huangyhg

lifting loads - proper way to account for in design
i have been asked to design a steel frame to facilitate lifting a piece of equipment (approx 5,000 lbs.) to the rooftop of a building. the frame will be fabricated around the equipment, then the cranes will be rigged to the frame. direct rigging of the equipment isn't practical. frame will likely be w6x25.
what is the proper way to account for the effect of lifting on the frame members? it seems to be it would be like an impact load. i have seen references to adding 25% (or 10%) to the static loads. are there commonly accepted (and valid) approaches?
thanks in advance.
find a job or post a job opening
the typical factor of safety used for rigging is 5 based on the ultimate capacity. there is a nice article in the 4th quarter/1991 of the aisc engineering journal.
usually, there's not an allowance for dynamic loading, but fairly high safety factors are used.
you might check out asme b30-20 which is applicable to spreader bars and similar below-the-hook devices. (i think it specifies fs of 3, based on yielding.)
you might want to check out asme b30.20, standard for below-the-hook lifting devices. this code specifies safety factors but does not require application of impact factors.
i am designing a lifting strongback frame and am using asme b30-20 (not free by the way). i am limiting the bending stress to fy/3.
i have used 1.5 to 2.0 as impact factor in the past. it depends on what level of confidence you have on the crane operator. i have seen many crane operators, most of them excellent, a few of them terrible. impact or dinamic load will deppend on how he drives the crane.
i also include a 60%/40% distribution factor between slings unless the load path is very clearly determined and a 75%/25% when the lift is done with more than one crane hook.
dead load factor between 1.1 to 1.4 deppending on how well calculated it is and do not forget to include the weight of the rigging. sometimes it is significative.
it might appear very conservative to some, but i have seen a couple of crane operations gone south and it is not pretty when it happens.
normally for onshore structure, the dynamic amplification factor is taken as 1.25.
you also need to consider how many points of lifting are going to be used. usually, 4 lifting points for typical equipment's protection frame. bear in mind, when you design the padeyes, you also need to consider the condition where at the loads are not equally distributed. ( let say 2 lifting points carry 75% of the 1.25 x weight).
as zhuge says there is inevitably one sling longer or shorter than the others. this will increase the load in the sling set and therefore increase the load on the relavant lifting point. in addition, the skew effect will tend to twist your spreader frame, thus increasing stresses.
make sure to add your assumptions, assumed rigging, and maximum loads to your drawings. you might want to add a sketch showing assumed rigging, angles, etc. also consider having the maximum load stenciled onto the frame.
zcp
analysis or actual max working stresses must be determined based upon working conditions including impact or other factors pertinent to the application. actual max working stress must be equal to or less than the allowable stress.
asme b30.20-2003 20-1.2.2
"the load-bearing structure components of a lifter shall be designed to withstand the stresses imposed by its rated load plus the weight of the lifter, with a minimum design factor of three, based on yield strength of the material, and the stress ranges that do not exceed the values given in ansi/aws d14.1 for the applicable condition. ...."
in the us the following may help guide guide you:
asme b30.20a-2001 structual and mechanical lifting devices. section 20-1.2.2 construction
cmaa crane requirements
aasho 1.6.17
steel construction manual-aisc
occupational safety and health administration (osha) 29 cfr standard 1926.753(e)(2) and 1926.251
american society of testing and materials specifications a391
naval facility command navfac-307
american national standards institute, "below the hook lifting devices"
granted, this may be a little late, based on the date of the original posting (i don't get a chance to log on much), but . . . it can assist anyone who may be searching later for info.
the best information for design of the lifting frame is an article entitled
"design and construction of lifting beams",
by david t. ricker, published in the
aisc engineering journal, fourth quarter/1991.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]