Eternal Mystery of the Universe #1 - 1-1/4" Guardrail 1

[Guastavino]

This issue comes up all the time for me and how it came about is one of the eternal mysteries of the universe in my mind. I always tell folks they can't use 1-1/4" STD pipe guardrail because of the 200# concentrated force at 42" makes it such that it just doesn't work in bending. Then, I get the "well it's the way we've always done it". Code officials never seem to catch it, but it doesn't work unless you start to count on a FE model that spreads the load between posts, etc. And then it's still questionable at best IMO.

Then, I see arch's spec 1.50" O.D. handrail/guardrail, which of course doesn't exist in the Pipe shape realm. So I'm curious, A) how people justify using 1-1/4" (1.66 O.D.) std pipe guardrail, and B) what do you all tell clients if you're on the same side as me, that it has to be 1-1/2" STD Pipe (1.90" o.d.)

Thanks!

 

[sbisteel]

NAAMM has published a pipe railing manual (AMP 521) that gives reduction factors based on how many posts are sharing the load.

After that you either need to take advantage of an allowable stress increase (IBC 1607.7.1.3) with a working stress design, or use the newer ASD method with plastic section modulus.

It's still cutting everything down to the razors edge. I'll push for 1.5" Dia. std pipe whenever I can get it. The worst is when architects absolutely require a 1.5" stair stringer, which is smaller than the handrail. The weld detail is awful.

 

[DaveAtkins]

IBC permits the allowable stress to be multiplied by 1.33 (one of the few times the 1.33 increase is still allowed). However, most of us don't use allowable stress design anymore (and I am not sure the 1.33 increase can be applied to Mn/Ω).

In my opinion, the only way to justify it is to spread the load to more than one vertical post.

DaveAtkins

 

[JStephen]

One way that comes up is that in the OSHA rules, they specified a minimum pipe (and angle) size and maximum spacing. Then the loading was a separate requirement, so the presumption is that using the minimum size member and maximum spacing meets OSHA. And, then OSHA has an official interpretation that indicates that 1-1/2" pipe refers to pipe OD, not nominal size, so 1-1/4" meets the size requirement.

In the older OSHA specs, loads were given without any indication of acceptance criteria, "shall support XXX lbs", not "shall support xxx lbs without exceeding some arbitrary allowable stress". That is, specified loads could be interpreted as failure loads, not service loads. So if they specified "shall support 200 lbs" and your handrail supports 201 lbs, you're good.

And lastly, handrail specification indicated "or members of equivalent bending strength", so using closer post spacing allows you to use smaller rails, etc., if desired.

Anyway, it's all a mess, and you can work the numbers a lot of different ways. I'm not aware that handrail failure has been a big problem, though, rather lack of handrails or falling through handrails, etc.

 

[nutte]

First, the posts probably never see the design load. Second, the pipe almost certainly has a yield stress much higher than the design yield stress of 35 ksi. When I have seen mill certification reports for pipe material used for handrails, they have been triple-certified (A53 Gr B, A500 Gr B, and A500 Gr C) and had an actual yield stress of close to 70 ksi.

This doesn't help the designer, unless you call out a higher material grade, which will probably cost the fabricator more money in material costs. Ask for A53, you get A500 for nothing extra. Ask for A500 and there's a premium.

 

[BUGGAR]

For what it's worth, the handrail failures I've seen have been ductile and the posts bent over but not enough to allow someone to go over the top. Must have been some crushed ribs, though. I never saw a weld failure at the base which surprises me.

 

[Guastavino]

Thanks all. The NAAMM document is very helpful. Note that the stress increase in the IBC was removed in the 2009 IBC.

 

[Triangled]

42: The answer to life, the universe and everything

 

[mtu1972]

Agree with JStephen. Used to be "against failure" back when we only did Allowable Stress Design.

Back in the mid-70's on the MN Iron Range. The Pipe-Fitters would claim the handrail work because "they are pipes". We had several projects where we changed to rod handrails and flat bar posts. Because it was so different, we had testing done on both the posts, the rods, and three post assemblies to prove that they could carry the required loads.

In theory, we may have spent more on material, but the field fabrication and installation labor was supposedly cheaper.

gjc

 

[cliff234]

Considering that multiple posts will share the load will help, however if the 200# load is applied at an end post (and if the handrail does not turn 90 degrees at that end post) then I don't see how the load will distribute to other posts. A related issue I run into is that when the handrail is welded to the top flange of an MC12x10.6 stair stringer (web thickness = .19") I can't figure out how the moment at the base of the handrail post can be resisted by the stringer. Any suggestions?

 

[canwesteng]

1-1/2" round bar. Problem solved, and you keep the iron mines open.

 

[sbisteel]

cliff234 - As a rule, we supply web stiffeners underneath any posts for MC12x10.6 stringers. A bevel weld is also required, but again the flange width is smaller than the post OD. It's a really bad detail that I always try to avoid.

 

[mtu1972]

We did not use 1 1/2" bars. I don't remember exactly what size we used, but our rational was if it satisfied the load criteria it was OK.

I have also worked at a place where we did the calc's and computer analyses and determined that schedule 40 pipe did not work so we went to schedule 80 for the posts. We limited the post spacing so that the handrails could be schedule 40. Not my idea. I bet the steel fab shops loved working on those handrails, and I bet more often than not the posts were also schedule 40.

gjc

 

[JStephen]

If I remember correctly, MSHA specifies solid bar guardrails rather than pipe. Not necessarily round bar, though.
Either OSHA or the building codes may specify a minimum size where also used as a handrail, regardless of strength.

 

[XR250]

1 1/2" Square tube FTW!

 

[TLHS]

I tried to prove our handrail standards for the general case. There are a bunch of typical connection details and guardrail arrangements that don't seem to technically meet strength requirements. They should go back to having a prescriptive standard design or two, and then have loading if you want to vary from it.

 

[TLHS]

It's also irritating because it's one of those things that punishes you for trying to be a good engineer. If you try to follow all the requirements, you come in more expensive than the people that don't. It's an extra kick in the teeth, though, because there's no practical difference in performance between the engineered to code version and the typical installation. They both adequately protect workers. So you're doing your due diligence and cost more, but don't feel like you're actually adding value.

 

[SteelPE]

Eh.... I just use 1-1/4" sch 80 and move on with my life. I design these all the time and most of the guys I work with don't mind. They are also now using mc12x14.6 for stringers.

There was a Modern Steel Construction article a few years ago that addressed this issue. If I remember right, they made an argument that music iron doesn't fall under the AISC spec. However, I still design them this way.

 

[Ron]

SteelPE.... I have an "official" email from AISC that says stairs are not "structural steel"

 

[Guastavino]

When AISC pays me it will be official. Until then I'm with you Ron, it's "official".

My issue is that if I don't check it and there is an issue, I'm at fault. So I figure I might as well do it. And before anyone says "if you exclude it in scope", I just say, a lawyer won't care. And I know the first question, "rail loads are in the structural chapter of the code and you say you are a structural engineer right?"

So I usually do the basics and move on and just say 1.5" std. If they use something else and don't tell me, at least I tried.