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[SEAOC] Footings' Tie Beams

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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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