Light Gage Stair Framing

[XR250]

I have been asked to design some light gage stair stringers. Never been asked to do this before and it seems strange. Anyhow, this is what I have come up with (see attached).
Here is my question - Should I design the horizontal track for a 250lb point load or do you think the tread and underlayment material will distribute the load enough so that it is not an issue. When I check 20ga track using "CFS" software, it fails miserably. Even 18ga is overstressed by 70%. They are likely using 1" Armoroc

http://ameriformllc.com/armoroc.asp

or 3/4" plywood for the tread underlayment and then a wood tread on top.
I have talked to the author of the software in the past and he claims that the calc. for weak axis bending of a track is very conservative.
I hate to go to 16ga material as it intuitively seems overkill for this situation.

Thanks

 

[thaidavid40]

Unless you are doing dozens of similar structures from this design, why would you not want to upsize the gauge of the material? The labor costs to put these together probably dwarf the materials costs. If the heavier-gauge material works, then why not use it? You never know what might be done with - or to - these treads in the future anyway. Some material reserve might come in handy at that time.
Dave

Thaidavid

 

[XR250]

@thaidavid40:

We are 20 of these in a condo loft building. Same ol' story- contractor did not have it in his budget so wants to save as much money as he can. (he thought they were going to be wood)
I guess the nice thing about 16ga is that they can use winged reamer screws to attach the underlayment that require 16ga material to work properly. I am not sure if the wings would break off prematurely in the Armoroc however.