Rigid Vs Flexible Diaphragms 1

[ljk80oze]

Hi

I have read a lot of threads referencing diaphragm classifications and the respective design approaches required by foreign codes. In summary, it seems that lateral load distribution should be taken as a ‘tributary area analysis’ for flexible and a ‘stiffness analysis’ should be adopted for rigid diaphragms.

I was after some insight as to the existence of an Australian standard or design approach for this issue.

Generally speaking, I have pretty much adopted flexible diaphragms in most cases (ceiling, roof, timber floor) with reinforced concrete slabs being the only exception.

Is this a conservative assumption? Is there an Australian standard or publication that defines the diaphragm types?

I don’t have access to foreign standards so any advice would be appreciated.

Thanks.

 

[csd72]

ljk80oze,

I dont think there is any australian standard on this.

This distinction doesnt matter as much for wind as it does for seismic.

The codes in the US are more detailed and comprehensive than pretty much anywhere else. They have codes for everything.

 

[ljk80oze]

Thanks for your response csd.
Are the assumptions that i have been making regarding diaphragms accurate?
Can you direct me to any literature that would help me with this?
Do you think it is worthwhile purchasing the US code?

Also do you do detailed calculations for validating your diaphragms? In particular gyprock ceilings to trussed roofs (which have zero shear capacity in cyclonic conditions).

Apologies for the barrage of questions, but this a fundamental concept that i don't have my head around yet.

Many Thanks.

 

[csd72]

ljk80oze,

The assumptions that you make seem to be resonably accurate.

Generally I would expect concrete/composite slabs to act as rigid diaphragms and lighter floors/roofs to act as flexible.

I would not recommend buying the US code as very little of it is relevant. I will try to write you a brief summary of the diaphragm rules e.t.c. this week. I will see If I can find some references.

But as I said above, the diaphragm analysis is not so critical for wind and low seismic analysis. The rules of thumb you are using should be more than adequate.

Calculation checks are different for each material, but I wouldnt recommend relying on gyprock for diaphragm action. Best to put some metal straps in if need be.

If you still want to research it, then here are a few links to look at:

http://www.ce.memphis.edu/WTNSEA/Diaphragm Design.pdf

http://www.seaintarchive.org/group/seaint/mailarchive/2003c/msg00988.html

http://www.cssbi.ca/Eng/_pdf/Facts9.pdf

http://www.apawood.org/level_c.cfm?...s&pD=Yes&pF=Yes&CFID=6646949&CFTOKEN=81228814

http://www.skghoshassociates.com/sk_publication/PCI-NovDec02_Untop_diaph_high_seis_app.pdf

http://www.njb-united.com/usd/dpdfs/dp63.pdf

http://www.supremusgroup.com/images/structural-engineering-design/Structural-Calculation-Report.pdf

http://www.seaoc.org/seismpdfs/Positions/diagrigidity_inlightfram.pdf

http://www.awc.org/pdf/WDF16-12005SDPWS.pdf

 

[ljk80oze]

Thanks for your time and effort csd!

There are some excellent references and examples in there! I will get stuck into it!

I think the basis of my confusion with this issue is with gyprock ceiling diaphragms. I have seen many examples in residential construction of trussed roofs with no ceiling level bracing. It is my understanding that (using reinforced masonry construction as an example) if the combined bondbeams (on both sides of the building) cannot span between bracing walls, additional ceiling bracing is required (and additional compression is put on the roof trusses). I feel that even this assumption is a pretty weak basis for design given the large deflections that would occur at serviceability loads would result in significant damage to the extremely stiff and extremely weak ceiling linings. I find it hard to validate the standard and prolific practice of residential structures without any ceiling bracing or additional lateral (compression loads) being nominated in the bottom chords of the roof trusses. The minimal reinforcing to the bondbeams could not span between bracing walls. Am I missing something here? Is the roof sheeting being used as the diaphragm? I have seen enough examples of this type of construction surviving cyclones…..

Anyway I will have a read of the literature you have provided and see if I can get my head around it.

Thanks again.

 

[csd72]

Have a good look through the timber framing code, they cover a lot of it in there.

Particularly the first part that cover fundamentals and calculation methods.

A lot of what is standard practice in residential construction is hard to justify by calculation.

The code does let you use non engineered partitions (such as gyprock walls) to take a proportion of the wind loads (25% I think).