Design of stair tread with single stringer AUS - Tread is timber with steel plate under

[James_Gleeson]

Hey guys what are the key considerations for designing a stair tread which is supported by a single hollow section central stringer.
-Do you design the tread as a cantilever that is fixed to the stringer?
-What loads, load combinations and deflection limits are to be considered?
-Is 2.7kn point load at the end of the cantilevered tread and 2kpa UDL in accordance with AS1170.1 Table 3.1 appropriate?
-Should we consider the balustrade horizontal loading (say 0.6kn top edge loading - as1170.1 T3.3) which results in a point moment being developed at the end of the cantilevered tread?
-Is span/300 an acceptable deflection limit or is it more appropriate to consider an absolute deflection (max 5mm?)
-I have ran a check on the balustrade loading affects on the tread (steel plate component only) which results in a 0.6knm moment being generated at the edge of the stair tread (assuming cantilever length is 0.6m for a 1.2m wide tread and steel plate is 195mmx10mm) and the deflection is 24mm for permanent and short term load combo (600/24 = deflection limit of 25). Indicating that I need to reduce the cantilever span and thicken up the steel plate, does this sound normal because all photos of these stair configurations I see online do not appear to require plates much thicker than 10mm and they all look to cantilever approx 500mm?
-Is it reasonable to consider the combined I and Z of the timber and steel component of the tread for deflection and strength checks or alternatively is it better to utilise a thick bit of timber to resolve the loads rather than the steel plate?

Thanks guys, anything you can provide would be appreciated.

 

[human909]

A quick reply because that is all I have the time for....

For something like this I'd be quite conservative in you serviceability criteria. Over and above the code minimums. All well and good meeting code but if it is bouncy as somebody walks down one side or leans against the handrail then they won't be happy. And you might get complaints from your client.

I'd be designing that to be damn stiff.


On the topic of steel and timber. I'd be considering the combined stiffness otherwise you'd end up with something insanely thick.

Though done discount 12 or 16mmm plate if that is what you need. Be damned what others do, make yours fit for purpose. The cost of the thicker plate isn't huge.

 

[bugbus]

I almost never deal with stairs but if there's one thing Eng-Tips has taught me it is that the balustrade is pretty useful in sharing the load between treads and can control deflections effectively. I will always keep that in mind if I ever need to design some residential stairs.