Subject: RE: IBC, Rho Factors, 10/lw, and Influece of FEMA/NEHRP Folks
From: "Ron O. Hamburger" <ROH(--nospam--at)eqe.com>
Date: Wed, 26 Apr 2000 07:18:25 -0700
This message responds to recent posts by Ben Youssefi and Roger Turk. The posts
addressed both technical and nontechnical issues. Technical issues were the
basis for the 10/lw factor, in calculation of rho, and its application to wood,
masonry and other elements. The discussions as posted were correct -
a) the original intent was not to have 10/lw ever result in a reduction in the
effectiveness of a peer or wall as an element. Thus 10/lw was not originally
intended to be taken as less than one.
b) certain folks, including members of the SEAOC Seismology Committee and S.K.
Gosh believed that in addition to causing higher design forces, this could be
used as a way of encouraging people not to design short peers and walls.
Apparently this group was successful in the recent code hearings, and it will
remain as a penalty for other than wood frame construction.
The second technicl issue was with regard to the application of rho to
When originally developed (as noted in earlier posts, I was a member of the
Seismology sub-committee that did that) it was intended to apply to all elements
of the structure, including diaphragms. In fact, at one point, we considered
including in the rho formulation , a consideration of diaphragm configuration
relative to placement of walls, but could not figure out a way to do this. The
behavior of diaphragms in non-redundant structures is significant. Failure of a
number of parking garages in the Northridge earthquake occurred because the
diaphragms were asked to work too hard to bring shear forces to relatively few
walls. This was specifically what the committee had in mind when the rho
provision was first developed.
Now - if the rho applies only to vertical elements of the structure, and not the
diaphragms as proposed by the current SEAOC Seismology Committee, you run the
risk of creating relatively strong walls (designed with a large rho) and
relatively weak diaphgarms (designed with rho =1). Since the load applied to
diaphragms is a direct function of the strength of the walls, this can lead to a
situation of diaphragm failures, just like the Northridge structures. The IBC
action was, in my opinion appropriate.
Finally, the non-technical issue-
Roger Turk and others have initmated that the FEMA/BSSC group have too much
influence on the IBC and that they are not Californians so they don't know
anything. This is an incorrect understanding. More than 50% of the membership
of the FEMA/BSSC committees are Californians, many of them members of SEAOC's
Seimsology Committee, including such people as - Bob Bachman, David Bonneville,
Richard Phillips, Tom Hale, Bill Holmes, Tom Sabol, and many many others. The
FEMA/NERHP committees are truly comprised of some of the top earthquake
engineering talent in the United States, and for that matter, the world - and
they do know something about the way real buildings behave in earthquakes.