Angle Iron Analysis

[lexengr]

I'm looking at hanging a large industrial fan from 3 x 3 x 1/4" angles. I have the static load of the fan and the torque from the manufacturer. In terms of stress analysis to make sure my angles are sized right for the 8ft span, I'm looking at bending stress (Mc/I) and shear stress (3/2 V/A). I'm not sure how to think about the torque, would that basically just be another bending stress at the connection point? The angles are bolted to bar joists. Thanks in advance.

 

[rb1957]

yeah, i expect that the torque would be reacted by couples, assuming the torsion axis is normal to the angles (the fan is attached to the angles at two points on the upper edge and the lower edge ?). i expect that the fan is slung off to one side of the angles, so there'd be a bit of torque into the angles.

isn't there some gyroscopic force associated with rotating elements ? maybe it's small, but ...

what about vibration, the natural frequency of the support structure compared with the speed of the fan ?

 

[lexengr]

There are actually 2 angles that support the fan, which is mounted in between them so there shouldn't be a torque on the angle per se. I'm thinking the torque acts to rotate the angles, which is resisted by the clamp load of the bolts.

 

[nuche1973]

Lexengr:
You’re on the right path. Your stresses also are also dependant on the type of connection. I am assuming a welded connection, which would be best. You’re moment diagram would be similar to the moment diagram of a horizontal cylinder mounted on two saddle supports. If you’re bolting, then you’ll need to account for the lost steel due to the holes.

 

[lexengr]

If I consider the bending stress due to the weight, and the bending stress due to the motor torque then to combine them would I simply add them? Von Mises wouldn't apply here would it?

 

[rb1957]

ok, you've got an angle on the front and the back, so no offset. each angle is bending under the weight of the fan, and the torque ... the maximum moment in the structure is the moment due to weight + 1/2 the torque (the torque is reacted by a couple at the angle reactions)

 

[nuche1973]

Yes to both. AISC simplifies it into calculating the bending moment and calculating the appropriate plastic modulus to accommodate the imposed stresses. And, I apologize for missing the fact that you did mention that the angles will be bolted to the joist. You’ll have to account for the lost steel due to the bolt holes. Treat it like a bolted flange connection, minus the temperature variable, and you’ll be conservative.

 

[dhengr]

Lexenger:
Before you try doing a structural design problem like this, you may want to get out your old strength of materials and machine design textbooks, and then take a short course in formulating a good question. Some sketching might help too. With the little info. you have given and lack of a sketch, imagine the number of different hanging configurations we could conjure up, each involving a slightly different approach to the analysis, stresses, deflections, vibrations, etc. We can’t see what you’re looking at from here and you don’t tell us.

The advice above isn’t bad advice, but we are all guessing at what you are really doing; and given the way you asked the question, you don’t really know either. You should be talking to your boss or a local, mentor, engineer on these kinds of questions; where you are both looking at the existing bar joist conditions and fan product sheets, and doing schematic sketches together, and some of your questions would probably become self evident, or maybe you shouldn’t be doing this design problem yet. Where are you loading the bar joists? Are you overloading them? An 8' angle span btwn. bar joists seems kinda long to me. How do you attach the fan frame to the angles and what orientation do the angle have? Angles are an unsymmetrical bending shape so they can be subject the torsional issues, so you could be right on that account, but I’m not sure that’s the torque you’re talking about. But, this may not be a big problem depending on the connections and their relative locations. Mc/I & 3V/2A may not be exactly appropriate, but I don’t think von Mises would give the problem a second thought.

You’ll learn a lot more, and more quickly, if you can ask these kinds of questions of someone who is knowledgeable, and right there, to work intimately with you on these projects. If you can find those types of people, mentors, it can be quite a rewarding experience and exchange. Furthermore, your boss should know what you do and don’t know, so he/she can keep you all out of trouble on this stuff. Good Luck.

 

[BAretired]

Sketch, please.

BA

 

[desertfox]

Hi lexengr

Agree with BA a picture speaks a 1000 words.

desertfox