Equiv Lat Force Procedure Modal Mass Participation Minimum
Equiv Lat Force Procedure Modal Mass Participation Minimum
(OP)
Some questions about period/mass participation when using ELF:
- I've always assumed that where your analysis model gives worse (shorter) periods than the code approximate periods you use the analysis periods
- If doing the above is there a min mass participation for the mode?
In particular my question applies to something along the lines of this:
I have building that is SDC B so low seismic and no real restrictions. The lat system is essentially a big 'C' from walls so very torsional but stiff enough to make it work overall.
I can use the code approximate period but this appears to be too generous compared to the analysis model. However if I want to use the analysis period then the 1st mode is clearly torsion. The following modes are not clearly x or y so to find an "x" period for seismic loads I can look at the participation, but how much do I need? To get a good degree of x participation I get to a very short period, especially compared to the code approximate period. Hate to screw myself if I don't need to but at the same time the code approximate period seems silly to use if I know that it's way off. Is there any guidance on this? (and no I don't want to do rsa, junky little building that I don't want to go nuts with - just wondering if there's some guidance/code commentary on this).
- I've always assumed that where your analysis model gives worse (shorter) periods than the code approximate periods you use the analysis periods
- If doing the above is there a min mass participation for the mode?
In particular my question applies to something along the lines of this:
I have building that is SDC B so low seismic and no real restrictions. The lat system is essentially a big 'C' from walls so very torsional but stiff enough to make it work overall.
I can use the code approximate period but this appears to be too generous compared to the analysis model. However if I want to use the analysis period then the 1st mode is clearly torsion. The following modes are not clearly x or y so to find an "x" period for seismic loads I can look at the participation, but how much do I need? To get a good degree of x participation I get to a very short period, especially compared to the code approximate period. Hate to screw myself if I don't need to but at the same time the code approximate period seems silly to use if I know that it's way off. Is there any guidance on this? (and no I don't want to do rsa, junky little building that I don't want to go nuts with - just wondering if there's some guidance/code commentary on this).






RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
Analysis models can also be sensitive to a bunch of assumptions regarding effective stiffnesses, element stiffnesses that get neglected/included, etc. So just because the model says something doesn't necessarily mean it's any more right than the approximate value from the code. For instance, a lot of people/firms will omit floor out of plane stiffness in lateral models because they don't want any frame action but this can artificially increase your period and thus lower your seismic demand. Can also swing the other direction if you haven't applied ultimate level crack factors to a concrete structure when analyzing for an ultimate level event like an earthquake.
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
Listing all of the other stuff that we assume and get wrong doesn't sell me much. You could chase that one forever, might as well just give up. I do see the argument that the code allows it and it saves the client money, fine. I'd still like to know if this is what everyone does.
The torsional building is one situation that doesn't sit well with me. The other one is that I often end up with loads of wall in one direction that I don't need because of the site layouts in my area (usually lot line construction and often get full walls for simplicity/logistics etc) and in the other direction have to fight for every inch of wall. So I might end up with 3x the stiffness in y vs x, doesn't seem right to assume same period in both for elf loads.
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
The number I have typically heard over the years is about 90%.
Of course, if you have a FEA model......you can take all the guess work out of it by using the Modal Response Spectrum procedure in the code. It accounts for mass participating in different modes. (I think a requirement of that procedure is enough modes to get 90% of the mass participating per direction btw.)
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
I'm also particularly bothered by this when you've got building torsion as your first mode. It seems to me that the simplified period estimates must have been based on some simplifying assumptions. I don't know exactly what those were but I'm guessing something like:
1) Regular-ish building.
2) Torsion not as first mode.
3) Shear building lateral deformation behavior.
Where much of that is violated, so are the estimates in my estimation.
I very much agree with MrHershy's general skepticism regarding accuracy though, especially with shorter buildings. Once you get into mixed lateral systems, gross estimates of cracking, and considering the shear deformation of shear walls, it starts to feel as though you could drive a snowmobile through your wannabe result bulls-eye.
You should really consider offshoring your sexy NY lateral work to a lowly paid foreigner who lives for this stuff. I know just the guy.
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: Equiv Lat Force Procedure Modal Mass Participation Minimum
I assume that the elf has assumptions baked in as you've said, was wondering if anyone had that off the top of their head or how they've dealt with (or ignored) the above situations.
They're not sexy and I doubt your lowly paid but if you want/need/have interest in work let me know.
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
If you want to stick with ELF, I would pay careful attention to your modeling assumptions and use the shorter of the actual and approximate periods. The intent of the approximate code equations is to provide short periods for conservative base shear estimation. If you take the time to model the building, you are rewarded by being allowed to use the actual period (assumed to be longer) as long as it doesn't exceed the upper limit of Cu x Ta. If your modeled period is shorter than the approximate period, I would interpret that as either (1) inappropriate modeling assumptions or (2) a shortcoming in the approximate code equations. The following articles might be helpful:
Link #1
Link #2
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
Like most things, it's in the eye of the beholder. Sometimes you uber-urban environment structural engineers tend to have kind of a skewed perception of what constitutes a cool project. I see that a lot with my Toronto cohorts. In my neck of the woods, an eight story building has a pretty good chance of being in the top five of your projects for the year with respect to building height. I spent a little time in Dumbo last weekend, along with all the other tourists. I thought that there was quite a bit of interesting construction going on near by.
Put me in the want/have interest in work category. I currently have a relationship with a Midwestern US engineering firm that I'm enjoying. While I don't know all of your specifics, I wonder if we might do something similar. I'll describe what I'm doing and let you ruminate on it's applicability to you.
- The Midwest firm is a couple of buddies that I used to work with who've gone out on their own. They're doing great but they're not yet at the point where they'd be comfortable adding senior level engineers to their staff. And sometimes, you want the help of a senior engineer. They could certainly handle any of the work that they send my way themselves but they need to spend their time at project meetings and doing business development stuff.
- I cherry pick the work that I take on. I'm trying to position myself so that I have two kinds of work: 1) interesting and marginally profitable 2) boring and very profitable. The design work that I do with the Midwestern firm is the former. If they've got ten thousand wood headers that need designing, they don't come to me unless they're in a bind (thankfully). However, I'm happy to help out with all kinds of things that interest me.
1) Structural modelling, particularly multi-story lateral.
2) Design of long span systems and trusses.
3) Design of transfer elements.
4) Design and detailing of complex connections.
5) Pre-stressed / Post-tensioned anything.
6) Quality control on contract documents.
7) Contract administration to the extent that I can participate remotely. I like developing solutions to field issues etc.
Basically, all the fun stuff that they'd love to do themselves if they could spare the time.
- I do charge a healthy rate for my services on an hourly basis. That said, I charge in Canadian dollars which typically makes as US dollar go about 33% further than it does state side. That allows me to do fun design engineer stuff while still earning a decent hourly rate. Also, since I'm cherry picking the work that I find interesting, I tend to budget my hours pretty competitively. I go over budget a fair bit but don't mind eating it as I'm doing interesting work and doing it for people that I like.
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: Equiv Lat Force Procedure Modal Mass Participation Minimum
One other alternate if you're worried about the major changes in stiffness in each direction is ASCE 7 gives you an alternate procedure for shear wall buildings (based on the Chopra article that Deker posted above) that would yield different periods in each direction. It's a fairly long process if you've got more than a few walls though, but would give you another alternate if desired.
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
Doesn't ASCE 7 have a test for torsional irregularity? In Canada, there's a torsional stiffness test. If you fail it, your building is classified as irregular and you have further seismic analysis restrictions.
You end up not being able to use the static force method for structures that are torsionally irregular unless you're in a lower seismic area. In a lower seismic area, I'm sure the irregularity would be just as big a problem for wind and I'd probably be okay with using a more approximate seismic method, as it may not be the governing concern.
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
Robert Hale, PE
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
Thanks for the opinions/input - doesn't sound like there is anything that I was missing. In my area it's sdc b 90% of the time and therefore wind almost always controls design. Seismic is usually a formality only. In the cases that I was referring to the choice of analysis can be enough to make seismic control and change the design. Everything is usually modeled in etabs so I agree that it's not much of an extra step to do rsa, but the less I have to do the better so was just trying to iron out any documentation/commentary that limits ELF or lays out the assumptions since it's explicitly permitted in the code even with torsional buildings for sdc b. (rob hale - I think i'm the oddball but I almost never worry about lawyers with this kind of stuff. I'm pretty sure that if anything gets me it will be something frivolous such as a slip and fall in one of my buildings, or it will be water infiltration- its' always water)
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
I've been stung, when using AS codes, that the fundamental period for ELF methods only consider height and general lateral system. This particular building was much stiffer (lower period) than predicted by code equations, therefore attracting much more seismic load. I always like to compare the results of ELF and RSA, allows you to sleep better at night.
RE: Equiv Lat Force Procedure Modal Mass Participation Minimum
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: Equiv Lat Force Procedure Modal Mass Participation Minimum