Question about Bar Gratings

[NewEngineer]

Hi everyone. I am a new engineer, and I am having some trouble doing a hand calculation. (My mentor at work gave me this assignment and said a simple hand calc would suffice)

The problem I am working on is that we have a Bar Grating installed spanning two ledges, almost making a bridge. It is about 4ft long (also the way the bars run) by 2ft wide. However, there is a spot that is 9 inches wide, and 8 inches deep, where there is a cut out. I figured out that 5 of the bars will not be able to span to both ledges, making 5 cantilevered beams.

I am supposed to figure out a way to make it take a certain load, around 250-300lbs per square foot. I can do this however I want, but he said probably either by welding a bar to the ends of the 5. Or by welding an angle somewhere.

As a new engineer I really need some guidance. I am not sure how to go about assuming things and being able to do a hand calculation. Ive been trying to do some beam analysis, but am not sure how to do it for more than one bar in a row. Any help or tips would be great

 

[dik]

There are many bar grating suppliers; I typically spec Fisher and Ludlow grating with a note "or an approved alternative". The suppliers have standard load tables that can be used as well as details.

Check to see what standard widths of gratings as well as standard lengths and try to accommodate these into your application.

Else, you can calculate the strength using the plastic section modulus and check deflection.

Dik

 

[NewEngineer]

Forgot to mention, this will be the grating.

http://www.mcmaster.com/#6498t18/=gm8tz

So do I use the load tables to come up with an answer? And how would I calculate the strength of 5 beams in a row / and when there is an angle or gusset welded to the bottom?

 

[SnTMan]

NewEngineer, remember composite beams?

Regards,

Mike

 

[NewEngineer]

Do you mean make a composite beam and have some of the sections be air? (for the cantileverd situation)

 

[civilperson]

Outline the cutout hole with sufficient steel to replace the cut bars. Weld continuously to each bar that is cut and add steel parallel to the bars at the side of the hole.

 

[kslee1000]

Basically you have a simply support beam with varying moment of inertia. Say at each end, there are 16 bearing bars accross the width, then I1 = 16xI(individual bar), and after cutting 5 at the opening, I2 = 11xI. Do an analysis on this beam by trial to derive an uniform, or concentrate load, that meets both strength and deflection criteria (see manufacturers' manual for info).

If the resulting load is acceptable, weld a piece of plate (same depth and thickness as the bearing bar) accross the 5 cut-off's to the adjacent full length bars (total 6 spaces). Do the same at opposite side to form an enclosed cut-out.

If the resulting load is inadequate, you may provide an angle accross the 2' width somewhere in the beam to make it a 2 span beam. You need to provide end anchorges for this case.

 

[Tmoose]

http://c.managemyhome.com/Images/Structure/Floors/0329_L.gif

 

[Einsteim]

Why don't you post a quick sketch of your situation? It would help to understand it better.