STAAD STAIR MODEL

[Orsted]

Hello,

In modeling a stair in STAAD, I'm wondering if I need to input the treads, especially when design to seismic/wind loads (using any beam size or plate), and have it's material property as shown below.

Please note that per my understanding the density value should be 0 so that it would only provide the bracing effect of the treads in the stringers. The loads transmitted by the treads to the stringers will be by manual input.

ISOTROPIC STEELD
E 4.176e+006
POISSON 0.3
[highlight #F57900]DENSITY 0[/highlight]
ALPHA 6e‐006
DAMP 0.03
TYPE STEEL
STRENGTH FY 36 FU 58 RY 1.5 RT 1.2

Is this acceptable? or Is there a another way to provide the bracing effect of the treads to the stringers in STAAD analysis?


Thank you for your time.

 

[TLHS]

I treat the stairs as providing stability bracing against lateral torsional buckling but I don't rely on them to brace lateral forces. So when I model stringers in STAAD I don't model the treads at all and then set the stringers as being fully laterally supported.

I'm comfortable treating the treads as stability bracing because if it didn't work there'd be stairs collapsing all over the place (and the NAAMM stair manual agrees).

To actually transmit non-nominal forces though you'd end up having to analyze it as a vierendeel truss and then verify the connection capacities. Given that the treads aren't even square to the stringers, I wouldn't take that check lightly. There's obviously some capacity there, but quantifying it would be a bit tricky.

 

[SteelPE]

How complicated are these stairs? Why do you need to run a computer analysis on them? It's incredibly rare for me to run a computer analysis for a stair design like the one in the attached photos.

If you are having issues with diaphragm capacity just tell the fabricator to stitch weld the tread/risers together.

I don't understand why we need to make these items so complicated.

 

[TLHS]

That's a good answer as well. I'm normally doing industrial stairs without a riser. In that case I'll run angle braces under the stringers if I really really need lateral capacity.

 

[SteelPE]

My industrial stairs are commonly done utilizing grating treads..... in that case there is no lateral restraint for the stringer and I design the stringer with an unbraced length from header to header or support to support (where I put C8 headers).

 

[Orsted]

So when I model stringers in STAAD I don't model the treads at all and then set the stringers as being fully laterally supported.

Yes, I do this most of the time but when I look at the nodal displacement at the x and z axis of my stringer some values are greater than my allowable values (mostly deflection) which makes my design fail but when I add the treads for stability bracing it now meets my allowable values.

Thanks for the input.

It's incredibly rare for me to run a computer analysis for a stair design like the one in the attached photos.

Yeah, I use STAAD mostly to design stairs like whats in the attached file but I don't design the treads there for stability bracing since I'm not sure if it would meet design requirements

 

[TehMightyEngineer]

in that case there is no lateral restraint for the stringer

Interesting; I've always considered the grating as providing lateral restraint. You're probably right I shouldn't consider it as lateral bracing but I want to hear your rational before I start worrying about double checking all my stair designs.

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

 

[Orsted]

TehMightyEngineer,

Per AISC Design Guide 34, the author also suggest to treat gratings treads as unbraced (see attached file). Although this is from his/her experience and it's difficult to determine per author.