Concrete Stairs 1

[HeavyCivil]

At the risk of asking a vague, open ended question, does anyone have any basic information they'd like to share about C.I.P. exterior concrete stairs?.. I guess I will elaborate.

Doing a small (but wide) set for an existing building - landing is a suspended slab which is being removed and replaced with area beneath backfilled and ex. foundation walls to remain with a new SOG poured for landing. Question about stairs is about typical reinforcing, nosing, forming:

How is distribution steel requirement met in treads, risers? Are the bars (say, #5 @ 12" o.c. EW) 3" clear of sloped backfill going to help control shrinkage cracking at the surface? That reinforcing may be enough for the cross sectional area, but given the geometry will it really help limit crack widths at the surface? Climate is very cold, local Pg is 80 psf so there will be a lot of chloride exposure from deicing.

The existing set has deteriorated so badly there is a wood overbuilt set (and even that has been there over a decade). Want to get this one right.

For a very wide set can riser form boards simply be "braced" off each other and cleats at the bottom with 2x lumber perpendicular to forms? Otherwise, even with a stiff mix, they would bow badly I would imagine. On that note, will a 3" slump work? (to pour, not asking for a mix design). Are they typically vibrated? It would seem necessary to avoid honeycomb, but puts more stress on a potentially weak form frame.

I don't see the need to reinforce aside for temperature and shrinkage, although there is an obvious shear-plane. I doubt a pedestrian could cause a failure. Is this pretty typical?

 

[Teguci]

I have eliminated the nose bars from my designs without any consequences (10 degree F winters) other than not having rebar exposed to chlorides. Figuring that the concrete will be cast in warm climates and concrete surface will contract in cold, there is a potential for splitting. I would probably put in additional cold joints rather than place epoxy bars in the nose.

 

[connectegr]

May I suggest steel, instead of concrete? Steel stringers and gauge metal pan treads with concrete.

http://www.FerrellEngineering.com

 

[doka1]

In Northeast(NYC area), we usually build the walls all around the stair, with the face of wall being the last riser. Most engineers require a wall underneath the first riser(end of platform). You therefore have three walls parallel with eachother in one direction, and of course two walls in the direction of the stairs. When you put up the forms for the wall strike the grade, and layout the stairs. Make a 5" inch slab below the intersection of the tread and the riser. Some engineer require this slab to be keyed and doweled into the foundation wall, doweled in at a minimum. (this is if the stairs are adjacent to a building) We like to use 3/4" chamfer strip at this joint to make a nice clean break between the slab and the wall.

Once you have stripped the walls proceeed to backfill. If there are many stairs, some contractors may opt to place a mud mat to be able to retain the backfill material better. (Usually its just some left over concrete from some other side of the job). To form the risers, rip 2X8 down to the riser height, make sure you chamfer the form as it makes finishing a lot easier. If your stair are wide, like 10' then nail a 2X12 following the slope of the stairs on each side of the wall. Layout the stairs on these outer 2X12's...nail your rises to the 2X12. Now to brace the middle of the riser lay oa 2X8, 2X6 or even 2X10 down the slope of the risers. This should catch the corner of every riser if they stairs were laid out correctly. Then just nail cleats down from 2X8, 2X6, or 2X10.

Oil forms well and place concrete from the bottom up. Vibrate as needed, but make sure to hit face of forms with hammer. Dont forget to drop in the nosing bars as required too. Then wait for the concrete to set up some and proceed strip the forms and finish the riser portion of the concrete.

As far as design goes, I have seen some engineers from up north design the slabs as suspended slab, requiring us to use corrugated decking to span between walls, and leave a 6" air space. But in our area, between backfilling with a pourous material, and doweling into the walls should suffice to prevent any frost heave.

I would use epoxy bars, and make sure the platform and treaders have pitch.

 

[JAE]

Pan stairs in an exterior situation? With chlorides applied? I don't think so. You'd have a rusty, failing mess in two years.

If chlorides are applied by the owner, you might look into a corrosion inhibitor (see DCI Inhibitor here: DCI Link

Also, some have used silica fume to densify the concrete and reduce water infiltration (water carrying the chlorides to the rebar).

 

[jike]

Definitely use epoxy coated rebar and ties...no steel stringers. To prevent heave, use non-frost suceptible backfill (less than 6% passing a no. 200 sieve) or 2" styrofoam under the slab or provide 6" gap under the slab.

 

[HeavyCivil]

Thanks for the help.

Doka1- great explanation of forming.

I will specify epoxy bar. I will also use a tight mix design, and may specify a coating or corrosion-inhibiting add mix.

JAE - I am not sure whether or not the local batch plant can get silica fume. I will look into it.

I attached a PDF of how I think the stairs should connect to the existing foundation wall (stone masonry). Existing space is being backfilled. I think it makes sense to overpour the slab to form the first riser and tread. No visible joint and there is a "grade beam" or "cap" the whole perimeter of the ex. wall so I'm not worried about additional restraint on the slab.

Not sure why it didn't come out black and white -sorry. There will be fill below stairs.

 

[doka1]

Make sure to specify the slope you want on the tread, and do you really want a 12" tread? Detail the 1" batter on the stair rise, which will get you the 12". Always go for 7" riser, 11" tread. I did not answer your first question regarding slump...a 4-5 inch slump will work, very difficult to pour concrete that needs to be finished at a 3" slump.

 

[HeavyCivil]

What is the general consensus on aluminum nosings? My feeling is that embedding anything with a coefficient of thermal expansion that is 3x that of the surrounding material is not a good idea in a location that will experience temperature swings.

Has anyone used these with success or failure?

 

[EdStainless]

Not a structural, but some comments.
Change the step dimensions. Carry your tape measure around and check various stairs. You won't find 6x12. These are too shallow and too long.

As far as nosing I have mixed feelings. You are probably better off creating a notch for it and adding it afterward. Look at brass or CopperNickle. I wouldn't expect Al or stainless to stand up in this application.

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Plymouth Tube

 

[HeavyCivil]

EdStainless -

6/12 is architects call, not mine, and I agree.

Not sure how much I like a post installed nosing though... Create notch with wood chamfer strip and tapcon in?

 

[hokie66]

Some of you may be confusing exterior stair dimensions with interior. 6" risers with 12" treads has long been the standard for monumental stairs. Very comfortable.

 

[doka1]

hokie, yes you do have a point, although you typically see that for the main entrance of a building when it comes to concrete stairs, where the handrails are far apart. Do you not like tool grooves into the treads? hokie you know when i say 7" riser and 11", you ultimately get a 12" tread with the batter.

 

[apsix]

Re aluminium nosings; there is also the issue of accelerated corrosion for embedded Al in the presence of moisture.

 

[hokie66]

doka1,

No, I did mean actual 6/12 stairs. The nosing doesn't count. I thought we were talking about entrance stairs. There are a variety of treatments for the treads to make them look good and be slip resistant.

 

[cvlenv]

to doka1, what is a mud mat?

 

[hokie66]

I can answer that for doka. A "mud mat" is a layer of concrete, not structural or reinforced, which is used solely as a working platform for placing the structural slab. It gives a more solid base than the ground for support of the reinforcement. Another use for a mud mat is to serve as the substrate for installation of a waterproof membrane under a structural slab.

 

[doka1]

hokie is correct, i will add its not something you would see called out on the drawing when it comes to concrete stair construction, its just part of means and methods. I would not do that for a stair that is made up of 7 risers, but if it were like 15 risers and a wide stair it makes it easier to work off of rather than the stone continually falling.