Safety factors for lifting beams

[weron4u]

I have been analyzing lifting beams for their load limits for several months. I deal with single and double girder trolley beams, lugs welded to the bottom of beams, and slings around beams. I was hoping that someone could give me an idea of what type of safety factors are expected for these points.

My starting point for allowable bending stress is 0.60Fy for beams with the required unbraced length. I assign safety factors from here. I have been using deflection limits as follows. L/602 for single girder; L/888 for double girder and L/400 for jibs. If the deflection limit is reached before the beam reaches capacity, I rate the beam for the load that reached the deflection limit and then assign the safety factor with respect to the 0.60Fy.

If the beam does not reach the deflection limit before reaching its yield, then I assign a safety factor of approximately 2.0 with respect to the 0.60Fy.

Does this sound accurate?
Any suggestions?

Thank You

 

[boo1]

In the US the following may help guide guide you:
ASME B30.20a-2001 Structual and Mechanical Lifting devices.
main section Requires sf=3
padeye Requires sf=5

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"

 

[weron4u]

Thank you boo1, but what is the safety factor based off of? Fy; Fu; 0.6Fy? And when dealing with beams that have 20ton capacities, it is not practical to have a safety factor of 3 because the cost increase would be immense. How do you handle those cases?

 

[Tankman650]

Your approach is basicly correct. Determine the normal capacity of a beam and divide the load by 2 to get the factor of safety of 5 (A36 material). There are other factors that need to be addressed.

The following paper has numerious examples:
"Design and Construction of Lifiting Beams" David Ricker, Structural Engineering Journal, Fourth Quarter/1991 of AISC

http://www.aisc.org.

 

[boo1]

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 applicacable condition. ...."

This paragraph determines the maximum allowable stress for materials used as load-bearing structural components. 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.

The intent of Section 20-1.2.2 is that the load suspension parts of a lifter shall be designed so that the static stress calculation for the rated load shall not exceed 33% of the yield strength.

 

[boo1]

I would like to get a read that article, "Design and Construction of Lifiting Beams" David Ricker" , is it available?

 

[Tankman650]

The paper is available for download by members under epubs at

http://www.aisc.org

.