Exterior Stairs Design Questions

[kxa]

I have a couple of questions that would appreciate help on:

1. For the northeast area where the frost line for foundation by code is 42” , I see that the as-built plans for the exterior stairs footings which are part of the sidewalk slabs are only trench footings 18” deep x 6” wide. Don’t the frost depth requirements apply here?

2. If the treads are concrete or stone, what is the best way to seal the edges so that the water and salt don’t get under the stone/concrete and cause corrosion? I intend to use galvanized steel.

 

[dik]

To minimize movement of the bottom of the stair and stringers, they can be supported on a foundation wall constructed of either concrete or masonry depending on local custom. The foundation wall can be supported on a strip footing founded 42" below grade. Alternatively, the foundation can be protected by using suitable extruded polystyrene insulation and founded at a more shallow depth.

If frost heave is an issue, then if the soil is free draining or if it is a clay material, then it is possible to construct it at grade as noted but there is a risk of frost heave and your stair may move. Talk to your local building inspector.

I gather from the reduced depth of frost penetration, that you have frequent freeze-thaw cycles and that salt is often used. You should provide good quality concrete with adequate air entrainment. In these environs, we use 35 MPa concrete and 5 to 7% entrained air. Check local useage.

I would not normally use pan stairs for exterior stairs exposed to salt... too many problems.

Dik

 

[JAE]

Frost heave occurs.

If the stairs can move upward without distressing themselves or an attached building then the 18" would be OK. However, in most cases, the stairs are attached and are not designed to move 1 to 2 inches upward. Therefore a deeper footing is warranted.

 

[kxa]

Dik,
I agree that pan stairs are problematic but since I can not have any open risers due to the ADA regulations, I have to go w/ that option. Also, I have to make sure that water can not drain through the stairs on the area below where 1st floor people access their apartments.

Any suggestions or reference material will be helpful.

Thanks.

 

[hokie66]

Dik is suggesting that you use concrete stairs rather than steel, and I agree.

 

[dik]

Typ detail... modify as req'd...

 

[hokie66]

I don't think you can build a set of steel pan stairs which won't allow water through and also won't have corrosion problems unless you build the whole flight as a sealed unit and hot dip it.

 

[kxa]

dik,
Thanks for the detail but it is not that clear to me. Is the notch under the tread concrete/stone so that water drips down instead of rolling down the riser?

There was a comment why it can’t be made of all concrete? I am replacing the existing pan tread stairs that have excessive rust. People live in the 1st & 2nd floor apartments and con not use poured concrete. I am looking into pre cast but I would need additional support post/columns. I don’t believe the client will go for that. The idea was to use something similar to the existing to reduce cost and construction time as there are 40 of these stairs. It is very hard to find any reference material on this subject. As simple as it is it is difficult to come up with something that can last a long time in this harsh environment.

 

[dik]

I tried to show a single bent plate and a combination to show how they are assembled. The treads should have a slight cast to allow water to shed. They are relatively waterproof after fabrication and casting the concrete treads. If the notch you are referring to is the one at the back of the riser, it's only to provide a bit of a finish for the tread; this can be omitted.

Dik