Truss Bracing

[akastud]

I am in the middle of designing exposed Glulam trusses. These trusses are parallel chord flat trusses about 7'-6" in depth, and are top chord bearing. The bottom chord is entirely in tension, and I am trying to determine if and how to brace the bottom chord. The trusses will span 66' and the bottom chord is about 55' feet long, and the truss overall looks very similar to a steel bar joist, at least in shape. The 1997 NDS (ASD) Appendix A (A.10 part B) says 'Also bottom chord lateral bracing should be installed in the same bays as the vertical sway bracing, where practical (I hate that phrase), and should extend from side wall to side wall.' I cannot picture what this sentence is telling me to do, this bracing is different than the struts between bottom chords which should extend from 'end wall to end wall' which I can picture. Can anyone explain this sentence to me, and can anyone provide any experience in the bracing situation I have as a whole?

 

[flamby]

How far are the trusses spaced apart?

 

[akastud]

24' O.C.

 

[ishvaaag]

It is just an opinion but may indicate that when feasible (in your case it would be, seeing you have your big trusses @24') the inferior chord -on account of horizontal sidesway maybe occurring?- must also be braced. For the bays that are braced, both top and bottom bracing must be brought to the walls, surely understanding the walls or other stiff devicie is placed there to pass any bracing loads to the ground.

However the "where practical" as you indicate leaves the thing loose, since many won't want what to them may look unnecessary, once all is properly accounted for.

 

[akastud]

Feasible does not necessarily mean the same thing to me abd the architect, but we all know that is not a new occurance.

 

[ERV]

Generally lateral bracing is required if there is no rigid ceiling applied to the bottom chord of the truss (perpendicular to the truss), either on top of the bottom chord, or the bottom of the bottom chord. According to the NDS it should extend to, and be attached to the end walls for added lateral support (restraint).

 

[flamby]

akastud,
Your trusses are large and any out-of-plane load may fail the trusses so the answer to your simpler question 'if' to brace the bottom chord is a definite yes.

Since the spacing of the trusses is also large at 24', I feel, it will not be possible to use single members as tie rods between the bottom chords due to slenderness difficulties. You need another trusses from each vertical of the left-truss to corresponding vertical member of the right-truss to take care of lateral loads.

Weld perpendicular gusset plates to each gusset plate at ends of each vertical member. You can then use these perpendicular plates to connect your lateral trusses. In the end you will have a grid like pattern of trusses in plan view. Remember to stiffen the right angle between the gusset plates.

HTH

 

[akastud]

Let me just say I have every intention of bracing these trusses at the bottom chord, I have even checked to see that a 5 1/8" x 13.5" glu-lam beam will suffice for the bracing member. I am more confused by what the other statement in the NDS is saying, I understand what it refers to as 'struts', but what I am curious about is the 'lateral bracing'. Wouldn't theses 'struts' serve that purpose? If you read the section I referenced above, aparently there is a difference because one should extend from 'side wall to side wall' whereas the struts should extend 'end wall to end wall', the later I understand, but the first I do not.