When to and when not to use the overstrength factor... 4

[vato]

I am venturing more deeply into higher seismic zones and I could not get printed clarification on when a strucure, or member is exempt from having to use the overstrength factor. I was advised that it comes into account on structures with lateral irregularities, but still no direction, that I could dig up, in the code ASCE7-05. Any direction would be appreciated.
thanks

 

[frv]

I'll preface this by mentioning I have never done a seismic design.

But I recently attended a seismic design conference and, if I remember correctly without looking at my notes, the overstrength factor has nothing to do with lateral irregularities. It has to do with the fact that materials (concrete and steel specifically) have more capacity than specified. The overstrength factor takes this into account, as you want your "fuses" to be ductile; i.e., you want them to yield. Consequently, you need to accurately estimate the strength.

There's a distinct possibility I pulled that out of my rear and that the factor I'm talking about is called something else. In any case, this is the type of topic a lot of people will probably chime in on.

 

[JAE]

It is required where it is explicitly stated in the code.

ASCE 7 says, "where specifically required" so you simply have to keep your eyes open for references to section 12.4.3.

In ASCE 7-05 look here for a few:
12.10.2.1
12.13.6.4
12.13.6.5
12.13.6.6

Also, in AISC's 341, there are numerous reference to use of the overstrength factor in special load combinations - they term it "amplified seismic load". Here are a few sections in 341:
Part I
8.4a
9.6(a)
9.7b.(1)(a)
13.2c
14.4

I'm sure I've missed a few. The key is to read[/] through the codes prior to design and understand all the sections that apply to your particular case.

 

[JAE]

frv - you are correct in my view.

 

[JaredS]

I am pretty certain frv is correct,

like frv I have never performed a seismic design before however, I only recently wrote a research paper for my advanced steels class on steel plate shear walls.

As I recall, when the design intent is to yield something, in my case a giant steel plate, as it reaches its capacity the overstrength factory Ry should be used so that the elements connected remain elastic.

 

[strguy11]

Frv is correct.

I was at the steel conference and they specifically discussed the nature for this in one of the seminars. It has to do with the fact that we design steel, say for Fy = 50 ksi, but in reality, the steel has a Fy greater than 50. This "extra" strength capacity, actually reduces the ductility of system. When an R>3 is used, the seismic forces are "reduced" due to the ductility of the system. Since the ductility of the system is reduced for an R>3, you need to be "add" force back into the design to account for the loss of ductility. This is an oversimplification, but it is basically what I understood from the seminars.

 

[sandman21]

Ry is a material overstrength due to strain hardening effects in steel. The overstrength which he is talking about is omega, it is applied where stated. Horizontal and vertical irregularities are covered under the Rho factor found in 12.3, there are also increases in forces due to some irregularities.