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Using FEMA 356 to Size New Footings

Using FEMA 356 to Size New Footings

Using FEMA 356 to Size New Footings

(OP)
Has anyone heard of this? I'm being told to use this method to size footings. New footings for a new building. Seems weird, and unconservative (multiplying qallow by 3.0, and using no load factors).

Thoughts?

RE: Using FEMA 356 to Size New Footings

Doesn't make sense...

Dik

RE: Using FEMA 356 to Size New Footings

Assume qallowable = 2 ksf; then required area of footing = 6 S.F. It could carry 12 kips allowable load. Conservative if P < 12 k. Unconservative if P > 12 k. Seems weird in either case.

BA

RE: Using FEMA 356 to Size New Footings

FEMA 356 calculates the demand differently. Unless you are using its method to calculate the demand, do not use its acceptance criteria.

RE: Using FEMA 356 to Size New Footings

FEMA 356 is a performance-based document, has actually been superseded by ASCE 41. It's more geared for existing buildings but can certainly be used for new as well. The reason that q-allow is multiplied by 3.0 is because the seismic demand in this document IS NOT divided by an R factor, you're using the full seismic demand. Using the 3.0 multiple for q-allow while not also using the much greater seismic demand would be very unconservative, as you've noted.

Seems odd to me if only foundations are intended to be designed for FEMA 356 rather than the whole building. Regardless, I would also tend to check foundations for gravity and wind demands using just straight IBC/ASCE 7 still. FEMA 356/ASCE 41 aren't really geared to look at gravity/wind.

RE: Using FEMA 356 to Size New Footings

But qallowable*3.0 gives units of pressure, not area, so how is this a way to calculate footing size?

BA

RE: Using FEMA 356 to Size New Footings

Unless it is an unusual case not covered by current new building code and referenced standards, I would be concerned that mixing an outdated existing building standard would be rejected by a building official during permittimg or by the building inspector during construction.

RE: Using FEMA 356 to Size New Footings

(OP)
So I've talked with the engineer in charger about my concerns. His reply is something along the lines of...

"We're doing performance base design, not "enveloping". I.E. we're trying to see exactly what's going to happen. We're finding out what size the footings have to be in order to assume full fixity for the column bases for a Life - Safety perfomance level. So we do not factor our loads, and we use the full expected bearing capacity of the soil. Afterwards for our final design checks we use ASCE 7-10."

I still have a lot to learn about performance based design, but in reading FEMA 356, I see what many of you have stated - there is no R value. My question is why not? IF a system is detailed to behave inelastically then why can we not take advantage of that?

RE: Using FEMA 356 to Size New Footings

Still doesn't make any sense...

BA

RE: Using FEMA 356 to Size New Footings

That makes more sense, they're doing it for everything and not just the foundations. And they're checking with ASCE 7-10 as well, which takes care of gravity/wind.

To your question, you are taking advantage of the inelastic behavior. That's why you're multiplying your allowable bearing by 3. If you look in other areas of FEMA 356/ASCE 41, you'll see other elements get their capacities inflated as well. This inflation factor in ASCE 41 is called an 'm-factor'.

For linear design ASCE 7 and FEMA 356/ASCE 41 take a similar approach with one distinct difference. In ASCE 7 you reduce your loads for the entire structure with an R factor and use uninflated capacities. In FEMA 356/ASCE 41, you don't reduce your loads but use these 'm-factors' to inflate your component capacities. Answer should be about the same but you can tweak things a bit more with the second approach as the m-factors vary by component type (yielding/non-yielding), detailing level (compliant/semi-compliant/non-compliant), and sometimes even load level. A lot more differentiation than straight ASCE 7 where you just have one R factor and one overstrength factor for the entire building.

RE: Using FEMA 356 to Size New Footings

(OP)
Thanks. What I still don't understand is he is still using an R value.

Unfortunately he's not being much of a teacher. :(

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