Grade Beam with Stepped Under-Pour
Grade Beam with Stepped Under-Pour
(OP)
I have designed a stepped grade beam alongside an existing strip footing. The top of the grade beam is at the same elevation across its entire length but the bottom makes two steps to match the bottom of existing footing elevation. The grade beam starts at 36" deep, steps down to 44" deep, and then makes a final step down to 60" deep. The contractor is asking if he can split up the pours with the first pour bringing all the steps up to the 36" depth. The second pour would then be a continuous 36" deep pour across the entire length. The reinforcing and 36" depth would work for design across the entire length if no steps were to occur. Do you think there would be a problem if I allowed an un-reinforced under-pour to act as a kind of bearing mat? I would then leave the reinforcing for the 36" depth across the entire length. I have attached a sketch of the proposed grade beam prior to the two pour modification.






RE: Grade Beam with Stepped Under-Pour
RE: Grade Beam with Stepped Under-Pour
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Grade Beam with Stepped Under-Pour
RE: Grade Beam with Stepped Under-Pour
I see nothing wrong with that, but would want to do a sulfate test before recommending low strength concrete. Without further test results, I suggest keeping concrete strength the same as originally specified.
BA
RE: Grade Beam with Stepped Under-Pour
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Grade Beam with Stepped Under-Pour
RE: Grade Beam with Stepped Under-Pour
On my very first project in private practice, the contractor poured a two foot thick vault wall against the neighbor's building using the existing wall as a form. The wall collapsed and everyone was sued (the owner was a lawyer). Best to avoid that possibility.
Perhaps, I am no expert. But I recall a materials engineer telling me years ago that the best resistance to sulfate attack is high concrete strength. There may be other measures such as blends with fly ash.
BA
RE: Grade Beam with Stepped Under-Pour
RE: Grade Beam with Stepped Under-Pour