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SEISMIC DESIGN to UBC'97
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- Subject: SEISMIC DESIGN to UBC'97
- From: Julian Birch <julian(--nospam--at)mbplon.demon.co.uk>
- Date: Wed, 1 Sep 1999 09:53:25 +0100
Could anyone offer advice on seismic design of a 10 storey rc building in Kuwait to UBC'97, particularly the following: 1. Thermal expansion joints / movement joints. The floor plan area of the building (approx 4000m^2) is such that several expansion joint are required. The UBC does not cover joints specifically but reading between the lines I assume that either joints should be avoided or they should wide enough to avoid pounding of one part of the building against the other eg. double columns with wide gap or wide corbels say. Is correct? 2. Consideration of uplift effects (UBC'97 1633.1) The UBC calls for consideration of uplift effects (including overturning) but does not qualify. Am assume this to mean the application of the "lateral force" (Section1632) to beams & slabs in addition to columns is that correct? Also is the "load effect due to the vertical component of the earthquake ground motion" ie. Ev (Section 1630.1) to be taken as both positive and negative thereby increasing/reducing/removing the gravity compression effects. 3. The structure, which incorporates a double basement, is to be built on a reinforced concrete raft founded at 10m below GL. I believe that the site comprises dense sand and SPT values are greater than 50 at the founding depth of 10m. Also the water table is at ground level. The high SPT values lead me to believe that liquifaction MIGHT NOT be a problem. However, I assume that the soil layers in Kuwait are hydraulically deposited which would appear to be more prone to liquifaction than non-hydraulically deposited layers. Any comments? 4. One thing that does have me slightly puzzled (although I think I know the answer) is the application of the base shear when using the static lateral force procedure to UBC. The UBC states that the BASE is "the level at which the structure as a dynamic vibrator is supported" - and that level can make a lot of difference! ie all mass above that level is used to calculate the total design lateral force ie base shear. My interpretation s this: If both basements are infinitely rigid then the base would be ground level since structure above this level would oscillate at whatever frequency the earthquake excited. Therefore the basement floors would not be included in the calculation of lateral force. Similarly, if both basements and the ground floor were infinitely rigid then the base would be taken to be first floor level and all floors below this level would not be included in the calculation of total lateral force and so on. If my logic is correct then joints of course become very important when deciding whether a floor can distribute load to many walls and so be taken to be infinitely rigid relative to the floor above (and hence the joint query above) Of course it would be easy to consider the base to be the underside of foundation slab but this would mean including the mass of all floors above this level in the calculation of lateral force and even at 0.049g this is a huge force. Any comments? Regards
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