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RE: Need Help for the Seismically Impaired

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Jim,
The comments you received are correct. Let me be a bit more simplistic
in the method. You can assume the top of the foundation wall to be a
point of fixity or the level where the base shear is distributed.
>From a purely simple aspect, the distribution of shear will not deflect
the solid masonry wall unless there are openings and the wall is high
enough above grade to consider the distance from the soil to the top of
the masonry. 
Otherwise, you need to accumulate the base shear and design your
connection of the mud-sill (sole plate) to the masonry - in other words,
design your anchor bolt connetion and assume the top of the masonry wall
to be fixed or supported laterally.
When designing the masonry basement wall, I would first design the wall
without consideration of the first floor to brace the wall. This brings
the moment down into bottom of the stem causing bending of the wall at
the connection of the stem and the footing.
Then I would design the stem for the assumption of the wood diaphragm as
braceing the top of the wall. The moment will rise to the center of the
wall and I would add reinforcing steel at the center to control the
tension side of the moment induced at this level. 
By doing this, you cover all conditions of bending. I would do this
whether you are in a seismic zone or not as the design does not actually
change. Of course the condition of the soil, the drainage behind the
wall all adds to the equation, but if  you consider the two conditions
during the backfilling of the wall and then the installation of the
bracing at the top, you will have covered all conditions.

Unless a portion of the diaphragm at grade level is a slab on grade, you
won't actually be able to use the friction of the soil to control the
lateral force at the top of the stem.

The distribution of shear is tributary to each level, but cumulative
down to the top of the stem. The force normal to the stem will generally
be resisted by the chord created at the top of the stem at the opposite
side between walls perpendicular to the direction of force.

It may sound confusing, but I think it is very easy if you consider the
top of the stem and the installation of the diaphragm as a simply
supported beam. The moment occurs between the perpendicular resisting
masonry stem walls and the top of the stem creates a chord that you will
design. If the aspect ratio is large enough, you might consider a
concrete beam at the top of the masonry stem to act as a chord that
resists the calculated tension in bending.

The rest can be calculated from the roof down. If the shearwalls stack
around the perimeter of the building you have an easy chore of carrying
the calculate lateral load down from the roof to the foundation.
Interior shearwalls without basement stem walls will create a greater
challenge.

I would highly recommend Breyers book on Wood Design (Donald Breyer) as
it will cover lateral loads.

Finally, the lateral loads that govern - especially in your area will
most likely be the forces due to wind - not seismic and will be treated
very much the same way.

Dennis S. Wish, PE

-----Original Message-----
From: Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com] 
Sent: Friday, October 04, 2002 11:50 AM
To: 'SEAINT'
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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