Aluminum Guards in Water Treatment Plants - Aluminum Post Question

[Cambridge99]

He have been designing aluminum guards in water treatment plants using posts that are 1.9 Sch 80 Pipe at 4'-0". The aluminum is 6061-T6.
We reviewed the design loads and we noticed that these posts are overstressed. He have seen this design on other drawings and we are wondering how other engineers are making this calculation work.
We were also wondering what other engineer use for aluminum guard post. To make the guards work we could install a solid bar in the base of the post to make it work, but this is alot of fabrication.
The design loads for the guards are typical as per code.

 

[JAE]

A lot of vertical guard posts are "on other drawings" prepared by an architect or others who don't do the required calculations.

In the US these vertical posts must resist a 200 lb. load at 42 inches above the floor (in any direction) or a 50 plf load on the top horizontal guard rail.

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[TehMightyEngineer]

I suspect that the other engineers are in fact not making this work.

Maine Professional and Structural Engineer.

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[Ron]

If you are welding the pipe at the maximum moment (bottom) of a typical 42" high guardrail, then yes, it will be overstressed because of the weld reduction required. If you bolt the post to the support or set the post in a pocket, then it will not be overstressed. Your allowable stress is almost 25 ksi for the unwelded condition. It drops to about 17 ksi for the welded condition. Your computed stress is about 20.5 ksi., based on the info you gave and considering a 200 lbf point load applied at the top of the post. Also, for a welded rail, there is load sharing between posts, so instead of applying a 200 point load at the top of the post, consider the top rail to be a multi-span beam with a concentrated load. Then you can use the reaction at the post to compute the moment at the base....

 

[JStephen]

For handrails, the design loads vary depending on whether it falls under OSHA or IBC or some other code. If it falls under OSHA, OSHA specifies design loads, but then also specifies member sizes and post spacing, with the requirement of using these "or members of equivalent bending strength", and that will give different results than designing for the specified loads. The OSHA requirements are that the handrail must be able to withstand that load, not that it must do so at a specific stress level. Net result is that there is a lot of variation in how the requirements can be interpreted. Also, some of these requirements can change as they go along, so what was okay ten years ago might not be now. Design for what you feel is reasonable for the situation and go with it.

Note that in some cases, upsizing a pipe/tube may be more economical than using extra joints or extra posts.

 

[Gumpmaster]

The typical railing at water treatment plants is screwed together rather than welded. We specify some of the major railing vendors such as Thompson. The vendors typically have their own calculation packages and some of the pieces are tested rather than calculated.

This route seems to work well. I recommend avoiding welded railing.

If your height gets much above 42" (such as side mounted railing) then you may need a solid bar at the bottom or upsized posts even with un-welded railing.