A Pennsylvania style Petit-Pratt truss has curved cords. See the various bridge construction methods at this site:
http://pghbridges.com/basics.htm
"The Parker truss is named after Charles H. Parker who patented the design on February 22, 1870. It is a variation of a Pratt truss but differs from the Pratt in that the top and bottom chord are not parallel."
http://bridgehunter.com/category/tag/parker-truss/
Since model railroaders generally tend to be rather picky about prototype "correctness" and "getting it right", I thought posting a topic on this forum would be better than sending a private email, as here it will get more attention, and private emails can languish in someone's inbox. If anyone was offended by my choice, I do apologize.
John Mock, P.E.
P.S. By increasing the depth of the middle spans of the bridge, the load carrying capacity is increased. It was not just purely for aesthetics.
Paul Mallery's book Bridge & Trestle Handbook pages 50-51 illustrates the Pratt with the straight top chord, but then goes on to say that with longer spans needing greater depth, "on through bridges this is often accomplished by curving the top chord as shown by the curved-chord Pratt on the left in Fig. 3 (emphasis added). This type of truss has been called a 'camel-back' or 'Parker.'"
Interestingly on page 52 Mallery shows what he calls a "Petit truss with the three end panels as Pratt" -- and it has a curved top chord as well.
Dave Nelson
The ad for the bridge is on the right side of the MR Forum, in case you haven't seen it. The ad goes:
"CORNERSTONE Beautiful HO scale Arch Pratt Truss Bridges," then a pic of a silver Pennsylvania truss bridge, then "Coming soon!"
It looks to be a single-track bridge.
Maybe the OP was trying to point out an error to forum members, and possibly reach Walthers at the same time.
PRR8259 To whom it may concern: In professional civil engineering a through truss bridge having a curved or angled top chord is called a Parker Truss. A Pratt Truss by definition always has a straight top chord. Respectfully submitted. John Mock, P.E.
To whom it may concern:
In professional civil engineering a through truss bridge having a curved or angled top chord is called a Parker Truss.
A Pratt Truss by definition always has a straight top chord.
Respectfully submitted.
Rich
Alton Junction
PRR8259A Pratt Truss by definition always has a straight top chord.
Almost always.
https://en.wikipedia.org/wiki/Fair_Oaks_Bridge
Could you post a link to the offending advertizement so we can see the error of their chords? I appreciate the information...
Thanks, Ed