Steel Bar Joist, Maximum Bridging Force Clarification 1

[SRO]

I'm trying to design a new roof system with the Steel Joist Institute specifications, and need to design the connection from the bridging to the wall. However, I'm a little confused on my maximum bridging force is.


-For a 14K6 joist, Table 5.4-3 gives a Pbr = 340 lbs., which isn't effected by angle size or spacing of bridging.

-For joist spacing of 4'o.c., & a 1-3/4"x7/64 angle, Table 2.7-1c gives a maximum Pbr = 2,960 lbs.


What do I use to design the connection to the wall? Unfortunately I can't even go back & use the equations given in the specifications since they are all essentially based on the radius of gyration for the top chord of the 14K6 joist, which I can't find anywhere. Can anyone clarify this for me and/or provide me with advice on how to proceed?

Thank You.

 

[SandwichEngine]

Not sure how helpful this is but you can't find the radius of gyration because the joists are different every time. All a joist manufacturer has to do is provide a joist that meets the depth, moment and deflection tables listed in the SJI for that particular joist. One day, they may make a 14K6 joists with rod web members. Bent cold form pieces the next day. Top and bottom chord sizes change accordingly.

What you can do though is back calculate the required moment of inertia from their deflection tables. You could use this to calculate what sized angle pairs for top and bottom would give you that moment of inertia with top and bottom each being 7" from the centroid. By neglecting the web members, you should be conservative. Then you've got your I and A to determine a rough radius of gyration.

Also, not able to open your document.

 

[phamENG]

SRO - there's a lot that goes into that beyond the two pages you posted. If I remember correctly there's 10-15 pages on various bridging requirements for construction loading, in service loading, etc.

To those two tables specifically - I believe the singular force is the minimum strength of the connection of the bridging angle to the joist chord. The other table has to do with design of the bridging angle itself and connection to the wall. For exactly how you use each one, you have to read the text.

 

[SRO]

SandwichEngine; I love the thought, but I need the radius of gyration for the weak axis "ry", and there is no posted deflection tables. Any other brilliant thoughts?

phamENG; All I see SJI 100-2010 is Section 5.4 (5 pages mostly tables), Section 104.5 (3 pages), Section 2.7 (6 pages mostly tables). I've heard about Technical Digest #2 from SJI which covers everything about bridging, but I can't find a copy of it anywhere, including on their website. Is that what you are referring to?

 

[SandwichEngine]

SRO,

Without weak axis deflection information, I'm don't see how you could come up with anything.

Maybe look at it this way. The bridging is just nodal bracing per AISC Appendix 6. Wouldn't be hard to calculate the bottom flange force and use the 1% rule.

 

[phamENG]

It's not quite so cut and dry. On one hand, SandwichEngine is right that you won't know what the actual chords are until the joist is installed. But I think they are mistaken when they say there's nothing you can do - or that you should use appendix 6.

You need to understand the entire joist specification (SJI 100). It forms the basis for all the load tables, bridging requirements, etc. Every joist manufactured by an SJI member company has to meet it.

So you can determine the minimum r[sub]y[/sub] for the chords based on the specification which defines it as a function of depth and span length. The lower the radius of gyration, the lower the buckling load and greater the bridging requirements.

From that minimum r[sub]y[/sub], you can use the specification to determine the minimum bracing load.

 

[SRO]

phamENG; The only equations I see are 4.4-1(a-c), and 5.4-1(a-c), but they are the same thing except one solves for the radius of gyration, and the other is solved for brace spacing. Am I missing something? I have been watching webinars, scouring the web, to try and figure this out and there is nothing.

 

[phamENG]

For starters you're using a spec that's 10 years old. You'd be better off getting the 2020 version as it has expanded tables and more explanation and includes new research.

Did you find this presentation? What about this one?

Take a look at the code of standard practice, tables 2.7-1 and 2.7-2 (I'm looking at the 2020 version, which is free on steeljoist.org)

Table 2.7-1. If you study the equations in the specification, you'll see that there's a direct link between the radius of gyration of the chords (or more generally, the out-of-plane axial stiffness of the chords) and the the required strength/stiffness of the bridging. If you need more info on that, google the name "Yura" or read the AISC commentary for appendix 6 which is largely based on his work. So this tables gives you a nominal P[sub]br[/sub] to use that will satisfy the brace force calculation for any joist manufactured to the specs.

Table 2.7-2. This gives you the maximum bridging force you can have in each angle size based on the unbraced length of the bridging angle (spacing between joists). This is important because bridging forces are cumulative. The importance of the accumulated bridging load will depend on a number of factors, like attachment to a roof diaphragm, construction sequencing, etc. So if you have joists that are depending on top chord bridging for in service strength (if they have standing seam directly fastened or some covering that doesn't brace them sufficiently), a design snow event could push all of the top chords past their unbraced axial buckling load and every joist will load the bridging line. If that happens, you have to make sure all of the brace loads get resolved at the anchorage. A similar situation could happen during construction when they stack bundles of roof deck. Depending on how they put them up, you could end up with loads in bridging coming from multiple joists. Since you don't usually have control over that, it's often best to assume the worst case.

 

[SRO]

Thank you both for all the helpful input on this one.