Utilization Ratio Not Working?
Utilization Ratio Not Working?
(OP)
Hi all, i'm having an issue with using the utilization ratio for a concentrated force on a square HSS section. I am currently analyzing a HSS square spreader beam and am treating the lug plates as concentrated forces on the HSS. I am checking the capacity of the spreader beam and have checked flexural capacity, axial, and interation of the two according to AISC 13th edition. Now I wanted to check for concentrated forces on the HSS flange. To do this I am using AISC equation K1-9 and want to check Rn against the "pull" or axial force on the lug plate. My issue happens to be when trying to calculate 'Qf' which uses the Utilization Ratio found in equation K2-12. My utilization ratio becomes greater than '1' which does not work in the Qf equation under EQ. K1-9 since I will have to be taking the square root of a negative number......I checked all of the limits of applicability and nothing. Also looking at the equation for flexure, since my member is compact, the controlling equation for moment capacity was M=Z*Fy divided by the appropriate Factor of Safety (using ASD). Yet when you look at the utilization ratio you are dividing your required moment by the section modulus and Fc which is .6*Fy. So the result of dividing by these smaller number is what is giving me a Utilization Ratio greater than one. How is this possible as the beam passes all Flexure, Axial, and Interction checks? Has anyone ever run into this issue before? I should be reducing the capacity of Rn as the Flanges of the HSS that is connected to the lug plate is experiencing axial as well as flexural loading...
Thank you in advance for any help!
Thank you in advance for any help!






RE: Utilization Ratio Not Working?
I have analyzed the HSS square spreader beam and let's say it has a flexural capacity of 5 kip-ft. This is simply a global check on the beam, so sure it can handle 5 kip-ft globally, but looking at local checks on the beam if the lug plate is added parallel to the beam direction, I am getting a local effect on the flange so great that I will actually have to reduce the flexural capacity of the beam (this is shown by a utilization ratio that is greater than one that nullifies the equation). One way to fix this is to have the force distributed in a different manner locally on the flange of the HSS so that the beam can finally reach it's global flexural capacity original calculated per AISC.
Am I understanding it correctly? They should could mention something in the code or guides about what happens when your utilization value is greater than one...sheesh
RE: Utilization Ratio Not Working?
RE: Utilization Ratio Not Working?
I don't have a copy of the AISC 13 Ed., so I can't be of much help there. It may be a good guide for the general design of your spreader beam, but it was not developed with spreader beam design in mind, and it may not be the last word for loads below a crane hook in many jurisdictions. Good clean detailing of lugs and welds, and direct load paths, almost always trumps general stress and buckling considerations on this type of equipment.
RE: Utilization Ratio Not Working?
RE: Utilization Ratio Not Working?
RE: Utilization Ratio Not Working?
Good Luck