Diaphragm Chord Forces

[ars001]

Here's my deal. I basically have a square steel framed building with a metal deck. On two sides of my building(about in the middle of the building) the outside of the building jogs in approximately 2'-0". Therefor my chord forces are broken or my continuous load path is broken. Do I try and provide some sort of flat diagonal plate from one chord member to another? Or, do I justify my diaphragm as rigid and forget about my chord forces?

 

[jike]

The approach that I would take would depend a little bit upon which way the secondary framing runs. If secondary framing is parallel with the chord can you just use that framing as the chord and let the outside line go along for the ride? Or, if the secondary framing is perpendicular to chord, then you may just have to create diagonals to carry the chord force inward toward the cutout to carry it around the discontinuity. I probably would use an angle or WT in lieu of flat bar since the chord may have to carry a compressive force depending upon which way the wind is blowing.

 

[msquared48]

I would set the chord tie 2' in on the projections, in line with the interior portion.

Mike McCann
MMC Engineering

 

[JKStruct]

Essentially, you have a non-prismatic diaphragm. Another option is to design one set of chords for the full diaphragm depth, and design a second set of chords aligned with the inset that that run parallel to and overlap the first set for a sufficient distance to transfer the axial forces -- similar to the way you would reinforce a steel beam with a web penetration. I'll try to "dash" a schematic, but I'm not sure how it'll turn out. The "-" is a chord.

|--|=> overlap

----------| |----------
| ---------------- |
| ---------------- |
----------| |----------

Make sure you calculate your maximum diaphragm moment and ensure your chords can take the load.

On another note, "forgetting about chord forces" implies your diaphragm is infinitely rigid... which cannot be the case. If you have a metal deck acting as your diaphragm, consider the diaphragm deflection vs. story drift for the rigid vs. flexible determination. In any event, your diaphragm will flex to some degree.

As an aside to everyone... wouldn't it be nice to have a simple AutoDesk (or similar) plug-in on eng-tips? Nothing fancy... just the ability to draw lines, basic polygons, leaders, and text. The advent of uploading files was a tremendous improvement -- now if the powers-that-be gave us the ability to draw in the message box, wow... I could probably just go out on my own and rely on my 650,000 strong e-office for advice and reassurance.



Best Regards,

JK

 

[ars001]

Thanks for all the great replies. Unfortunetly my sub framing runs perpendicular to my chord framing, which makes things a bit harder.

JKStruct, Is it safe to say if I meet the criteria in the ASCE7-05 Section 12.3.1.3 and Fig. 12.3-1 that I can assume the my metal deck is semi rigid? The kicker in the whole deal is can I really be in section 12.3.1.3?

 

[haynewp]

I think the same as jike on using the diagonal to connect the chords. Make sure all the local forces are resolved around the diagonal.

JK: You can draw something in AutoCAD and plot it directly to pdf using PDF995 which is sponsored free. Then upload the pdf. Pretty easy.

 

[ars001]

Thank you all for the informative posts. After looking at it a bit and actually seeing what sort of chord forces I have which were very minimal. I've decided do as JKStruct suggested and overlap my chords.