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[SEAOC] Re: [SEAOC] Footings' Tie Beams

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Both the 91 NEHRP and the 90 SEAOC Blue Book, specify that the ties are
required when using deep foundation systems, such as piles, caissons, and
piers.  If you are using spread type footings, it is good practice to tie the
footings together but it is only required in zones of highest seismicity or
for buildings of essential characteristics, such as a hospital, school,
police and fire stations, and buildings with large number of occupants,
founded on bad soil.

The requirement really is intended for deep foundation types.  The reason
being that in the eventuality that there is normal soil settlement,
earthquake induced consolidation, or liquefaction, the top portions of the
piles and pile caps will be exposed with no lateral support; by providing the
ties the piles and caps act more like columns and with the tie beam they will
act as a frame.

Another reason for the ties is to ensure that the building foundations act as
a unit, minimizing the differential movement.  If you are on good soil, such
as bedrock, the supporting soil will act as unit and there is no need for
ties, especially that in good soil you will not use deep foundations.  Also,
the slab can provide that lateral support, with or without additional (slab
embedded) reinforcement to create a tie beam.

Janah A. Risha, S.E.