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21 Oct 2007 03:26 UTCSun 21 Oct 2007 - 3:26 am UTC
I'm planning a lumber sawmill operation as a second (semi-retirement) career. Back problems prevent me from lifting heavy loads, so I want to build an overhead traveling crane to move logs onto the sawmill. I need an engineering analysis to tell me beam sizing for the following load parameters:
1) vertical supports at four corners of a 24 foot square supporting cross beams 12 feet above the floor
2) fixed cross beams on two sides of the square, 24 feet between supports (all working load will be between the supports however the cross beams might need to extend two or three feet beyond each support post depending on how the traveling beam is mounted)
3) traveling beam rides on the fixed beams, 24 feet in length. No dead load (except the beam itself), live load carried by two trolleys suspending each end of a tree trunk:
3a) max length: tree trunk 21 feet long by 2 feet diameter => about 5,300 lbs supported about 3 feet from each end (=> 15 foot trolly separation on the live beam, each carrying half the load)
3b) max load: tree trunk 12 feet long by 3 feet diameter => about 7,000 lbs supported about 1 foot from each end (=>10 foot trolly separation on the live beam, each carrying half the load, assume evenly spaced from each end of the beam)
[load calculated based on wood <= water = 80 lbs. / cu. ft. ]
A) size of 4 corner vertical members, assume they are set in concrete foundation 6 feet deep by 2 feet diameter (or adivse if more is needed)
B) size of two fixed beams for the traveler to ride on
C) size of the travelling beam
Usage will be infrequent, loads indicated already have a 10% safety factor built in and represent maximums not normals. I don't want to spend more than necessary but I want to be safe. Based on other posts and answers I would appreciate hearing from Redhoss but will take the advice of any licensed engineer.
21 Oct 2007 07:15 UTCSun 21 Oct 2007 - 7:15 am UTC
Clarification: minor deflection of the beam is not a problem as we are simply trying to raise the load a foot or so off the carrier and then move it horizontally to set it down on the bed of the sawmill. The carrier would be in the middle of the side span and the sawmill at one end near or under the support posts.
22 Oct 2007 15:05 UTCMon 22 Oct 2007 - 3:05 pm UTC
Hello Dave, always glad to help a semi-retired person. I haven't really done any crane design, but I have done sling design for lifting heavy loads. Normally a safety factor of 5:1 is used. In this case that would be 36,000 psi/5 = 7,200 psi. Using your load numbers, the beam required for the two fixed beams would be a W14x30. The maximum deflection would be 0.25 inches. The allowable deflection is 288/360 or 0.8 inches. This looks like a good choice.
I tried the same beam for the traveling beam and got a maximum deflection of 0.38 inches. The W14x30 is a good choice for both beams.
Now for the tough part of the question. The columns can be subjected to large bending moments due to the inertia of the load being stopped quickly. The flange width of a W14x30 is 6.733 inches. I would use 6 inch standard pipe which has a diameter of 6.625 inches. Kicker braces should also be added.
I think this will give you a safe crane design without costing too much. Please ask for a clarification if I have missed something or there is something you don't understand.
Good luck with your new career, Redhoss
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