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[SEAOC] [SEAOC] Footings' Tie Beams[Subject Prev][Subject Next][Thread Prev][Thread Next]
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- Subject: [SEAOC] [SEAOC] Footings' Tie Beams
- From: "Edward R. Haninger" <75331.1705(--nospam--at)CompuServe.COM>
- Date: 16 Jun 96 14:27:14 EDT
My 2 cents on the subject of tie beam requirements: The only code requirement I know of is the 1994 Uniform Builiding Code (UBC) section 1807.2 which states in part "Individual pile caps and caissons of every structure subjected to seismic forces shall be interconnected by ties....designed to 10% of the maximum column vertical load." Many engineers may not be aware of this requirement. Pile foundations are probably the worst case because the lateral capacity of piles is very small compared to their vertical capacity. In an earthquake, any lateral movement of the pile caps could cause great damage to the superstructure. Foundations on soil will see the same type of effect, although to a lesser degree. Many engineers like to add tie beams, without including them in the formal analysis, just for overall structural integrity. Ed Haninger ---------- Forwarded Message ---------- 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 dont 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 yoall think ? Thanks Mony Serhal ============================ ...
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