Steel Guardrail Design

[spats]

I am designing a steel guardrail under the Florida Building Code, although this same question likley applies to the International Building Code as well.

The 2007 FBC and 2006 IBC both reference AISC 360-05 for steel design. They also both allow a one-third stress increase for guardrail design per Section 1607.7.3. For design of the pipe uprights I have been using AISC 360-05 Section F8, where the nominal flexural strength is FyZ, and consider Omega(bending) = 1.67 x 0.75 = 1.253 to account for the one-third increase.

In checking the 2010 FBC, which goes into effect on March 15, 2012, I notice the one-third stress increase is gone. I assume this is the same for the 2009 IBC, although I don't have a copy yet. Does anybody know what the deal is? Was it an oversight that the codes still allowed a one-third increase in stresses for guardrails in conjunction with AISC 360-05 since design is based on plastic rather than elastic section modulus?

 

[nutte]

This article has a brief discussion on the one third increase. As you note, it has been removed from the Florida Building Code, as well as the IBC.

http://www.modernsteel.com/Uploads/Issues/February_2009/022009_Feb09_Handrails_web.pdf

 

[woodman88]

Yes it is the same for the 2009 IBC. The attached thread has more infomation about it. Check out the link to modernsteel.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Ron]

spats...one thing to recognize is that AISC does not consider stairs and rails to be "structural steel". I recently had reason to question serviceability requirements under AISC 360-05 being applicable to stairs and rails and sought an opinion from AISC. Here is the response I received:

[highlight]"Stairs are not generally considered structural steel per Section 2.2 of the Code of Standard Practice; therefore, AISC does not have very much information on stairs and their applicable material specifications. The National Association of Architectural Metals Manufacturers web site (

http://www.naamm.org)

may be one place to start. You may be interested in 2 documents in NAAMM’s AMP technical literature section(

http://naamm.org/amp/amp_technical_literature.aspx).

Although they do not specifically address your question, both the pipe railing manual and the metal stairs manual discuss structural considerations for the design of stairs and handrails."[/highlight]

As for the 1/3 stress increase, I rarely use it anyway. But yes...it is gone now.

 

[spats]

Thanks guys. Ron, I do have those NAAMM documents, but I considered that they were out-of-date... 10 years old for the Pipe Railing Systems Manual. Allowable stress in that document was 0.72Fy, with elastic section properties based on nominal rather than design wall thickness.

It's interesting that you're saying that AISC really doesn't apply to stairs. I guess that means it doesn't apply to guardrails as well? Should I be designing per NAAMM with properties based on nominal thickness? NAAMM is not a code, nor is it included by reference in either code.

 

[connectegr]

I agree that the 1/3 increase is no longer recommended. But you can use Zx instead of Sx, which is nearly the same result.

http://www.FerrellEngineering.com

 

[Ron]

spats...I realize that NAAMM is not code referenced; however, it does play to the "standard of care" issue we all have to meet. Just because AISC 360 is code referenced, doesn't mean we have to meet it for all considerations...particularly since AISC seems to conclude that stairs and rails (including guardrails) do not qualify as "structural steel". They are doing the same thing we are...limiting liability to only those structural steel items that support the building.

When our supporting organizations do not support our design intent, we are left only with engineering judgment and standards of care.

As outdated as they appear to be, the NAAMM documents are "industry standards" and as such would comply with our "standard of care" compliance. The only other consideration is whether YOU consider the NAAMM requirements to be sufficient to accommodate your conditions. If not, then increase them...no problem there.

 

[spats]

Thanks again, Ron. You make some valid points.

Essentially, I'm just trying to come up with the best "standard of care" scenario, since the process doesn't seem to be well defined when working with the latest codes. I was hoping that AISC 360-05 would help to justify lighter guardrail uprights.

I personally feel that the perscribed code loads are pretty high for service loads. Effectively, the one-third increase at least provided some recognition that this load is transient in nature. Personally, I think it should be considered an impact load.

I'm trying to get the most out of it, since guardrail uprights won't work in a lot of instances if you don't go with a 1 1/2" x-strong pipe. I've seen plenty of guardrails in the past that were "designed" by an architect or steel detailer that used 1 1/4" standard pipe. I fight the "that's what we've always done" perception constantly.

I'm currently teaming with a steel detailer and a fabricator in bidding on steel stair work, and I design quite a few of them, so the topic is important to what I do. Don't misunderstand, I'm not looking to cut corners, but we do need to be competitive against other bidders who may have found a better way to skin the cat.

 

[connectegr]

I am frequently faced with your concerns. Our firm designs stairs and handrail for several fabricators. And I am constantly getting the "we have done it before" argument. I also frequently see handrail and other exposed miscellaneous steel, that I would never approve. Even in the stairwells of my own office or our local shopping mall, there are welds that cannot calculate or details that I have rejected in the past. We refer to NAAMM, AISC 360, and use structural analysis software to optimize our design. I have also worked with fabricators to develop weld details, that although are not the easiest option, they satisfy the design and AWS requirements. With regard to the posts reinforcement or adjustment of the spacing are also discussed with the fabricators. We also find that analyzing continuous rails as a system can also take advantage of increased interaction of the components.

http://www.FerrellEngineering.com

 

[Ron]

connectegr... Your statement "We also find that analyzing continuous rails as a system can also take advantage of increased interaction of the components." is almost a necessity in most railing analysis. Single post analysis is conservative, but requires much thicker sections, more embedment if posts are embedded and larger baseplates/fasteners if that's the attachment method, not to mention more weld.

I also use FEA software in an attempt to optimize the design. One issue that I have no good answer for is the amount of lateral displacement of free ends of stair rails. I know of no limiting criteria, but under code loading, the free end (usually only occurs at bottom floor)lateral displacement can be upwards of 2 inches (though in actuality, the fabricated rail system seems to displace very little under reasonable side loading).

Any thoughts?

 

[connectegr]

Ron
FEA software is what I was referring to. For a simple straight rail, we have written software using the charts in NAAMM for interaction. Similarly we have written programs for most of the NAAMM examples. In some cases these can be overly conservative. As with most things we have to balance the engineering time with the actual material and fabrication savings. But we are using the FEA software more and more, and we are getting more efficient. We have found that the NAAMM examples ignore several factors and resulting forces, such as thrust in the stringer analysis. The FEA software can be more thorough and still produce better results.

http://www.FerrellEngineering.com

 

[spats]

How does FEA software help in the design of posts? Usually the 50 plf requirement controls, so load distribution does not help.

 

[Ron]

spats...load distribution helps a great deal. Run an FEA analysis on a line of rail with, say, 4 posts. Compare the post stresses to a single post analysis using a 200 lb lateral load.

You'll see the difference.

 

[spats]

Ron,

I understand the distribution of a concentrated load. It's the continuous 50 plf load that most ofter controls, and distribution of a line load gains you almost nothing. An end post will help the second post in, so if you have a three-span guardrail or less, distributing the line load definitely helps. Once you get past three spans, distribution does little for the most inward posts. I've never specified any intermediate post being heavier than the others, it's not practical.

 

[Ron]

spats...50 plf only controls if the post spacing exceeds 4 feet. In that case, it is hard to get the posts to pass. For the company that I do analysis, I have them keep all post spacing at or below 4 feet; whether line rail or stair rail.