Metal Building Thickened Slab Foundations
Metal Building Thickened Slab Foundations
(OP)
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!
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!






RE: Metal Building Thickened Slab Foundations
RE: Metal Building Thickened Slab Foundations
DaveAtkins
RE: Metal Building Thickened Slab Foundations
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
RE: Metal Building Thickened Slab Foundations
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."
RE: Metal Building Thickened Slab Foundations
RE: Metal Building Thickened Slab Foundations
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.
RE: Metal Building Thickened Slab Foundations
RE: Metal Building Thickened Slab Foundations
RE: Metal Building Thickened Slab Foundations
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.
RE: Metal Building Thickened Slab Foundations