Seismic Weight Rule of Thumbs

[strutandtie]

Any experienced engineers have rule of thumbs for seismic weights of buildings? I would like a couple values to gage the accuracy of my calcs.

e.g. Weight per square foot for single story commercial, two story commercial.

 

[EngDM]

Floor construction? Hollowcore, owsj with topping? Masonry? It really depends on your construction.

 

[milkshakelake]

The seismic response coefficient (Cs in ASCE 7) is a good place to start. For example, if your Cs is 0.07 and your DL+SDL=60 psf, you can approximate using 0.07*60=4.2 psf. This doesn't account for a lot of things (like facade loads) so you might want to use something like 0.09, which will get 5.4 psf. This will get the seismic base shear. You can run a quick calculation to get Cs using a spreadsheet; it's just inputting a few values, like importance factor and R. Or you build a library of a bunch of Cs's you get from similar buildings based on experience and just use that. I prefer to calculate Cs because it's very quick and low effort, but sometimes for a back-of-envelope type of calculation, I use a typical Cs (slightly increased for facade loads) times building footprint times number of stories times total DL, ignoring things like setbacks and balconies.

Distributing that approximate base shear is a different story. I've never needed to approximate that, so I don't have a rule of thumb for that. I only use it to check the base shear of my FEM model.

Of course, like with any rule of thumb, there are numerous caveats like when you have significant live loads that need to be included.

 

[strutandtie]

EngDM: Yeah, I know there are a lot of factors which is sort of why I'm interested in a rule of thumb for building weight at least. Someone told me 200-250psf is a good ROT but that seems high to me. Generally I'm interested in single-story, light frame construction with typical fiber cement panel siding. I would just factor up for brick facade since there's a published dead load in ASCE 7 compared to siding (48psf, 12psf respectively). I just calced a building of this type with half FCP siding and half brick, and seismic weight came to about 400k, which is an average of 40psf (50psf is total DL considered) which obviously is much lower than 250psf, but I have historically gotten much lower number compared to the 200-250psf ROT.

MilkShakeLake: I believe you need to review the concept of seismic weight and Cs. Cs is the seismic response factor for the equivalent lateral force method, which cannot be used on all building types. ELF multiplies Cs by the seismic weight for the equivalent base shear. Live loads are not considered in seismic weight except certain conditions per ASCE 7 (eg 25% of storage loads). I only say this because it is a public forum and want to be clear to others who may look this up. Also, I highly recommended calculating your distribution of base shear...I have never not done that on a project.

 

[milkshakelake]

@strutandtie Sorry, I totally misread your post. Somehow I thought it was about base shear, not seismic weight! I guess base shear has been on my mind lately. Yes, I mentioned the live loads for that reason; there's also a provision for snow, if I recall correctly.

I don't have a great answer for you about how to lump in facades and dead loads with a general psf for your type of building. For the stuff I do (multi-family residential, about 40'x60' footprint) I just increase the actual dead load by 25% to account for the facades (actually I increase the Cs, but it amounts to the same thing). Heavy facades, increase by 40%.

I do calculate distribution of base shear for very complicated projects but mostly rely on ETABS to do it; the results are fairly close in testing with hand calculations.

 

[Deker]

Best thing to do is review the old calc packages in your office for similar building types and see what weights were used. You'll likely find that weights are essentially constant for the roof and that most of the variation comes from the exterior walls and size/quantity of pop-ups on the roof. When you do enough of these, you should be able to ballpark the seismic weight for any building in less than 10 minutes.

I've found that single-story wood commercial tends to come in at around 40-50 psf total. If you have a lot of pop-ups with clay tile roof it can be higher. 200+ psf is concrete building territory, so there must be a misunderstanding surrounding that number.

I'll also add that having a rule of thumb seismic weight won't do much for assessing the accuracy of your calcs, but it can be valuable in schematic design when you're laying out the lateral system.

 

[strutandtie]

Thanks, Deker. Great idea - I'll review old calc files (we're a small business with basically one PE and one EIT, so it's limited). Glad to know I'll hopefully be able to estimate soon for SD...thanks for the insight!