Point Load on Angle 1

[teddyt11]

Hello,

Really basic question, I'm just new to engineering and wanted to see if this made sense.

I have a 24" simply supported 3x3x1/4 angle flipped that one of the legs is taking a 500 lb point load at the center.

I went through the equations and it seems fine, but just thinking about little 3x3 angle holding a 500 lb load seems like a lot.

Let me know if you think this makes sense, I just don't think I've seen enough of these to really know the strength.

 

[jayrod12]

24" isn't a large span. When you say you went through the equations, which ones. There aren't many that directly deal with a single angle in bending.

 

[rb1957]

max moment = PL/4 = 3000 in.lbs = 250 ft.lbs

on just the standing leg alone ... bending stress = 6M/(wt^2) = 6*3000/(1/4*3^2) = 24/9*3000, about 7500psi ... yawn.

it's always a good idea to get a physical sense of the load ... 500 lbs ... two big guys (or 3 girls), but only 24" span. try to see if you can get an angle piece to mock-up.

something to think about, particularly as you're starting out ... what is the allowable stress (maybe fcy, but you should work through the crippling calc) ? just so you can see how little of the section's capacity you're using (only about 20%). also, look into the twisting (ie torque) you're putting on the angles (angles don't like torque ... but I doubt it's a Problem in this case).

another day in paradise, or is paradise one day closer ?

 

[KootK]

These two articles will help with evaluating the localized effects:

Link
Link

As rb1957 mentioned, torsion is often very important, both directly and in terms of lateral torsional buckling. Google Dr. N.S. Trahair and you'll find some great information on this. If you tell us a bit more about your application, we can probably give you some more detailed recommendations.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MikeHalloran]

Angles have a nasty habit of buckling when a bending load is applied. ... and similarly under an axial compressive load.

They are, however, relatively easy to roll, which makes them cheap.

You will find angles in historic structures, as the corner elements of columns latticed with flatbars.

... and in less historic structures, as elements of roof trusses, usually paired.

... and in railroad bridges, reinforcing the edges of plates before large rolled I-beams became available,

You will also find angles in electrical transmission towers, as the corner members and also as the bracing that stabilizes the corner members. I think the bracing/diagonals are cleverly designed long enough to buckle in compression, so they are only loaded in tension. Angles are lightweight, resulting in towers that can be placed by helicoper, and angle structures don't tend to collect rainwater, improving the life of structures that are infrequently maintained.

You will hardly ever find single angles stressed in bending in properly engineered structures. Start running numbers for your example with progressively longer spans, or just stand (carefully) on the middle of an old angle bedrail, and you may see why.



Mike Halloran
Pembroke Pines, FL, USA

 

[TLHS]

You'll often see them as bending members in small scale industrial applications. Pipe and tray supports especially. They aren't pretty, but they're cheap and easy to fab if you arrange them properly. Normally you're looking at span lengths where lateral torsional buckling won't apply. You may also see them in bending as lintels.

Jayrod, AISC-360 directly addresses angles in bending for both laterally supported and unsupported cases. So I would assume that those are the calcs in question. I feel like it's a fairly recent addition, though.

 

[TLHS]

But certainly consider how it's being loaded. If the load is spanning over top and is supported in a few spots then you're basically loading through the centre of the member. If you're hanging something or only have one support point you may be applying a torsion. If you're hanging, you've likely bolted through the horizontal leg and may have a tearout/local bending failure in the leg (you almost certainly won't, given how thick angle legs are but you should check it if you don't have a good feel for it)

 

[Ingenuity]

Dr. N.S. Trahair

Dr Nick, my old prof from Uni of Sydney. Great lecturer!

Some of his research papers on steel angles, as follows:

Trahair, N. (2004). Biaxial Bending Of Steel Angle Section Beams. Journal of Structural Engineering, 130(4), 554-561.
Trahair, N. (2003). Lateral buckling strengths of steel angle section beams. Journal of Structural Engineering, 129(6), 784-791.
Trahair, N. (2002). Bearing, Shear, and Torsion Capacities of Steel Angle Sections. Journal of Structural Engineering, 128(11), 1394-1398.
Trahair, N. (2002). Moment Capacities of Steel Angle Sections. Journal of Structural Engineering, 128(11), 1387-1393.

Some of his papers are free downloads here: Link But you will have to use the site search function to get those papers.

 

[KootK]

I'm jealous. Trahair = next gen Galambos.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

It also makes a difference if the leg tip is in tension or compression.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[desertfox]

Hi teddyt11

I just wanted to add that unless the point load passes through the shear centre of the angle then the angle becomes subject to torsion.


Have a look at this link and look for sample problem 6.5-3

http://cfile8.uf.tistory.com/attach/15234543507FF9C4019C51

 

[transmissiontowers]

I've been working on single angles since 1973 in lattice towers and they are horrible in bending but usually work fine as truss members when used in a T-Line tower. The unique aspect of our industry is after we design a new tower, we do a full scale test to destruction to show we knew what we were doing. I don't think the bridge or building people would build the structure, load it to failure and study where it failed (probably too expensive). Also probably why they use relatively large safety factors. Our towers use very low factors compared to buildings because we are willing to accept a few failures during an extreme wind or ice event. We use Tension-Only angles with L/r around 500 so they buckle out and the tension member takes the load.

Angles in bending are not used much in the T-Line world but at the most extreme event after a few members in the tower have failed, the legs will start taking bending loads which usually lead to total collapse but there are many load paths from the wires down to the foundations and you might have an arm buckle and have electrical contact but the tower can sometimes be repaired.

_____________________________________
I have been called "A storehouse of worthless information" many times.