Steel Stairs In A Wood Structure 2

[spats]

I am designing steel stairs that are being installed in a wood-framed multistory building. The intermediate landings and floor landings are also framed in wood. The edge of all landings that I need to attach the stairs to are framed with (2)1.75"x11.875" LVL beams. The landing decking is simply 3/4" OSB with 1" of gypcrete. The stairs themselves are 4' wide with C12 stringers and 3" thick precast concrete treads bolted to carrier angles welded to the stringers.

The situation I describe is common for apartment and townhouse buildings, but my research has been unable to turn up any suggested details for attachment of the stairs to the LVL beams. The ends of the stringers can be coped out so that bearing plates can rest on top of the OSB. However, I'm concerned about the crushing strength of the OSB. I also do not want to count on the bearing connections only. One idea is to weld a continuous steel banding plate across the 4' stair width at the ends of each stair run so I can face bolt or lag screw the plate to the side of the double LVL. This also assures everything is well tied together, and gives me the "belt and suspenders" design I'm looking for. However, if I do that, then face-of-landing to face-of-landing has to be the perfect distance.

I feel certain someone out there must have dealt with this situation before. Any ideas are welcome. Maximum stringer reaction is 2.8K, but is more typically a little less than 1.5K.

 

[oldrunner]

I would discourage using OSB in any part of a stair landing detail if these are exterior. In a recent inspection, we found that OSB had been used as shims to get the steel stairs to fit the already built structural wood framing. (In this case glulams.) Water got into this area and eventually two different stair sections failed. The first did not cause any injuries but the second one killed one person. Also don't use lag screws. Use through bolts. Do a personal inspection to make sure that there are no "accidental" holes in the waterproofing. You might need that inspection report twenty years from now.

 

[KootK]

12" long, horizontal seat angle welded on the stringer web, vertical leg down. Notch the top of the LVL a bit to receive the horizontal angle leg and spread the load some. To hell with belt and suspenders. Few thing are as reliable as just stacking stuff on top of other stuff. It was good enough for the Romans and it should be good enough for you.

 

[CANPRO]

I came here to post my sketch, but I like kootk's idea better. Sketch still attached.

Also, I agree with oldrunner about the OSB and lag screws - although my sketch shows the opposite, notching the OSB out of the way and using thru-bolts would be more reliable.

 

[Ron247]

KootK, I am having trouble visualizing your angle orientation. The 12" length I assume runs horizontal. Is is centered on the stringer or to one side? I think I am more struggling with the length of 12". I think you are proposing creating a "ledge" at the end of the stringer to set on top of the LVL.

I also agree about not setting on OSB.

 

[KootK]

I came here to post my sketch, but I like kootk's idea better.

I dunno, usually the best sketch wins. And you've got me there. I think your detial would be awesome if:

1) It were a plate between stringers and shear tabs rather than angles and;

2) The bolts could be done up slip critical.

Unfortunately, I'm not sure that it's wise to assume you can get SC on a residential project.

I think you are proposing creating a "ledge" at the end of the stringer to set on top of the LVL.

Sounds like you've got it.

The 12" length I assume runs horizontal. Is is centered on the stringer or to one side?

Shifted to one side so that, outboard, it would not project beyond the flanges of the channel. I'd want to weld an end plate over the end of the channels connecting the flanges. That way, eccentricity in the angle brackets could transfer through the channels in torsion back to the nearest treads. Although the treads themselves don't sound great for this purpose.

 

[BUGGAR]

"Notch the top of the LVL a bit..........". No further comment on this.
Any other comments on rigid stairs collecting unintended seismic loads?

 

[KootK]

"Notch the top of the LVL a bit.........

Don't be coy. What's the problem?

 

[KootK]

I recently convinced myself that I could do a light duty moment connection like this on one of my own projects.

 

[spats]

Thanks to all for the responses. I need to digest this. I will respond tomorrow. Again, thanks to all! Who's watching The Masters?

 

[BUGGAR]

When I say notch, the Contractor sees chain saw. That's why I asked about stair lateral resistance for seismic. When the Contractor is through notching, the stair stringer is usually holding up the lvl, if you don't let the bottom slide out. I'm sure you have met my Contractor.

 

[KootK]

Ah... gotcha. Another possibility is to use a smaller LVL and build up from it with plate to create the notch.

 

[spats]

One aspect of having the continuous vertical plate across the entire width of each end of each stair run is to create a rectangular stair assembly composed of both stringers and the end plates. The fabricator plans on adding a couple of temporary intermediate steel plates, tread plates if you will, that bolt to the carrier angles on each side, for handling and erecting. This will help keep the assembly from racking. The intermediate plates are removed once the stair is in place, and the precast treads are bolted to the carrier angles. We lose that with the details suggested. I guess they could use a temporary end member for shipping, handling and erecting that can be easily removed, but that will definitely add cost.

These stairs are interior, so I'm thinking the OSB should be OK for bearing, or should I insist the stair landings be plywood? KootK, you seem to be indicating a cutout in the plywood in your sketch, at least for the bent plate. Is the bearing plate sitting on the plywood beyond the bent plate? Also, which element is taking the vertical load, the bearing plate or the shear tab? Since there's only one thru bolt per shear tab, I assume it's the bearing plate... Or are you giving me my "belt & suspenders"? Certainly I want to bear (as the Romans do). Why is the bent plate 4' long? Doesn't that place the shear tabs at the very ends of the bent plate? Is the shear tab connection slip critical? (I assume not). I have to field weld the shear tabs to the bent plate, right up against the engineered wood beam? A lot of heat applied to the LVL?

Sorry for all the questions. I'm just trying to understand your concept, and CANPRO's, which is similar.

 

[oldrunner]

Have you checked the capacity of either the OSB or plywood or the solid wood for bearing perpendicular to the grain(crushing)?

 

[KootK]

Or are you giving me my "belt & suspenders"?

That.

Since there's only one thru bolt per shear tab, I assume it's the bearing plate...

I was seeing several bolts being installed along the 4'+ plate length.

Is the bearing plate sitting on the plywood beyond the bent plate?

I have no strong feelings on this and would just do whatever suits your preferred detailing. Allowable bearings on OSB seem to be on par with everything else in wood and, even at that, these are deformation based allowables which I violate regularly.

Why is the bent plate 4' long? Doesn't that place the shear tabs at the very ends of the bent plate?

It's 4' long because:

a) you said the stair was was 4' wide.

b) I'm not drawing the fabrication drawings for you man, add the 2" yerself! Gawd...

Is the shear tab connection slip critical? (I assume not).

Not slip critical. That would be awesome but, as a I mentioned earlier, may not be realistic for res. That said, if the bolts are nice and tight, I'll happily take whatever incidental SC we can get.

I have to field weld the shear tabs to the bent plate, right up against the engineered wood beam?

I was seeing the tabs as shop welded.

 

[oldrunner]

And what is the full live load plus dead load and some sort of impact? Looking for P/A and a D/C check. As the stairway is inside, I wouldn't be concerned with elongation of the stair stringers thermal movement unless the interior has a large temperature range.

 

[spats]

Thanks for the response KootK.

Oldrunner - the loads were given in my original post. I'm not sure what a D/C check is.

The stair detailer is telling me he can't have bolts exposed on the inside of the stringers.

 

[KootK]

You're most welcome spats.

Demand/Capacity. Or derelict concubine, it's hard to say for certain.

 

[KootK]

The stair detailer is telling me he can't have bolts exposed on the inside of the stringers.

In that case, I'm leaning towards shop welding the shear tab to the stringers and living with a shimmed gap between the connection plate and the LVL. It's just a modest sized, non-feature stair. Forward...