Monolithic Concete and Slab Foundations 2

[metalman]

I have seen a lot of metal building designed with a footing integral with concrete slab (monolithic). Most metal buildings that are designed for southeast locations have a large uplift on the foundation due to wind. I usually design the footing heavy enough the handle the uplift without taken into account any effects due to the slab. I have found that some engineers use a portion of the slab as well as a perimeter grade beam to resist uplift and use a hairpin to resist the horizontal forces. Does anyone know of any published literature other than a book called "Metal building Systems" that Clearly states the design assumption and amount of slab that can be used to resist uplift.

 

[Ron]

Since uplift resistance by code does not have bending or other analytical structural implications, it is a matter of how much slab you can justify as being "tied" to the affected uplift member. For instance, I typically consider that the slab area around a column contributes to uplift resistance for half the column spacing on either side, as well as half the span of the beam/bent, as long as the slab is tied together for these respective areas. Uplift resistance is like a boat anchor. It doesn't matter what form it takes, just that it has sufficient mass to be effective. [sig][/sig]

 

[paulomanuel]

I dont know of any literature that might help but maybe i can. It might become against safety to consider the half span and half the column spacing, just check the principals of composite structures and you will realise that not all of the concrete widt on top of a stell beam contributes to the resistance off the composite beam. There is a reduction factor! It is also reasonable to consider the bending of the interested area of the slab as a limit state, because after that limit state some portion of slab considered might not be acting against lifting at all. [sig][/sig]

 

[paulomanuel]

I dont know of any literature that might help but maybe i can. It might become against safety to consider the half span and half the column spacing, just check the principals of composite structures and you will realise that not all of the concrete widt on top of a stell beam contributes to the resistance off the composite beam. There is a reduction factor! It is also reasonable to consider the bending of the interested area of the slab as a limit state, because after that limit state some portion of slab considered might not be acting against lifting at all. [sig][/sig]

 

[paulomanuel]

I dont know of any literature that might help but maybe i can. It might become against safety to consider the half span and half the column spacing, just check the principals of composite structures and you will realise that not all of the concrete widt on top of a stell beam contributes to the resistance off the composite beam. There is a reduction factor! It is also reasonable to consider the bending of the interested area of the slab as a limit state, because after that limit state some portion of slab considered might not be acting against lifting at all. Do something that is coerent with the model. [sig][/sig]

 

[Ron]

For uplift resistance, most codes require that the resisting dead load be at least 1.5 times the uplift load. This requirement is phrased differently in different codes, such as "the overturning moment cannot exceed 67 percent of the dead load resisting moment", but the bottom line remains the same.

You may only consider the concrete between the columns as contributing if the concrete is integrally tied and also tied or keyed to the footing. [sig][/sig]

 

[tps]

I am not quite sure of what you mean by footing integral with slab, I hope you don't mean thickened edge.

The column should be supported by a footing at some depth. In northern regions the frost depth is the minimum. The plan of the footing is projected up at 60 degrees. The weight of the soil, concrete, etc. enclosed by the 60 degrees is effective in resisting the uplift. The submerged unit weight should be used and factored down to determine the resistance in accordance with your building code. This applies to cohesionless soils only. It follows that the deeper the footing the more mass you will be able to use to resist the uplift.

The hairpins usually wrapping around the column anchor bolts going into the slab are designed to resist the kick-out (horizontal forces) of the frame. The length required is a function of the slab friction with the soil, steel in tension, development length, mass of slab to be mobilized, etc.

There are publications available for designing foundations for uplift and horizontal load. The foundation design type depends on the magnitude of uplift vs horizontal. I use to have some tables with standard designs, now where did I put them....


[sig][/sig]