W-type lifting beam - bottom flange loading - unbraced length

[weron4u]

I am analyzing lift points at a company with many of them. I have been following the AISC Manual of Steel Construction ASD 9th Edition with regards to beams with unbraced lengths greater than Lu.

I am currently analyzing a W18x55 that was put in place for a specific application. It is 40.5ft long with an unbraced length of 28ft. The person who designed the beam for the application was a highly competent structural engineer, so I trust his calculations. However when I use Equations F1-7 and F1-8 in ASD, It is determined that the maximum allowable stress in the beam is 9.35ksi. The 4ton rating that the structural engineer put on the beam creates a stress which is greater than this.

I was hoping that someone could inform me of possible exceptions to these equations that I am using. Are there different criteria when loading with a trolley from the bottom flange?

Any help would be greatly appreciated.

 

[weron4u]

also, the beam is set up as follows



| |||||||||||||||| |
| vvvvvvvvvvvvvvvv |
|------------------------------------------------|
| 27.5ft * 13ft |
| |

The horizontal dashes are the lifting beam. The vertical lines at the ends are columns. the Asterisk is a lateral support. And the arrows indicate where the trolley is allowed to travel.

Do I calculate the allowable load in the beam using an unbraced length of 27.5ft or 13 ft? Or do I use both separately and calculate the maximum moment in both the 27.5ft and 13ft sections due to the load?

 

[JAE]

With a moving load, you will create an envelope of maximum shear and moment along the beam.

Now, once you have that envelope, you would then check the maximum moment in the 27.5' zone, against an lu of 27.5. Then you would check the maximum moment in the 13 ft zone with an lu of 13'.

You would never take a moment from one zone and use an lu from the other zone for your check.

 

[weron4u]

Thanks JAE

I wasn't sure if the localized buckling happened near the loading point or if It could occur anywhere on the beam. I am interpreting from your message that localized buckling occurs due to solely the moment in the beam and the point of load application has no bearing on when the beam buckles.

If I am incorrect, please let me know.

Thanks a bunch,
Weron4u