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RE: Need Help for the Seismically Impaired

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Title: RE: Need Help for the Seismically Impaired


California engineers are still operating under the 1997 Hammurabi Code. Let me help.

They call it base shear because it is at the ground level.  Framing below the base just needs to be detailed the same and be as stiff and strong as the super structure to transmit the force to the final point of resistance [IBC 1616.1 & 97 UBC 1633.2.10].  The UBC had framing below the base in the detailed system requirements but the 2000 IBC seems to assume that everyone must know this.

If the first floor is at the ground surface then the code does not require you to include it in the seismic weight.  You should include the first story framing  weight to design the connection to the foundation, but you would only need to include the upper half of the first story framing weight with second floor seismic weight to calculate the story forces and base shear. 

For walls you figure Vbase=sum(Fstory)+(wall weight)*Cs.  It is quick and conservative to add the wall weight in the direction of loading to the story weight.

Scott Haan

-----Original Message-----
From: Jim Kestner [mailto:jkestner(--nospam--at)]
Sent: Friday, October 04, 2002 10:50 AM
Subject: Need Help for the Seismically Impaired

Here is a question from a seismically impaired engineer.....

Since I rarely have to deal with seismic, but now are required to do so
because of the IBC 2000, I have a few simple questions for you California
types......  Please bear with me.

When dealing with a building with a full basement below grade, does the
design "base" shear occur at the first floor level? When calculating the
design "base" shear does it need to include the first floor?

Does the design "base" shear include the basement walls? What about lateral
soil pressure on the basement walls?

When dealing with lateral seismic forces generated by the weight of masonry
partitions, it seems logical to assume forces parallel to those walls would
all be transfer to the floor. For forces at right angles to the masonry
partition walls, it seems logical that 1/2 goes to the level above and 1/2
to the floor (assuming the wall spans vertically). Where the wall spans
horizontally between cross walls, it all should go to the floor.

Jim Kestner
Green Bay, WI

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