Handrail Post to Tread Connection

[Heldbaum]

Hello folks, has anyone ever seen a connection as per attached sketch ? This is a middle handrail attached to a steel tread..I don't see how this group of welds can sustain the bending moment of approx. 7.5 k-in ( posts spacing is around 4.5' therefore 50 plf horizontal load governs). Does 2" of topping concrete play any significant role here ? Any comments much appreciated.

 

[r13]

The minimum weld size for 5/16 plate should be 3/16". The weld pattern looks ok, unless strength calculation indicates otherwise.

 

[SteelPE]

Is the concern you are having of welding the plate to the 10ga tread?

In regards to the welds, unless I am doing something wrong, you have 7.5 in-kips w/an 8x8 plate.... 7.5in-k/8in= .9375 kips/2 inches of weld = .46875 k/in of weld. This is easily resisted by the weld (minimum size excluded).

However, I am not sure about the bending in the 10ga tread. I have no idea how you would prove the tread is strong enough. Those Z shaped treads are quite strong (Sx = 4.5 in3 Ix =29.3 in4) about their length but I am not sure about the torsional capacity of the tread. You could just request a center stringer be applied under the rail (what I normally see).

When designing stairs, I have yet to even thing about relying on the topping.

 

[XR250]

The detail passes my gut check. Possibly the topping may crack due to the warping of the tread under a design rail lateral load.

 

[Heldbaum]

SteelPE - yes you are right, I ran quick calcs and indeed the group of weld easily resists the applied bending moment. Yes my concern was the tread itself but I don't think it's reasonable to check that tread resisting this fore alone since it will be welded to both stringers and connected with other treads/risers so it will work together..?
Yes cracking of the topping might be a problem in the long run

 

[SteelPE]

Heldbaum,

I have designed my fair share of stairs for clients. I can't say that I have ever run into your detail. As XR alluded to, warping of the tread will be the main concern. I have no idea how you calculate that easily on this system. I'm sure you could make a fancy computer model of the entire stair, but then again, how much time do you want to put into this. If you are that concerned about it, have the fabricator install another stringer under the center rail and weld the post to that stringer. This is the what I have see from my clients. It's not easy (as they need to cut triangular pieces of channel attach the treads to), but it can be done.

 

[phamENG]

I'd start by looking at the tread as a simple beam with a concentrated moment in the center (or, better yet, a force couple with 8" between to better approximate the reactions from the base plate). The cross section of the beam would be the "L" of the tread with its edge stiffeners - I don't think I'd try to incorporate additional treads unless you're really confident in the connections between them. That'll give you a deflection of the steel, at least. Though it won't consider warping or any distortions within the shape - that's going a bit above and beyond a practical analysis.

But more importantly, you may want to look at the concrete instead. It's going to be stiffer than the steel tread I would imagine - for the tread to deflect the concrete would have to give way (by which I mean crack). Maybe check the concrete to ensure you stay below the modulus of rupture. At service level loads, the concrete and steel will probably behave in a pseudo composite manner, though it may be tough to quantify it.

 

[Heldbaum]

Thank you guys. I appreciate your help.