Foundation design for uplift during construvtion period

[killi]

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

 

[civilperson]

Leave the roof fabric off until the floor is constructed.

 

[killi]

It rains a lot here, and the customer wants to cast the floor with the roof on and walls up. The specified floor surface tolerances are quite strict.

Any support for the customer here? Am I worrying too much? Can the whole thing just lift off?

 

[civilperson]

The contractor "owns" the building until certified complete. If this gamble on low winds is chosen by contractor, then you should c.y.a. with a letter stating this gamble is on the contractor's risk.

 

[killi]

This is probably what I'll end up doing. But I really want him to accept my big pad solution. Others do.

 

[killi]

He accepted the big pad design.

 

[BSE05]

Are you worried about floatation from your foundation displacing a volume of water such as a closed basement or tank?

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.

 

[killi]

Sorry, I don't seem to have stated that the uplift would be due to strong winds.

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.

 

[BSE05]

I recently worked on the foundations for a pre-engineered one story warehouse building. The uplift loads given to us were very large. It was not practical or economical to make the footings heavy enough to offset this uplift. I asked the building manufacturer how they did this.

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.

 

[valleyboy]

Here in the UK the wind code represents a worst case wind load criteria that a permanent structure will be exposed to.

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

 

[JoeTank]

I worked for a tank construction firm for about 15 years. Our practice was to check the foundation and structure for lower than design wind speeds. This was considered appropriate since the construction periods were very short. I believe the general rule was to check the foundation and structure for 70 mph wind loads during construction, not 100 mph as was typical for the design code.

Joe Tank

 

[cvg]

essentially what you are doing is reducing the factor of safety (in a round about way). Why not use the actual design loads, uplift etc. but assume FS=1? Might be easier to defend this than by somewhat arbitrary reduction of the applied loads.

 

[Taro]

Using reduced loads during construction is standard practice in the United States. A good reference is ASCE 37-02 "Design Loads on Structures During Construction".

 

[JoeTank]

cvg,
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

 

[killi]

I spoke to the person behind the standards, and the following was his way of thinking:

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?

 

[cvg]

my point is that even in a 6 month construction period, you may still see the full design wind load applied to the structure. Granted, the probability is lower - but if it happens, how will you defend your rationalized 2% probability. Standards are expected to be a minimum condition, not necessarily the absolute requirement. Granted, the Contractor "owns" the building until substantial completeion, however the engineer may still get blamed. Require the minimum that both you and the owner can agree to so that you can both sleep at night.

 

[JoeTank]

cvg,
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

 

[csd72]

Heyda Killi,

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.

 

[killi]

The uplift per pad is in the order of 100kN, and the pad area 700x700mm. Height 300mm. Top of pad flush with backfill. That isn't a whole lot of friction. If the pad was an empty tub I could lift it myself.

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.

 

[mitchelon]

Think about reality and not about what the code says. There are few codes for temporary structures. In the US we have ASCE 37-02, which is still not extremely popular. ASCE 7-05 recommends a basic wind speed of 140 mph for the coast of NC. ASCE 37-02 reduces this BWS to 90 mph. In addition, it reduces the BWS depending on the duration this temporary structure is going to be up. Basically, you can go from a BWS of 140 to 67.5 mph. Is this unreasonable? No. The probabilities of experiencing a wind speed of 140 mph during the six months you will be building the structure are low. Plus, we are talking about hurricane winds and not an earthquake. You should know when a hurricane is coming and should have enough time to properly brace the structure or do whatever you have to.