metal building thickened slab foundations

huangyhg

metal building thickened slab foundations
i am often asked to design metal building foundations using thickened slabs. contractors and architects love it, primarily due to cost. it is my contention that monolithic, thickened slab footings will not work in most instances.

metal building foundation design is virtually always controlled by wind uplift, at least in my state of florida. with a thickened slab sidewall column foundation, the center of the thickened slab weight is not even close to being centered on the column, since the column is generally right at the edge of the thickened slab. since the load and the resistance are not concentric, significant bending resistance must be developed in the slab beyond the thickened portion at the column for stability. recent changes in the aci code greatly reduce the amount of bending resistance you can assume from the slab without special reinforcing extending into the slab beyond the thickened area. you can't even reduce the bending resistance required very much by increasing the thickened area, since the resistance generally gets more and more eccentric from the center of load.

during my tenure with butler manufacturing company, we hired one of the most pre-eminent experts in structural design, dr. james fisher, to write a foundation design manual for the company. long story short, there is not one mention anyplace in his book about thickened slab footings! i believe the concept was developed by contractors for their convenience, and promoted for use by architects. i have seen many buildings over the years calling for relatively tiny thickened slab footings that were "designed" by architects (no offense intended), that have only survived because i'm sure that they were never loaded to anything close to the uplift numbers the code requires.

i challenge any structural engineer to prove the mathematics of a thickened slab footing for high uplift loads. i have spent hours searching the internet to see if i could find any technical information on the subject, and there is none that i could find. i think there is a good reason for this; it can't be structurally justified!
if anybody out there can shed any light on the suject, it would be greatly appreciated!
i would never consider using a thickened slab footing alone for a pemb.  i have used and have no problem using a thickened slab turndown along the perimeter with spread footings and pilasters at each column.  but in my opinion and past experience the uplift loads are just too large to make a thickened slab footing work by itself.
i have done many of these--the unthickened slab needs to be reinforced for the moment induced by the eccentricity, as you mentioned.  and you need enough slab weight to counteract the uplift.  the ones i have done have been in 90 mph wind load areas, so the uplift is probably not as high as what you are looking at.
daveatkins
i have used thickened slab moonopour footings for over 20 years with no adverse effects, but only on mainframe spans in the order of 35 to 40 feet max.  beyond that, the loads dictate that the footings be concentric with the mainframe column to avoid eccentricity.  hence the monopour option is lost at these longer spans.  
as butler does not really deal with the smaller span metal buildings, i can understand dr. fisher's argument and agree.  i have done foundations for butler buildings in the past, as well as soule', varco pruden, and the like.  however, i cannot agree with the same argument being applied to smaller metal buildings.   
mike mccann
mccann engineering
spats, i'm in florida and have had the same gripe and opinion as you.  i would be very interested to see who designs their thickened slabs for uplift and properlyfff"> accounts for the eccentricity and bending.  when i have asked about it in the past, i get a lot of arm waiving and "we've done that for xx years" bull-loney.  numbers talk.
i think you hit it right on the head when you said "i'm sure that they were never loaded to anything close to the uplift numbers the code requires."
i agree with daveatkins,  thickened slab with sufficient reinforcemnet for the moments and shears is very doable.  
thanks for the comments, everyone. maybe this is more a question about how different engineers deal with convincing architects & contractors that monolithic footings are not a good idea except for the smallest spans. frankly, i don't run into many 30' & 40' spans.
then there is the question of slab control and construction joints when you're using the slab for resistance. obviously, you don't want joints through your "footings", or even close, for that matter.
my experience has been that in this case a turn down would not be the most applicable solution, i would use a solid slab though out with 2 mats of rebar. this maybe alittle pricey but will function well. make the slab thickness that of frost depth. this sounds like your over designing but i believe its your soultion.
i use to live and design butler bldgs. in florida and lost more jobs than i got because i could never get a sufficient monolith slab to work with any spans in 140 mph winds.
we are in 140 mph florida and just happen to be designing the foundations for 130' rigid frames with interior pinned columns and spaced at 50' c/c. this is a very timely conversation.
i was always told when i was younger and working for someone else not to worry about "e" (were smaller buildings). now work for myself and want to be accurate. can't really use seperate footings because everyone else in this area uses mono footings. has anyone tried to use rectangular mono footings? the weigh of the mono edge grade beam works for you with the footing and attached slab working against.
this is all made much worse with 0.6d+w.
i've used rectangular footings, and concentric footings at columns under the thickened slab at the perimeter.