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[SEAOC] [SEAOC] Footings' Tie Beams

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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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