Double angle steel truss bottom chord in bending

[PostFrameSE]

I am analyzing an old steel truss. The bottom chord consists of a 2L2x2x1/8 member. The span length is 15' between any support, and there are no web stiffeners or stitch welds. The customer has added a ceiling which adds 30plf to this member.

Seeing that this is primarily a tension member, the shape doesn't matter that much, but I also have a bending load applied uniformly to it. How do I determine my allowable stress Fb on this member? If I knew that, I could determine whether the unity equation is ok.

Thanks.

 

[slickdeals]

You will have to use the equations in Chapter H of AISC to computer the unity equation for a member in tension and flexure.

 

[Stillerz]

Yep...chapter H.
Should be rather straight forward.
I'd automatically place some spacer plates in between the angles at the third points as a minimum.

 

[PostFrameSE]

OK. I understand the unity equation. What I'm struggling with is how to calculate Fb. Do I need to go to Table B5.1 and see what the limiting Width-Thickness ratio is? That would be 76/(sqrt Fy.) On a 2x2x1/8 member, my outstanding leg would be 1-7/8" x 1/8" (or would it be 2" x 1/8".) Therefore my b/t ratio is 15 and my limiting ratio is 12.67 for Fy=36ksi.

At this point, what can I do? Does this mean that these members, regardless of how many spacer plates that they have, cannot carry anything in bending because the b/t ratio is too high?

OR............since the unity equation calls for bending tensile stress, do I not worry about the compression side of those angles? It seems to me that the higher the tension effects, the less effect the bending would have on those members.

I'm confused.

Thanks.

 

[jberg]

Could you add a plate to the top of the angles? In essence, you would make it a I-shaped beam. I have done this with WT members several times, but I have never done it with double angles. With this method, you also need to take into account "locked-in" or residual stresses from dead load and maybe a portion of the live load, etc.

JWB

 

[bootlegend]

You need to look in chapter H of the AISC 13th edition. Section H.1.2. does concerns flexure and tension. It looks like you can only increase Cb for bending based off of the axial tension load.

I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will.

 

[slien]

Why not use vertical ties from top chord to bottom chord in ordder to apply the ceiling loads to the upper chord. These ties could be threaded rods or even cable/turn-buckle.

 

[paddingtongreen]

Does any part of the angle ever go into compression? bear in mind that wind uplift reduces the bottom chord tension. If not, just combine the tensions.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[PostFrameSE]

My tension stresses under dead load alone are around 6,800psi. My bending stresses under dead load alone are 21,600psi. So yes, part of the angle is in compression.

I appreciate the ideas that each of you have. However, I want to understand how to calculate the allowable capacities so that I not only know this time how to look at this, but next time as well.

Maybe I should re-phrase my question. Given a 2L2x2x1/8 x 3/8" space between the back-to-back legs spanning 15' with no additional spacers or anything.......how much uniform load could that section carry in bending?

Thanks.

 

[bootlegend]

Maybe I am missing something here, but why can you not just calculate the axial (tension) strength, calculate the bending strength, and then use the interaction equations to see if the chord is okay?

I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will.

 

[PostFrameSE]

Johnnycash,

Perhaps I'm a moron, but the allowable bending strength is what I'm struggling with. I understand the unity equation. What is my Fb for the aforementioned angles given 36ksi steel using ASD? Do I need to use that Q factor as found for single angles? That would knock my Fb down to around 19.7ksi. Would that make sense?

 

[bootlegend]

I don't think you are a moron, but I think you might not be using the AISC 13th edition. I can't remember using 2L's for bending but it looks like this would be the procedure:
1. Axial tension strength from Ch D

2. Bending strength from Ch F section 9 based on Yield, LTB, or Flange Buckling. It does matter what part of the section is in compression, but you ignore the axial load while calculating the pure bending strength. That is my interpretation anyway.

3. Interaction check from Chapter H is just a plug and chug equation using those first two strengths. It says you can increase Cb for bending, but there doesn't appear to be a Cb for LTB of 2L's now that I look at it.

If you aren't using the 13th edition, maybe at least download the spec part from AISC and look over it to see if it would help.

I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will.

 

[ishvaaag]

According to AISC 360 05, class your section as compact, noncompact or slender (with slender elements), then determine limit axial strength as per chapter E, and limit flexural strength by chapter F. Then chapter H indicates how to proceed. In all cases the safety factor is 1.67 respect limit load for allowables.

 

[bootlegend]

By the way, isn't Q just for the compression checks? I think so. If you section needs that you would use it compression calcs. But you said the chord is in tension so you shouldn't have to fool with it.

I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will.

 

[Stillerz]

2005 code...Chapter F, section F.9
Tees and Double Angles Loaded in the Plane Of Symmetry.
This gives the bending capacity

 

[Stillerz]

I think you could probably just pull the compression and tension capacities traight out of the tables in chapters 4 & 5 respectively.
Gather up your numbers and head to Specification chap H for combined forces.

 

[Stillerz]

I think that no matter what you do, you're going to have to assume that the double angles are working together like a tee section to get your design strength and then in reality make these two sections work together.
I have had simliar situations in double angle truss compression members and I put a 3/8" plate between the angles for their full length.