 | References for the diaphragm
In this article:
Resistance properties for steel decking are based on a calculation method established for North America. The method is published in the United States by the American Iron and Steel Institute (AISI) under the name “North American Specification for the Design of Cold-Formed Steel Structural Members” and in Canada by the Canadian Standards Association (CSA) under the same name with the designation S136-07.
Resistance factors
The CSA standard contains formulas for establishing: 1) the properties for decking under gravity loads, and 2) the shear and tensile strength of welds and screws for attaching steel deck to the supports and at the side-laps.
In addition, Table D5 (Table 1 below) shows the factors to use for calculating the factored resistance from the nominal (rupture) strength in comparing the factored resistance to the factored loads.
| Ωd = | As specified in Table D5 (ASD) |
| ɸd = | As specified in Table D5 (LRFD or LSD) |
Safety factors and resistance factors for diaphragms
| Load type or combinations, including | Connection type | Limit state |
Connection related | Panel buckling* |
Ωd = (ASD) | ɸd (LRFD) | ɸd (LSD) | Ωd (ASD) | ɸd (LRFD) | ɸd (LSD) |
| Earthquake | Welds | 3.00 | 0.55 | 0.50 | 2.00 | 0.80 | 0.75 |
| Screws | 2.50 | 0.65 | 0.60 |
| Wind | Welds | 2.35 | 0.70 | 0.65 |
| Screws |
| All others | Welds | 2.65 | 0.60 | 0.55 |
| Screws | 2.50 | 0.65 | 0.60 |
Table 1
Note: Panel buckling is out-of-plane buckling and not local buckling at fasteners.
Source: Canadian Standards Association
It should be noted that the 2007 version of the CSA standard shows an increase in some of the factors for rupture modes involving connectors. Since all of these factors were 0.5 in the 2001 version, the screws and other mechanical fasteners can now be used with a coefficient of 0.6 for earthquakes and 0.65 for wind. In the case of welds, the coefficient remains 0.5 for earthquakes but increases to 0.65 for wind.
Calculation methods for deck diaphragm
The deck diaphragm calculation is based on two methods in North America:
The Tri-Services Laboratory (American Armed Forces) method, which is only applicable for welding to supports and for crimping or welding of longitudinal side-laps; and
The Steel Deck Institute (SDI) method, developed by Dr. Larry Luttrell of the University of West Virginia, which is more general and makes it possible to calculate and thoroughly evaluate the resistances for various types of connections to supports and along longitudinal side-laps.
Publications and software application for establishing resistance tables
Three publications and one software application that use these methods to establish resistance tables are presented below.
Publications
This publication contains tables developed entirely on the basis of the SDI method. The values in the tables are nominal strengths, which must be multiplied by the resistance factors depending on the type of load producing the stresses in the diaphragm.
In addition to the tables for the connection rupture mode, there is a table, at the bottom of every page, that shows the nominal shear strength limit of the diaphragm for rupture caused by buckling of the deck between supports. The user must determine the minimum resistance values of the two types of rupture.
The tables, in imperial units only, show the values for different spans and patterns of connections to supports and for a given number of longitudinal side-lap connections per deck span.
This publication contains tables developed according to the SDI and Tri-Services methods. The factored resistance values in the tables were multiplied by a single resistance factor of 0.5 for the connection rupture mode and 0.75 for the buckling rupture mode, with only the minimum value of the two types of rupture shown.
The tables, in metric and imperial units, show the values for different spans and patterns of connections to supports and for different spacings of connections along longitudinal joints.
 | Steel Deck Diaphragm, 2007
Canam Canada |
This publication contains tables developed entirely on the basis of the SDI method. The factored resistance values in the tables were multiplied by a single resistance factor of 0.5 for the connection rupture mode and 0.75 for the buckling rupture mode, with only the minimum value of the two types of rupture shown.
The tables, in metric and imperial units, show the values for different spans and patterns of connections to supports and for different spacings of connections along longitudinal joints.
 Download the Canam publication Steel Deck Diaphragm
Software application
This software application is used to establish diaphragm resistance tables according to the SDI method only. The factored resistance values in the tables are multiplied by a resistance factor selected by the user for the connection rupture and buckling rupture modes.
The tables, which can be generated in either metric or imperial units, show the values for different spans and patterns of connections to supports and for a given number of longitudinal joint connections per span. The connections to supports are limited to welds, screws and Hilti mechanical fasteners.
Download the software application
Comparison of values (SDI vs. CSSBI)
The comparison between page AV-49 of the SDI manual and page 204 of the CSSBI manual (Figure 1) shows the resistances for a standard steel deck 38 mm (1½ in.) deep and 0.76 mm (0.030 in.) thick utilizing Hilti ENP2K fasteners for the supports and #10 screws for the longitudinal joints.
Figure 1
Sources: Steel Deck Institute (left) and Canadian Sheet Steel Building Institute (right)
For the same spans and the same number of longitudinal joint connections per span, corresponding resistances can be found by multiplying the values in the SDI table by a resistance factor of 0.5 and comparing them with the values in the CSSBI table.
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