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Brace Free Trusses?

Brace Free Trusses?

Brace Free Trusses?

(OP)
Some local truss companies are offering "brace free trusses." Is this possible? It looks like they are adding a 2x4 block to the compression members to eliminate a brace. But if memory serves me correct, 1.5" thick lumber in compression has a length limit that requires a brace no matter how wide the lumber is. Has anybody seen this or questioned this?

RE: Brace Free Trusses?

Is the "2x4 block" full length of and nailed to the compression member?  If so, that is an L or T brace.  Bracing out of plane is now as strong as in plane.

RE: Brace Free Trusses?

It's possible to add a member to the compression diagonals, as structuralaggie noted, but I would check the costs... the added material would have to offset the labour costs for and material for providing the horiz bracing.  It might be an issue for longer span (deeper) trusses.

Dik

RE: Brace Free Trusses?

Check out http://www.allpan.com/res/SpecialtyBracing.pdf to Alpine's Web-Block

Also, sometimes bumping the web material grade from SPF #2 up to MSR can eliminate the need for bracing.  Lumber costs a little more, but saves the contractor time in the field.

RE: Brace Free Trusses?

(OP)
The requirement for web bracing on compression members is also dictated by the slenderness ratio (I/d ratio) of the member. Where I, is the effective length (actual length divided 0.8) and d is the depth (1.5" for 2x members). This ratio must not is exceed 80. So, for compression web
members, the maximum unbraced length is 93.75", regardless if it is 2x4, 2x6, 2x8, etc., or if it has a Web-Block applied to it. This requirement is specified in Chapter 3 of NDS-2001 and is referenced by the ANSl/TPl 1-2002 truss design standard. So in other words, the Alpine web block does not work on webs over 93.75 inches long, and a brace is still required. Yet I have drawings with webs over 132 inches long in compression, with the web block added, and no brace is on the engineering.

RE: Brace Free Trusses?

Isn't the maximum slenderness ratio 50? NDS 3.7.  I think the limit is actually 75".  

I am not sure what you are refering to as a web-block, but if it is a 2x nailed flat to the compression member and full length of it, the max length would be 50*width.  So 175" for a 2x4.  If the member is is bracing is also a 2x4, then 175" is also the limiting slederness length of it.
 

RE: Brace Free Trusses?

I is effective length (actual length {Lu} TIMES 0.8)

I=0.8Lu

I/d < 80

I < 80 * d (or 80 * 1.5")
I < 120"

I = 0.8Lu < 120"

Lu < 120" / 0.8 = 150"

Looks like the limit is 150" so 132" is OK

RE: Brace Free Trusses?

TPI 1-2002 7.3.6 limits compression L/d to 50 and tension to 80 (including tension members subject to stress reversals from short term loading).  so 93.75" would be correct for compression web and 150" for tension.  


See the link i posted above to see what Web-Block is.

RE: Brace Free Trusses?

Sorry, I didn't see the link before.  I don't see how that accommodates the NDS slederness requirement either.  Intuitively though the slenderness requirement is really just a guide line.  Columns that exceed this do have some capacity, but the equations given in the NDS should not be used to estimate it.  My guess is Alpine has done some testing to verify that their block works.

RE: Brace Free Trusses?

(OP)
I am really starting to worry. I just reviewed an entire structure. There are 115 webs in compression that have the Web-Block attached. After some calculations (I was bored) I figured 113 of the webs still needed a brace. Some webs exceeded 160" in length. I based my calcs as if the braced web was a 2x8 member. (Actually a 2x8 exceeds the web block values, but lets give them something.)It looks as if the Alpine guys are treating this member more like a 3x8 web member. Could the plates be adding additional value? I have a call in to Alpine.

RE: Brace Free Trusses?

L/d for construction is 75, but the link to that pdf you posted didn't say anything about construction bracing.  I think those guys might have been smoking crack if the compression members are as long as you say they are.

I looked at a hotel that was about 5 years old that had insufficient bracing. Several compression members were braced horizontally to each other - they ALL buckeled together.  The roof didn't fall down because of the interior partions.

I would be nervous too.  Are you the structural engineer for the job?  

RE: Brace Free Trusses?

(OP)
Okay, just to fill you guys in. Alpine says the TPI rule applies to dimensional lumber only, and not to their web-blocks. They mentioned that there has been a request to update TPI as the formula is based on a 2x4 retangular section. Pretty loose interpretation.

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