Why do engineers flip the last diagonal in trusses?

[gharli]

Hi All,

Please can someone shed some light on a peculiarity with regards to pratt trusses.

I've often seen the last diagonal on either end flipped, i.e. instead of it being in tension it is forced to be in compression.

What is the rational behind this?

Thanks.

 

[human909]

Probably because that they want the load to a particular support. Thus your design a load path to achieve your intention.

Eg there not flipping the last diagonal in this example would require 2 extra members on each end.

 

[1503-44]

If not flipped, the top chord and the vertical end member would both be in compression, so no real gain in load carrying efficiency there and, as human909 implies, eliminating both of those members probably costs less.

I want to know why the diagonals change direction as you cross the centerline.

 

[rowingengineer]

We normally continue the top cord down not the angle being flipped if the compression is high.

Normally you try to keep the diagonals in tension in the worst case loading, and only have them in compression in the reverse load case.

 

[1503-44]

Yeah, but that only happens when the truck is at the middle of the bridge.

 

[gharli]

Thanks, I get all that, but I've often seen the top chord continuing to run horizontally (not omitted), and the end diags flipped. See below.

It also is not limited to mid span situations where there is continuity across the support, like below. I've seen at the end supports as well.

It seems to be popular amongst conveyor gantry designers...

 

[Lomarandil]

I do it sometimes to ease the connection design at the support node, where shear/diagonal demands are the highest.

This logic mostly makes sense when some degree of diagonal-to-support bearing is assumed. Depending on tolerances, scale of strains, ductility and acceptable deformations, etc., that may not be a valid assumption.

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just call me Lo.