Elastic Seismic Design
Elastic Seismic Design
(OP)
Interesting question: if someone actually wanted an elastic seismic design (i.e. where there wouldn't be any damage in a seismic event).....how would you go about it? The first thing that comes to mind (if you are using the ELF method of load calculation) is to NOT divide your load by R (i.e. the response modification factor).
Another interesting question is: if you are doing an elastic design, would the building system height limitations of Chapter 12 in ASCE 7 still apply? After all, they are built around controlling damage (and locations of said damage).
This thing came up in my mind because a client asked about the feasibility of a damage free seismic event. (Obviously cost prohibitive.) And it got me to thinking that if someone actually asked for it: legally I'm not sure how it should be done.
Another interesting question is: if you are doing an elastic design, would the building system height limitations of Chapter 12 in ASCE 7 still apply? After all, they are built around controlling damage (and locations of said damage).
This thing came up in my mind because a client asked about the feasibility of a damage free seismic event. (Obviously cost prohibitive.) And it got me to thinking that if someone actually asked for it: legally I'm not sure how it should be done.






RE: Elastic Seismic Design
First off...."damage free seismic event" is like saying concrete doesn't crack. You really got to define what your client means by a damage free seismic event....does it mean 1/16" wide cracks or hairline cracks...or even equipment damage inside the building.
Try looking at some of Englekirk's work on precast moment frame systems.
Also take a closer look at the ELF method assumptions with regards to distributed yielding (Read NEHRP)....I don't think it applies if you have an elastic system.
If I remember correctly if your system is to remain "elastic" you would have to use modal analysis with an unreduced design spectrum. You could even modify the spectrum for damping provided by soil-structure interaction at the foundation. Furthermore, you could run a few time histories.
Also remember that your floor acceleration levels for your equipment and non-structural items will be much higher and could damage them. The contents of your building may be worth more than the building itself.
Also think base-isolation....look at the work of Forell-Elsesser of San Francisco.
RE: Elastic Seismic Design
A second thing to consider on the economic end of it is that it makes a big difference where you are. In an area where seismic loads are pretty low anyway, it might not be that big a deal to do your design, versus, say, California.
(A couple of years ago at a seminar put on by FM, the speaker mentioned that the way the seismic factors in ASCE were calculated, the probability of a overload/failure wasn't necessarily uniform across the country like you might expect. Essentially, the factors are tweaked so that seismic design is less conservative in high seismic area. I forget the details, though.)
Lastly, consider that the success of designs has to be confirmed somwhat by experience. If there is a large body of experience showing that buildings built to certain standards don't fall down, that's taken to mean the method is essentially correct. On the other hand, if nobody bothers tabulating data on zero-damage buildings, you might be missing the historical background to confirm your analysis.
RE: Elastic Seismic Design
1. The value of R as defined in Chapter 17 is taken as 1.
2. For OMFs and OCBFs, design is in accordance
with AISC 341.
So, you can probably get a taller limit, but nonetheless there will be one.
Also, remember R=1 gets an elastic design for assumed earthquake spectra of code, but it doesn't necessarily mean thats the actual earthquake that will happen.
RE: Elastic Seismic Design
irizarry, that's an excellent point you made. If I had remembered that part of the code, I could have answered part of my own question.
RE: Elastic Seismic Design
RE: Elastic Seismic Design
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