Thin stair concrete design

[Euler07]

Hi All,

I'm designing the refurbishment of a 60 year old building (1960's). Part of it involves replacing a 50mm thick concrete stair landing and tread. The concrete is undamaged. See the photo below.

I'm just wondering if anyone knows how this was designed? There's no corrosion damage or cracking. For 50mm thick this would not be possible with the current standards for reinforcement cover that needs min 30mm cover for 40MPa. Do you think this is unreinforced or is there some type of structural additive that was commonly used in the 1960's?

Thanks.

 

[HTURKAK]

Check the bottom of the treads and landing.
I guess this stair has concrete filled metal pan treads and landing.

 

[phamENG]

No, there's no metal pan under any of that.

I doubt there were any additives. I think this one just got lucky. Maybe the plant where they were made (I find it hard to believe it's anything but precast) used an especially good mix, or really low water so it's really strong, or something. Maybe the owners took care of it? Or it's sufficiently sheltered (or exposed?) that it didn't stay wet for long periods of time? Hard to say.

Most of the rules about thicker cover, etc. are because this sort of condition is the exception. Sure, thinner can work, but the number of variables that have to align are hard enough to control that when any of us see this, we're shocked it's still standing.

 

[Brad805]

Given it is from the 60's, I would expect a fairly simple design using welded wire mesh.

The treads appear to be precast as there appear to be bolts. Those could be a rivet for the support tab.



I am not sure if the landing was precast. That would be not be as easy to transport and I do not see any bolts on the tabs. This part would be very easy to form.



I believe I can see a form line here as well.

 

[JAE]

The treads may be 50 mm (2") but the landing looks a bit thicker doesn't it?

 

[TRAK.Structural]

Maybe check stresses from 2-way bending/yield line analysis. The aspect ratio looks a little square-ish.

 

[Ron247]

It looks like the top step aligns with the landing. If so, are they the same thickness? Like JAE notes, it does look a little thicker but not much.

My guess, high-strength mix, really good placement of thicker wire mesh and a lot of luck over the years. Even a precast from the 60s could have a form joint, especially if the slab sizes were per order and not a standard offering. Ignoring the newer clearance requirements, how does it calculate out for a modern code assuming you knew the original parameters.

The one thing it does have some advantage with is concrete gets a little harder with time. Disadvantage, big modern gun safes, I know many who own one and do not have any guns.

 

[canwesteng]

Why even replace this with concrete? Grating is cheap and easy, maybe too ugly, wood could be used and have some kind architectural cover. If you need something that looks exactly like this, then just leave it in place.