Handrail 200 lb. concentrated load, is there a built-in safety factor? 2

[onlineeng]

When designing handrails with the 200 lb. c.l., does the allowable stress need to be divided by 4? Or, can the moment caused by the 200 lb. load be compared to the yield allowable of the material?(Florida, to conform to FBC.) Thanks in advance.

 

[Lion06]

It's just like any other service load. Design with it straight up with ASD methodology or factor it with LRFD.

 

[onlineeng]

Thanks for the response. Can you give me an example using the building code load of 200 lbs. concentrated load? (With 6063-T52 aluminum.)

 

[Lion06]

Well, there are two components to the design the handrail and the post.

For the handrail, just see if the 200# controls over the 50 plf (it usually does).

For the post, if the posts are spaced less than 4' use the 200#, if they are spaced greater than 4' then use the 50 plf. If you need the help, you can reduce the load on a post through some loadsharing factors that you can find in the Julius Blum handrail catalog, but I don't typically use that unless I absolutely need to (not that it matters, I usually only checks other engineer's designs for this).

As far as the design goes, just track the load through.

I use PL/4 (or (wl^2)/8)for the moment in the rail (I know it's not pinned-pinned). Use that moment with the section properties of your section to compare to the allowables of your material.

Same goes for the post. 200# at the top of a cantileverd post (Pl). Moment at the base with section properties to compare actual stress to allowable of material.

 

[dhengr]

YES there is a built in safety factor if you only weigh 50lbs. But, NO there is probably not if you weigh 400lbs. and push/lean real hard, or if you are three down linemen all showing off for a cheerleader at the same time. Decks and handrails usually don’t fail from normal use or 40lbs./sq.ft. LL’s., They fail frequently from poor design, detailing and construction. And, ledgers, guardrails and posts poorly designed, detailed and constructed, w.r.t. reasonably expected loadings caused by crazy partying (stupid) people are often the cause. It says apply a 200lb. load, anyplace, any direction, and it should not fail.

 

[wannabeSE]

NAAMM has a manual that might be useful

http://www.naamm.org/landing_pages/AMP_521-01.pdf

 

[Ron]

Lion06 is correct.

When analyzing the components of a railing system (keep in mind that if it is attached to the stair section, it is a "handrail". If it is not part of the stair section, it is a "guardrail".), isolating the components is a conservative approach. For instance, to determine the moment at the bottom of the post, if you apply 200 lbf to the top of the post alone, you are not gettig the benefit of any load sharing. In most cases, a typical 6063-T52 square post will fail under ASD. Load sharing typically begins with the continuity of the top rail. It is a pinned, continuous beam in most cases.

To develop the weld at the baseplate for the post, you consider that the post-to-plate connection is fixed, yet the plate-to-concrete attachment is not really fixed, but the resulting couple from the base plate is the load on the anchors.

Further, many codes still allow a 1.33 overstress for stairs and rails. Check your code.

 

[spats]

As wannabeEIT says, use the NAAMM pulication. There is load sharing betwwen posts for concentrated loads, and you can design for 65% for a two span rail, and 60% for a three span or greater rail.

I would use 6061-T6 aluminum. It's yield strength is 35 ksi, with an allowable stress of 24 ksi, as opposed to 16 ksi/11.5 ksi for 6063-T52. Also note Table 1 in the referenced publication, allowable stress for aluminum is greatly reduced within 1" of a weld. Unfortunately, that means a big reduction at the base of the guardrail posts where the stress is the highest.

The FBC allows a 33% increase when using allowable stress design.

 

[Ron]

The NAAMM document is a good one; however, it is not a code-referenced document (either in the International Building Code (IBC) or the Florida Building Code (FBC)), so the provisions of the code, particularly the reference to the Aluminum Design Manual (ADM) will apply.

Heed spats' warning about the reduction in allowable stress near welds. That's what usually kills an aluminum handrail calc.

 

[JStephen]

"It says apply a 200lb. load, anyplace, any direction, and it should not fail."

If I remember correctly, OSHA has similar wording for industrial handrails, and also specifies minimum member sizes. But the way that is worded, I would take it to already include factors of safety associated with the loading. If it fails at 201 lbs, that's okay, just as long as it holds 200 lbs. "Should not fail" is not the same requirement as "Should not result in an overstress condition under Code XYZ".

 

[BAretired]

The NBC makes it quite clear that the load is a service load.

BA

 

[gwynn]

JStephen

The problem with OSHA, unless it has changed recently, is that the member sizes it specifies actually fail even if the 200lbs is taken as an ultimate loading.

Last I checked, OSHA called for 2x4 railings and posts at 8' on centre for guardrails used on construction sites. These fail when taking the 200lb horizontal loading as an ultimate load. As another side note, OSHA and WCB guidelines are usually meaningless as they provide no details on the anchorage of posts, which is where the failures are most likely to occur from my experience.

This is not meant to be a jab at you, so I hope it is not taken that way. I was hoping to point out the failings of some safety codes. And I will refrain from commenting on the continued use of ASD as well.

 

[Lutfi]

FBC permits increasing the allowable stress by one-third if allowable stress design is utilized. I am not sure are aware of FBC provision 1607.7.1.3

“1607.7.1.3 Stress increase. Where handrails and guards are designed in accordance with the provisions for allowable stress design (working stress design) exclusively for the loads specified in Section 1607.7.1, the allowable stress for the members and their attachments is permitted to be increased by one-third.?

Good Luck.


Regards,
Lutfi

 

[slickdeals]

@Lutfi,
I don't think that 1/3rd increase applies in the HVHZ zone though (aka South Florida)

 

[Ron]

slickdeals...yeah it does...not wind related.

 

[slickdeals]

Ron, I may be mistaken, but I remember seeing a reference for no allowable stress increases in FBC. Not sure if it was in 2004 or 2007.

 

[slickdeals]

1618.9 Load combination.

The safety of structures shall be checked using the provisions of 2.3 and 2.4 of ASCE 7 with commentary.

Exception: Increases in allowable stress shall be permitted in accordance with ACI 530/ASCE 5/TMS 402 provided the load reduction factor of 0.75 of combinations 4 and 6 of ASCE 7 Section 2.4.1 shall not be applied

Doesn't this mean that 1/3rd stress increase is not permitted unless it is for the exception?

 

[Ron]

slickdeals....

1607.7.1.3 Stress increase. Where handrails and guards
are designed in accordance with the provisions for allowable
stress design (working stress design) exclusively for
the loads specified in Section 1607.7.1, the allowable
stress for the members and their attachments is permitted
to be increased by one-third.

 

[slickdeals]

Right..........but isn't that in the non-HVHZ portion of the code?

 

[onlineeng]

Lion06, dhengr, wannabeEIT, Ron, spats, JStephen, BAretired, gwynn, Lutfi, and slickdeals:

Thanks much... very helpful suggestions and discussion. btw, this particular case is not HVHZ.