Geofoam Under Stair Detail

[IsaacStructural]

This is more of a general construction practices question. Please see the attached image. Just wondering, when building a stair over geofoam, is it common practice to leave the geofoam pieces rectangular and have the stair bottom follow this form as the architect is showing, or is the foam generally cut back like I am showing in red? I'd like to get at least some reinforcing into the stair and it seems like it would be a pain in the butt to bend the WWF to follow those stair contours.

Thanks guys.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[structSU10]

In my office we have kept the blocks rectangular. We set WWF for the tread, and have a bent and lapped #3 bar that goes up the riser. I would think if they cannot pour the whole stair at once, if the foam was sloped you end up with a knife point construction joint, which is probably not desirable.

 

[RHTPE]

You don't say how the stair slab was designed. Does it span from side-to-side, or from landing to landing?

Why not do as I've shown on the attached?


Ralph
Structures Consulting
Northeast USA

 

[IsaacStructural]

Thanks for the input. I think the idea with the geofoam is that the slab doesn't really span in either direction, it is just fully supported, isn't it? I was thinking of the reinforcing as primarily T&S steel. Stepping the foam at each course is a good suggestion.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[RHTPE]

If the slab bears on concrete walls along each side, then it's really supported by the walls. The geofoam will 'give' much more than will the walls, thus the walls will carry most of any load imposed on the stairs. If the foam deteriorates over time, then the walls will indeed support stair. If this were an outdoor installation and carpenter ants found a way in, they would have a field day in the foam.

Ralph
Structures Consulting
Northeast USA

 

[IsaacStructural]

Hmm, I see your point that walls will always be stiffer than the foam. My intent was to have the stair not cast integrally with the walls. As for carpenter ants, Isn't geofoam used as a support base for roads and other structures?

See this image. If deterioration of the foam is a concern, how can it be used in this sort of application?

http://www.geofoam.org/road-construction.asp

Thanks,

Isaac

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[SteelPE]

The use geofoam for stadium seating in theaters as well:

http://www.stadiumsavers.com/

I've worked on two projects that used this type of system. One was a theater the other was a church. In both cases, geofoam proved to be much more cost effective that the alternative proposed steel framing.

My guess is if you are concerned with the wall being stiffer than the foam (a legit concern) then you need to detail the stairs accordingly (if that is even possible).

 

[RHTPE]

Isaac: If the stair is cast monolithic with the walls, it's really supported by the walls. If the stair is cast after the walls and over-pours the walls (probably the contractor's preference), it's still really supported by the walls. If the stair slab is cast against the vertical wall face along both edges, then it may behave as free-floating and supported by the geofoam (except at the top & bottom).

As far as ants go, my experience with EPS insulation close to the ground surface or above finds that they love the stuff because it's easy to burrow through. While the manufacture may state that EPS geofoam offers no nutritional benefit to insects (which is true), they often make their homes in materials that suit their needs. I suspect that roadway applications of geofoam put the product far enough below ground so there really is no attraction to insects.


Ralph
Structures Consulting
Northeast USA

 

[IsaacStructural]

Ralph, thanks for the helpful discussion. The detail is for the slab to be cast against vertical walls, so it will be supported on the foam. I'll provide reinforcing to resist any local spots that might lose support, from carpenter ants or otherwise.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)