in case B should be about double those in case A. Instead, when I increase q, shear forces also increase, which is the opposite of what I expect.
Could this be due to a mistake on my side, or is this the regular way SeismoBuild handles q?

[seismosoft]

In the case of q=1 the seismic forces are not divided by the behavior factors, thus the demand on the members is obtained directly from the analysis. On the contrary when q=2, the shear forces are calculated from capacity design considerations, i.e. from the bending moment capacities at the edges of the member.
Seismosoft Support

[fsapostolou]

Thank you for your detailed response. According to Annex 9A of KAN.EPE., if a member is classified as quasi-ductile, a capacity design check is not required (for RSA with the global factor q). SeismoBuild automatically determines whether a member is quasi-ductile (can I see this result?), and then decides if the check is performed using the shear force from RSA divided by q or by comparing the bending moment capacity at the edges of the member.

Thank you in advance!

[seismosoft]

Shear is a brittle type of action and should always be checked with capacity design considerations in linear methods. It is not in the discretion of the engineer to decide whether a specific action is brittle or ductile.
Seismosoft Support

[paulidi]

I understand that the capacity design considerations must always be checked even in old buildings where the capacity design was not actually in use. So the only way to deal with this is using q=1 (elastic actions). We use q=1 for performance level B Life protection. At the table 4.1 it says we can use for performance level C Collapse prevention q*=1.40*q=1.40 (if q=1). I ran the analysis (for performance level C,q=1.40(and ag=0 to be sure of the results) and i see that the capacity design considerations made many members to fail. To make a long story short shear capacity design is always checked even in performance level C ?

[seismosoft]

According to Annex 9A of KANEPE, with the q factor approach for the linear methods and for limit states B and C, capacity design should be applied for the 'brittle elements', which is for the elements where brittle modes of failure (like shear) are critical. To the best of our knowledge, there is no mention in KANEPE that one can employ these increased q-factors in limit state C (from Table 4.1) without capacity design considerations.
Seismosoft Support