Rectangular HSS truss

[structuralnerd]

I'm designing a rectangular HSS truss with welded connections. I put together a RISA model to analyze the overall design of the truss. I am now to the point where I am checking localized failure at each connection point. I am using Chapter K of the 13th Edition of the Steel manual. In Chapter K2.3a there are Limits of Applicability for the HSS-To_HSS Connections section, and number (2) states that the truss branch angle needs to be greater than 30 degrees. However, the branch angle I have is a little over 20 degrees. In order to make it 30 degrees, I would have to add quite a few more branches to the truss, and it would increase the cost significantly. Does anyone know of the provisions for the condition I have? Any suggestions on what I should do?

 

[twinnell]

contanct the AISC solution center for direction.

 

[civilperson]

Add 90 degree posts at panel points.

 

[nutte]

If you do ask the Steel Solution Center at AISC, be sure to post back with their response. I have found them to be totally useless when it comes to questions like this that are out of the box.

Here's the response I bet you'd receive: "The equations in the specification are based on testing which was performed within the limits shown, in this case the 30 degree minimum angle. For your case, the equations are not applicable. Good luck figuring out a solution."

 

[msquared48]

Add another diagonal 90 degrees to the main axis of the one the one with the shallow angle, or whatever angle would get you within the 30 degree limit. Let RISA do the rest.

Think out of the box.

Mike McCann
McCann Engineering

 

[structuralnerd]

I already have 90 degree posts at panel points.

msquared48 - I'm not undertanding your suggestion. How would adding a perpendicular member to the main axis change its angle?

I suppose I could widen the posts that are 90 degrees. That might get me to my 30 degree angle...

 

[msquared48]

I wqas assumint the angle of intersection of the web member to the bottom chord was 20 degrees. Cutting the web member short by adding another diagonal at 90 degrees to it should eliminate the 20 degree problem. As in ...

_____________________ top chord
\ /
\ / Problem diagonal
Added diagonal \/ \/
\
\
______\______bottom chord

Pardon my drafting...

Mike McCann
McCann Engineering

 

[msquared48]

Gee...That really printed well...

I won't try that again!

Mike McCann
McCann Engineering

 

[civilperson]

With the 90 degree posts, the angle of the web members can be modeled as 70 degrees to the posts. Build the connections of the webs to the side of the vertical posts, (at the extreme top and bottom), rather than directly to the chords.