Evaluation of Top Chord in OWSJ - Limit State of Buckling 2

[Deener]

Hey guys,

I'm evaluating the capacity of an existing OWSJ and am determining whether or not reinforcement will be required. Client wants to give occupant access to the roof area (additional 4.8 kPa loading). I'm using Skyciv to determine the factored loads in each member. A coworker has me referencing Example 3.1 in "Technical Digest 12 - Evaluation and modification of open-web steel joists and joist girders" which I have attached. I don't believe this example is checking the proper limit state. Hoping to get the opinion of others on the matter. Here's a bit more information:

-Top chord for my case and in the case of example 3.1 are back to back angles.
-example 3.1 determines the composite section properties of the two back to back angles along with the proposed reinforcing. These section properties are used to check for buckling of the top chord assuming an effective length equal to the panel width.

Here's my question:
The back to back angles are not continuously connected along their length (atleast the examples has no mention of this). Should we not be calculating section properties of a single angle with reinforcement and checking buckling using the minimum radius of gyration?

Thanks in advance!

 

[JoshPlumSE]

Should we not be calculating section properties of a single angle with reinforcement and checking buckling using the minimum radius of gyration?

Refer to section E6 of the latest AISC manual. That's the section for built up shapes. It gives you formulas to adjust the L/r of the built up member based on the spacing between connectors.

 

[DaveAtkins]

The top chord cannot buckle sideways since it is braced by the deck. In my opinion, that means it is also braced against buckling in a diagonal direction, so rz will not control.

I would check it for buckling up and down, between panel points, and use rx.

DaveAtkins

 

[KootK]

Should we not be calculating section properties of a single angle with reinforcement and checking buckling using the minimum radius of gyration?

Generally no. As Josh implied, enough interconnection is usually supplied between the angles to preclude single angle failure modes. This is often provided in two ways as required:

a) the natural interconnection that will occur at webbed panel points and;

b) stich plates provided between the angles between panel points.

The top chord cannot buckle sideways since it is braced by the deck.

This is complicated by the fact that one can usually not expect both angles to be fastened to the deck. Or, at the least, not as far as I know. When this is the case, the limiting unbraced length for the individual angles becomes the spacing between the points of interconnection between angles for vertical, horizontal, and torsional buckling, regardless of the deck fastening spacing.