Foundation design for uplift during construvtion period
Foundation design for uplift during construvtion period
(OP)
I'm a Norwegian structural engineer working in Norway, and have run into a "we have always done it this way" theme.
The building is a regular single storey steel column and lattice truss design. Truss span is 25m, 75'. Soil is rock fill. Uplift in the order of 100kN per pad foundation.
I have insisted on making all pad foundations sufficiently heavy to resist uplift during the construction period, prior to the concrete floor slab (300mm, 1', thick) being cast, the customer quotes common practice locally which seems to ignore the effects of uplift during construction.
Two questions:
1) Does anyone know of a clever and contractor friendly method to constrain the bnuilding during construction?
2) Are there any dramatic pictures of building foundations lifted out of the ground by uplift? These to show the customer who is a major local contractor.
Thanks,
Dag Alan Killi
The building is a regular single storey steel column and lattice truss design. Truss span is 25m, 75'. Soil is rock fill. Uplift in the order of 100kN per pad foundation.
I have insisted on making all pad foundations sufficiently heavy to resist uplift during the construction period, prior to the concrete floor slab (300mm, 1', thick) being cast, the customer quotes common practice locally which seems to ignore the effects of uplift during construction.
Two questions:
1) Does anyone know of a clever and contractor friendly method to constrain the bnuilding during construction?
2) Are there any dramatic pictures of building foundations lifted out of the ground by uplift? These to show the customer who is a major local contractor.
Thanks,
Dag Alan Killi





RE: Foundation design for uplift during construvtion period
RE: Foundation design for uplift during construvtion period
Any support for the customer here? Am I worrying too much? Can the whole thing just lift off?
RE: Foundation design for uplift during construvtion period
RE: Foundation design for uplift during construvtion period
RE: Foundation design for uplift during construvtion period
RE: Foundation design for uplift during construvtion period
I have had projects where I have put notes on the drawings stating that the water level must be maintained at a certain level during construction to prevent floatation until heavier elements were in place to offset buoyancy or that temporary openings be made to allow the infiltration of water into the structure.
The down side is if it floats or rolls, then what? what if it only moves a little? whose call is it to say it is okay?
Not a good position to be in as the engineer of record.
RE: Foundation design for uplift during construvtion period
The customer changed his mind again. Small pads, his responsibility.
Man, I'd like to know of a case where a building took off due to insufficient anchorage.
RE: Foundation design for uplift during construvtion period
They typically take the weight of soil on the footing and the slab as a square mat that when lifted at the center by the footing has a width equal to the point of failure in the concrete. In our case this was a 25 foot by 25 foot square with enough weight to offset uplift.
Since I have not worried about this uplift force for the last 40 years and never had a building float away, this solution was fine with me.
RE: Foundation design for uplift during construvtion period
The code does have a statistical factor for temporary conditions which reduces the wind velocity and hence the applied pressures (and uplift).
I dont know if that helps, but you could check whether your code has such a provision for short term exposure - using this method it is possible that the smaller pads will suffice until the floor structure has been put in place.
Another possible solution if your building layout will allow is to introduce further braced bays to reduce the net uplift at each braced bay, which means you will need less mass to resist uplift.
Regards
VB
RE: Foundation design for uplift during construvtion period
Joe Tank
RE: Foundation design for uplift during construvtion period
RE: Foundation design for uplift during construvtion period
RE: Foundation design for uplift during construvtion period
There was no attempt to reduce the factor of safety. The factor of safety was maintaned, but for a different load expectation that was consistent with the period over which the structure was built. For gulf coast projects during the hurricane season such reductions were not considered.
Joe Tank
RE: Foundation design for uplift during construvtion period
For a 50 year life span: During any one year there should not be a greater than 2% probability of design wind being exceeded.
My understanding is: If construction time is three months, I need to ensure that the 2% probability is not exceeded. Is it as simple as applying the 50/4 = 12,5 year wind?
RE: Foundation design for uplift during construvtion period
RE: Foundation design for uplift during construvtion period
I understand your point. I was just explaining the long standing practice that is often followed during construction. I wonder how the owner or EOR defend themselves against the rationalization of the 2% probability of exceedance? Why not a more stringent criteria? I guess we rely on the rational judgements of experienced folks and back that up with some level of success? Someone has to take a stand. It looks like owners and EORs have the shield of a design code. Contractors have to make judgements. Although I am not familiar with it, an earlier poster cited an ASCE document for construction loads.
Joe Tank
RE: Foundation design for uplift during construvtion period
Have you taken any cohesion/friction on the sides into account? I would think using these would be justifiable for short term loading.
We have used something like 5kPa side friction to reduce the footing size. I dont like using it for permanent loads though as you dont know when you may have a flood/drought.
RE: Foundation design for uplift during construvtion period
There are however 4 large pads, one per side, to anchor the wind bracing in. If the wind takes a hold of the roof, it will pull the small pads up, but the allround "feeling" is that the roof of insulated corrugated steel will stay together, kind of like a sail and transfer loads to the big pads. If it rips, the roof will be damaged, but the loads eased. The science isn't exactly precise.... Any other "feelings" out there.
RE: Foundation design for uplift during construvtion period
RE: Foundation design for uplift during construvtion period
I don't think there is an economical way to anchor the structure before the slab is placed (see my previous post). Until recently myself and other engineers I know never designed for wind uplift at the footing level. (NE US)
Damage to the structure from high winds during construction would be covered under the Builders Risk Insurance. So if there is concern make sure such a policy is in place.
Good Luck