Overstrength and Inelastic Response
Overstrength and Inelastic Response
(OP)
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?






RE: Overstrength and Inelastic Response
For example, connections have to typically be way over designed so that they aren't the weak link because it would be sudden. Yielding of steel is what you want, not connection failure, etc.
RE: Overstrength and Inelastic Response
DaveAtkins
RE: Overstrength and Inelastic Response
RE: Overstrength and Inelastic Response
Imagine a braced frame where you design the brace to deform inelastically. Now imagine you didn't design the connection to transfer that higher load. At that point, who cares about the brace if the connection fails and the brace can't absorb the energy?
And remember, structures don't know what code they are designed to. R factors and over strength factors are approximate and educated guesses. Point of overstrength factors as I see them is to make sure the load gets dumped into the main lateral system, and that the lateral system is designed and detailed in such a manner to behave inelastically in a controlled manner, again the chain analogy. Connections, collectors, etc. that dump load into the system are critical, because if the load can't get into the lateral system it won't matter if you have a "weak link" there
RE: Overstrength and Inelastic Response
1) We usually need inelastic response in order to dissipate seismic energy.
2) Inelastic response implies damage so we want inelastic response concentrated in certain places where that damage will not result in instability and hopefully be straight forward to repair. Typical choices are plastic flexural hinges in shear walls and moment frame beams and axial yielding in cross braces.
3) Seismic demand on our structures generally hits the lateral force resisting system first and then flows out to the collectors, diaphragms, gravity framing etc. By dealing with the bulk of the energy dissipation in the lateral system, close to the source (foundations), the structure further down the load path is shielded from having to sustain significant damage.
4) The primary way that we ensure inelastic damage is concentrated in the locations where we want it is by a)using R factors to ensure that these locations have nowhere near the strength required to resist the elastic level earthquake response and b) ensuring that these locations can sustain cyclic, inelastic damage without creating instability.
5) As designers, our goal generally is to keep our diaphragms and their connections to the lateral systems elastic. Two of the reasons for doing this are a) it's often hard to design collector/diaphragm path to be highly ductile and b) if our diaphragms yield then our assumptions about load distribution between lateral elements goes to hell in a hand basket and we may lose the ability to stabilize our gravity columns.
6) We do our best to ensure elasticity in the structure beyond the designated inelastic zones by designing the important parts of those systems to a load level intended to represent that corresponding to complete, whole system mechanism formation (the point where the structure would stop absorbing additional seismic load).
7) Recognize that elastic earthquake loads reduced by R represent the load at which the first designated inelastic zone in the structure yields. The structure as a whole does not yield until all of the designated inelastic zones yield which will occur at a load significantly higher than (Elastic Level EQ / R). How much higher? ((Overstrength - 1) x 100) percent higher.
In summary, we use overstrength factors to increase the seismic load to maximum level we expect to see in the system and thereby ensure that the critical parts of the structure structure beyond the designated inelastic zones remain elastic. It is, of course, a rough science.
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.
RE: Overstrength and Inelastic Response
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.
RE: Overstrength and Inelastic Response
"It is imperative Cunth doesn't get his hands on those codes."
RE: Overstrength and Inelastic Response
I am wondering about the source for KootK's figure. Somethings are unclear. 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. Perhaps the commentary that goes with the figure would clear up my misunderstanding.
RE: Overstrength and Inelastic Response
njlutzwe I starred you. For payment I'm stealing your analogy of a chain with a plastic link. That's such a fantastic explanation of seismic R factors.
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Overstrength and Inelastic Response
"It is imperative Cunth doesn't get his hands on those codes."
RE: Overstrength and Inelastic Response
I think the source of KootK graphics is the Structure Magazine article entitled "A Brief Guide to Seismic Design Factors", authored by the SEAOC Seismology Committee back in 2008. Link
Tom Paulay used the chain analogy way back when...but no 'plastic' links in his!
RE: Overstrength and Inelastic Response
RE: Overstrength and Inelastic Response
RE: Overstrength and Inelastic Response
"It is imperative Cunth doesn't get his hands on those codes."
RE: Overstrength and Inelastic Response
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Overstrength and Inelastic Response
@wannabeSE: agreed on all counts regarding the superiority of the ASCE version and the flaws in the SEOC/structuremag doc. Nothing in the text of that reference improves matters. Many of the dimensions are sloppily presented by omitting the primary multipliers. Even at that, the definitions of omega and Cd are incorrect, not just sloppy.
Thank you for pointing out the errors and supplying the better version. The inclusion of CuTa adds value to the ASCE version as well. I wonder if there was an editorial issue at Structuremag. It's hard to imagine SEOC mucking seismic up that way.
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.
RE: Overstrength and Inelastic Response
Cast-iron = brittle, mild-steel = ductile.
RE: Overstrength and Inelastic Response
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.
RE: Overstrength and Inelastic Response
Hehe. No, but just in case it was 'lost in translation' from the Kiwi authors. Eh!
RE: Overstrength and Inelastic Response
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 Vs x (omega-1), Vy x (Rd-1) and Vs x (R-1).
What they actually meant, of course, was Vs x (omega-1), Vy x (Rd-1) and Vs x (R-1).
Or, equivalently and far more simply, Vs/omega, Vy/Rd and Vs/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.
RE: Overstrength and Inelastic Response
Actually no, it's correct as shown below and as you wrote in this statement:
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.
RE: Overstrength and Inelastic Response
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.
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.
RE: Overstrength and Inelastic Response
Thanks, I see it now. Apologies to AISC/SEI for not looking at it carefully enough and for my little tirade.