Cantilevered stair vs masonry wall 5

[Jay_]

Hello all,

As the subject says, i am in one of those situations.

There is an existing solid masonry wall that need to support a one floor cantilevered staircase structure.

For the fixing of the cantilevered treads structure to the wall i am thinking of two options.

Option 1) using chemical anchors with anchoring adhesive but all these chemical anchor suppliers recommend one bolt per masonry cell and thats an issue since the anchor bolts spacings are going to be less than a masonry cell.

Option 2) using a sandwich connection between the treads beams and the other side of the masonry wall; basically welding steel rods on a plate, inserting it from the opposite side of the wall and fastening the structure on it from staircase side.

I think option 2 can be better than option 1, but i need guidance, advices, and even other options but taking into consideration that it’s an existing solid masonry wall and it is not under construction.



Detailing is a hobby,

 

[milkshakelake]

Sketch would be helpful. First thing I'm thinking about is the flexural moment coming from a cantilevered staircase. Will the CMU be able to support it? Maybe some reinforcement to the CMU is required, like grouting the cells. But maybe your arrangement doesn't induce any moment except a little eccentricity, in which case it might work.

Option 1:
You might be able to get epoxy anchors to work. The manufacturer guides should have a reduced capacity for when they're close together. And if you can't, consider using a WT or something to spread the anchors out. I can send a sketch of this if you want, but I'd need to see your sketch first to see if I have the right idea about what you're doing.

Option 2:
I'm not sure what option 2 is. I don't think you'd necessarily need to weld the bolts to a plate. You can just use thru-bolts, which would avoid some shop welding. The only concern I'd have is bending in the bolt itself. The inherent eccentricity of the connection means one side will be in compression (bearing on the CMU face or brick wythe) and the other would be in tension (on the other side). I've never analyzed something like this; I'm mostly posting this musing to see if others have the same thought.

In this case, bearing stress has to be checked because it will be higher than the epoxy anchor case. Also, I'm not sure how you can make the connection tight. Meaning the hole through the masonry will be larger than the bolt itself. It won't be a slip-critical connection in masonry, so there will be a bit of slippage. But once the load is set, if there's no load reversal (loads will always be gravity), then that's not a concern.

For these reasons, I tend to go with Option 1 because it's simpler, with less things to think about.

 

[JAE]

Is it "Solid' masonry? Or hollow block? Or fully grouted and reinforced masonry? Or partially grouted?

I think your ideas (1 or 2) probably won't work as you'd probably crank on each block at the attachments with repeated load applications from the treads being stepped on and the block would begin to crack.

 

[Jay_]

I draw a sketch by hand.
More than bolt in a masonry cell i inevitable in option one because the beam is too small compared to the masonry size, its 400x150x200mm solid masonry. The stair tread is composed of light hollow section trusses.
Anyways here is the sketch.

Detailing is a hobby,

 

[HTURKAK]

Still it is not clear for me what is solid masonry . Probably fully grouted hollow blocks . Even if RC wall, i would not suggest cantilevering every each step.

There should be other better options . If you provide a sketch , probably you may get better options. However, if the only option is cantilevered staircase , i would suggest you to cantilever from 10 mm thick stringer fixed to the solid masonry wall.

If you write ''cantilever step stair .. '' and search the web, one of the outcomes which may be an option in this case;

Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[milkshakelake]

I don't think the flexural moment will work in the CMU/solid masonry itself.

 

[hoshang]

its 400x150x200mm solid masonry.

He stated solid masonry. Where I practice, solid blocks are manufactured only with dimensions 400x150x200mm.

 

[Jay_]

Yes, sorry all for the confusion, its a solid block of 400x150x200mm.
What if i did option 2 but i filled the hole after drilling with anchoring adhesive prior to fastening each step?

Detailing is a hobby,

 

[BAretired]

How do you know the wall can take the fixed end moment from the cantilevered treads? If you don't know, you should use a support at the other end of the treads. Doesn't have to be a masonry wall; could be a beam spanning the length of the stair.

 

[Jay_]

I am going to connect all the treads together at the free end. The concrete contractor said that the wall is well constructed, and he said it can take the bending moment. By issue is still how to fix the treads.
Option 1
Option 2
Or option 2 with anchoring adhesive inside the drilled hole.

Detailing is a hobby,

 

[Eng16080]

The concrete contractor said that the wall is well constructed, and he said it can take the bending moment.

I guess it's good to go then!

 

[BAretired]

With the accent on guess!

If the ends of the treads are going to be tied together, tie them with a stringer capable of spanning the stair length. Then the tie to the existing masonry is nominal, carrying only vertical load but no moment. Beats the hell out of guessing.

 

[milkshakelake]

If you have a stringer on the other side that's connected at the top and bottom, they're not cantilevered treads. They're pinned. Totally different discussion. But it's a LOT better of a design. It'll actually work, as opposed to not working.

It's really hard to recommend option 1, 2, or something else because I don't have the full picture. Just go with option 1. I explained the pros and cons.

 

[BAretired]

The concrete contractor said that the wall is well constructed, and he said it can take the bending moment.

It looks good from the outside, but unless the contractor has X-ray vision, he can't see whether or not there is any reinforcement. If it doesn't, or if the rebar is spaced too far apart, the wall could crack from the applied bending moment.

Without evidence that the wall has been adequately reinforced to sustain the fixed end moment from the stair, it makes sense to support the opposite end of each tread. To do otherwise is a foolish gamble with little to be gained and plenty to be lost.

 

[Jay_]

I can do that,
Pinned connection sound like a good idea,
I think i can achieve a pinned connection by using two anchor bolts in the direction of the stair.
Below is a new sketch.

Any comments?

Detailing is a hobby,

 

[Lomarandil]

But it's a LOT better of a design. It'll actually work, as opposed to not working.

Thanks for the Monday morning laugh!

 

[BAretired]

BA[/color]]
Below is a new sketch. "At right is a new sketch".

Any comments? I don't understand it.

Detailing is a hobby, Keep at it!

I suggest a ledger fastened to the wall at perhaps 1200mm centers (measured horizontally) and a stringer of similar depth at the free end of the treads. The stringer should be supported by posts, one at each end of the stair. A handrail (or perhaps a plexiglass wall) is required at the free end to prevent folks falling off the end of the treads.

 

[JAE]

The concrete contractor said that the wall is well constructed, and he said it can take the bending moment.

If I heard this spoken I'd immediately be much more inclined to triple check my calculations. My my.

If the treads do indeed cantilever off the wall (where the other end of the treads are totally free in the air) then my view would be any sort of masonry would have trouble maintaining the possible bending and/or tension forces generated.

Think of a 250 lb. well-fed engineer jumping up and down on the end the tread.

And on top of that your connection is not redundant....the connection or wall fails and there's no other load path - people get hurt badly.

The only cantilever tread design I've done is where it's connected to an embedded steel tube in the wall and the embedded tube is fixed to significant columns each end.

 

[milkshakelake]

@Lomarandil In terms of zingers, BAretired has a nice one in there

 

[HouseBoy]

Do you folks ever work with architects? The suggestion of a stringer at the free end makes me think you don't! (jk)

No doubt it would be much different (simpler and better performing) with a stringer at each and of the treads but that is not at all the look that is depicted in the sketch.

I've done this before. Some kind of torsionally stiff system is needed inside the wall. Tough to do with the existing "solid" masonry there already but... Much will depend on how much compromise is possible with the aesthetics.