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- To: <seaint(--nospam--at)seaint.org>
- Subject: re: guardrails
- From: "Andrew Kester" <akester(--nospam--at)cfl.rr.com>
- Date: Tue, 7 Apr 2009 12:04:01 -0400
Thor (sorry this got a little long),
You guys design all guardrails for a good hockey check up there, eh? That is pretty robust:
"In Canada, for locations of assembly/egress, horiz load >= 206 plf and vert
load >= 102plf.
In "normal" circumstances the horiz load need only be 52plf or point load of
Per IBC, it is the worse case of 50plf or 200lb, applied at the top in any direction. The other components are to be designed for 50lb horizontally over an area not to exceed 1sf. I am guessing you guys have similar though slightly higher loading.
For the posts, this means under 4' oc, 200lb always controls, and for a post this will be in the lateral direction as this creates the highest bending stress. We have two steel/alum fabricators as clients and do an unusual amount of stair, platform, and handrail design. My personal opinion having gone through the ringer to make every architectural version of a guardrail and handrail you can imagine work, is that 200lb seems a bit high for most circumstances (just using some common sense), especially handrails. I cannot imagine how one would go about applying a 200lb force in normal situations to a handrail. When people use a handrail, they just use it to balance themselves, so outside of a skateboarder doing a grind on the top rail, it will not see much in terms of a lateral force.
However, if you are designing a guardrail for a balcony at a school, sports arena, bar, nightclub, theater, etc. in the event of a fire or other assembly, you could see a crowd perhaps pushing against a guardrail with this amount of force, probably more in the 50plf format. Also handrails and guardrails in schools take a lot of abuse, as kids will be kids....
With this being said, deflection is all about servicability and perception. I do not believe a normal handrail/guardrail will ever see the code maximimum load or anything close to it (especially the Canadian load), so we don't really consider deflection to be a critical item. I hardly think anyone exiting a building because of a fire will write a letter to the building dept the next day indicating they observed a 1/2" of deflection of that guardrail they were shoved up against.
The other thing I have observed on as-built handrails is the deflection is often controlled by what the posts are mounted to- a channel flange (stair stringer) is particularly flexible, and you may need to add a flange/web stiffener just for performance. However, if it is a guardrail around the perimeter of a floor, and it is mounted to a wide flange or a bent plate, as part of a concrete slab system, this is less likely an issue.
Be cautious as to what you are mounting to, often the EOR or architect has given this little thought and you as a specialty engineer may be asked to make something work that is not possible. This is a very common issue with us, and we have to get pretty creative....
Andrew Kester, P.E.
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