Overstrength and Inelastic Response 1

[awa5114]

I'm a bit confused on the intent of overstrength provisions in the seismic AISC manual. Basically, I'm not sure if Seismic force resisting systems such as Moment Frames, Braced Frames...etc are used to take more inelastic response or to prevent it. In other words, are we including these systems to make the structure more inelastic or do we apply overstrength and demand that they carry more load so that the system can remain elastic throughout an event?

 

[Archie264]

I prefer Figure C12.1-1 in the Expanded Seismic Commentary to ASCE 7-10.

WannabeSE,

If you found that chart easier to understand than the one Kootk posted then it's because you actually understood the principle in the first place. I did not, and possibly never would have, but for Kootk posting his chart a while back on a different thread. If you're wondering what the distinction is just look at the figure you referenced.

In my copy it reads as follows V_s x (omega-1), V_y x (R_d-1) and V_s x (R-1).

What they actually meant, of course, was V_s x (omega^-1), V_y x (R_d^-1) and V_s x (R^-1).

Or, equivalently and far more simply, V_s/omega, V_y/R_d and V_s/R.

I find it exceedingly frustrating. If I prepared construction documents with such seemingly little care for or understanding of what they communicated you would be reading of me in the the inevitable litigation that would follow. I wistfully pine for the days of old when drafting and presentation were treated more seriously and I dream of future days when code-writers have to practice the same standard of care as do the practioners charged with implementing their decrees.

I know they [the code-writers] know what they're doing. Contrary to how it may sound I am in awe of them and their research and I am very grateful for it. I am standing on the shoulders of giants without whose work I would not be able to design so much as a swing set. I know they are brilliant; I just wish they would make it easier for little people such as myself to follow along. Or "climb aboard", if I'm to stick with the "shoulders of giants" metaphor.

 

[KootK]

What they actually meant, of course, was ...

Actually no, it's correct as shown below and as you wrote in this statement:

In my copy it reads as follows Vs x (omega-1), Vy x (Rd-1) and Vs x (R-1)

Give it another stare and, if it doesn't come together for you, report back and we'll get it sorted.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

For instance the "x Ω0" dimension. Shouldn't the dimension be from 0 to Vy , rather than Vs to Vy? As it stands, the figure makes it look like Vy = Vs + Ω0Vs instead of Vy = Ω0Vs. I have similar problems with the other dimension shown.

So it turns out that the graph below is quite correct, if somewhat oddly presented. The trick with the dimensions is to realize that there are no dimensions! Very Matrix-esque, I know. If you look closely at the "dimensions", you'll see that they only have arrows at one end. What the dimensions are, in fact, are the ratios required to get you from the starting values to the ending values. For example:

Vs = Ve x 1/R
Vy = Ve x 1/Rd
Vy = Vs x Omega

There, all is right with the world.

I wonder if there was an editorial issue at Structuremag. It's hard to imagine SEOC mucking seismic up that way.

Naturally, I rescind these libelous statements.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Archie264]

Kootk,

Thanks, I see it now. Apologies to AISC/SEI for not looking at it carefully enough and for my little tirade.