Hollow Core vs CIP Concrete

[Juanchoganda]

I need to design a concrete roof structure for a residence in South Florida. The dimensions are 26ft x 40 ft, and I'm trying to get some opinions on the most economical option. With a cast-in-place option I'm using intermediate beams running the short direction to act as joists, with the HC planks this would be eliminated but would this be a cheaper build, and also do I need specific requirements to prevent water leaking?

 

[JedClampett]

I do quite a bit of design (but not residential) in Florida, and even though I suppose that cast-in-place has some advantages, I can't think of them. No formwork, no shoring, a platform safe for workers immediately after installation, quality provided by the precast concrete manufacturer, etc.
I'm not sure it's cheaper, but since I get very little squawking from the contractor when I use it, I'm pretty sure it is.
As far as waterproofing, your architect should do that, but there's a zillion roof systems, like membrane, MBR etc., as many as your roofing supplier can build. Someone needs to take care of sealing (and insulating) the roof, but you'd have to do that with cast-in-place, too.

 

[JLNJ]

The only argument against precast is that you have such a small quantity that you are losing the economy of scale.

If cast-in-place: 3" metal form deck. 8' spans. If you are bearing on CMU you will need just 3 beams (and a smaller crane). Plus, you are not at the mercy of the precaster's schedule.

If you need the DL for uplift, you might get a little more from the hollowcore.

 

[Juanchoganda]

thanks for the input guys! I have a question to follow up on the HC slab, is it possible to have a 2ft overhang in both directions? I'm thinking in the span direction I can hook some bars into the cavities but how do I deal with the other side? I'm thinking of adding a beam and change the direction at the ends but I will still run into a portion of the HC slab hanging on its side

 

[jayrod12]

Generally speaking, no. You would be expected to cantilever your beams/walls out to grab the hollowcore panels. 2 feet past the supports is also a bit far for non-reinforced cantilevers, but as you indicated if you bust out some of the cores, put in some top reinforcing, the cantilever can be conventionally reinforced.

 

[KootK]

FWIW, I've been a precast engineer at various points in my career.

I'm thinking in the span direction I can hook some bars into the cavities but how do I deal with the other side?

That can be done but I'd consider it a last resort for such a short span and cantilever. Most precasters will be able to put a couple of strands in the top of the plank to deal with the cantilever. This would be much cheaper than any post-casting bed operation. I know, you'd think that the top strand would neuter the bottom strand but it doesn't work out that way.

I'm thinking of adding a beam and change the direction at the ends but I will still run into a portion of the HC slab hanging on its side

Again, the best option probably depends on how aggressive you and your precaster feel. The ballsier options include:

1) Let a 4' plank cantilever 2' in the transverse direction relying on plain concrete in flexure. Keep an eye on hold-down detailing.

2) Have the precaster make the end pieces plant cast solid slabs that have top steel in the transverse direction to deal with the cantilever.

 

[jayrod12]

That can be done but I'd consider it a last resort for such a short span and cantilever. Most precasters will be able to put a couple of strands in the top of the plank to deal with the cantilever. This would be much cheaper than any post-casting bed operation. I know, you'd think that the top strand would neuter the bottom strand but it doesn't work out that way.

Honestly this is news to me, and shocking. I thought that they absolutely hated messing with their hollowcore casting beds.

 

[KootK]

I thought that they absolutely hated messing with their hollowcore casting beds.

Oh, they do. They shouldn't have to mess with their beds though. Adding top strand should just mean using some of the default strand positions located above each of the interior webs at minimum.

Adding top strand is more common that you might think. A lot of long span, hard working plank will be so heavily stressed at the bottom that:

1) The tops of the planks might crack during handling or when the strands are released.

2) The excessive camber eats up to much of the topping or mucks up side lap connections to walls etc.

In these situations, adding some top strand to bring the camber down is a useful trick.

Where I've struggled with top strand is with the utilization of the stressing head mounts. Most precasters think of those in terms of XXX kips but it's really XXX kip-in measured from some datum a bit below the bottom of the bed. Because of this, one top strand can tax the stressing head mounts as much as several strands placed at the bottom. It always surprised me that the mounts had so little reserve that this was an issue. I suspect it mostly comes down to a conservative presentation of capacity on the part of the equipment manufacturers.

 

[Juanchoganda]

I appreciate everyone's input on this topic, I'm fairly new here and it has been so helpful to bounce some ideas with different engineers. After reading the different responses I'm leaning towards letting the pre-caster add this to their planks but I just had one last question.

How do I attach the cantilever plank to the top of the wall? it seems there is not much room to add rebar hooks once the plank is in place. Can I just drill through and hook a bar into the topping concrete?

 

[KootK]

I had not realized that you had a topping. That's uncommon in my experience but, then, my experience doesn't include Florida hurricanes. Having a topping affords you another way to handle the cantilever where it would be perpendicular to the plan span: put top steel in the topping there. Certainly, most precasters would prefer that to having to manufacture solid planks which are very costly compared to the extruded planks.

There are several ways one might make the connection:

1) Install dowels through the plank keyways and into the bond beams.

2) Drill dowels through the plank voids and into the bond beams.

3) Have embeds installed in the plank soffit that get welded to a connection angle that gets bolted to the inside of the walls.

Note that neither #1 nor #2 is going to create a connection that provides a direct lap between your wall vertical bars and the dowels. This might matter if you're dealing with an appreciable net uplift.