joist bridging design for uplift

[sonnyson]

I am evaluating existing roof joists that will be subjected to large net uplift loads due to inclined solar panel arrays being attached on the roof. I calculated the axial compressive stress in the bottom chord,the lateral r value of the double angle bottom chord, and determined a KL/r value for that maximum stress. This enabled me to determine a spacing for the bridging. I realize this is conservative, since the compressive stress varies across the span length (zero at ends and max at center). Has anyone come across a methodology to account for the varying load?

 

[msquared48]

You speak of roof joists (rafters) which means solid sticks to me, but later, you also speak of "bottom chords" which makes me think trusses.

Do you have roof trusses and not rafters?

Mike McCann
McCann Engineering

 

[hokie66]

I think he means open web "steel joists", as you folks in the States call them. Now that I have offered that opinion, I will leave the advice to people who work with steel joists.

 

[msquared48]

You're probably right...

Mike McCann
McCann Engineering

 

[sonnyson]

Sorry,
These are open web steel bar joists

 

[frv]

The Cb factor in AISC accounts for the variation in moment between brace points. Although I am unfamiliar with how this would translate in ASD.

 

[Lion06]

I would also check the bottom chord for in weak axis bending to brace the compression diagonals.

 

[UcfSE]

If in the US, you should be using the SJI specifications, not the AISC. Have you checked the SJI spec for what you want?

 

[vincentpa]

Go to SJI and download the SJI Digest on modifying open web steel joists and joist girders. I think it is only $25. Also, Vulcraft has a book by Fisher on Steel Joists and Steel Deck that everyone should have. It's only $15. Typically the manufacturer designs the bridging for new buildings. All we do is require SJI bridging and put the loads on the drawings. SJI also has a "Digest" on designing bridging for I believe $25. You can also call the manufacturer of the joist you are analyzing and ask for help. They are usually very helpful.

 

[sonnyson]

I have ordered the SJI guide for bridging design. I was wondering if anyone had any comments or thoughts on my approach to determining the bottom chord bridging spacing, since it is conservatively based on the maximum compressive load in the bottom chord assuming it is constant along the chord. In reality, it varies parabolically from zero to max value at center span

 

[vincentpa]

If you take the maximum, you are being conservative and that is good. Load on joists change over time so conservatism is good with them.