Stress in bottom flange of beam

[PEVT]

I want to calculate the stress in the bottom flange of a beam that supports a trolley and hoist. I have already checked the overall beam for bending, shear and bottom flange bending per CMAA #74. The problem is that there is an existing group of bolt holes immediately adjacent to where the wheel passes, so I need to look at a VERY small area and am not sure how to proceed. Now I need to nail down the point at which the load is adjacent to the bolt holes. I have attached a sketch which shows the scenario. The classic kicker is that this hoist has been operating like this for 20 years or so without any visible deflection or damage adjacent to these bolt holes. Please help.

 

[StrykerTECH]

I will run a quick FEA on it. give me the max p load value with safety factor in lbs and the support length of the of the beam. This will take like 10 minutes.

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[corus]

I'm not sure why you need to calculate the stresses if it's been operating for 20 years, unless the load is going to change. However, I would have thought you'd have to combine the overall bending of the beam with the localised bending of the flange. On top of that you'd apply a stress concentration around the hole, and a stress concentration at the fillet radius of the beam. Use Peterson's Stress Concentration Factor book, or there may be something in Roark's Formulas for stress and strain. I don't see the need for FE in this case.

Tara

 

[rb1957]

yeah, like Corus .. normal bending + flange bending. I'd account for the holes as missing area (net section) but probably wouldn't bother with the Kt ... so what if a very small amount of material yields.

If you do worry about fatigue, then I'd keep the combined bending stress below the endurance limit for Kt = 3.

 

[PEVT]

Thank you all for your input.

StrykerTECH, the load on the trolley is 4.3K, which translates to 1.08K per wheel passing by the holes. The allowable stress is .66fy or 23.8ksi. The beam span is 63".

There are two reasons why I want to know the stresses, even though it has been operating for 20 years. The first is so that I understand why it works, when it looks so suspect in application. The second is because I am load rating this frame and so I want to be sure I am confident in its continued safe operation.

Unfortunatey I do not have a copy of Peterson's book. If you think it a valuable work to have on hand I will pick one up.

 

[rb1957]

if you want to know more than you ever did about stress concentrations, then Peterson's is the place to start.

but these days there are plenty of (free) on-line Kt calculators.

 

[Tmoose]

It might be possible to reduce the stress concentration by drilling additional holes is strategic locations. Extra points if in the form of the company logo.

 

[GregLocock]

I'd just look at it as a little shear section 1/2" wide by whatever it is deep. SCF doesn't really apply in that case it is the local collapse of the material outboard of the hole that is the most likely failure. Does it go "Baddum baddum" as you push the trolley over the holes?

Simple solution would be to weld a doubler plate underneath, and then shoot whoever didn't stagger the holes.

Cheers

Greg Locock


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[flash3780]

That's a fairly complex loading situation. FEA software handles this sort of analysis very well. I'm not aware of any hand calculations to handle the roller-to-flange loading/waffling, so I'd end up making a gross approximation for a hand calculation.

The bending load, however, is fairly straightforward - I'd start with that for a hand calculation and then do an FEA for a more refined answer.

 

[StrykerTECH]

you're fine, the majority of the stress is in the area where the web contacts the flat portion of the beam, for some reason it isn't let me upload a pic. You have like a 5x safety factor, for static loading. that doesn't take into fatigue or dymanic/impact loading.

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[chicopee]

Attachment has a quick basic hand calculation with an assumption that the N.A. does not shift where the holes are located. Also assume simply supported beam between the rail supports. Other hand calculations can be made by moving the trolley with one set of wheels between the holes and over the holes to get an idea how shear and bending stress are affected.