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Fw: FEMA 310: Force-Controlled Action onto Column Axial Load

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Eddie:

OK, to begin with I am sorry to say that I don't (gasp) have a copy of FEMA
310 to refer to, but perhaps I can throw out a couple of points that may or
may not assist.

Assuming that you have a frame structure, without discontinuous shearwalls
for example, the maximum axial load induced by lateral motion will be
limited by the capacity of the beams framing into the column.  This capacity
limitation could either be controlled by the shear capacity of the beams, or
the flexural capacity.  Either way, there will be a maximum Mo and
associated Vo in the beams, and that Vo will be the limit as far as axial
load input into the column.

I am assuming that you are looking at a corner or edge column, since an
interior column, with the same beams framing in from 4 directions, would
have little or no net shear=axial load input at each floor, since the beams
on opposite sides would cancel each other out.

Regarding the torsional effect you mentioned in your post, I don't feel I am
fully understanding where you are coming from.  If one has already
considered the case wherein all of the beams at all levels have yielded,
then the resulting axial load (which will be due to the lateral movement of
the building, not gravity loads) is all that will be manifested.  Additional
movement beyond this displacement level, be it torsional or translational,
will not result in additional axial load on the column since a mechanism has
already been formed. Since the beams can't increase their Vo, they cannot
increase the column axial load, and since they are the only members that CAN
increase the column axial load, the column axial load cannot increase.

That said, there can be a very grey area wrt just what the "beam" is.  How
much slab one considers as contributing to the beam Mo, and hence Vo, as
well as slab steel etc, would affect the maximum Vo and hence axial load in
the column induced by lateral movement.  Paulay and Priestley's text gives
some values on assumed bf of slab.

Interestingly enough, P&P as well as, I think, the NZ code, allow designers
to take a portion of the total Vo from all beams yielding in tall buildings,
the rationale being that in such a structure it is unlikely, because of
higher mode effects, for ALL the beams to be yielding simoultaneously.
Hence, the portion taken is a function of building period among other
things.

Finally, if the structure being evaluated has 2 way frames, then
consideration needs to be given to diagonal movement which would cause
additive Vo's in two beams for say a corner column.  This would be the
controlling condition both in terms of tension and compression induced in
the column.

Regarding flat slabs, I would probably be inclined to make some assumptions
as to effective "beams" to model the slab action, then proceed with the
above.

Not sure this answers your question, but let me know if it does.

BTW, thanks for the heads up on the concrete stress strain article in ASCE's
Journal of SE.  Got a copy of it a couple of weeks ago.


T. Eric Gillham PE
GK2 Inc.
PO Box 3207  Agana, Guam  96932
Email - gk2(--nospam--at)kuentos.guam.net
Ph:  (671) 477-9224
Fax: (671) 477-3456
-----Original Message-----
From: Eddie Gonzalez <Eagonzal(--nospam--at)ENG.CI.LA.CA.US>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Tuesday, February 02, 1999 7:08 AM
Subject: FEMA 310: Force-Controlled Action onto Column Axial Load


>Can anyone clarify for me how the axial load in a column of a frame
building is
>a force-controlled action per FEMA 310 (pg 4-6, 4-7).
>
>I would grant that there is definately a limit on the portion of the axial
load
>contribution coming from the shear of the beams framing into the columns.
>Nevertheless, the lateral forces also cause a torsional effect on the whole
>structure, which inturn, will cause large axial loads onto the columns,
>undoubtably larger than the phiVn of the beams.  Since these lateral forces
>are, by definition, deformation controlled forces, directly, why consider
the
>axial loads onto the columns only force controlled.
>
>Now granted, the beams can have an axial load limitation, but how about
flat
>slabs of the 1950's with drop panels and no beams?  What am I not seeing?
>
>
>Mr. Eric Gillham or Mr. McClure are you there?
>
>Regards,
>ed gonzalez
>
>
>
>