From: "Sprague, Harold O." <SpragueHO(--nospam--at)bv.com>
Date: Wed, 23 Feb 2000 08:45:30 -0600
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.
> -----Original Message-----
> From: marfra(--nospam--at)opus.co.tt [SMTP:marfra(--nospam--at)opus.co.tt]
> Sent: Tuesday, February 22, 2000 11:11 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: aci 318 Seismic provisions
> I have a concern about ACI 318/318r-296 regarding clause 18.104.22.168.
> 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