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Re: Seismic - UBC 1997 Application

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Richard Lewis wrote:

> I sent a similar message to the list yesterday, but only received one
> response.  I am still confused.  No offense Ben Yousefi, I did very much
> appreciate your input, I'm just still confused.  Now somebody has got to
> understand this code and be able to explain it to an average engineer like
> me!

As one who understands the equations, I like Ben Yousefi's response, but let me
see if I can clarify it further:

> Some background, I thought 1997 UBC took care of the change from ASD to USD
> through the R factor and other adjustments.  I don't understand why Equations
> 30-1 and 30-2 are there.  Do they apply to all designs?

Yes, the base shear produced by the 97 UBC equations is automatically factored.
That is why the R value differs from the 94 UBC Rw by roughly 1.4 times.  I
personally think this is a somewhat backwards way of handling it - since
everything else that you plug into a USD equation (LL, ADL, Self-Weights, Etc.)
needs to be factored, why set the new base shear apart?  Just my 2 cent soap
box.  But, it is indeed pre-factored.  If you wanted to do ASD work (old-style
steel or wood), you would need to "remove" the 1.4 factor as noted in 1612.3.1
and 1612.3.2.  Since you are designing a concrete SMRF, you can take the base
shear more or less as it is.  Don't forget redundancy, though, which has a trick
for a SMRF.  For a SMRF, there is a cap to how high your redundancy factor can
go.  If it is above the cap, you need more frame.  See the latter parts of
1630.1.1 for more info on this, or just start a new email discussion if it's
confusing.

> Equation 30-2 multiplies Eh by the omega factor.  I am designing a concrete
> SMRF structure.  Omega is 2.5.  Therefor Em is equal to 2.5 Eh, which I
> believe is the base shear.  So does this mean I multiple the base shear by
> 2.5 for my member design?  If I am running load combinations in my frame
> analysis, do I add this factor to the combinations?  It seems I am increasing
> the shear quite a bit, although it was already suppose to be at strength
> design.

The Omega factor is something like the 3/8 Rw factor in the 94 UBC.  It is
intended to address special elements that are not ductile or that you need to be
particularly sure will not fail for whatever reason.  Omega is defined as a
"structural overstrength" factor.  It estimates how much greater the resistance
of the building to lateral loads might be compared to what you designed it for.
In a concrete building, much of this overstrength would be due to yielding.  So,
if an item can't yield, or if you don't want it to, it may need to be designed
for 30-2, because everything else will be yielding and it will get dragged along
for the ride.  As Ben Yousefi pointed out, collectors are one example of this.
Unlike a beam that may be one of several in a line, if a collector starts to
fail, there just isn't much ductile redistribution that can happen.  Only use Em
where it is specifically called for.  For typical members (ductile beams, shear
walls, etc), use equations 30-1, which is basically just the base shear with
redundancy and vertical forces thrown in.  Nothing too exotic there (making sure
to note the cap on redundancy noted above).  For a concrete SMRF, you probably
won't use 30-2 much if at all, but you will use 30-1 constantly (30-1, as
previously stated, isn't really as complicated as it looks).

I hope this helps.

Paul Crocker
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