Pile Cap Strap Beams

[msegerman]

Hi,

How does a strap beam for a pile cap differ from a traditional strap beam for spread footings? Since I have a short, deep cantilever, do I need to design using the 'strut and tie' method? What would I use for the width of my struts/nodes if this is the case? Also, would I have to adhere to the phi*vn max <= phi*10*b*d*sqrt(f'c)? ... This will drastically increase the size of my strap.

Any input would be greatly appreciated. Thanks.

-Matt, EIT

 

[WARose]

I don’t think I’ve ever used a strap to connect pile caps (I have used grade beams). Straps between footings have traditionally been used to connect an eccentrically loaded footing to another footing so that uniform (or close to uniform) soil pressure can be achieved. An eccentrically loaded pile cap would be more of an issue (I would think) of load distribution in the cap itself. (Or (in the case of lateral load) the lateral capacity of the piles.)

 

[msegerman]

Warose,

Thanks for the reply. I have designed traditional strap footings like you have said and I have a pretty good feel for that type of structure. The project I'm working on now is an 8 story concrete building in NYC. It's in the east village so the soil is bad and the bedrock is approximately 100' below grade. In order to reduce the # of piles, it was decided to use 240 ton piles. The geotech said we need to go 24" from the centerline of the piles to the property line to avoid damaging the existing adjacent building. For our edge columns, we generally only have one pile and about 14" of eccentricity. I need the beam for stability. We have a 12" cellar slab so I'm not concerned about stability in the other direction.

I was thinking I could use 'b' as the width of the pile cap for confinement (to get around the Vn max = 10bd*sqrt(f'c) ) as that will be the actual width in the critical section and just size my beam slightly wider than the column.

 

[WARose]

Ok I think I see your reason for the strap now. I think the limitation (for shear) you are talking is applicable for deep beams…..and I don’t know the distance from cap to cap but I would think it would be far enough where the deep beam assumption wouldn’t be valid (ergo you would have to use a lesser value for maximum allowable shear). I’ve found that the controlling values continue (for all practical purposes) for a distance away from the caps.

 

[msegerman]

The interior column is an appreciable distance away and I see your point. There exists other criteria for deep beams though, such as if a concentrated load is applied very close to the support allowing compression struts to form... I feel like my situation falls into this category.