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Re: bridging to floor jo

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Brian Viet wrote:

. > I have heard a 2% rule for bracing -- two percent of the load on the beam
. > laterally is adequate to be called "supported laterally".  (This seems to 
. > make sense, but does anyone care to confirm/deny this?) 

It is 2% of the *compression force in the member,* not the load on the beam.  
I saw a derivation of this by Dr. Bruce Johnston using an axially loaded 
column supported by a pin on the bottom and a horizontal spring at the top.  
The force in the spring required to prevent rigid buckling came out to be 
about 2 % of the load.  I still have my notes around, but since it has been 
more than 30 years since I last looked at them, I don't know where they are 
and it would take a long time for me to interpret them.

However, I believe that the question had more to do with distributing the 
floor load between joists than it did with stability.  With the joists 
closely spaced (which they would have to be for diagonal bridging to even be 
considered), I would think that a good floor system would do more to 
distribute the load than 1.5" X 1" diagonal bridging.  (I have seen 1 X 2's 
used as diagonal bridging.)  It would seem that the bridging is there only to 
provide lateral support to the floor joists *until* the floor system is 
installed (and in that case, they would/should be designed to take 2% of the 
force in the member), much the same reason that blocking is installed in 
wood stud walls to prevent buckling of the studs before sheathing is 
installed.  (It seems that frame buildings are essentially completed before 
sheathing is installed.)

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