AASHTO Limiting Slenderness Ratio for Tension Members

[BridgeEngineer21]

I have a question about AASHTO 2020 Section 6.8.4, snip below for convenience:

I am designing a temporary tower for support of bridge erection. The tower will be a truss tower consisting of HSS members for the primary longitudinal legs and lateral bracing, and with thin diagonal lattice plates intended to act in tension only for out of plane load stability. Crude sketch below:

In my analysis model I have set the plates to have a compression limit of 0, i.e. they are only active in tension. They pass all design checks since the lateral loads on the tower, and resulting tension on the plates, is relatively small. However, they fail the limiting slenderness ratio check since r in the thin direction is tiny. However, I'm not sure the intent of the first sentence of the AASHTO clause above - does it mean I can just ignore this slenderness limit for plates acting in tension only? Has anyone dealt with this before?

 

[BridgeSmith]

I think the reason cables, rods, bars, plates, etc. are exempt, is that buckling due to unintended axial compression or out-of-plane forces (wind, etc.) are either not possible or are inconsequential to the tension capacity (only minor yielding occurs and fatigue is not a consideration).

By a strict reading of the spec, I would agree with your conclusion. However, if it were me, I'd strongly consider using angles that would be effective in tension and compression.

 

[centondollar]

Tension-only lattice members are "plates" or "rods" described in the spec. If they never see compression, they are not subject to buckling checks, i.e., not subject to slenderness checks. Also consider that if lateral load is reversed (wind and movement of superstructure can act in almost any direction), the top diagonal (and other diagonals) will be in tension and the horizontals will be in compression, making them prone to buckling.

Using HSS profiles for all members and welding the joints would seem a more obvious choice to me.