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RE: aci 318 Seismic provisions

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Mark, also please note that the beam moments due to seismic loads do not
cancel out, rather, they are additive, and the seismic loads can be much
larger than the gravity loads in seismic zones 3 and 4.  At the level of
seismic force where the beams would yield, the gravity loads could be
negligible.  This may be obvious to engineers that design for high seismic
regions, but not so to others.

Joyce Fuss, Structural Engineer
Lionakis Beaumont Design Group
Sacramento, CA

> -----Original Message-----
> From:	Sprague, Harold O. [SMTP:SpragueHO(--nospam--at)]
> Sent:	Wednesday, February 23, 2000 6:46 AM
> To:	seaint(--nospam--at)
> Subject:	RE: aci 318 Seismic provisions
> Mark,
> The whole section of 21.4 is for special moment frames.  The ACI 318-99
> added a lot of commentary to this that may help explain the provision.  
> Me and Mg are the flexural strengths not the applied loads.  What this is
> intended to assure is that the first failure mode from ground shaking is
> in
> the form of bending in the girder.  This type of frame assumes a lot of
> inelastic yielding as it absorbs energy from an earthquake.  If the column
> yields first, you could get a collapse.
> Regards,
> Harold Sprague
> > -----Original Message-----
> > From:	marfra(--nospam--at) [SMTP:marfra(--nospam--at)]
> > Sent:	Tuesday, February 22, 2000 11:11 PM
> > To:	seaint(--nospam--at)
> > Subject:	Re: aci 318 Seismic provisions
> > 
> > 
> >  I have a concern about ACI 318/318r-296 regarding clause
> > Sum Me must be greater than sum Mg
> > 
> > For most internal columns , dead and live load moments partially cancel
> > each other out. Therefore the design moment capacity of the column is
> > naturally less than that of the girders it supports. If the moment
> > capacities of the girders are added without taking into account the
> > partial neutralizing effects of their directions, the columns become
> > much larger than they need to be without the provision. Did i miss
> > something , this provision  does not seem logical for internal columns.
> > 
> > mark francois engineer
> >