Definition of Rigid Diaphragm 1

[dcarr82775]

I think I am having a memory lapse. I can find the code definition for the determination of a flexible diaphragm, but I can't find a similar explicit definition of a Rigid Diaphragm. For flexible, if the maximum diaphragm deflection is more than 2* the average story drift if it flexible. At first I would think if it isn't flexible is may be considered rigid. But, the Code also says diaphragms that are between Rigid and Flexible shall be treated accordingly. I can't find a specific definition of a Rigid Diaphragm in the Code other than a material description. Is there one similar to the 'hard number' definition of flexible? I thought there was one, but can't find it.

 

[JoshPlumSE]

ASCE 7-10 section 12.3.1.2:
Diaphragms of concrete slabs or concrete filled metal deck with span to depth ratios of 3 or less in structures that have no horizontal irregularities are permitted to be idealized as rigid.

 

[dcarr82775]

Josh,

I know of that definition, but it is vague and frankly useless in my application. I have a wood diaphragm that I think is fine to use a rigid assumption. The local code official is asking for calculations to justify my assumption. So I need and actual 'hard number' type of definition that I can use in a calculation. They give one for flexible, but I can't find a similar one for rigid.

 

[slickdeals]

Without knowing the entire details, I think you will be in the minority of people considering a wood diaphragm as rigid. I am not sure a hard formula exists to prove it is a rigid diaphragm,it would have to do with an in plane stiffness value.

 

[dcarr82775]

Ahh but therein lies the problem. How can you assume something isn't rigid when no definition is given? You can only actually show something is flexible. Often times if you ran the numbers you find that the flexible assumption allowed doesn't come close to satisfying the definition of flexible.

 

[WillisV]

Decks are either flexible per the definition you gave, rigid per JoshPlum's quote, or semi-rigid. There is no deflection limit for saying a deck is rigid - it either meets the criteria of construction type and span to depth ratio per JoshPlum, or it is classified as semi-rigid or flexible.

 

[woodman88]

All diaphragms are semi-rigid. The codes and engineering standards allow one to use either a flexible or rigid assumption if certain conditions are met for the simplicity of designing.
The 2009 IBC and ASCE Standard 7-05 sets some of these conditions for use as flexible or rigid. The 2009 IBC also permits some additional ones (over the ASCE) to be applied to wood structural panels and steel decking diaphragms. The simplest is that a 1.5” non-structural concrete topping of more than 1.5” can not be idealized as flexible. See section 1613.6.1. Whether this is a condition you can use and/or would allow you to use a rigid assumption over a semi-rigid, would have to be looked it more closely.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[structSU10]

Check out the ICC reports for various steel deck manufacturers (vulcraft is ICC-ESR-1227). It details some things about diaphragm flexibility; span and aspect ratio limits and such. Don't know if its useful from a code standpoint.

However, I would think that the wood diaphragm is difficult to quantify its stiffness; you have nail slip for the plywood field connections along with the chords, on top of standard shear and flexural deformations, some of which may be difficult to define accurately. Also, you would have to make sure the diaphragm could take torsion, and would have to design the field connections and such accordingly.

You could always use the semi-rigid diaphragm definition, and try to develope a reasonable element in FEA software to estimate what may happen. Or you can check both flexible and rigid conditions and their effects on the shearwalls.

 

[dcarr82775]

Thanks Woodman. I knew there was something about topped wood diaphragms, I couldn't find it.

 

[charliealphabravo]

I suggest you contact the APA. I suspect that they are very well aware of the issue you are facing and could help you with a defensible approach.

 

[focuseng]

I think you are looking for this from IBC 1602.1 (Definitions). See the last one for "Diaphragm rigid".

DIAPHRAGM. A horizontal or sloped system acting to transmit lateral forces to the vertical-resisting elements. When the term "diaphragm" is used, it shall include horizontal bracing systems.

Diaphragm, blocked. In light-frame construction, a diaphragm in which all sheathing edges not occurring on a framing member are supported on and fastened to blocking.

Diaphragm boundary. In light-frame construction, a location where shear is transferred into or out of the diaphragm sheathing. Transfer is either to a boundary element or to another force-resisting element.

Diaphragm chord. A diaphragm boundary element perpendicular to the applied load that is assumed to take axial stresses due to the diaphragm moment.

Diaphragm flexible. A diaphragm is flexible for the purpose of distribution of story shear and torsional moment where so indicated in Section 12.3.1 of ASCE 7, as modified in Section 1613.6.1.

Diaphragm, rigid. A diaphragm is rigid for the purpose of distribution of story shear and torsional moment when the lateral deformation of the diaphragm is less than or equal to two times the average story drift.

MAP

 

[dcarr82775]

Excellent. Thanks Focuseng