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Re: Pier and grade beam foundation systems

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I don't know how you cannot consider the pier and grade beam system as part
of the lateral force resisting system.  If you do your analysis correctly,
the point of pier fixity is somewhere below grade level, which means not
only do you consider the weight of the house, but you have to include the
weight of the all the grade beams and the portions of the piers above the
point of rotation.  In the past, I've ignored a "frame" system and just
considered the tributary weights and lateral forces to the a line of
cantilevered piers; then determined the depth of the piers per the UBC
non-constrained formula.  From there, I considered that the participating
pier weight is .35 of the calculated depth plus the height of where my
passive pressure begins, as determined by your soil engineer.  (This is
assuming the 1st floor is wood).

This means that the weight in formula (28-1) includes the weight of the
grade beams and portions of the piers and the house and that the base level
is some estimated level below the ground that is used in formula (28-8).  

Note that sometimes, the passive pressure may not start for 3 or 4 feet.
Where this also gets tricky is when the building is on a sloping lot.  


Neil Moore, S.E.



>Try to convince the plan checker or building official that the pier and grade
>beam system is not part of the "lateral-force-resisting system" (which is
>defined as "that portion of the structure composed of members proportioned to
>resist forces related to earthquake effects").
>
>For residential jobs, the main purpose of the grade beam and caisson
system is
>to transfer the vertical dead and live loads to the ground. If there are
>seismic forces resisted by this system, they are small and has only minor
>contribution. 
>
>An exception is when the system carries a significant portion of the seismic
>force. Houses with steel frames or narrow shear walls with heavy uplift and
>downward loads resisted by the grade beam or even if the upward and downward
>forces goes diredtly to the piers, if they are heavy loads compared to the
>dead and live loads, then, using one's engineer's judgement, it may be
>considered as a "lateral force resisting system". 
>
>I had a plan check correction before on a small house with a medium uplift
>load on a grade beam/continuous footing. He wanted me to design the continous
>footing/ grade beam per this code section but I convinced him that the seimic
>related loads and stresses are small enough that  this should not be
>considered as part of the lateral force resisting system. 
>
>But it depends on how reasonable the plan checker and building official is
>when it comes to city policy. In my experience, if they insist, try a
>compromise where you will add more rebars or ties and increase the concrete
>strength without having to go full compliance with the code section.
Otherwise
>you can try fighting it out agrressively and risking getting nitpicked on
>other issues.
>
>Ernie Natividad