mtl.tread detail

[AdM22]

Hi everyone, I need assistance to properly frame a cantilever steel tread; I would like the wall to be as slim as possible due to space constrains. Attached is a simple drawing of what I had in mind, but I am completely open to suggestions. However, the tread length must remain and I prefer if we could avoid a triangular support bracket underneath it.

Any help will be strongly appreciated!

 

[IJR]

Your two angles will probably support this tread well. However, your connection to wall matters

A simple calculation is possible. Determine distributed load on one angle and check stresses and deflections assuming the wall end fixed.

KEEP DEFLECTIONS VERY LOW. People tend to walk away from the wall toward the freer side, and they will feel the springiness(sort of vibration)

WORRY ON THE FIXITY ON THE WALL SIDE.Is it a concrete wall or a masonry wall?

Nice tread

respects
ijr

 

[JAE]

It isn't only the vertical cantilever deflection (and the resulting springiness) that is a concern here. Note that your framing is NOT a closed section but rather two separate angles connected by small plates.

When someone steps on the tread, you will get TWO kinds of deflection - the basic downward, cantilever deflection, and a twisting deflection as a foot hits, perhaps, only the front angle.

This twisting might be substantial. I would rather see a horizontal tube as the structural element, or some other kind of closed section.

The treads you show look to be very susceptible to twisting.

 

[AdM22]

Thanks gentlemen for both responses.

Answering to IJR:
Indeed connection to wall is fatal here. We plan to have a 6"block wall with a strip footing following its perimeter to transfer loads. My question is if we ought to use a diagonal conc. beam acting as a stringer and filling the cells reinforced w/re-bars underneath OR use a steel channel stringer to bold and weld plates to the tread....

Answering to JAE:
I see you point, but a horizontal tube won't satisfy the design intent, perhaps a C12x25 section?

We only have 8 cantilever treads up to a conc.landing and all remaining steps are regular conc. treads w/closed risers.

 

[msquared48]

Your 1/4" vertical center reinforcing plate will see in excess of 50% of the load, but has much less sedction modulus than either exterior angle. The stair will bow inward as JAE suggests and the tile will crack.

Stiffen the interior vertical plate substantially. Deflection IS critical here due to the tile.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[AdM22]

Thanks msquared48, what do you suggest for that int.vertical plate?

 

[IJR]

JAE has a point: your angles are free to rotate.

Here is what we do: we would either bend a plate into a U with lips 2" high(5cm) and fill the U with plain concrete, then place the tiles. This looks clean from the bottom

Otherwise you could use 4 angles and make a frame, and then place the tile in the frame.

I think your diagonal concrete beam should work. It is perhaps best to embed plates well anchored into the beam by dowels(reinforcing bars hooped and welded to plates). then you can weld your tread to the plates for better fixity and clean look

Sure, check deflections and the twist as JAE suggested.

respects

IJR

 

[AdM22]

Thanks once again IJR.

I agree about a single plate in lieu of the angle for a cleaner look from the bottom. A "C" section should work fine, but why filling it with concrete?

regards,
AdM

 

[JAE]

AdM22,
The "C" type section will not work as well. The "open" section of a "C" or a "U" does not have but a fraction of the torsional stiffness of a closed section (such as a rectangular tube, pipe, etc.

You'd have to go pretty heavy with a channel to avoid twisting deflections. Now a heavy "C" section might work.

I'd try the concrete fill idea if you must go with a C.