Flat Plate Stringer Design - stairs

[engstructural]

I am designing a stair with flat stringers using BS5950 and the SCI advisory note for the flat plates.

In a stair design the stringers extend across the landing at the top and have checker plate welded to brackets bolted to it at the top. I have analysised this section using software assuming the flat plate beam is fully restrainted (compression flange tied - this is using the principles of the SCI advisory note) there is no restraint at the btm of the section. As ths is in tension am I right in assuming the desing assumptions used are OK?

 

[ishvaaag]

Normally the steps are in between and also coerce rotation of the stringer plate. If such is the case you can at least assume the step length as unbraced length for the stringer plate. I wouldn't assume fully restrained top in compression if it is not so restrained.

 

[engstructural]

It is really the landing area I am concerned with. This has a continuous connection at the top of the member but no connections at the btm.

 

[ishvaaag]

I wouldn't be very concerned as long as you design it as simply supported; if continuous restraint atop there length between braces, 0.

Another issue that may happen if the stringer plate is TOO thin is that may sway laterally unnervingly; but this you can control with a 3D model (stiffening steps included) not so difficult to make and using a notional lateral load at a percent of the load to be sustained vertically, quite likely 10% of it is more than enough since operating on the total (unlikely to be present). Then make all stout enough to ensure very small lateral displacement even when the people is going down in mass through the stair.

If you are worried by some residual compression at the bottom flange appearing even in a setup where the stringers are held mainly from above, place a plate-like division to brace also the bottom and use the resulting length between braces. You can forfeit this behaviour by creating a gap between the bottom of the end of the stringer at the landing and by using some fork support where the stair starts.