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RE: Question on wood Roof Trusses

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Neil Moore wrote:

>>Roger:

Either you missed the point or I didn't explain it well.  Web bracing
suggested by the truss manufacturer is usually using a 1x4 or a 2x4
connecting a series of webs together.  Now if the ends of these
1x4's(2x4's) are not connected to anything else, when the failure happens -
they all fail at once.<<

Neil,

I think that there might be two of use that may be missing the point or not 
explaining it well.  Never did I suggest that the web bracing not be 
connected to anything.  What I said was that web bracing can be 
self-equilibrating.  Let's see if I can illustrate:
      
    /--CL slender member
   |   Connecting strut             Braced Bay
   V   |                         <----------------->
   |   |         |              |                   |              |
   .   V         .              .                   .              .
===|=============|==============|XXXXXXXXXXXXXXXXXXX|==============|===
   .             .              .X  X           X  X.              .
   |             |              |X     X     X     X|              |
   .             .              .X        X        X.              .
   |             |              |X     X     X     X|              |
   .             .              .X  X           X  X.              .
   |             |              |XXXXXXXXXXXXXXXXXXX|<---Bottom    |
   .             .              .                   .     Chord    .
   |             |              |                   |              |

The braced bay is rigid.  It will not permit the braced point to displace 
horizontally and a series of like slender member bracing forces can be 
dragged into it.

The bracing is internal and therefore does not require the base structure for 
an external support.  Since it is an internal force, there is no increase in 
the truss reactions.

NM> By the way Roger, the L/d of a 1x4 connecting webs at 24" on center is
NM> about 32; I sure that that's good for something.

As the Euler buckling formula does not apply to steel plates subject to 
compression, I would imagine that the same thing would apply to a 1X wood 
member.  This is in addition to boards not being rated for structural 
performance.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona