Steel Mono Stringer w/Steel Treads

[jdgengineer]

I think there have been similar discussions in the past, but I didn't see the exact answer to my question. We are working on a stair for a single family residence. The stair will be a central mono stringer with steel treads wrapped in wood. The tread build up will be a steel plate with 1 1/8" plywood above and 5/8" plywood below. The whole system will then be wrapped with 3/4" architectural wood. The stringer right now is an HSS 8x6 with 6x6 stanchions to support the treads and landings.

I'm trying to design the treads and not sure what deflection etc is reasonable. When we have done these in the past often the tread is a solid 3x or 4x so I am not as concerned with deflection. With the plywood buildups, I'm tempted to design the steel plate alone for the loading as I'm not as confident in the multiple layers as I am in a single solid tread.

If I analyze the stairs with 100 psf or 300# concentrated point load the stair tread deflection is about .4" with 1/2" and maybe 1/8" with 3/4" (I don't have the exact numbers in front of me).

How do you all typically design these? Am I being too conservative by ignoring the wood build-up for added stiffness? I'm not sure how they will be attached, likely a combo of adhesive and small screws. The stair fabricator initially suggest 1/2" plates for the treads and the landings. The landing design currently is a single post in the center of a 42" square plate. I'm not crazy about that either and may add additional posts from the stringer below to support the landing. (posts are only ~4" tall).

Any thoughts?

 

[TehMightyEngineer]

I'd ignore the wood; the stiffness differential between steel and wood is too great IMO to easily account for them together and your connection method is likely insufficient for true composite action. However, I would rely on the wood for some peace-of-mind damping of vibration via it's added mass and misc. stability support.

100 PSF is a lot of people; that's a stairway crammed with people escaping a fire or whatnot. 100 PSF isn't reasonable for a service load for deflection in a single-residence home IMO. Nobody would be worried by feeling the stairway sag a little bit (if they even notice 0.4") under a 100 PSF load. What you want to ensure is each tread feels sturdy and has minimal deflection. With a central stringer each tread cantilevers and relies on the torsional stiffness of the center girder.

I'd actually do this if this were my client. I'd prepare a quick demonstration by getting some cheap wood and mocking up a few stair treads with varying stiffness such that you have a range of tread deflections under the clients weight. Have the client step on each tread and tell them the thinnest is the code minimum and the thickest is your recommended thickness. This does two things; makes the decision the customers and not yours, helps both you and the customer understand the "feel" of these treads, and would be a great demo of your structural prowess for both this client and future clients.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[jdgengineer]

Thanks tehmightyengineer. I'm not concerned too much about the overall stair deflection, really just the individual treads. At 42" wide 100 psf is only 350# uniform across the tread. That doesn't seem too crazy to me. The 300# point load at the end is governing.

Interesting idea about the mock ups, but I'm not sure how feasible that is for us.

 

[T_Bat]

jd - what code are you designing to? For a single family residence, I would expect the stair live load to be less than 100 psf (e.g. the IRC prescribes 40 psf for stairs).

 

[StrucDesignPE]

What building code are you using?

IBC 2015 and ASCE 7-10 ony call for 40 psf uniform load and 300 lb concentrated load for stairs serving a single family residence.

Edit - T Bat beat me to it.

 

[jdgengineer]

Designing to 2016 CBC. Would only be required to use 40 psf for stairs. We typically use 100 psf to account for the fact that these types of stairs can be used for "prom photos" and to make them a bit more rigid.

Regardless for the individual tread design I think 40 psf wouldn't govern anyways. That would only be a force of ~140 lbs. The 300# concentrated load governs.

But back to the original intent of my question, those who have designed these stairs, what criteria do you use for the treads? If I go steel only, I think I would want a 3/4"-1" thick steel tread which feels thick.

 

[StrucDesignPE]

Well, assuming the stair width is 42" and supported in the center of the tread, then a typical cantilevered deflection limit would be 2L/360 = 0.116 inches. This is not too far off from the 1/8" you calculated for a 3/4" plate. I think that I would look at the steel plate alone and neglect the plywood as others have mentioned.

 

[TehMightyEngineer]

That's very thick IMO. I'd look at using an angle or channel or otherwise providing a vertical leg for flexural stiffness. It could be a short enough leg that it's hidden by the wood but should do more for you than trying to add thickness to the steel plate.

I'm not surprised 40 PSF doesn't control for the treads; that's what the 300 lb. load is for. Keep in mind that 100 PSF is not a "prom photo" that's a jam packed elevator in terms of personal space. It's very difficult to conceive of that many people in a residence on the stairway.

What does the 0.4" deflection relate to. Is this the deflection at the cantilever of each tread? If so I'd say that's unacceptable.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[StrucDesignPE]

I agree with TME that welding several steel plates or strips on the bottom side of the tread plate to act like "spines" would help. The underside will have 5/8" plywood, so maybe welding two or three "spines" of 1/4" or thicker steel along the bottom at third points would reduce the tread plate size and deflection.

If the total wood thickness and a 3/4" steel plate are added together to get a sandwich, its only about 4 inches thick. This doesn't seem unrealistic to me.

 

[Agent666]

If you've done a few previously but just in wood, and they felt acceptable in practice, then just use a similar deflection criteria.

Rather than welding "spines" (a lot of work), is folding a small downturn each edge of the treads in a thinner plate an option to in effect create a thin channel section?

Also are there any thin/wide RHS sections available in your area? Even if you had to stitch a few together to get the width it could be an option vs a solid plate?

If 3/4" plate is required to achieve acceptable results and the architectural vision, then that's what it it even if it feels thick. Provided there's a reason to justify it to those who might say you're being over the top or anything, then I wouldn't worry about how it looks or feels. Saving a small amount of material for the sake of creating a deflection or vibration problem down the road is just false economy.

 

[MotorCity]

I would only consider the contribution of the steel plate with regard to the tread strength.

Given that your treads will be cantilevered, I would be inclined to consider the force due to a "heel drop" of someone going down the stairs. Seems like the tread could get a little bouncy. Inevitably, this may lead you down the path of comparing the natural frequency of the tread to the comfort level of the users.