Steel Guardrail Design
Steel Guardrail Design
(OP)
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?
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?






RE: Steel Guardrail Design
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RE: Steel Guardrail Design
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.
RE: Steel Guardrail Design
"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 (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."
As for the 1/3 stress increase, I rarely use it anyway. But yes...it is gone now.
RE: Steel Guardrail Design
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.
RE: Steel Guardrail Design
http://www.FerrellEngineering.com
RE: Steel Guardrail Design
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.
RE: Steel Guardrail Design
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.
RE: Steel Guardrail Design
http://www.FerrellEngineering.com
RE: Steel Guardrail Design
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?
RE: Steel Guardrail Design
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.
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RE: Steel Guardrail Design
RE: Steel Guardrail Design
You'll see the difference.
RE: Steel Guardrail Design
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.
RE: Steel Guardrail Design