Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Golf Course Bridge Design & AASHTO

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Ed,

A7 steel was essentially replaced with A36 in the 1960's and the spec was 
discontinued in 1967.  The 1965 ASTM spec I have lists the following 
mechanical properties:

Tensile Strength:

   Shapes of all thicknesses:          60,000 to 75,000 psi
   Plates and bars <= 1.5 in. thick:   60,000 to 72,000 psi
   Plates and bars > 1.5 in. thick:    60,000 to 75,000 psi

Yield Point, min.                      33,000 psi
Elongation min. in 8 in.               21 percent
Elongation min. in 2 in.               24 percent

While A7 steel has been welded successfully, the spec does not have any 
limits on carbon content and that was one of the big reasons to switch to A36 
steel.  (36 ksi is not much different than 33 ksi.)  I think that careful 
evaluation of the compatibility of currently used electrodes with the A7 
steel would be appropriate.  Are the existing trusses welded or riveted?

A pony truss is a shallow thru truss (typically a parallel chord Pratt Truss) 
with the roadway surface at or near the bottom chord and the top chords not 
connected.  Top chord bracing is usually accomplished by extending the 
roadway beams (at the panel points) beyond the plane of the trusses and 
installing diagonal bracing from the extensions up to the top chord.  The 
angle between the bracing and horizontal should not be greater than 60 
degrees.  If your bowstring trusses spanned 100 ft. and conforms to the depth 
to span ratio of 1:8, the trusses should be about 12 feet deep.  (If the 
depth to span ratio is less, then you will not have uniform force in the 
chords and deflection might be a problem.)  For a 60 degree angle, your floor 
beams would need to extend out about 7 feet on each side of the truss at the 
point of maximum depth and the bracing, itself, would have an unbraced length 
of almost 14 feet.  While pony truss compression chords are designed to be 
laterally unsupported between panel points, I would doubt that a building's 
bowstring truss compression chord was so designed and would need to be 
investigated.  BTW, L/r limit for steel construction (buildings) in 1950-60 
was 120 for main members, 200 for secondary members.

BTW2, AASHTO specs only recently became essentially the same as AISC specs, 
with AASHTO previously being considerably more conservative.

Hope this helps.

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

Ed Fasula wrote:

>>   >HOWEVER, if these bowstring trusses (steel or wood?) were
   >salvaged from a *building,* the compression chord probably was
   >"continuously" braced...Bracing would probably have to be installed
   >similar to the way compression chords of "pony" trusses are braced.

Sorry to omit that the trusses are steel.  The Forensic Engineer said that
since they are pre-1950, Fy of 30 ksi would be must likely.  I assume he was
thinking of A7 which has an Fy of 33 ksi (I don't mind being on the safe
side though).

I am surprised that A7 is not listed in the oldest manual (~1970) we have.
I would like to know what the value of Fu was if someone has it.
Furthermore, are there special considerations for using A7 such as
weldability?

I don't know what "pony" trusses are, but what we are thinking of doing is
running outriggers from the deck stringers at with a diagonal panel points
(10'-0" o.c.).  If this does not pan out in analysis, I was thinking of
building up the members.  I would be interested to know what type of "pony"
truss bracing you are referring to.

   >Hope this helps.
   >A. Roger Turk, P.E.(Structural)
   >Tucson, Arizona

It does, and I'd like to thank everyone who has responded and pointed out
several issues of substance.

Ed Fasula<<