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Fw: What's so special about a Special Moment Resisting Frame

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The difference can be most quickly illustrated by the magnitude of the
seismic design lateral force reduction factor (R in UBC97, Rw in UBC94 and
previous).  In UBC97, R for a steel SMRF is 8.5, while R for an OMRF is
only 4.5.  

Not to get into a lengthy dicussion of seismic design philosophy, but
suffice it to say that the UBC envisions both "elastic" and inelastic
action of the part of a building system when it is resisting the effects of
a very large magnitude earthquake.  The difference between the OMRF and
SMRF is mainly the relative amounts of elastic and inelastic action.  The
SMRF is expected to have a much higher inelastic performance capacity than
the OMRF, hence the higher R factor (resulting in a much lower Vb).  This
higher inelastic performance capacity is the result of more stringent
detailing requirements, which allow for more plastic curvature/rotation
capacity while still keeping the member (and system) stable.  The OMRF does
not have to adhere to the seismic detailing requirements (and limitations)
as the SMRF, so it cannot be expected to have the same inelastic
performance.  It is supposed to behave predominately in an "elastic" manner
(I put quotes around elastic because if one is using strength design, LRFD,
the design state really isn't elastic, but it is considered as such because
it doesn't involve really huge plastic rotations).

Hope this helps.

T. Eric Gillham PE  
> From: Kathleen  A. O'Brien <wildwoman1(--nospam--at)>
> To: SEAOC Forum <seaoc(--nospam--at)>
> Subject: What's so special about a Special Moment Resisting Frame
> Date: Thursday, May 07, 1998 4:30 PM
> Can anyone tell me the difference between a special and an ordinary
> moment-resisting frame?
> It's not clear to me from the Code, the Advisory or anything else I have
> read
> Thanks!
> Kate O'Brien
> Simi Valley