Stair landing cantilever

[ztengguy]

See attached. I have a concrete landing to design, supported at the top and bottom by a steel angle. I am worried about the extensive cantilever at the bottom, usually I design as a cantilever from the front of the landing, but the backspan is usually bigger. I can have a support angle at the back of the landing (right side) and use that too, and sort of design a diagonal beam. What do you guys think?

I can update later with loads if need be.

 

[CELinOttawa]

Please post an elevation, for clarity... However, generally the issue with long span staircases and cantilevers is that they by necessity include changes in the "beam" shape of the staircase, necessitating shear reinforcement.

There is an old thread here which discusses the issue. May not be applicable to your case, but I have a feeling it may be what you need.

http://www.eng-tips.com/viewthread.cfm?qid=231262

 

[ztengguy]

Thanks, I will look thru it. Elevation attached.

 

[Once20036]

I`m not quite following your description. Are you designing the landing at the bottom L-6? The stairs in between ST-2? The top landing L-7? Or all of the above?
You say the landing is supported top and bottom by an angle, but it looks like the landings are supported by an angle at the bottom only and the stairs are supported by the landings.

At a glance - I'd worry about rotation of the low landing due to the eccentricity of the stair load. Is it possible to tie the landing into the wall or add an angle along the top left side of the low landing to prevent this rotation?

 

[CELinOttawa]

Yup, that's a nasty one... I'm not comfortable with the angles supporting the cantilever either, not when the cantilever is then required to support the staircase segment above! Also, a segment of angle supporting loads that arrive parallel to the angle means that the fastener in the angles farthest to the right is going to take the bulk of the loading. Not a good detail at all.

I would suggest that you intoduce a beam underneath the landing as far right as possible, perpendicular to the current angle seen in the elevation, and then use the cantilever action in the landing slab only to permit the offset of where the staircase lands.

Alternatively you can cast the landing and the staircase as composite (even with a cold joint) and you need to refer to the thread I posted above in doing so.

 

[ztengguy]

@ Strucutral20036, yes I am designing the stairs and landing. When I said they are supported top and bottom, I ment that there is a bearing angle at the top and bottom of the landing in my first sketch, but on the underside of the landing.
I can add an angle along the long dimension of the landing (perp to the stair) at the back below, that would help capture the landing and be supported 3 sides, and if need be, provide some anchors up into the landing for resisting overturning.

I am going to ask if the stairs can be shifted, i.e. the stair coming up be moved left, and then the stair continuing up moved right. Currently, as you come up the stair, there is a 2'-11" landing left and then it will be even less with the wall studded out (2'-5"). Im not sure what code requires there. See attached.

@ CELinOttawa, I have designed these before, and the angles support fasteners are always a problem. Thats designed by others right now (usually comes back to me)((Im working for the precaster, designing the stairs, landings, usually support by others)) and is always a problem. I have warned my customer already about that.

Usually I do design as a cantilever to pick up the stairs, and have an intergral beam in the precast. this one is just too extreme as shown for that. With a support angle along the back side, it sort of creates a beam diagonal across the front, but nothing I am comfortable with right now.

I will see what my customer says, and where it will end up going. Thanks.