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The Effective Compression length of Pony Portal Truss internal members

The Effective Compression length of Pony Portal Truss internal members

The Effective Compression length of Pony Portal Truss internal members

(OP)
Hi EngTips team,

SOME BACKGROUND:

In a pony portal, the top chord acts as the main compression element resisting vertical loading. The bottom chord acts as the main tension element. The internal (vertical and diagonal) members, of each truss, act to transfer loading to achieve equilibrium.

Unlike many other truss types, the top chord of a Pony Portal truss is not directly restrained. An inverted portal U frame (a Pony Portal) [comprising of the bride's transom and vertical posts] acts to restrain the bridge at intervals. Unlike restraints that work in tension or compression, these restraints are highly flexible. Because the Pony Portal restraint is flexible, under high compression loads, the top chord deflects to an effective length that is smaller than the length of the chord but larger than the spacing of the pony portals that act to restrain it. If all members are sized correctly, the primary failure mode of the bridge under vertical loading is the buckling of the top chord.

MY PROBLEM:

I have begun designing Howe Truss, pony portal bridges over the past year. In design, my superior has asked that all connections apart from the portal connection [between the structure's transoms and vertical posts] be considered pinned. Under this assumption, what would people assume is a reasonable effective length for the diagonal members in compression?


MY VIEWS:

Under the critical vertical loading, the frame of the bridge deflects inwards, towards the center of the bridge (much like a sway frame). The bottom chord, that the diagonals will connect to, will deflect in response to the pony portals' flexural restrain of the structure's top chord.

Should the compression effective length of the diagonal compression members be in accordance with sway action (ke = 2.2) or does the assumption of pinned end connections make more sense (ke = 1.0 )?

I could determine whether both ends of the diagonal internal members could be assumed as fixed. This would dramatically reduce the effective length of the member in my local code (NZS 3404 - 1997) but this could be more effort than what is warranted.

Happy to hear all critiques. Just another Graduate trying to figure it all out.

Cheers.





RE: The Effective Compression length of Pony Portal Truss internal members

I'd have thought that the (small) rotation wouldn't affect the diagonals. The compression is due to truss action rather than a gravity load applied at the top so stays in the truss plane. Pick your favourite effective length factor from 0.7 to 1.0 from guidelines.

Quote:

If all members are sized correctly, the primary failure mode of the bridge under vertical loading is the buckling of the top chord.

Why is this an outcome of correctly sized members? Why not yielding of a tension member? I prefer to have a larger margin on buckling failurr than section failure.

RE: The Effective Compression length of Pony Portal Truss internal members

Quote (steveh49)

I'd have thought that the (small) rotation wouldn't affect the diagonals. The compression is due to truss action rather than a gravity load applied at the top so stays in the truss plane. Pick your favourite effective length factor from 0.7 to 1.0 from guidelines.
I agree. What matters here is the truss plane to which the diagonals presumable are staying aligned. The rotations needed to start significantly affecting the diagonals would have long started to cause issues to the top cord.

To put it another way. The sway frame assumption assumes the point of axial load on a member is allowed to sway away from the line of axial action with respect to the system being considered. The system being considered is the truss and you effectively have a coordinate system rotation as the top cord sways.

RE: The Effective Compression length of Pony Portal Truss internal members

(OP)
Thanks for these responses!

They are really helpful.

Good point about tension failure over buckling failure steveh49.

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