Residential Concrete Cantilevered Stairs 1

[Buleeek]

Hi everyone,

I am currently working on residential concrete cantilevered stairs and landing. The architect wants to keep 4" stair thickness, but my calcs show about 1/2" deflection. By changing to 5" thickness I could bring the deflection down to 0.2". I usually do not design concrete cantilevered stairs so I would like to ask more experienced people if I am missing anything in my design (see attached) and if 4"-5" thick stairs make sense at all. Each step is 12" wide. All dimensions are shown on the attached drawing. Thanks for any advise.

 

[TidePoolJunkie]

Hooked bars sticking out of the formwork with 2x knockouts for a shear key. Here's a close-up of one of the photos above.

 

[MotorCity]

When you calculated the deflection, did you base it on a cracked or uncracked section? It could make a big difference. That said, calculating concrete deflection is a best guess (more magical hand waving than calculating steel deflection). I was more concerned about your retaining wall and footing. The detail looks incomplete since there is no soil, pavement, etc shown on the other side of the wall. At first glance, your footing width looks a little small for what I was expecting. Did your stair deflection account for any wall rotation?

 

[TidePoolJunkie]

Motorcity--I don't have access to my calcs/drawings anymore and I can't recall unfortunately what I accounted for in deflection. The retaining wall is buttressed by jogs at the top of the stair, and it's a centered footing.

 

[Buleeek]

Okay, so the architect has agreed to reduce the cantilever stair to 3'-6", but it needs to remain 4" thick. They want it to look just like on the picture attached. Would you proceed with confidence or argue that 4" is still too little ? How much cover would you recommend?

Thanks,

 

[jayrod12]

Tell the architect that the stairs they are showing you an example of are thicker than 4". very much thicker. What is the obsession with the 4" thickness?

Are these exterior stairs? If so doesn't your local concrete code dictate clear covers to reinforcing? I doubt that you'd be able to get clear cover above and below and the thickness of the steel. The steel would need to be at essentially mid depth. Then when working the stair so hard, it's likely that the neutral axis depth would be almost half of the section depth. That would render the steel essentially useless. You'd have almost no strain at the steel meaning no yield.

 

[BAretired]

The architect is pushing the limits. The stair in the photo has a cantilever of 3'-6" less 8", or 2'-10". For a solid slab, the code calls for a thickness of L/10 which, for 42" would be 4.2", so this stair is likely to be a little springy. Maybe the handrail could be designed to spread the live load over several steps.

Looks like the photo I was referring to has been deleted (see below).

BA

 

[EdStainless]

Buleeek,
Either my eyes are way off or the dimensions on the photo are a dream and not the actual for that stairs.
They sure look 6" thick to me.
Look back at the photos from TidePool
This is the kind of hand rail that it takes to get real load sharing.
I could see going 5" with that style of hand rail, but not with posts.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[hokie66]

They are not 6" thick. Look at the gap. If they were 6", the height of the riser would be about 10". My guess is they are 4" thick, 3" gap, for a 7" riser. Maybe 4.5", 3" gap, 7.5" riser.

 

[GeorgeTheCivilEngineer]

I was looking at the bricks - that'd suggest 5" thick (slightly less than two courses)

 

[dik]

With a thick tread (even 4") there is a bit of a safety hazard. Walking down the stairs, it's possible for your sole, at the heel, to catch on the underside of the tread and cause a bit of a tripping hazard.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Brad805]

I think this will come down to your comfort level. Based on my past experiences arguing about concrete deflections with a precast manufacturer, we are not great at estimating concrete deflections. I ran a cracked analysis one evening and I more in favor of the idea than others. That said, I think there are a couple of hurdles highlighted by jayrod that could be challenging. I should preface my comments below that the residential concrete trades I work with vary from crap to ok. Our commercial projects involve professional rod busters and detailers that add more confidence to a problem like this.

1. If cast in two pours I suspect they would want the construction joint at the bottom of the treads. How one bends the primary rebar into the tread will be important, and unless you have a good trade this could be a problem for you to deal with on site.
2. As Jayrod mentions, where the rebar ends up is important and if poorly done this would not work.
3. Is the landing a cantilever design as well of the same thickness? If so, that tends to be a location where more visitors stand.
4. The railing in the as-built pictures above would likely help in your design, but the last sketches seem to have posts on treads. Your railing loads may be a problem you need to look at.
5. A hanging rod railing would be easy to help solve your problem. I would get the architect to give you some parameters for that part.

 

[Brad805]

Dik, I built my own open tread stairs, and I am not a huge fan of the open idea. The cantilever stairs like this look cool when you exclude the railings, but if you use too many large posts the airy look is lost.

 

[dik]

I've never had great confidence in concrete deflections... a guesstimate at best. With the cantilevered stair treads, you have to use pickets... guard posts will put too much load on the one with the post. I like the idea, and depending on the climate the thin treads look good. If using salt, one might consider HDG rebar. With the span shortened to about 3' the 4" tread is workable, with the longer span it's not. My $.02...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BAretired]

I've never had great confidence in concrete deflections.

I agree, and with such a thin slab, there is considerable doubt about strength calculations...doesn't take much to change the effective depth by a sizeable percentage.

BA

 

[dik]

With the 3' cantilever... I would be OK with it. Isn't that what safety factors are for?

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik