Emt support

[gadfly56]

i'm feeling stupid at the moment, but i'm assuming you are talking about combo truss with wood stringers
and thinwall tubing for the lattice.....:dunce:

i live in earthquake land, and pretty much everything here gets hung from the bottom stringer.
where you have an all steel truss, made of back to back angle iron for the stringers, with angle
iron at a 45 degree angle for the segments, dropping running thread from the lower one is the
norm.... and not trivial loads, either. half inch rod, and racks of 4" EMT, and a beam clamp is
what is used around here. the only time i've used the top, is if i throw a piece of unistrut across
the top of the truss, in the cavities of the robertson decking, to drop a rod in a specific place.
and i'll seldom use a beam clamp, just use square strut washers between the angles of the bottom
stringer, and put the rod in between them.

i've never had a do over in 40 years of this... :huh:

We call that "bar joist" construction back east. I've never understood people requiring supports from the top chord of the assembly. Mechanically the joist doesn't care if the load is on the top chord or bottom chord. The only thing I can think of that might be an issue is if the load set up a bending moment perpendicular to the long axis of the joist. Most joists I'v seen of this type are structurally attached at the top chord, and it might (emphasize "might") make a difference then.

 

[Carultch]

The only thing I can think of that might be an issue is if the load set up a bending moment perpendicular to the long axis of the joist.

That's precisely what a bending moment is by definition. If the internal load in units of foot-pounds is parallel to the long axis, it is a torsional torque. The axis of moments and torques is perpendicular to the rotation such loads would cause, by convention.

Bar joists are built to withstand bending moments in the horizontal axis perpendicular to the length, due to loads that are either distributed, or applied immediately nearby the joints. This is usually what comes from vertical linear loads, like weight. Bar joists are much less effective at withstanding torsion and vertical axis bending moments.

In any case, I would think that the structural integrity is an issue for the structural engineer of the bar joist to approve or reject, and not an issue for the NEC.

 

[gadfly56]

That's precisely what a bending moment is by definition. If the internal load in units of foot-pounds is parallel to the long axis, it is a torsional torque. The axis of moments and torques is perpendicular to the rotation such loads would cause, by convention.

Bar joists are built to withstand bending moments in the horizontal axis perpendicular to the length, due to loads that are either distributed, or applied immediately nearby the joints. This is usually what comes from vertical linear loads, like weight. Bar joists are much less effective at withstanding torsion and vertical axis bending moments.

In any case, I would think that the structural integrity is an issue for the structural engineer of the bar joist to approve or reject, and not an issue for the NEC.

Bending moments can be either in-plane or out-of-plane. The usual bending moment (in-plane) is the point or distributed load placed somewhere on the top or bottom chord and implicitly symmetric about the long axis. If you were to cantilever a load off the joist perpendicular to its long axis then you have an out-of-plane bending moment, and now the ability of the joist to resist that moment could be different if the load is off the bottom or top chord.

 

[mwm1752]

Bending moments are referred to a "deflection"in these here parts

 

[gadfly56]

Bending moments are referred to a "deflection"in these here parts

While bending moment can cause deflection, you can also get deflection from shear loads.

 

[Carultch]

Bending moments are referred to a "deflection"in these here parts

Bending moment is the cause, deflection is the effect.

What you are telling me, is "bending moments are known to be in the same direction as deflection in these here parts".

 

[mwm1752]

Yessir -- Most framing under girders & prestress beams have allowances for movement, & top track for metal studs is not attached to the sheetrock to compensate for deflextion -- building movement under live conditions & your creates deflection would most likely be center point between supports of the lateral support - it would be an active bend under all sorts of live conditions throughout the life of the building.