Pipe spreader bar for cranes 1

[TylerS]

I'm currently an engineering intern at a crane company. We use schedule 40 steel pipes for our spreader bars. I have already rated 4 and 8 inch piping for spreader bars and how much load they could lift at various lengths and with various length slings. I am now faced with rating some homemade adjustable spreader bars that consist of one 2 and 3/4 inch bar inside of a 3 and 1/2 inch bar. I was curious if anyone knew of any software that could simulate this and allow me to see how much they could handle before they failed? I need to look at the pipe expanded from 8 to 12 feet and with slings at 10-50 feet.

 

[TGS4]

How exactly do you "rate" the existing design? What failure modes are designing against? What design margins are you using in these calculations?

Follow the same approach with your new designs.

 

[keyepitts]

I'm curious how you'll get a 2 3/4" bar INSIDE a 3 1/2" bar?
Whatever computer/pencil result you come up with, I would probably additionally want to see a proof loading as a final check.

 

[TylerS]

By rated I meant that I found the load that could be put on the spreader before it deformed in anyway either by bending or compression. I was just told to treat it at as a 3 inch pipe to stay on the safe side and just use the thickness of the inside pipe which is 3/16" I also use a safety factor of 3.

 

[TGS4]

OK. Since the bar will be in compression, what design margin are you using for buckling? How are you checking buckling? Your "safety factor" is for what loading and against what (yield, ultimate, etc)? How do you define "deformed in anyway"? 0.001in? 1e-6in?

 

[TylerS]

For the original 4 and 8 inch pipe I was using a program that showed me the bending moment and loading.

http://www.maximumreach.com/SpreaderCap.asp

. The safety factor is against yield strength which is 30000 psi. Deformation would be 0.001 inches.

 

[fegenbush]

I think this needs to go back to TGS4's original comment of how designs are rated. For designing such beam-columns, I would use the formulas available in the AISC Steel Construction Manual (14th Edition is current). It covers the design of such members that are subject to simultaneous axial loading and flexure in Chapter H.

In designing items which are critical to the safety of the public (including your workers), it is necessary to use established standards and design methods.

 

[TylerS]

Thank you fegenbush, I'm currently downloading the book now and hopefully it will have all the equations i need.

 

[racookpe1978]

What strengths are you assuming for the "pipes" - Pipes are rated for service against internal pressure (which tends to straighten the ovals and reduce out-of-straightness in length), NOT external pressure as a collapsing column under end buckling loads. Pipes usually with a side seam and poor "straightness" and poor strength characteristics should NOT be used as structural material, but rather "structural round shapes" which ARE made to better specs AND of better material should be purchased.

I strongly recommend you NEVER assume load or strength factors for a "below the hook" design using ANY "roadside-leftover-scrap-pipeline" material of ANY capacity!

 

[rumandcola]

You may also want to review ASME BTH-1: Design of Below-the-Hook Lifting Devices. This will provide you with an appropriate factor of safety and address the possible failure modes in your design.

 

[chicopee]

"I am now faced with rating some homemade adjustable spreader bars that consist of one 2 and 3/4 inch bar inside of a 3 and 1/2 inch bar" so that could possibly mean that holes are drilled in both pipes(internal and external) thru which pins are inserted to adjust lengths. There is a reduction in the moment of inertia so watch out as these holes can lead to early buckling failure.

 

[TylerS]

Chicopee, yes it does have holes and a pin. Thank you to everyone who is posting, they are great suggestions. I will continue working on this tomorrow and will ask any more questions I come up with. Thank you all again.