Slabs integral to pile caps or on top

[sybie99]

Good Day Guys and Girls

I am designing a suspended slab, spanning between pile caps. For this kind of design/construction I have seen that either the slab can tie into the top of the pilecap, ie be integral with slab reinforcement running through the top of the pile cap reinforcement, or the pile caps can be constructed and the slab sit on top of the caps, with no rebar tying them together. When do you use each option? Does it have anything to do with whether you are using a gas membrane or waterproofing membrane?

Obviously you use more concrete for the 2nd option, and more steel. Maybe someone knows the reasoning behind each case.

Thanks

 

[vandede427]

based on typical construction sequencing, the cap is usually below the slab, whether the slab is on grade or suspended.

 

[sybie99]

vandede427, I have seen an equal amount of cases where the slab is integral to the slab, there must be benefits and reasons for using either case?

 

[mijowe]

I agree that it is a construction sequence issue. The contractor would need to be ready to put down the vapor barrier, gravel, slab steel, at the same time that they pour the pile cap. As you indicated the cost savings would be a reason to pour them together.

 

[normm]

I think it will also depend whether in an accidental case upward water pressure can build up underneath the slab. If so, there will obviously be need to tie the slab to the cap with rebar.

 

[rowingengineer]

There are a lot of factors at work here that an engineer must process to determine what is best for his client. The simple reo or no reo v's cost is not a good indication. the pile connected allow the engineer to model the pile as taking moments from the slab possibly reducing the requirements for deflection ect, however the slab on top with the pile embed only makes it easier for the contactor with concrete driven piles. At the end of the day you would need to develop a more realistic question rather than a blanket to get any real answers, as the answer will change from pile type to pile type. For instance timber piles cannot be connected for moments.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[doka1]

I have seen it either but prefer to build the pile caps first. It allows the grading of the slab to finished properly, and cuts down on wasted concrete. Otherwise the ironworkers destroy the sloped grade required a the grade beam locations. Recently I was on a project where the grade beams were integral with the slab and we requested to lower the top grade beam height to slab subgrade height. Instead the engineer opted for a horizontal joint in the beams, allowing all the grade beams to be poured up to the height of subgrade and then follow with the slab on grade pour. We did have to be careful with splatter on the rebar from the first pour.

Depending on the type of building, one concern might be future changes of use of structure. If any plumbing has to be changed its hard enough to do it in a structural slab, now add grade beams, its becomes a structural nightmare.