Aluminum Stair Rails

[spats]

I've come across a problem with what should be a standard handrail connection using aluminum. See attached sketch. The handrail is attached to the vertical guardrail posts with a bent rod bracket. It's common for steel stairs to use a 5/8" to 3/4" solid rod. Handrails, per the Code, have to be designed for 50 plf or 200 lb. concentrated load. The 200 lb. concentrated load is the problem. If the load is applied horizontally, it seems there will be a decent distribution of the load through the guardrail and handrail system, so that a single bracket will not experience the entire 200 lb. load. When the load is applied vertically, only the handrail can act to distribute the load, and it seems a single bracket will see most of the 200 lbs.

The rod bracket needs to be cold-bendable, so 6063-T5 aluminum is specified... Fu = 22 ksi, Fy = 16 ksi. A 3/4" diameter rod isn't even close to working, especially since the 2005 Specification for Aluminum Structures says Fu = 17 ksi and Fy = 8 ksi within 1" of a weld. I could possibly justify a 1" rod working, but it would essentially need to be full-pen welded to the post. The post itself could be the weak link in terms of tear-out of the face of the pipe. Also, a 1" rod is not practical for attaching 1 1/4" and 1 1/2" O.D. handrails... it's almost as big as the handrails and creates an issue of passing your hand over the bracket without having to let go of the rail.

I don't know how to make this work without inventing a whole new type of bracket. Do others have experience with this problem? Do you let the bracket connection yield, but not break? Maybe the Specification for Aluminum Structures does not actually apply to handrails (I don't think it does), but you can't ignore the reduced strength at an aluminum weld. Any insight would be greatly appreciated.

 

[JedClampett]

Code only requires 1 1/2 inch clear from the handrail to the guard. You have 2 1/4 inch. Even though it seems like a small adjustment, it should reduce your moments by 1/3. Will that help?

 

[spats]

Jed,

Thanks, but reducing the moment by one-third is not enough to help.

 

[JAE]

I think the problem here is the use of aluminum. Unless you can re-configure the rod geometry to reduce the bending in the rod I'm not sure what else you can do.

Also - are you planning on welding aluminum rods to steel posts? Rust issue?

 

[spats]

JAE,

No, the guards and handrails are all aluminum.

 

[racookpe1978]

I don't follow your your logic:

The same rod needs to withstand the vertical 200 as the horizontal 200 lb load. With the same distance and offset, why would one fail under vertical load (bend the rod) but not under horizontal loading? (By the way, I only know of horizontal loads in the Code - not a vertical load of 200 lbs.) If anything, since this is a stair, a 200 lb load down would be sliding down the rail .... 8<)

Also, how do you intend to mount the Al welded handrail system (rail, rods, posts) to the channel? Is the channel also Al?

 

[spats]

racookpe,

The 200 lb. load is to be applied in any direction per the Code. Per the sketch, the moment arm of a horizontal load is 3 3/4", where the moment arm for a vertical load is 3". There is distribution of the 200 lb. load based on stiffness/spring constants for the various guard and handrail elements. Your "sliding down the rail" comment doesn't make sense, when you think about it. As for the post attachment to the stringer, that was another research project. I'm using solid stainless steel rods welded to the steel stringers that sleeve into the posts.

 

[Ron]

Based on your design configuration, you can mitigate the 200 lbf load in the vertical direction by providing the support brackets at closer intervals.

Consider using a tube with the same diameter as the rail for the bracket instead of a rod. This will help with the weld issue.

Keep in mind that stairs and rails are life safety issues, so don't expect much help with marginalizing designs.

 

[SAIL3]

check out the site below , they have aluminum H/R, fittings, etc...

http://www.juliusblum.com

 

[spats]

Thanks guys. Excellent suggestion Ron on using the tube bracket. The numbers work. Hopefully we can get the brackets fabricated that way and looking right.

 

[NHstructural]

The aluminum design manual, 2005, defines weld affected zones. In section 7.2, #4 "Columns or beams of tubes or curved elements with transverse welds affecting their entire cross-section"

I have considered the exact situation you are describing and rationalized the curved bracket as a beam with welds but the welds do not affect the entire cross section if the bar is solid. Maybe I'm incorrect and others will discuss further?