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.
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.
