Subject: Appropriate R factor for restaurant addition
From: Albert Meyer <Ameyer(--nospam--at)martinaia.com>
Date: Mon, 17 Jan 2000 18:14:52 -0500
I need some direction as to how to model a particular building
seismically. Since I am from the Northeast, I don't have the
opportunity to do extensive seismic analyses on buildings, and my
knowledge in this area is weak and limited mostly to static analyses. I
do have a special interest in seismic design though, and do my best to
make a correct seismic analysis on the buildings I design, although most
portions of lateral design are controlled by wind in my area.
I have a two story restaurant addition which consists of a second floor
office space partially open to below. The construction is lightweight
roof trusses, wood framed bearing / shear walls on three sides on a
lightweight wood floor truss system with OSB floor sheathing and gypsum
concrete. The shear / bearing wall at the front of the addition is 12"
CMU with various openings, and this wall is the full height of the
structure. The front to back shear / bearing walls will be supported by
steel beams which also support the second floor system. At the left
side I will probably tie the second floor diaphragm to an existing 8"
CMU wall. I'm not sure how I'm going to handle the right side since I
have a 1' drop down from the second floor trusses to the low roof
trusses and this is the bearing location for the wall. The low roof
trusses are supported at the exterior on an 8" reinforced CMU wall. The
first floor area is larger than the second floor area. The first floor
perimeter walls looks like a square which has had the upper right corner
chooped off at a 60 degree angle from the midpoint of the right side
I apologize for the rambling description of the building. Since I have
two different lateral force resisting systems not in orthoganal
directions but in different stories, what is the best approach to
modeling this structure? Should I consider the upper floor
independently with its appropriate response modification factor ( R )
and then apply line loads to the lower diaphragm and structure with its
appropriate R factor? I am only doing an equivalent lateral (static)
force analysis here. Should I model the structure as a whole using the
lower R value (generating a higher base shear)? The 1996 BOCA code
states that the lower floors may not have an R lower than the lowest R
value above, but what is the case for a higher R above? I know that
most of you use the 1997 UBC, but any advice and comments are welcome
and greatly appreciated.
Albert J. Meyer, Jr., P.E.