Metal Stair and landing 2

[EDub24]

I'm designing a new concrete tank at a wastewater treatment plant in California. The client would like to have a stair going from the top of an adjacent building to the top of the tank (my vote was a spiral stair but I was vetoed). There isn't a problem with doing this however the vertical height change is about 15' (horizontal distance is about 21') which according to the California Building Code requires a landing (every 12'). The client would strongly prefer not having posts support the landing as there is truck traffic in that area so I'm wondering if the stair and landing can be designed to cover this span without additional intermediate supports? Normally we push the stair design onto the contractor and I don't think this should be a problem for the Contractor to design/construct but I'd like to verify before I tell the client we can do this.

I'd imagine the stringers would have to have be welded together or have moment connections and be stronger than the typical stringer to cover this span.

 

[dik]

How far is the adjacent building from the tank? 15' or so, so that traffic can pass between the tank and the building?

Dik

 

[dhengr]

EDub24:
The stringers should be full length, continuos beams, with full strength moment joints at the landing slope changes (btwn. the stair stringers and the landing stringers). Some people call these cranked beams. The entire stair stringer frame should be attached to one of the structures and allowed to move laterally on the other, for lateral loadings like wind and EQ’s. You might want to check the industrial sections of the codes and OSHA to be sure you cover their bases too, RE: landings, handrail heights, tread and riser dimensions, etc..

 

[dik]

Can you use a 'cranked' channel section going from roof to top of tank? Any parapets to worry about?

Dik

 

[Bagman2524]

Agree with dhengr. C12 x 20.7 stringers should work.

All I know is P/A and Mc/I

 

[JedClampett]

If it's a concrete tank, can't you cantilever a concrete landing off the tank?

 

[JLNJ]

If it's just an equipment access stair you might not be subject to the 12' maximum rise between landings.

Make sure you look at the forces in those mitered joints rather than just weld up a dogleg where the "cranked" stringers meet. Stiffeners will likely be in order.

And lateral forces can be an issue for something so long yet so narrow. Stairs without plan bracing get wobbly quick and adding a landing complicates the brace action.

 

[BridgeSmith]

Can you get the required rise/run at 15' vertical and 21' horizontal, with a landing in the middle? That will put you at nearly a 1 to 1 slope, right?

 

[dik]

Do you require the rise:run for an industrial stair?

Dik

 

[BridgeSmith]

Dik, I wasn't sure of the requirements for the application, but I thought it might be useful to mention. Likely it's both or neither for the landing requirement and the limits on rise:run.

 

[dik]

Landing would be stair width or 3' minimum?

Dik

 

[EDub24]

Sorry for the late reply. I was in the field all day yesterday. To answer all the questions, the tank is about 21' feet away from the building so there's plenty of room for traffic (nothing larger than a pickup truck). The stringers will definitely need to have moment connections between the stringers and landing if there are no intermediate posts. I just wanted to ground truth this to make sure i'm not crazy by specifying no intermediate supports for the Contractor's designer. As far as cantilevering off the tank, I did consider having the stair go a bit lower on the tank and then having a landing there and a 2nd shorter spiral stair going up to the top of the tank. It wouldn't be as clean as a straight run but that's definitely is something to consider. I'll also check with OSHA to see if they require a landing. I'm not sure if the client would allow that but if OSHA doesn't require it we might make a case to ignore it. Thanks everyone!

 

[JedClampett]

As a caution, EDub24, if this project goes through any plan review, you might have trouble. Plan reviewers are creatures of routine. All stairs need to have maximum 7 inch rises, and all flights must not exceed 12'-0". It doesn't matter what the stairs are for. You can explain until you're blue in the face, but they know what they know, and they know chapter 10 in the IBC.

 

[Bagman2524]

OSHA does not have a maximum vertical height (i.e. 12 ft. or any other value)for a stair at which a landing is required. Wasn't in the pre Jan-2017 OSHA and it isn't in the post Jan-2017 OSHA.

All I know is P/A and Mc/I