really basic, kinda stupid seismic question

huangyhg

really basic, kinda stupid seismic question
a little background first -- our firm practices in an area where seismic was not really a design issue until the new ibc 2003 was adopted statewide (western pa) less than a year ago.
so, here's the dumb question -- where is the "base"?  now, i know the official definition is "the level at which the horizontal seismic motions considered to be imparted to the structure" -- but is that bearing level of the foundations or exterior grade?  all of the diagrams in the various literature we've reviewed show structures with nothing below grade -- does that mean below grade is ignored or is the grade line meant to infer the bearing elevation of the building?
we are currently starting a project that has 3 levels below grade and only 2 above grade -- so the location of the base is going to have a significant impact on the seismic loads (using equiv. lateral force -- design category b).
thanks for the help
generally speaking you should consider what the effect of the soil surronding the structure is.
can it be considered to actually restrain the lower sections?
is it engineered fill?
is the structure on or in rock?
is the structure on soil that will poorly attenuate the wave motion?
if you can find an arguement for the bottom two floors having only ridgid body motion (motion following exactly that of the ground in this case) then perhaps you'd fix the base at the mudd line, dredge line or existing grade line.
to ignore the potential effects of the lower levels you'd better have a good reason other than not finding an example.
regards,
qshake
eng-tips forums:real solutions for real problems really quick.
just to clarify -- this isn't a case of not finding an example -- it's more a case of just wanting to better understand the intent of the code.
while i like the easy answer much better, i rather expected more along the lines of the 2nd answer.  we're in site class d soil, no rock.
i suppose for a low rise bldg w/ a single story basement, an argument could be made that the foundation walls would act as shear walls and could lock the 1st floor down -- causing it to move together with the foundations, but i'm a little leary saying that would be the case when we go down 3 levels.
any thoughts?
i would say that your base is the bottom floor of your structure.  normally the ground and the bottom/first floor are the same, so this doesn't come up.
when doing your analysis, you'll need to analyze the building as a 5 story building for seismic, and only a 2 story building for wind.  seismic will probably control, but its not quite that simple.  to be more realistic, use the soil as a force acting against the seismic force.  its possible that the soil force will be greater than the seismic force.  if this is the case, you will need to analyze the structure as a 2 story structure only.  if the soil force is not greater than the seismic force, use the seismic force minus the soil force as your total seismic force on those stories.
thats how i would do it.
billyclyde,
great suggestion!  thanks
actually, thinking more in depth about it, if the soil force is higher than the seismic force for the levels below the grade, you will still need to ensure:
1) the floor is a diaphragm and the loads can be transferred to the soil on the "far" side of the building, and be resisted by the soil there. or
2) if the floor isn't a diaphragm, ensure there are enough braces, shear walls to transfer that lateral load to the "far" side of the building, where the soil will resist any movement.  again, ensure that the soil will be able to resist that force.
you will obviously have retaining walls on each side of the building, but the floor diaphragm force will be transferred to the soil through the exterior beam, or an equivalent depth.  the whole story depth won't exert an equal load on the soil.  this is getting long and pretty complicated, but to use the whole story depth as your area acting on the soil from seismic forces, you would have to design your retaining walls to resist the seismic lateral force distributed over the wall area, as well as the soil pressure force.