Mass Source for Seismic Excitation 1

[DJSim]

Hi,

I am modelling a 37-storey reinforced concrete buildings in ETABS. Would like to ask if the superimposed dead loads such as floor finishes and internal brick walls should be included in the mass of the building when calculating the seismic base shear?



Thanks

 

[Enhineyero]

Yes

 

[JoshPlumSE]

Yes, they should.

Though I have a minor caveat. At times, we (as engineers) will make our Superimposed Dead Loads too large. Just to give flexibility for future tenant improvements in localized areas. You probably want to use the most realistic Superimposed Dead Load rather than the over conservative one when we're talking about seismic load.

 

[strucbells]

Josh- do you have any recommendations about "realistic SDLs" by building category or methods by which to determine?

Generally I have used conservative numbers handed down to me by my supervising engineers/prior example projects as we often don't have great information about piping/cable tray etc. when we are doing our calcs.

 

[r13]

strucbells,

Future addition consideration is usually requested by the owner, not dictated by the engineer. It is a contractual obligation to include the consideration in the present design.

 

[JoshPlumSE]

strucbells -

I'm really talking about cases where you're adding something that you know for certain is too high.

Let's take your piping / cable tray loads, for example. That number (let's say 8 psf) may be fine, a little bit conservative. But, we know it only applies to the location where the piping corridor is going to be. But, plumbing, electrical and HVAC haven't told you where this is going to be. To prevent a problem (like undersized beams) at a late stage in the project you just assume that this load is applied over the ENTIRE floor area.... in reality it's applied over something like 10% of the floor area.

Similar ideas when you design for other types of dead load (storage load?) to increase gravity resistance, but where that load isn't expected to exist over the whole floor area.

Those are the cases where you might want to use a reduced Superimposed Dead Load for seismic.

 

[milkshakelake]

@JoshPlumSE I found that it doesn't help to reduce loading to get a smaller weight for the client. I generally use conservative weights and apply it across the whole slab like you said. I might be able to save some reinforcement or concrete strength on a shear wall, or reduce some steel column sizes. But in case I made a mistake or I get a peer review, I'd rather be conservative. It's brutal when somebody else or a building department finds out that I underestimated the loads, and then I have to defend myself.

 

[mronlinetutor]

To DJSim,

I have the same trouble in estimate the load for seismic analysis too. But one thing I guess each structural analysis computer program should have a scale factor for the load and you can estimate the max load and the min load and use the scale factor to find the optimal loadings. The Min up curve and the Max down curve should meet somewhere where both the min and max curve agree with each other and that should be an optimal point. This is purely theoretical. Shame, I am still an EIT after thirty-five years out of school.

Another point is when you do your seismic analysis, you are prone to use large Earthquakes like the El Centro earthquake. If you use a large earthquake to do your design, the building would fail before you can analyze it by hand. So the solution is to use a scale factor for the seismic curve and scale down the earthquake to a 5 quake or use a 5 quake seismic curve if you have one available. You are not sure whether the scale down 8 quake is equivalent to a 5 quake that occurs naturally. So you scale down the seismic curve to the point where you are able to analyze the building without any member failure. This is a trial and error and it is tedious. But look, most engineering firms buy their own computer software so there are lots of computer times.

Finally, I double if you can pour a 37 storeys concrete building because although in the computer program, the beams and girders are momently connected, in real life, the moment connections behave like hinges in earthquakes for concrete. So if you were to use a moment connected concrete frame without any bracing, the building would move laterally unless you have to brace for each bent on each level. For 37 storeys tall buildings, concrete would be too heavy. I would rather use a moment connected steel frame with bracing in each bent on each level. Steel moment connections remain steel moment connections under earthquakes. I guess 37 storeys concrete building is okay because the Trump Tower, the tallest concrete building on the earth is 92 storeys.

From

Stanley.

disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.

 

[Agent666]

I guess 37 storeys concrete building is okay because the Trump Tower, the tallest concrete building on the earth is 92 storeys.

Lol, another trump lie?... I hope you're being sarcastic with that "fact"...

 

[Shu Jiang PEng SE]

Not only dead load of structure and equipment, mass shall also include some percentage of live load in some cases. You may go back to ASCE 7 for more information regarding seismic design.

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Shu Jiang, SE (Nevada). PE(Michigan, South Dakota), PEng (Ontario)
J&J Structural Consulting Inc.
Structural design, analysis, inspection, drawing review and stamping, and connection design