Truss tension chord bracing

[BAGW]

Hi,

Do we need to brace the tension chord of the truss based on AISC appendix 6 requirements to satisfy k=1 from compression web member design?

Was reading through the research paper from Fischer (1983). Seems very conservative to me as no tension diagonal contribution is considered. Wont the tension diagonal member stabilize the bottom chord against the compression vertical force member?

Any thoughts on bracing the bottom chord for the vertical member compression force?

 

[human909]

I don't understand the qustion. Pictures and links to the source documents would help.

 

[BAretired]

If the tension chord terminates at the end diagonal in a top bearing OWSJ, the node needs to be braced; otherwise two tension members meet at a point and are in unstable equilibrium.

 

[dik]

Yes... there was a really good article in the AISC publication about 40 years back that explained it... I'll see if I can find something on my hard drive archive. Maybe the same paper... The contribution of the forces within the web do little to stabilise it at right angles, I suspect.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1668291702/tips/Temson_Chord_Bracing_js9nwa.pdf[/url]

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

Wont the tension diagonal member stabilize the bottom chord against the compression vertical force member?

Nope. The tension member would only stabilize the joint if lateral movement of the joint caused the tension member to further elongate.

In many respects, I feel that it is useful to view tension chord bracing requirements as, effectively, the truss analog to beam lateral torsional buckling bracing. In both cases, you're trying to prevent the beam/truss from rolling over wholesale and deflecting according to its weak axis moment of inertia rather than its strong axis moment of inertia.

There are two ways to get around this for architectural applications:

a) Have the bottom chords of the truss laterally tied to the supporting columns such that the panel points needing bracing are braced by the lateral stiffness of the bottom chord.

b) Ensure that the bottom chord is heavy enough that it's own weight prevents it from swinging upwards and rolling the truss.

 

[jayrod12]

What would be the analysis procedure for your option b)?

 

[KootK]

Nowadays, I'd probably just skip to using Mastan2. Long ago and far away, I've attempted a work balancing between:

1) The work done in the bottom chord swinging upwards and increasing its potential energy and;

2) The applied load deflecting downwards under the weak axis stiffness of the truss and shedding potential energy.

The hand calc is a bit unnerving in that, while I feel that I can prove that the truss won't flip over wholesale, it may well rotate 20 degrees or whatever before it settles down. As you can imagine, the bottom chord doesn't put up much of a fight until the rotation starts to have a meaningful upwards component to it. Makes one appreciate proper bracing...

 

[KootK]

Also, there's still another option that I didn't mention:

c) Ensure that the torsional stiffness of the top chord alone is enough to LTB brace the truss. This might make sense with a beefy HSS chord and torsion connections to the supports.