Horizontal Bracing at top of Pratt bridge? 1

[EngineerofSteel]

I am designing a short, 35'8" span Pratt bridge (4 @ 9').

Simple enough.

But, I am unable to locate the engineering standard for the strength requirement of the horizontal bracing at the top.

photo of a Pratt truss bridge (also 4 section) is attached.

I am asking about the X-bracing which connects the top, horizontal compression members at the connections to the vertical tension members.

How is the force into these calculated?

 

[paddingtongreen]

For bridges like that one in the photo, we used to put 2% of the maximum force in the top chord at each panel point. Each bent was a rigid frame by virtue of the internal bracing, the diagonals were tension only with the top truss of the bent taking compression as well as bending.

There was much redundancy in this but there was no way to do an accurate calculation.


Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

The picture shows modified Warren trusses, not Pratt. Pratt trusses have all the diagonals in tension.

 

[bridgebuster]

Another method is to treat the lateral system as a truss in a horizontal plane. Apply the AASHTO wind loads to the panel points; assume the portals act as supports.