Hand rail and post design for metal stairs. 1

[amain002]

I am working on designing a post for a steel stairs and was wondering if I could use plastic analysis instead of elastic analysis. If so I have been trying to find a reference in the AISC or any other manual for provisions to do so. If you guys could help me out, it will be highly appreciated.

Thank you

 

[XR250]

Why?

 

[Ron]

I would not use plastic design for stair rails. One criterion for design is that there be "pedestrian comfort"....meaning the users have to feel a level of comfort with the load resistance of a rail. Plastic design will allow too much deflection, thus negating the level of comfort expected by the users.

AISC does not consider stairs to be "structural steel" and therefore does not address the design. Applicable building codes contemplate normal design procedures using ASD or LRFD, but not plastic design.

Check the NAAMM Stair Manual for guidance.

 

[amain002]

TO ExcelEngineering: I was looking for this method because I was going over some old calculations that I have collected, It uses plastic analysis to design it. So I was thinking about using it for my future calculations, but needed a reference to use it so. Anyways Ron, recommended not to use it and also couldn't find any reference so I guess I will be sticking with my ASD method.
Thank you.

 

[nutte]

You might want to elaborate on what you mean by plastic analysis. I suspect there is a miscommunication here.

 

[dik]

For some sections, I sometimes use the plastic section modulus to provide a slightly greater moment resistance (Zx vs. Sx) for some components (without compromising servicability issues as Ron noted). I wouldn't use plastic design as a general appoach because it relies on redistribution of moments for added strength.

Dik

 

[amain002]

To Nutte: Sorry I meant plastic design ( using plastic section modulus in lieu of elastic plastic section modulus).

To Dik: Same here I use elastic section modulus for designs but in one of my old collected sample calculation I found out that the other person designed his post using the plastic section modulus which gives him more room for allowed strength. So I was wondering if we could do that and if so are there any provision written anywhere if someone questions why did we used it.

 

[nutte]

"Plastic design" is not the same as "using plastic section modulus in lieu of elastic section modulus." Go ahead and use the plastic section modulus. That's what the current AISC specification uses.

 

[amain002]

I was looking in the AISC manual in section 16.1 F-8 where they mention that

If d/t ratio is less than 0.45E/fy then

Mn = Mp =FyZx. (for round HSS)

Using this equation (Zx) gives me more room for strength for the post. Please clarify if this is applicable .

 

[frv]

nutte is absolutely right. In fact, AISC has been indirectly using what you refer to as "plastic design" for quite some time. It was just buried as an increase in the allowable stress.

Actual "Plastic Design", as nutte, pointed out, is something entirely different. It's a way to take advantage of reserve capacity in compact, laterally braced members after the first hinges firm. I'm sure there you can find information on it if you do a google search.

 

[SteelPE]

I use Zx when I design posts, however don't forget the appropriate factors when running your analysis (omega or phi).

I have always found that the attachment of the post to the stinger is impossible to verify unless you use a stinger that has a large enough flange to weld the post to. We have looked at using other methods for attachment, none of which we have been successful with. Stair fabricators are just not used to the shapes required to make sure everything works properly.

 

[connectegr]

I also use Zx for the post design.

Previously IBC allowed an increase in allowable bending stress. With the current plastic design allowables in AISC, the stress increase is no longer allowed in IBC. But, the results are the same.

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[connectegr]

Ron
The allowable stress increase is recognized in the NAAMM standard. I would not use both the stress increase and Zx.

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[TDIengineer]

1 1/2" dia std pipe at 4'-0" o.c.

 

[slickdeals]

There is some good information in this article.

http://www.modernsteel.com/uploads/issues/february_2009/022009_feb09_handrails_web.pdf

I have reviewed a number of shop drawings that show an all-around fillet weld of a 1.5" diameter post on a channel with a 1.5" flange. You can never get the calcs to work out without an all around fillet, unless you do a CJP weld.

See WillisV's post regarding MC12x14.3 stringers in this thread thread507-237196

 

[dik]

amain... as long as the section is a Class 1 section, there is no issue, in these environs, with using Zx...

Dik