Effective Seismic Weight 1

[n39]

I was wondering for example I was modelling a 4 stories building in ETABS, why most people on youtube didn't design or model the tie beam/sloof in ETABS? My real question is, do you need to take account of the weight of the tie beam in determining the effective seismic weight for eartquake calculation? ASCE 7-16 didn't mention anything about including/excluding tie beam in calculating the effective seismic weight, however as I mention above, most youtube videos shown that the didn't model the tie beam or the neglect it even exist. I'll greatly appreciate an explanation with a trustworthy source from journal or other codes

Thank you

 

[KootK]

1) If the tie beam will exist in real life, then it's contribution to seismic weight out to be accounted for.

2) If the tie beam would represent an insignificant seismic weight relative to the other sources of seismic weight it is, of course, the engineer's prerogative to ignore it if they deem that appropriate.

 

[JoshPlumSE]

Isn't the tie beam part of the "dead load as defined in section 3.1"?

Sometimes, for expediency, we will say "for this type of framing we can expect framing weight to be approximately xx.x pounds per sf". That's usually based off of experience for the types of projects we've worked on before. Especially when we don't know the exact layout or beam sizes or such. That being said, I do like to check that assumption later in the process when the design is more complete.

 

[chris3eb]

You're talking about a subgrade tie beam that connects the foundations together, right? Foundations aren't typically modeled in ETABS (there would just be a restraint at the bottom of the column or wall) and foundation weight isn't included in the effective seismic weight because their interial forces are resisted directly by the soil, not by the building's lateral force resisting system

 

[JoshPlumSE]

If it was part of the foundation structure that the OP was talking about, that was not clear to me.

If that's the case, then that weight wouldn't exactly be "above the base", would it? Personally, I think this gets a little harry when you have multiple basement levels of a structure. If it's only one level of basement then I take it at the lowest level of the basement. But, if we had 4 or 5 levels of basement parking, I'm not sure what I'd do.

 

[driftLimiter]

I've always considered the displaced shape of the structure under earthquake loads. If the level in question doesn't really move relative to the ground, then I don't really consider that seismic effective weight. However, any and all mass that is elevated above the base aught to be considered in the effective seismic weight. Like others have said it is common for designers to smear out the specific weights of framing, columns, cladding, etc into a single uniform distributed load used for computing the seismic weight.

But generally for an intermediate floor level you have:
- Full Dead load of floor system including partitions, ceilings, etc.
- Full self weight of all floor framing (may or may not be included in the above)
- 1/2 self weight columns and walls(including cladding) below
- 1/2 self weight column and walls(including cladding) above

 

[n39]

When I compare the ELF analysis by ETABS and hand-to-hand calculations, I found a differences, in which although I modeled the tie beam, ETABS does not read the tie beam as effective weight, as to my hand calculation I did. Therefore the seismic weight of my hand calculation is bigger. But I think the seismic weight doesn't include tie beam/sloof in it's calculations. Correct me if I'm wrong

 

[Luceid]

If ETABS is not reading a modelled object as having weight - then it's most likely a problem in your inputs. Make sure your material has a self weight associated with it - make sure your "DEAD" load pattern has a self weight factor of 1, etc. If something has weight - and will be in your building - why would you ignore it (assuming it makes a noticeable difference).

 

[brTCP]

A lot of answers here already, but I think the correct answer is the following:
- the tie beam at the foundation level moves with the ground during an earthquake, so its mass does not produce effects on the structure. For the same reason we don't include the mass of the footings in the seismic mass. If you do so, you will never be able to mobilize that mass, no matter how many modes you include in your modal analysis.

 

[Luceid]

Oh - I hadn't read above - my mistake. If the tie beam is at the base of the structure - it wouldn't be considered

 

[n39]

So I get the conclusion that in that example above, the tie beam is not included in the effective seismic weight. I modeled an example of a simple building, with a basement but I didn't modelled the basement. I start my model from the 0.00 elevation as you can see above. My question is, in the bottom story for seismic load, should I input it as El. -2.5 (the base) or as El. 0.00???
**Elevation -2.5 from 0.00 was meant to be a basement