Need a book? Engineering books recommendations...

# R-values in seismic design

• To: seaint(--nospam--at)seaint.org
• Subject: R-values in seismic design
• From: terrence turner <tnturner(--nospam--at)glen-net.ca>
• Date: Sat, 26 Sep 1998 15:29:29 -0400

```	Excuse me for not explaining myself fully when I introduced the R-factors
without referencing their respective codes and location.  The R-factors I
was discussing were with respect to the National Building Code of
elastic base shear and design base shear.

Elastic Base Shear
Ve= v*I*F*S*W
v  is the zonal velocity ratio ranging from 0 to 0.4.  In my city Ottawa
our zonal velocity ratio is 0.1
I is the importance factor of the structure which varies between 1 to 1.5
F is the foundation factor which varies from 1 to 2
S is the seismic response factor which is dependent on the natural period
of vibration of the structure and the ratio of acceleration related to
velocity related seismic zones.  This factor varies form 4.2 to 1.5/T*0.5  and
W is the weight of the building (includeing 25 percent of the design

Design Base Shear
V=0.6*Ve/R
0.6 is a factor representing level of protection based on experience.  Our
R-factor is dependent on the type lateral force resisting system used.  Our
R-values range from 1.5 to 4.  To obtain a greater R-value more stringent
detailing requirements are required.  Some of the corresponding lateral
force resisting systems include:

STEEL
R=2		BRACED FRAME WITH NOMINAL DUCTILITY
MRF WITH NOMINAL DUCTILITY

R=3		DUCTILE BRACED FRAME

R=4		DUCTILE ECCENTRICALLY BRACED FRAME

CONCRETE
R=2		WALL WITH NOMINAL DUCTILITY

R=3.5		DUCTILE SHEAR WALL

R=4		DUCTILE COUPLED SHEAR WALL

A R value of 1.5, which corresponds to any system which has no ductile
detailing requirements, is permitted depending on location and the number
of stories of the structure.

Our load combinations from the NBCC 1995 are the following

1.0D + 1.0E
or
1.0D + 0.5L + 1.0E

My question was with regards to the detailing requirements for a steel
braced frame with nominal ductility(R=2.0).  Our requirements are governed
by CAN/CSA-S16.1-
M which states (clause 27.5.3) the connection must be designed to

"In velocity or acceleration related seismic zones of 2 or higher the brace
connections shall have a factored resistance at least equal to the axial
tensile yield strength of the brace (AgFy) unless the designer can show
that a lower resistance is adequate."

I find this clause leads to excessive forces for connection detailing and
that the final clause...(unless the designer can show...) is too open
ended.  Any suggestions?

thanks

Jon Turner

```