pemb spread footings

huangyhg

pemb spread footings
i'm working on the foundation design for a 130' span pemb with 30' bay spacings.  allowable bearing capacity = 4 ksf.  at column line a the columns are 4' above finished grade and at column line b the columns are 13' above finished grade.  bottom of footings are 5' below grade.  the max. "thrust" force for such a building is 68 kips, which i received from a pemb builder.  there is an interior retaining wall 14' from column line b, which forms a trailer bay.  i want to design the footings to resist the moment but the moment is so great at column line b that my footing is becoming too large.  i have thought about a tie beam at the base of the footing to the retaining wall and also about using a combined footing.  the combined footing also seems to turn out to be rather large.  
i would appreciate any tips on how to go about reducing the footing size i currently have.  
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ma'af - what is pemb?  not run into this term before.
pemb = pre-engineered metal building
if i understand you correctly, you have pinned pemb column bases supported above adjacent grade by a concrete column or wall, which would be fixed at its base.  the horizontal thrust * the 13' above grade is the moment you are having to resist, right?
i think the tie beam is the best solution.  are drilled piers an option?  if so, you might could design it as more of an embedded pole-type foundation.
taro - thanks; otherwise i would have swagfff">ged it.
our firm just finished a church project with a similar condition to what you are describing--and what a hassle!!!  is there any way you can talk the preengineered metal building manufacturer into extending his columns at grid line b down to grade?
if not, the tie beam is obviously the best answer, but can you span it across the trailer bay?  is there a headroom issue?  or an interference issue?
daveatkins
be aware that pemb's are typically designed assuming that the foundations do not deflect. a pier supporting a pemb column may have significant deflection that will have a weakening affect on the steel frame.
i like daveatkins suggestion.
thanks for the responses.  
drilled piers are not an option here.  i'm going to talk with the pemb manufacturer to see if the column can be extended down to grade.  if this is not a possibility than i will go with the tie beam.  unfortunately, there are headroom issues in the trailer bay so i will have to keep the tie beam at the footing level.  i will also have to talk with the pemb manufacturer about allowable deflections in the pier supporting the column.  
i'll post the answers i get.  thanks again!
a tie beam at the footing level won't do much good.  if you can't make the preengineered metal building manufacturer change his design, and you can't have a tie beam across the trailer bay, then you will need to design a large pier with a large rectangular footing to resist the loads.  or perhaps the entire wall along grid line b can have a retaining wall-like footing under it.
daveatkins
srbu,
we recently designed the foundations for a pemb where the client wanted a dirt floor w/o tie beams.  fortunately there was was an inner wall parallel to the foundation.  we tied the two together, using the conbined weight and soil between to resist sliding and overturning.  the span was a little greater than yours but we were nearly at grade.  can you tie to the truck bay walls and extend them a little deeper to get the resistance that you need?  if not i would look at drooping tie beams under the truck bay.