Stair support

[AK4S]

I am looking at a new/proposed stair on an existing building which goes from a low roof to a High roof along a sloped roof portion. Its a maintenance stair (~3ft wide), so live load is limited (60psf snow load governs)
The stringer is lifted about 12" above the roof surface using a vertical post. The stringer is welded to the post, the post welded to the base creating a welded framed system. See sketch below:

I am trying to confirm if what I propose is stable. Do you see anything which might indicate an unstable condition?

IMO, the gravity load should go directly to the supporting post and there should not be an inclined component which goes along the stringer creating a condition where the stringer will slide down the roof? is that a correct interpretation?

As an option I could probably get better stability if I anchor the stringer directly on the low roof and eliminate the bottom-most post.


[Edit: The stair treads are welded to both stringers]

 

[1503-44]

Have you never walked on a sloped roof? If it wasn't for the friction of your rubber soled trainers, you'd be sliding down the slope. While gravational forces are vertical, the roof's slope creates a plane where those forces are resolved into a normal and a shear force component. If you don't resist the shear component at that surface, through a nail in your foot, or friction of your rubber soled shoes, you're goin down. Yes, P cosX exists.

Yes attach it directly to the roof if you have any means to do so.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[AK4S]

@ax1e: To clarify, in my question I am not walking on a sloped roof surface. The stair treads are horizontal. How is the condition different than a regular stair where you go up a flight which is on an incline, but the stair treads are horizontal?

Edit: @ the interface of the post base to the roof surface, resolving the force into 2 components will give a component normal to the surface (bearing on surface) and a component along the surface (shear resisted by anchors). However, the post still gets axial load only.

 

[1503-44]

If you can avoid forces transiting through a slip plane, no problem. But in your detail in the first drawing, I see 2 bolts that have to be designed for a normal force and a shear force.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[phamENG]

AK4S - the difference is this: With a normal stair, the bearing/base plates are flat on the ground, and oriented 90 degrees to the gravity force and in line with the lateral force. You still transform them into components, but you're multiplying by cos(90) and sin(90), so it's a step that can obviously be skipped. Yours are not, so the step is necessary.

If you set your bracket on a frictionless surface with the slope of the roof, what happens? It will slide down the slope. That means you either need friction to hold it in place, or you need some other force along the movement vector (which will coincide with the summation of all of the force vectors making it slide). Your force will be shear in the bolts that hold the bracket. You need the components to determine that shear value.

I'm confused about that detail. It seems a bit...complicated. And your "weld all around" fillet weld is probably not going to be a prequalified weld per AWS. The angles of the pieces are a bit too extreme. And what are you proposing they weld it to, anyway? I don't see a cap plate on the lower HSS piece. If it's going to be a shop weld anyway, just bring the pipe down and land it on base plate. You'll probably need a PJP.

 

[1503-44]

phamENG, do you think that 1.5" ∅ x 12" long pipe is OK? Looks a touch skinney to me.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[phamENG]

It's plausible. 6' to 8' spacing, 3' wide - they're not carrying much. If this is a stair to a little maintenance area, he should probably using 60psf live load (says 60psf snow load governs, so that's probably fine). Might wobble a little if there's not lateral bracing.

Another thing to consider is the roofing around it. AK4S - talk to the architect about that. Some pitch pockets work best with larger bases. I once had to use 6" pipes on a little rooftop platform that only needed 3 or 4 because everything else on the roof was 6 and the contractor was already buying the pitch pockets in bulk. The extra few dollars for the extra steel was apparently much less than buying another pitch pocket size.

 

[AK4S]

@ax1e: yes, the anchor bolts are designed to carry the shear loads (and normal loads for wind uplift case.)
To get around local projections on the roof surface and conduits, the stringer was lifted up and not directly laid on the roof surface.

@phamENG: I agree with the need to resolve for 2 components at the interface with the sloped surface (HSS tube).
The post, HSS tube and base plate will all be welded in the shop. The connection between the post and HSS tube is proposed without a cap plate (won't this be similar to the case even if a cap plate is introduced as shown in the sketch below. i.e. the bottom of the post is cut at an incline and welded to the cap plate and the cap plate welded to the HSS.) I see the need for it if the cap plate was welded to the HSS in field, but if all of it is welded in shop, can't I ignore the cap plate

I am not sure if I am able to understand the concern regarding welding of the post/pipe to the HSS,the condition would be similar to welding a base plate directly to the post/pipe if the base plate was normal to the pipe? (for design purpose it is different, since the weld will need to resist the two components of the force, but what is different in the welding detail?)

 

[AK4S]

The stair treads are welded to both stringers here.

For the framing itself (post-stringer system along long direction and post-tread system along short direction) since they are welded, it is like a moment frame in both directions, so it should be stable against any lateral forces (wind/seismic). Would you agree?

 

[1503-44]

OK. Not much wind load I guess. Seismic? What Zone? And just thinking that they won't last more than 5 years if there is any corrosion at the weld joints. I'd tend to use 3.5"∅ pipe, half cut back at the stringer joint and welded all-round to the back of channel stringer and also to its flange, rather to just weldl onto their dinky little flanges alone that have little lateral resistance. As you say, maybe shakey if not Xbraced, but you can hardly fit Xbraces in that 12" height. Oh well. Not my baby. 1" diameter wouldn't fly offshore. It affects your view of how normal things should look after awhile.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[dylansdad]

AK4S,
Depending on your code, you may need an intermediate landing.

 

[AK4S]

@ax1e: Agree to your point regarding the welding of the pipe to the bottom flange of the channel stringer.
However, looking from a stability point of view, I could rely on the top connection of the stringer at the landing on high roof and the bottom connection on the low roof (stringer connected directly on the low roof and eliminate the bottom-most post). It should prevent the stair from sliding down the slope.
With the intermediate supports on the sloped roof portion added to relieve the gravity load from the stringers.

Lateral load is not significant. There is not much sail (Ice clad railings and stringer surface)in the system against wind and seismic force is low.

 

[phamENG]

AK4S - I misunderstood the orientation of the HSS. I like it even less now. Why is that HSS there? It creates more problems than it solves (for design, fabrication, installation, and maintenance). As for the wobbling - there's all sorts of things you can weld together, but it doesn't make them stiff. As ax1e pointed out - your pipe supports are pretty tiny. With that small of an I, stress will spike at lower lateral loads, and you may find yourself in a low cycle fatigue problem. Add corrosion to the mix and the whole thing is unpleasant. I'd use larger posts from base plate to stringer, and make sure the whole thing is galvanized after fabrication.

 

[1503-44]

Yeah, I really don't like the 1-½" stuff. Any savings isn't worth the potential trouble later on.
The rest; no problem 4 me, as long as you all are happy.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"