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[SEAOC] Footings' Tie Beams[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc <seaoc(--nospam--at)power.net>
- Subject: [SEAOC] Footings' Tie Beams
- From: Mony Serhal <mony(--nospam--at)destination.com.lb>
- Date: Sat, 15 Jun 1996 15:09:26 -0700
I'd like to get your input on the necessity of using tie beams for isolated footings for seismic considerations. The need for these tie beams was never brought up throughout my graduate studies at UT Austin, but here in Lebanon where I'm a practicing PE, tie beams are used very extensively, and this is probably a legacy of some French common practice since the French code has been the predominant reference for some time in this country. So I went back to my American books & codes and could only find that NEHRP requires that tie beams be used if the spread footings are not anchored into adequate soil and that they should be designed to withstand in tension and compression 15% of the vertical load carried by the column. I have so far always avoided using these tie beams for the following reasons: 1. Lateral load due to EQ is not necessarily proportional to the tributary gravity load on the column 2. Tie beams can induce complex stresses into the footings due to differential settlement, estimating these stresses would mean getting into the complex beams on elastic foundations calculations. 3. The coefficient of friction between Concrete and the worst type of soil is about 35% which is well above the 15% lateral force factor so I don?t see how the footing could slip (unless the overturning forces are quite large in which case the tie beams would probably not solve the problem) 4. Many buildings have columns that are sort of staggered and, therefore, tie beams would end up being skewed making it even harder to predict the behavior of such a system under seismic loads. This could also result in some undesired twisting action on the footings So what do yo?all think ? Thanks Mony Serhal ...
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