Concrete Slab on Grade Stair

[RFreund]

I'm curious to know how others do this -

A site detail for concrete stairs (not connected to the building):
Do you allow the stair to be 'floating' without foundation walls. Any particular added details for this (I'm used to option 2 typical detail)?
I could see the stairs being subject to movement from frost but so is the abuting side walk. Would you tie the sidewalk into the stairs or let them float individually?
Is this generally acceptable?
or
Do you provide foundation walls/footings around the perimeter of the stairs down to frost and tie the steps into the walls (fill with non-frost susceptible material)?


EIT

http://www.HowToEngineer.com

 

[EngineeringEric]

Did you mean to attach something?

 

[msquared48]

I generally run the foundation for the stairs down to frost level.

Mike McCann
MMC Engineering

 

[strucguy]

For the stairs that are closer to the building (say with in 10 from the building), we are used to providing cheek walls on the sides that extend down to frost level.

 

[EngineeringEric]

If the stairs are not for any structure, more like a walkway, the attachment to the lower walkway and upper walkway should be attached in such a way to limit the formation of a differential crack (those liable tripping hazards). I have sometimes turned down perimeter footings to almost act as confinement for the soil/fill below but i have not always brought it down to frost depth. My preference is to drop down to frost but sometimes that just looks crazy for a small cheap stairway.

 

[RFreund]

^ yes I am referring to something similiar to a walk way stair (not near the building). I am thinking that the floating slab should work but - adding a turn down "toe" at the bottom and also tying in the top landing (to prevent the tripping hazard). The bottom landing would float separate from the stair.

Thanks for the replies.

EIT

http://www.HowToEngineer.com

 

[racookpe1978]

How far do you expect the top to move? (Assuming"top" is attached to building and sidewalk/door.)
How far may do you think the stair base will move?
What is the stair length?

 

[jike]

If it is part of the sidewalk, then build it as the sidewalk so you do not get differential movement.

 

[KootK]

My thoughts:

1) Tie the stair to the building at the interface. For the sake of your own liability, I don't think that you want to encourage differential movement at the threshold.
2) For the most part, let the stair float. With the connection mentioned in point #1, I guess the stair is sort of cantilevering off the building.
3) I've seen folks detail some rigid insulation beneath the stair for a couple of meters off the building. I believe that the intent is to trap some building heat and prevent heave right adjacent to the building. I'm not sure how effective this is in practice.

The part that I get stuck on is what happens right at the threshold. Theoretically, if you get frost heave at that location, the upward force on the connection tying the stair to the main building will be extreme. A good argument for the insulation and/or non-frost susceptible back fill I suppose.



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[RFreund]

Let me repeat, this is for stairs not located near the building. It is a site detail. Say from an upper sidewalk/parking lot to a lower sidewalk/parking lot.
Although I would agree with the non-susceptible fill at stairs connected to the building.

Thanks again though for the thoughts.

EIT

http://www.HowToEngineer.com

 

[snowmachine88]

I have the same situation. I am designing architectural features on top of a dam. There are staircases from upper platforms to lower platforms on grade. I am using a 4'-6" frost depth everywhere else on the job (Frozen ground was found to depths of 9' here last year.) However, I don't want to go to that depth if possible for the staircases. I have multiple staircases of multiple levels on grade. I have attached the typical detail I am using. The staircases are bounded on either side by walls buried to frost depth and separated from the walls by 1/4" expansion joint material to match the slabs on grade above and below. Is this a good detail to use or do I need a top and bottom wall down to frost depth as well?

 

[RFreund]

My question would be what is the use of the concrete walls on the sides going down to frost? The concrete does not act like insulation so if the soil below the stair is susceptible to frost then the stair will still move. I suppose the frost walls keep the handrail from moving and they could fill with granular material. If you did tie the stair into the walls then the slab would be restrained from moving but again frost can penetrate into the soils below the stair and exert pressure (how much??) on the stair and possibly crack it.
If you ask, why not let the stair float? Well I suppose because it could move and it could crack. Well so can the sidewalk. I think if you can assume the stair may crack some but will as a whole remain intact and can be subject to some movement then maybe you don't need the frost walls. I think it would be important to toe in the bottom to help prevent sliding or creep over time (maybe?) and also have a landing at the top of the stair that is integral with the stair. This prevents any differential movement with the adjacent sidewalk that might cause a tripping hazard.

Again this is for a stair NOT ATTACHED TO A BUILDING STRUCTURE.

EIT

http://www.HowToEngineer.com

 

[snowmachine88]

The purpose of the walls on the sides going down to frost depth is to retain the soil under the stairs. Since a standard 7/11 stair is steeper than 2:1 I need the walls so that the grade next to the stairs can be flatter.

My feeling was the key at the bottom is only to prevent sliding so I will keep it at 1'-6". I will add dowels to the top expansion joint to prevent tripping.

Thanks!

 

[BUGGAR]

From a builders point of view, I've got to set the side forms for the concrete anyway, it don't matter to me if it's a thickened edge type footing or not, I just deepen the forms. I can go down six feet if you want. Do this everyday for buildings. But don't expect those inside walls for the footings to be vertical, though.

Go ahead and use lots of rebar. I hate that stuff but I hate it more when that rich guy who paid me cheap is calling me back every week because his little garden stairway is cracking up.