Double angle steel truss bottom chord in bending
Double angle steel truss bottom chord in bending
(OP)
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.
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.






RE: Double angle steel truss bottom chord in bending
RE: Double angle steel truss bottom chord in bending
Should be rather straight forward.
I'd automatically place some spacer plates in between the angles at the third points as a minimum.
RE: Double angle steel truss bottom chord in bending
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.
RE: Double angle steel truss bottom chord in bending
JWB
RE: Double angle steel truss bottom chord in bending
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.
RE: Double angle steel truss bottom chord in bending
RE: Double angle steel truss bottom chord in bending
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Double angle steel truss bottom chord in bending
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.
RE: Double angle steel truss bottom chord in bending
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.
RE: Double angle steel truss bottom chord in bending
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?
RE: Double angle steel truss bottom chord in bending
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.
RE: Double angle steel truss bottom chord in bending
RE: Double angle steel truss bottom chord in bending
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.
RE: Double angle steel truss bottom chord in bending
Tees and Double Angles Loaded in the Plane Of Symmetry.
This gives the bending capacity
RE: Double angle steel truss bottom chord in bending
Gather up your numbers and head to Specification chap H for combined forces.
RE: Double angle steel truss bottom chord in bending
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.