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- To: seaint(--nospam--at)seaint.org
- Subject: A seismic question with no (easy) answer?!
- From: Cliff Schwinger <clifford234(--nospam--at)comcast.net>
- Date: Thu, 12 Sep 2002 23:36:43 -0400
How do engineers working on projects in earthquake prone areas (California) handle expansion joints in existing buildings when additions or other modifications are made to the structures and the building code requires the existing structure to conform to current seismic requirements for new structures? Here is a fictitious example (but one that will be very common in the years ahead under the IBC Code in the northeastern U.S.): - A 20 year old eight story structural steel building was designed for future vertical expansion for 4 additional floors. - The building has a 1 ½? expansion joint down the middle and the two halves are separated by double columns. - The 4 additional floors are now being added ? but only on the north side of the expansion joint. No work is being done to the existing structure on the south side of the expansion joint. - This fictitious building is located in an area of low seismicity. When the original structure was designed the building code required no consideration of seismic forces. The present code is IBC 2000 and there are sizable seismic loads that must be considered because the building is an ?essential facility? and is founded on poor soil. - The buildings will ?bang? during the design earthquake. The ?banging? was not an issue prior adding the new floors, but now that the new floors are being added, IBC 2000 requires the lateral load resisting system in the half of the building with the new floors to be upgraded to meet IBC 2000 requirements. The building code does not require the structurally independent framing on the other side of the expansion joint to be upgraded because no work is being done to the existing structure on that side of the expansion joint. Even if we did upgrade the lateral load resisting system in this half of the building, the existing expansion joint is so small that the halves would still bang during the design earthquake. (All floor slabs align at the expansion joint, so there is no potential for floor slabs to ?knife through? columns.) The questions I have concern banging of the building halves at the expansion joint: 1. What do we do about the ?banging building? situation? Even if we were to cut the slab back and put in a big expansion joint, the double columns are close together and they?ll hit each other. In fact maybe ?banging (steel) columns? are not so bad due to the ductility of steel. 2. The code doesn?t prohibit buildings from ?banging? ? it just says that there can?t be any ?damaging contact?. If building segments bang together there may be lots of small damage, but I would think that if both buildings are of the same construction type, if all of the floors align on either side of the expansion joint, and if the structural framing is ductile, then banging action would not cause collapse. What is the definition of ?damaging contact?? If damaging contact means damage that causes a threat to public safety, then I would say that slab edges hitting each other are not a concern. On the other hand if facade components ?bang? at the expansion joint (same 1 ½? gap at expansion joint in facade), then that would be a threat to public safety and that condition would have to be corrected. 3. What is done in California when renovations or additions are made to old buildings with expansion joints and those joints are too small to prevent contact during a big earthquake? I heard that there is software that can actually model the ?banging building? phenomenon. Is this type of analysis common in California? Can anyone shed some light regarding practical solutions to this problem? As I said at the beginning of this post, the specific details that I outlined in this post all relate to a fictitious building, but this problem will have to be addressed in many renovation projects here in the northeast U.S. where I live when IBC 2000 kicks in. Thanks. Cliff Schwinger ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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- From: Cliff Schwinger
- RE: A seismic question with no (easy) answer?!
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