Handrail Posts 3

[Stenbrook]

I feel like I am missing something. I have run the calculations on a 3'-6" tall handrail post several times in this delusional hope that the calculations will come out differently but sure enough, they don't. The calcs point to needing a 1-1/2" STD handrail post (which has an O.D. of 1.9") to sustain the required load (50 lb/ft or a 200 lb concentrated load). But time and time again I check stair shop drawings where they call out a 1-1/4 STD pipe. Which, I have checked and absolutely does not work. I assume they do this so they can get away with a smaller stringer size (a MC12x10.6 as opposed to the C12x20.7 which has a larger flange width to accommodate the larger pipe), But I really don't know. Does anyone have any insight into this as to any other reasons why stair manufacturers use these smaller handrail posts ALL the time??

 

[racookpe1978]

What are your assumptions about how the smaller diameter 1-1/4 pipe vertical is mounted?
What are your assumptions about wall thickness and strength points?
Where is the failure point, and how is it failing? (Buckling at the lowest place where it hits a ring or mounting hardware around the vertical to the floor?

The 1-1/2 rail diameter "feels" much better to most people's hands - 1-1/2 is too big to grasp easily. But verticals? Have not heard before about 1-1/4 failing a theoretical 200 lb sideways force at 42 inch off the floor.

 

[KootK]

I believe that some of the relevant design guides give you factors for including system behavior as opposed to single, isolated post behavior. And that, I think, is what makes the 1.25" diameter fly.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Stenbrook]

1-1/4" pipe - OD=1.66 in, ID=1.38in
1-1/2" pipe - OD=1.9 in, ID=1.61 in

All of these values are listed in Table 1-14 in the AISC Steel Manual

it is a quite simple calculation:

1-1/4" STD diameter handrail post with a 200 lb point load at the top and "Fixed" at the base.


M= 200*3.5*12=8400 lb-in
V=200 lb

Design using Section F8 in the AISC Steel Manual
Yielding - Mn=Mp=FyZ

Z=0.305 in^3
Fy=35 ksi
Safety factor = 1.67

Mn=35*.305/1.67=6392 lb-in < 8400 lb-in

The pipe clearly does not work due to the loads required by the IBC, yet everyone seems to think it does.


The 1-1/2" diameter pipe in comparison has a Z=0.421

Mn=35*.421/1.67=8823 lb-in > 8400 lb-in so the design is ok for yielding.

The rails are typically 1-1/4 and I have no issue with that.

 

[Stenbrook]

KootK, Do you happen to know which Design Guide that might be in?

 

[KootK]

Just dug it up for you: Link. You are now indebted to the tune of 0.25 KootK Billable Hours. Not to worry though, it's in Canadian currency so, like, a slurpee or something.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Stenbrook]

I appreciate it! I will look into that. What flavor slurpee do you want?

 

[Archie264]

I've wondered this myself. I've seen handrails built both ways, for what it's worth. I assumed the 1 1/4" systems were built prior to the 200#, 50#/' criteria being instituted but it sounds like they are still being built.

This place is great for being force-fed humble pie so thanks for asking the question.

 

[stevenspm]

One way the 1-1/4" pipes work is until the 2009 IBC, the IBC allowed guardrails to have a 1/3 stress increase provided you followed the loads described for guardrails and handrails. I also have design guardrails using the bigger pipe only to get questioned by the architect and steel shop who have always used the smaller size before because "that is what we have always used".

 

[KootK]

What flavor slurpee do you want?

Due to some unfortunate features of my genetic heritage, sugary beverages are no longer on the docket for me. However, if you could convince somebody to invent a Diet Coke Slurpee, I would be forever in your debt. We put a man on the moon damn it! Surely a sugar free Slurpee can't be beyond our reach. And I'll digi-slap anyone who tries to sell me on those Crystal Light Slurpee abominations. Not the same.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TSCDman]

I am under the impression that the 200# OSHA load is to be compared to the yield strength of the material, not the allowable (see quote below). In Ontario, we compare 150# to the allowable. What code are you getting your design loads?

1926.502(b)(3)
Guardrail systems shall be capable of withstanding, without failure, a force of at least 200 pounds (890 N) applied within 2 inches (5.1 cm) of the top edge, in any outward or downward direction, at any point along the top edge.

 

[Stenbrook]

I am using the loads described in the IBC. And since my project is being designed using the 2012 IBC, the 1/3 stress increase is not allowed. I believe that the allowable capacity of the handrail should be compared to the full 200 lb load. I don't see any way to justify using anything less. Maybe there are specifications in the code in Canada that are different than here in Texas.

 

[Ron]

Stenbrook....AISC does not recognize stairs and rails as "structural steel" in the sense of building design. I asked of them several questions about serviceability because I was concerned about lateral deflection of rails near the end of a stair section that does not switch back to another floor. Here is the text of the email response I got......

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.

I already had checked the NAAMM document and go no help from that!

It is likely that the design was done by the prescriptive method in the NAAMM document. Another explanation is that some engineers use the height of the grip rail (34"), not the top rail height (42") as this is actually the line of action of the force.

I analyze stairs and rails for a local fabricator and we often run into issues such as this. We use heavier wall (3/16"-1/4"), 1-1/2" square tubing for the posts. The material has a yield of 45 ksi.

Check out the NAAMM document.

 

[andriver]

Interesting stuff,

on our offshore oil platforms, we use 1-1/2" STD

 

[Deadblow]

How do I archive this thread? Thank you!

EIT

 

[BSVBD]

Look near the scroll bar on the right.

I already archived this, as you can see, but, you will find it in the same area.

 

[Deadblow]

Done. Thank you. I was asked several months ago by a senior engineer to prove to him how the 1-1/4" posts work on paper.

EIT

 

[wannabeSE]

While we are on topic, has anyone seen or specified an MC12x14.3 with wider flanges? They were mentioned in a modern steel article

http://msc.aisc.org/globalassets/modern-steel/steelwise/2015/steelwise.pdf.

A Channel Especially for Stair Stringers: The MC12×14.3 that recently was added to ASTM A6 was conceived as a stair stringer. It has a 21∕8in. flange width, which is wide enough to accept the common handrail pipe size and fillet weld around it. No more crimping the pipe or goobering the weld!

 

[Hokie93]

An Engineering Journal paper (second quarter, 2002) by Thomas Sputo addresses guardrail post design and compares the results from five different methods: NAAMM, AISC ASD, AISC LRFD, AISI ASD, and AISI LRFD. The NAAMM method distributes the 200 lb concentrated load between adjacent posts. The paper is available on the AISC website (

http://www.aisc.org)

and is free of charge to members.

I also suspect that some engineers may still use the 1/3 stress increase that is no longer permitted by IBC.

 

[hokie66]

The size of handrail posts has been a point of contention for years. But I have always thought the weak link was the connection to the channel, and the distortion of the channel itself. I don't know if this connection has been subject to testing.