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Re: Steel Pan Stairs

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Scott thanks for the input. I've a few questions. Why did you reduce the moment by .85 and why is the yield stress only reduced by 75%?? Here's where I'm coming from I use ads so there would be a 60% or 66% reduction of stresses depending on the unbraced length and no modification to the service load. It seems that if your using strength design you should be increasing the service load and reducing the applied stress less (say 90%). Since your obviously not doing either of these perhaps you'll enlighten me.
 
Also the reason I said the reaction from the handrail is between 150 and 260 lbs is because if the handrail is simply supported then the reaction is 150# but if it is considered continuous then the reaction is about 260# at the interior supports. The pipe that the client purposes to use is standard weight which he tells me is what they use in all their stairs. So that's why I say the 1 1/4" will not  work. As far as moving the post's closer this obviously will help as Bill Allen on the list was nice enough to point out but in all honesty I've not seen many stairs that are any heavier than the one I'm dealing with now.
 
What I meant about the weak axis bending was the moment (torsional) caused by the handrails. In the case where there is no riser just a tread pan how do you resolve the torsional moment? As an aside the specification that my client is trying to conform with requires that the riser not be welded . Do you know why this might be? I certainly can't count on much torsional resistance If I can't weld the riser to the stringer.
 
Now the really technical part is when you include a compatibility analysis of all the connected parts of a stair system most of which are free from external restraints. The extra stresses induced by the deflecting supporting member i.e. the handrail in relation to the vertical post, the post (and handrail) in terms of the rotation of the stringer ect. The finite analysis of the whole assembly can yield very different results than considering individual pieces with simplified boundary conditions. Sometime the analysis of the whole structure does not show the simplified analysis to be conservative at all.
----- Original Message -----
Sent: Monday, March 03, 2003 10:33 PM
Subject: Re: Steel Pan Stairs

Actually, with the posts at 36", the 200# concentrated load will control, not the uniform loads.  The moment = 200*42" = 8.400 in-kips.  Using 1¼ø XS A53Pipe, S = 0.291, and accounting for the load redistribution to other posts through the rails;  fb = 0.85*8.400/0.291 = 24.53 ksi < 0.75*35 = 26.25 ksi  If you're really interested, I recommend the NAAMM stair design book AMP510-52, and its companion pipe railing systems book AMP521-95.  As for the weak axis bending in the stringer, the weakest axis is the vertical axis because the pans are all welded to the back of the channel resulting in very strong assembly with the stringers forming the "flanges" and the pans forming the "web" of a sloping, folded plate girder.
 
HTH,
Scott A. Dunham, PE
Dunham Engineering Services
Dothan, AL
334-678-6948
----- Original Message -----
Sent: Monday, March 03, 2003 5:20 PM
Subject: Steel Pan Stairs

Does anyone have any insight on steel pan stair design. I have trouble with an analytical design for such a structure. For instance an 1 1/4" diameter post supporting a handrail/guardrail with the prescribed load of 100plf vert. and 50plf lateral at 36" centers can have a cantilevered load of somewhere between 150lbs to 260lbs at a height of 42" (at the landing). This will cause a load well in excess of the strength of the pipe. Then there's the question of how you resolve the weak axis bending in the stringer. It seems to me that from most stair systems I've seen that there more empirically designed than analytically design.
 
Any input would greatly be appreciated.