Deflections under Seismic Load Effects

Question

Deflections under Seismic Load Effects

RVSWA (Structural)

(OP)

28 Apr 08 17:06

With regards to the seismic load combinations utilizing the overstrength factor (ASCE7-05 section 12.4.3), I am trying to find guidance on appropriate deflection limits. I suppose there are two classifications of response, 1) Code referenced and 2) Sound Engineering judgement.

1) I'm not finding any code reference aside from those in Table 1604.3 of the 2006IBC and they specifically mention wind, nothing about E.

2) Earthquakes are expected to cause damage, the extent of what is tolerable is left to the judgment of the engineer...

I wonder if there were a few opinion on this subject?

JAE (Structural) · Answer 1 · 2008-04-28 19:02:06

ASCE 7-05 - Table 12.12-1, Allowable Story Drift

You must determine your elastic story drift from your primary analysis and then apply the Deflection Amplification Factor, Cd, found in table 12.2-1. This per section 12.8.6, and 12.12.

RVSWA (Structural) · Answer 2 · 2008-04-28 19:52:10

I apologize for not clarifying; I have a cantilevered floor assembly. A "Rim" beam is supported at each end by two cantilevered beams. This "Rim" beam is subjected to overturning forces from the shearwalls it supports. Applying my overturning forces (horizontal seismic load effect) while using the overstrength factor and load combinations of 12.4.3 can generate deflections that exceed normal serviceability criteria. These are the deflections that I am trying to pin down. I hope the attachment helps...

http://files.engineering.com/getfile.aspx?folder=2408cd9d-d51c-4631-af10-95

WillisV (Structural) · Answer 3 · 2008-04-29 07:21:28

For seismic events there is no such thing as normal serviceability criteria. They are life-safety ultimate load effects and large deformations and force redistributions are expected. As long as your structure has the strength and stability to not fall down under seismic loads, you have met the intent of the code. The section JAE is referring to is actually a stability check as opposed to any sort of "serviceability" criteria.

RVSWA (Structural) · Answer 4 · 2008-04-29 10:36:34

WillisV thank you for this confirmation and I believe JAE was correct given my initial lack of information. I will have to adjust my allowable deflection criteria so the little red words don't draw unwanted fire. I thought I read something on this subject in ATC's "Built to Resist Earthquakes" but I'm not finding it right now. Having proper references when the question is raised is helpful.

civilperson (Structural) · Answer 5 · 2008-04-29 20:52:15

As stated, there is no deflection criteria for seismic events.

whyun (Structural) · Answer 6 · 2008-04-30 17:24:34

I am not aware of a "vertical deflection limits" subject to seismic forces in the code.

Your sketch shows a discontinuous lateral system which, in my engineering judgment, warrant standard beam deflection limits (Table 1604.3) on the support beams (i.e. the rim beam AND the cantilevered beams at the ends) subject to omega level seismic forces combined appropriately with dead and live loads. If the deflection limits are exceeded, I would recommend using larger beam sizes.

JAE (Structural) · Answer 7 · 2008-04-30 18:56:02

I don't know exactly what Seismic Design Category you are in, but it looks to me like you may have a vertical irregularity type 1a, 1b, or 4 (See ASCE 7-05, Table 12.3-2).

Be sure to check the requirements for those if they do apply.

RVSWA (Structural) · Answer 8 · 2008-04-30 19:41:24

Puget Sound, Washington, Design Category D, I have a flexible diaphragm so I believe I am exempt from 1a and 1b, but I land firmly in the middle of type 4, thus the overstrength deflection question. I appreciate the opinions. With regards to the load combinations for seismic overstrength, I believe I am going to start with a "double the normal deflection limit" for overstength and go from there...

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Deflections under Seismic Load Effects