lifting beam made of plate

huangyhg

lifting beam made of plate
i have been tasked with assinging a rating to a lifting beam to be made of 1-1/4" plate a572 gr 50. there will be two choker attach points on top 12ft apart. the lift points will be a pattern of hole 12inches apart with the outer most holes 24 ft apart. the overall length and height will be approx 25 ft x 8ft. following asme bth-1-2005 it looks like the rating will only be around 18 tons. the customer was hoping for something more like 100 tons (with nothing more than having seen something similar in the past as a reference). am i missing something? thanks  
hi jflusche
i am not familiar with asme bth-1-2005 however i would stress the plate as a been i presume you have done that.
what size are the lifting eyes won't the safe working load be limited by those.
desertfox
your drawing seems to indicate service level inclined shearing forces of 50 tons/sin(60 deg)=57.73 tons to be taken at 60 deg inclination, plus safety (and dynamic between holes needs be (between other things be checked), and interaction of stresses. some assumption of  initial deformed shape can be needed there, and anywhere the plate becomes compressed for some length and not properly restrained. given your drawing, it seems it will likely be controlled by the details of upper support.
retry better written i hope...
your drawing seems to indicate service level inclined shearing forces of 50 tons/sin(60 deg)=57.73 tons to be taken at 60 deg inclination, plus safety and dynamic impact factors. compression between holes needs be checked (between other things), and interaction of stresses. some assumption of  initial deformed shape can be needed there, and anywhere the plate becomes compressed for some length and not properly restrained. given your drawing, it seems it will likely be controlled by the details of upper support.
the maximum bending stress in the plate is about 3125 psi which is fairly low, but as ishvaaag has pointed out, lateral buckling is likely going to govern.
could you add a channel at the top and bottom edge to resist buckling?  then the depth and thickness of plate could be reduced as well.  the net result would be a lighter and stronger lifting beam.
ba
it can have also big forces for loads on a single hole at bottom so checking the maximum at any of these and the interesting combinations is also mandatory.
the most efficient structural shape to resist shearing and bending stresses is going to be a combination of vertical plates (the one you have) and horizontal plates welded to the top and bottoms of the vertical plate.  this you should have learned in mechanics of materials.  i have never seen a lifting beam without flanges.
other aspects not so apparent from mechanics of materials is flange buckling and web buckling.  the former will occur on the compression flange and will limit the load capacity of your lifting beam.  web buckling will also limit your capacity and can be dealt with easily by adding stiffeners to your beam's web.  
assuming you have the holes where you need rigging to bolt to (shackles, etc) then you should really have stiffeners right above the lifting points.  however, this may be impractical with so many different lifting arrangements.  so you may need to increase the web thickness.
  
regards,
qshake
eng-tips forums:real solutions for real problems really quick.

do you need a factor of safety to satisfy osha?
i appreciate all the help. i did not show that we will use "washers" around the lifting holes. i am confortable with the design of the lifting points. i am most concerned with the unsupported length and the plate buckling with a load at the outermost points. thanks for comments.
for what it is worth, the 13th ed. aisc code has a very nice section on the strong axis bending of plates.  you may find it helpful with your question about plate buckling.  page 16.1-60, i believe.  attached is a link to the spec.
jwb
something else i don't believe anyone has mentioned so far, normal beam theory does not apply to deep beams.  i believe the dimensions of this plate qualify it as a deep beam.
ba