Tension Chord Bracing-Short Discussion Requested 5

[IJR]

Pals

Fisher published a great article about this in one of AISC technical papers. I follow Fisher's advice whenever I design and detail a truss. I always provide lateral support at convenient locations to brace the bottom(tension) chord

Question is, do you know a different way beside physically bracing the chord?

There are cases I can not provide these braces, and I end up using heavy sections to limit overall slenderness to 300(lateral).

I have noticed however that guys who do cable kind of trusses never bother to brace their cables at all. Apparently it is difficult to detail the connection unless the cable system forms a grid, and aesthetically a grid becomes dense and too distracting. The easiest example I can give is the roof structure for the New Berlin Train Station(just type berlin Train Station or "Berlin + Hauptbahnhof" on your search engine,see also CE Magazine August 2006). I have seen many more of the kind

Your opinion requested.

Respects
IJR

 

[ctcray]

I believe the rationale for tension chord bracing in trusses is that you may have web members that are in compression framing into the tension chord and these web members must be restrained to prevent them from buckling out of plane of the truss. Since cables don't take compression loads, I don't think they would need to be braced.

 

[WillisV]

See this thread that I started on the same topic a while back for a discussion on this: thread507-124363

 

[IJR]

ctcray

What I actually meant is bracing the web members at their ends, and that means where they meet the tension member, cable or not.

respects
IJR

 

[RARWOOD]

I design a lot of heavy timber trusses, and have often asked for advice from one of the company's retired engineers. This engineer has between 35 to 40 years of experience in wood design.

His advice: IF IN DOUBT DON'T.

However, there is a major difference between the bottom chord of a heavy timber truss and a steel truss. The wood truss uses a solid section and the steel truss often uses wide flange beams.

 

[Tomfh]

If the system has enough rotational stiffness then you dont need bracing.

 

[IJR]

TomfH

How can you have rotational stiffness in a truss, usually very deep and web members pin-ended?

Do you want to elaborate?

Respects
ijr

 

[Tomfh]

The vertical web members will not be pin ended, but instead will be rigidly connected into the roof.

 

[KCRatnayake]

I agree with ctcray. You need bracings to prevent structural elements from lateral tarsional buckling. I.e. generally at the compression region.

Regards
CLEF