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# RE: Rw factor, UBC 97

• To: seaint(--nospam--at)seaint.org
• Subject: RE: Rw factor, UBC 97
• From: "Lutz, James" <James.Lutz(--nospam--at)earthtech.com>
• Date: Tue, 29 Apr 2003 08:29:13 -0700

```Rw and R are force reduction factors that allow you to design for less than
full elastic force levels in recognition that the ductility of the structure
will allow it to deform into the inelastic range, hopefully without total
failure in the time period assumed for the seismic event, generally around
half a minute.

The use of reduced forces for design is a tradeoff whereby you accept some
permanent deformation of the structure in case of earthquake, but you get to
design lighter members. You get life safety and better economy in exchange
for possibly having to write off the structure in case of a strong
earthquake.

R (as in reduction factor) reduces the force level to a value to be used
with ultimate strength design. Rw is a similar number used in earlier
versions of the building code, which reduced the force level to a value to
be used with working stress design. That's what the "w" stands for.

The R and Rw numbers are generally lower for connections and critical
lateral load resisting elements, both because they may be less ductile, but
also to force the permanent distortions to occur where you want them to. You
may be able to tolerate yielding in a beam, but you can't tolerate a broken
connection.

-----Original Message-----
From: Milton Veloz [mailto:m.veloz(--nospam--at)codetel.net.do]
Sent: Tuesday, April 29, 2003 2:02 AM
To: seaint(--nospam--at)seaint.org
Subject: Rw factor, UBC 97

Hello:

I am new using UBC 97 code. In 2108.2.5.6 Boundary members, it talks
about a Rw=1.5. Can somebody explain me what is Rw? Is a kind of
reduction factor?

Milton

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