Truss Design - Modeling and Chord Unbraced Lengths 1

[Simran7]

Designing a truss (~125ft) using ETABS. Need some clarification/confirmation on assumptions. The truss supports concrete roof/deck. In ETABS, I have top and bottom chords are modeled as short elements spanning between panel points (understand it is not necessary but for ease of load application did it this way). My query is related to defining unbraced length (effective length factor) in ETABS such that the software identifies the chords as continuous pieces instead of small elements (as modeled). Please advise if following effective lengths are correct assumptions.

Top Chord
In Plane: between panel points
Out of Plane: Continuously braced (concrete deck)

Bot Chord
In Plane: between panel points
Out of Plane: bridging spacing if allowed, full length otherwise

Bottom chord end connections provide restraint against vertical shear and hold in position against out of plane movement. Intent is to eliminate/reduce compression in bottom chord due to full length being considered for buckling. Appreciate your input.

 

[Simran7]

Truss Geometry

 

[ProgrammingPE]

Programs usually automatically assume that the effective length of a member is its full length, so by modeling the top and bottom chords as short segments between panel points, the program is assuming that the panel points are fully braced. It looks like you're already aware of this, but it's worth noting that you have to manually set the effective lengths otherwise the model is very unconservative.

The panel points would brace the top and bottom chords in-plane. The top chord is braced out-of-plane by the roof deck. I would use some value like 3 feet, though, instead of continuous to account for the spacing of the screws or puddle welds also accounting for if they miss some. The bottom chord is only braced out-of-plane if you add bridging, so this is the key effective length you'll need to manually set. It should be the distance between the bridging, assuming the bridging meets the stiffness requirements.

Will your bottom chord end connection actually be detailed so that no axial force is transmitted? Checking the truss with the bottom chord axial force released and also not released would make sure you're covered for however it actually behaves.

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[Simran7]

@ProgrammingPE - Thanks for the reply. Intend to specify long slotted holes (LSL) to release axial force for bottom chord connection. With the forces I have on the truss, normal strain is within the range of movement tolerances in LSL holes.

But for worst case scenario, if the bolts are already in bearing against the hole and don't have any room to slide, would check the bottom not released also. Just to cover, like you said.