ASCE 7-16 15.4-6 - Approx. Seismic Period Follow Up

[duckhawk]

This is a follow up question to the thread from 6/4 about ASCE 7-16 15.4-6

The commentary about the equation says to use lateral forces to determine deflection.

However, wouldn't you need to consider dead load in that load combo as well when deflection is calculated?

Asking because I've ran it with just lateral forces and lateral forces+dead and you get different deflections.

You can't act like dead isn't there and calc deflections based on lateral only, can you? It just doesn't sound right. Comparing this method with RISA's Eigensolution, it sounds like in RISA's general reference that mass is included.

In the example below I'm not quite sure if the seismic weight was considered in the same load combo when deflection are calculated.
https://res.cloudinary.com/engineering-com/image/upload/v1639177951/tips/Rayleigh_Method_m3ghfj.pdf

Appreciate it!

 

[driftLimiter]

It is odd that you are saying gravity loads cause lateral deflections. The only reasons I can think of that that would happen are if you have a lot of P-delta or If your lateral system supports dead load (thrust typically from non-vertical gravity load resisting elements).

First I think you need to investigate and understand WHY gravity loads are effecting your lateral displacement.

But yes lateral stiffness is simply F/d. Chopra's text for example has many instances where the mode shapes are calculated based on a simple determination of the stiffness of the MLFRS.

 

[HTURKAK]

....Asking because I've ran it with just lateral forces and lateral forces+dead and you get different deflections.

Could be two reasons ;

- P-Δ effect , ( as driftLimiter stated)
- If the structure and loading is not symmetrical .
Otherwise gravity loads will not cause lateral deflections.