Chord Forces

[Yousef ZAA]

Can somebody explain for me chord forces in diaphragm comprehensively .

 

[KootK]

Here's an article to get you started.

For a "comprehensive", explanation, you'll probably need to do some self study. The book by the author of the paper that I referenced is excellent in that regard.

Diaphragms act as lateral beams of sorts in their role of resisting in plane loads applied to the diaphragms. As such, they need flexural capacity in plane. That is often provided discretely by "chords" that act like the flanges of a beam or the top and bottom chords of trusses. An important part of the theory that I almost never see expressed explicitly is the "shear panel assumption". For diaphragms of wood sheathing, untopped steel deck, and sometimes even concrete, we routinely make the assumption that the diaphragm material is incapable of resisting in plane tension and compression forces (shear resistance only). And that creates a demand for collector/chord elements to handle the in plane tension and compression.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Yousef ZAA]

KootK, Thank You.

 

[HouseBoy]

(Not to hijack the thread but since we're on the subject.....
I get the impression that diaphragm chords are VERY often overlooked in either design or construction/installation but somehow we don't seem to have too many serious issues with chord failure.

Are my perceptions (overlooked in both design or installation AND not too many problems) correct?

Do you suppose the "shear panel assumption" (or the inaccuracy of it) is what may explain the generally good behavior of horizontal diaphragms?

Are there published case studies of how a lack of chords has caused poor building performance?

Does anyone provide guidance on how to assign T and C faces to the diaphragm itself?

Just wondering

 

[dik]

Not overlooked... often for irregular roofs on multi-storey work, I consider a conservative approach to a portion of it, and check that... generally OK since loading is minimal.

Dik

 

[XR250]

(Not to hijack the thread but since we're on the subject.....
I get the impression that diaphragm chords are VERY often overlooked in either design or construction/installation but somehow we don't seem to have too many serious issues with chord failure.

Are my perceptions (overlooked in both design or installation AND not too many problems) correct?

Do you suppose the "shear panel assumption" (or the inaccuracy of it) is what may explain the generally good behavior of horizontal diaphragms?

Are there published case studies of how a lack of chords has caused poor building performance?

Does anyone provide guidance on how to assign T and C faces to the diaphragm itself?

Just wondering

ds
I think what ends up happening is the building decides it wants to be a series of three-side structures that do not require chords or, as you stated, the "shear only" assumption is flawed.

 

[KootK]

Are my perceptions (overlooked in both design or installation AND not too many problems) correct?

Absolutely yes on both counts. Someone here once showed me a photo of an OSB diaphragm re-entrant corner that got a bit roughed up in a hurricane, that's it for problems.

Do you suppose the "shear panel assumption" (or the inaccuracy of it) is what may explain the generally good behavior of horizontal diaphragms?

Yup. That, combined with a couple other things that often take place in certain kinds of buildings:

1) you get unintended shear wall behavior from non-shear walls and that cuts the effective diaphragm pan down considerably.

2) your "chords" really end up being your walls and, in that sense, they don't need to be continuous in quite the same way as we normally consider.

In my opinion, you've basically got three classes of buildings in play:

1) Commercial, probably multi-story buildings with concrete diaphragms. Here, diaphragm capacity is easy to come by. And, frankly, "chord" design in these buildings is a bit of a joke unless the floor plan doesn't articulate at all. The portions of the deck between articulations behaves as a marvelous beams with or without the silly 2-#5 cont jogging about the perimeter.

2) Large scale commercial buildings, probably single story with very discrete lateral systems. Metal deck or plywood roofs. Here, decent engineers will usually do a little something with collectors into the big re-entrant corners etc. There are usually still some logical holes in the system though.

3) Anything residential with a choppy floor plate and/or a pitched roof. Plywood or metal deck roofs. For big developments with horizontal diaphragms, some folks in some regions pay a bit of attention to the chords. My experiences is that is not the case in most markets. And, even in the good markets, pitched roof diaphragms are still a complete mess. Check out the building below that I'm currently working on. Somebody tell me how to get diaphragm continuity in that without getting banished from construction forever? KootK loves ceiling drywall. Loves it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Interestingly, the Canadian concrete code gives you three options for dealing with diaphragms:

1) Strut and tie. Surely the most appropriate but who has the time except, perhaps, at transfer diaphragms.

2) Conventional, and surely flawed, flexural design. This is what the Romans are doing.

3) Shear panel assumption. Which surprises me mightily as it must be very far from the actual, expected behavior.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.