Truss Question 2

[bpstruct]

I have a project where an architect is wanting to use roof framing as shown in the pics attached. I can't see that these web members are doing much of anything....especially those in compression. The compression members don't seem stable since that bottom chord is unbraced. The top chords appear to be welded at the ridge and the bottom cable I'm thinking is doing nothing more than keeping the supports from spreading. I'm certain that, in an uplift situation, nothing is doing any work except the top chords in bending. Has anyone done anything like this before? I'm not too worried about the idea, as this is just a roof for an outdoor amenity center. I just want to make sure I'm not overlooking anything. I'm actually thinking that I would prefer to not have any web members. Under deflection, I'm thinking that any solid web member might have a tendency to push the cables out of alignment with the top chord above. I'll post a couple more pics in responses to this post.

 

[bpstruct]

Good stuff Hokie. Thanks for all of the responses. Picture 2 is how the architect wants this system to work. I think I can get it to work.

 

[msquared48]

I see the point here regarding the forces to keep the bottom cable in line, but it, to me, is still kinda like walking a tightrope - inherently unstable.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

Walking a tightrope is a good analogy, Mike.

BA

 

[msquared48]

BA:

Yep. Never done it, and I'll be damned if I want to either.

I look at the structure the same way.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

If you got a toy bow and arrow. Cut two of the arrows down to fit in the bow per the compression webs in picture 1. Pin attach them to the bow and string per the picture. How laterally unstable would the system be?
I say the system will work just fine. I will even go as far as to say that if you placed a pole jack between two of the trusses in picture 1 at the bottom cable to compression web points. You could push the points apart 2' without causing the system to fail under the design load conditions.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

Just make sure I'm not in the bulding when you do it!

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[woodman88]

"msquared48 (Structural)

Just make sure I'm not in the bulding when you do it!"

Okay. But can you give any examples or calculations to show that there may be a problem? Is there an engineering reason you do not like my the bow and arrow example?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

No Garth.

I know that it does work structurally as long as the system does not get perturbed. To me, it's just a "situation" waiting to happen. Cheers.

Interesting to note, though, that for the mobile radial sprinklers used in the midwest and in eastern Washington, the harped cable supports for the pipe are three dimensional in nature, one to each side of the pipe, and intermittently connected by a horizontal strut. This situation eliminates the potential lateral instability of the harping, and the perturbations due to the live loading conditions seen by the assembly, such as wind, and ground induced perturbations, such as the occasionally drunk roaming steer...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

I believe as follows:

1. If the tied arches in Pics 2 and 3 can carry design loads as built, adding web members to them would have little effect. They would continue to act as tied arches and the web members would be redundant so there would be little point in adding them (which is what bpstruct said in the first post).

2. If the chords had been hinged at all panel points, the usual assumption for truss design, the trusses could not act as tied arches. Web members would be needed and the bottom chord panel points would need to be laterally braced for stability.

3. If the chords had been hinged at all panel points, the trusses would be unable to carry uplift exceeding the dead weight of the roof structure.

BA