Spreader Beam Design
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Re: Spreader Beam Design
Re: Spreader Beam Design -- mluck	Post Reply	Top of thread	Forum

Posted by: swearingen ® 06/29/2006, 06:58:44 Author Profile eMail author Edit

Not quite. The moment is actually 20' x 12"/' x 11k = 2640"k, which is twice what you got. Using My/I, the stress comes out to 2640"k x 9.75"/2 / 121.53in^4 = 106ksi, which again is about twice what you had. Even if you had a high grade steel that could withstand this stress, your deflection would still be almost 1.25 feet! Pipe makes for a great spreader bar because it doesn't have any lateral-torsional buckling issues, but you're going to need a bigger pipe. I'd start with at least a 20" if not a 24". 40' is a long span... P.S.: Don't forget about impact factors. Most crane rigging is designed at 4:1 or higher safety factor.

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Re: Re: Spreader Beam Design
Re: Re: Spreader Beam Design -- swearingen	Post Reply	Top of thread	Forum

Posted by: mluck ® 06/29/2006, 11:43:46 Author Profile eMail author Edit

Thanks for the reply. Yes I was only using 10' for half of my length, not 20' .... it had been a long day at that point. Out of curiosity, how did you calculate what the deflections would be at the end? A thought I've been mulling over the last day or so ... could this pipe be used to hold a single cable 40' apart. As a member in compression between the two sides. In elevation view looking something like a capital 'A'. I've never done crane rigging design, but it seems to me that the compression forces from rigging it this way would be less than the moment forces created by two downward point loads at the ends and a single upward point load in the middle of the beam. Thanks in advance for any replys. ~Luck

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Re: Re: Re: Spreader Beam Design
Re: Re: Re: Spreader Beam Design -- mluck	Post Reply	Top of thread	Forum

Posted by: swearingen ® 06/29/2006, 15:10:50 Author Profile eMail author Edit

The A-frame idea would work much better, depending on the angle of the A. If it's too short, it won't work and you may run into the breaking strength of your cable. Of course, the taller you make it, the taller your crane must be to make the lift. Most rigging manuals suggest an angle of 30 degrees from the vertical for lines in this configuration. This means from your hook to your spreader bar will be about 35'. With 11k hanging on each end, this equates to a little over 6k compression on the pipe, which is fine at a SF of way over the common 4 or 5. Pay attention to the details at the ends and make sure you get good force transfer. I recommend through plates with padeye holes and shackles. I also suggest you at least have it looked at by a structural engineer before you go ahead with the lift... Max. deflection of a point load in center: PL^3/(48EI)

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