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.

 

[dik]

Not having done the calcs, 4" is way too thin for a 5.5' cantilevered slab if they are only treads. You don't have to worry about building a slope for water to drain away...

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

-Dik

 

[BAretired]

Maybe the steps could be tapered from 4" to 5".

BA

 

[hokie66]

My starting point would have been 6". Another thing, don't specify the bar spacing, say how many you want.

 

[dik]

If a cold climate and they use salt, not enough concrete cover... Hokie's suggestion of starting at 6", tapering to 4" is great...

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

-Dik

 

[LittleInch]

What's the "bounce" like when you walk up them?

Or a load of people leaning on the outer railing?

But I suppose it's only 4 feet high....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Buleeek]

Thanks for the replies. I agree, I would like to start with 6”.

How about a flat MC channel with poured concrete, welded to end plate? The end plate anchored to concrete wall. Has anyone done such a design ?
My concern is that some anchors at the top will be too close to the edge of wall.

 

[dik]

Why use a channel when you can use concrete? Channels unless treated, may provide a 'slippery' nosing?

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

-Dik

 

[Brad805]

What is the railing detail? I do not see any construction joints in the detail. We have done precast treads before, but if I recall correctly they were only 42".

 

[jdgengineer]

Completely unrelated to your question, but revise your back curtain steel hook to point towards toe to create a closing joint, and provide top steel in the heel as needed for moment capacity.

 

[canwesteng]

I'd would try and use a channel with some decorative concrete. I would weld bar to the channel instead of anchoring though.

 

[Buleeek]

Brad805 - There is no railing detail yet. Most likely 36" aluminum railings top mounted with 4 anchor bolts.

dik - because 4" concrete doesn't work and MC12x45 does.

I would go with the channel welded to plate and plate bolted to concrete wall, but as I get close to top of wall I do not think Hilti anchors so close to the edge can withstand the load.

 

[BAretired]

Forming is going to be a bit tricky. It is not practical to pour the treads at the same time as the wall, so maybe the wall should be poured with a stepped configuration to the underside of treads, then pour the upper portion of wall monolithic with the treads.

BA

 

[bugbus]

I agree with your calculations - about 0.2" deflection with a 5" slab.

I would be tempted to beef up the reinforcement somewhat to avoid any cracks showing on top of the treads near the wall. I assume this will be an exposed concrete finish?

 

[dik]

Thanks, Bul... Did you catch my earlier note, "4" is way too thin for a 5' cantilevered slab" The concrete stair can be far more attractive.

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

-Dik

 

[Brad805]

If any of the newel posts land on a tread I think that will be important. I suspect the treads will be cast in a second pour. If so, you could consider a higher strength concrete along with the other suggestions. I have argued about concrete deflections with one client in the past, and they have built test samples and proven my calculations were not accurate. If the architect or owner are adamant you could test a sample. I have also seen a highly simplified post tensioning system used as as well. PT Stair Tread

 

[TidePoolJunkie]

I designed something similar a few years ago that was built within the last year. It wasn't 5 ft if I remember correctly, more like 4'. But they were 4" concrete cantilever treads. They were poured separately from the wall.

 

[hoshang]

Hi,
You can use these (marked as red) or any suitable SHS beyond them (not to be visible) to connect two treads.

 

[MyCupboard]

I don’t do much custom concrete design like this, mainly wood framed residential. Would anyone be willing to share detailed calculations with me, possibly with notes on where formulas and values are found in code. I’m trying to learn as much as I can!

 

[hokie66]

TidePoolJunkie,
If the treads were cast separately from the wall, how were they connected?