Stiffening HSS Column 1

[UofIgrad17]

This is my first thread on here so I wanted to say hello to everyone and how much I appreciate the community.

I'm currently a structural engineering intern so I'm learning where real life and academia tend to clash. I'm working on a device that is going to be used to prevent falls from ladders. It is a 2x2x1/8" HSS tube with a plate welded outward from the top to give a point to attach to. The column has to withstand an axial force downward of 5k and, due to the attachment being away from the tube, it resists a moment as well. Preliminary testing showed flexural yielding in the member about 6" from it's support at 4.5k. After rechecking numbers and calcs, I realized that the member fails due to combined flexure and axial forces. The client does not want to go up a size due to weight issues, so I've thought to add rectangular bars to the sides of the HSS tube to stiffen it. My question is if I can use the nominal flexural strength calculated in Section F11 of AISC and simply add it to the nominal flexural strength of the tube itself. Similarly, use the nominal compressive strength calculated in Section E3 and add that to the nominal compressive strength of the tube. Some additional questions would be if the bar would need to span the entire length of the tube or if I could manage only having it for the section (about 18" long) of the tube that yielded during testing. I planned on stitch welding it as well. Any help would be greatly appreciated, thank you!

 

[BridgeSmith]

"HotRod10, as far as I know, channels typically are only made of A36 steel correct?"

Not sure for the small shapes like that; the bigger rolled shapes are mostly Grade 50 now. You'd have to check with a supplier for your area, I suppose.

 

[dcornelius]

Why don't you talk to some of your other engineers? I can't see why anyone would want this? In the US you need to attach to a non-moving part of the structure. You will also have to get the blessing of OSHA. You are attaching to the one most unstable part, the ladder.

 

[UofIgrad17]

CANPRO, I'm determining the maximum angle from the drop test (Edit: middle of the ladder rung, maximum of half a foot on either side). Real-life application will be different, I'll admit. However, in real-life application, there will be a device between the person and the post that is essentially used to dampen the fall, whereas this device is not present in the testing. It's all a bit strange to me, but the only requirements I've been told that need met are the static and drop tests with no real other criteria ranges or anything. I completely agree with you on some of the logistical issues, but it seems to me that a similar device is in use in other countries with daily applications so I am more confident in its use, but final responsibility lies on the client. I am solely checking design strength.

 

[UofIgrad17]

dcornelius, to clarify, the ladders are primarily installed in plants where they will be fixed to walls or large tanks essentially making them rigid. The client is dealing with all of the OSHA guidelines. I am strictly checking the design strength. Unfortunately, I'm the only one here with a structural background so it's me bouncing ideas off of myself I really appreciate all of the advice and guidance everyone has given! I feel pretty confident that the new 3 x 1 1/2 x 1/8 tube will be able to handle the drop test given that the 2 x 2 x 1/8 tube passed it. This section has the capacity for 5000 pound static load test, as well.

 

[GC_Hopi]

Sounds like you are designing a fall arrest system for a fixed ladder like this Link

 

[UofIgrad17]

GC_Hopi, yes, the design is quite different as mine is above the top ladder rung, but same basic idea.

 

[dauwerda]

UofIgrad17, my experience is that HSS being supplied now is typically dual certified as both grade B and grade C (this may depend on your local supplier though). The 15th edition of the steel manual shows that grade C is now the norm (table 2-4).

 

[StrucDesignPE]

GC_Hopi, yes, the design is quite different as mine is above the top ladder rung, but same basic idea.

So your device is cantilevering above the top ladder rung? Maybe i'm confused, but does it have a back span that keeps going down the ladder rungs?

 

[EngineeringEric]

If you are the only structural at your office... why is your company doing this type of job, and for life safety? Serious question.

As this is for life-safety, i don't want to tell you how to make a single element work as there are bigger issues than the bending capacity of the post.