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Re: Need Help for the Seismically Impaired

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Jim
Generally, the design base shear occurs at the footing,the basement wall
would still take forces relative to floor diaphragm and what evever
projected height of masonry element.  from a theroatical stand point, load
distribtion tend to get very small at low "h" values.  However, you still
have shear and moment on your basement wall that need to be evaluated.  As
far as Lateral load on stem wall, Lateral soil earth pressure, CALTRAN
guide lines is to consider earthquake soil pressures at walls 12 feet in
height or greater.  If you are building on hillside, and the relative wall
height from the upper side of slope to the lower side is six feet or more,
you need to consider seismic contributions from lateral earth pressures.
If the basement is supported on all four side, the basement has a period of
zero and thus moves along with the ground without any lag, therefore,
seismic accelerations are commonly ignored, if you do not have specific
geotechnical recommendations to account for seismic forces. For out of
plane forces, treat the wall as a pinned element at the floor and check the
force on the connection due to mimimum seismic force requirement. Hope that
helps.
Samir Ghosn,PE
Harris & Associates
800-827-4901 xt 360
At 01:49 PM 10/4/2002 -0500, you wrote:
>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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