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RE: Golf Course Bridge Design & AASHTO

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Ed Fasula wrote:


You mentioned a 60 deg maximum angle for "pony" truss bracing.  Is this an
AASHTO recommendation?  I'm curious where that comes from... is the concern
addition of tension to the truss due to the higher vertical component of
force in the brace causing a higher reaction at the deck beam/truss
connection?  It seems that the nominal lateral bracing force could be
resisted effectively at a steeper angle.



I thought that at one time, AASHTO had the 60 deg. to horizontal as a 
maximum, but right now I can't find it.  However, I think that whenever you 
have to brace a horizontal force, that 60 deg. to horizontal should be a 
reasonable limit.

The brace is to prohibit buckling of the compression chord, either inward, 
where the brace would be in tension, or outward, where the brace would be in 
compression.  The brace has to resist the buckling force axially and as the 
angle becomes steeper, the force component along the brace has to increase, 
and it increases in a non-linear manner.  Buckling is going to control the 
design of the brace.  If the brace makes a 45 deg. angle with horizontal, the 
force in the brace is going to be 1.414 times the horizontal force; at 60 
deg. it will be 2 times the horizontal force; and at 75 deg. it will be 3.86 
times the horizontal force.  Some compromise has to be made.

If you weren't using salvaged trusses, the verticals could be designed to 
have a rigid connection with the floor beam and provide the necessary bracing 
that way.

Hope this helps.

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