In the International Code Council’s 2003 International Building Code (IBC), there are two ways to assume a diaphragm is flexible. First in Section 1602.1, there is a definition for flexible diaphragm that states that a diaphragm is flexible when the computed maximum in-plane deflection of the diaphragm is more than 2 times the average drift of adjoining vertical elements. And secondly, in Section 1617.5.3, the IBC states that diaphragms constructed of untopped steel decking or wood structural panels or similar light-framed construction are permitted to be considered as flexible; however this provision only applies to the Simplified Analysis Procedure. As such, many structures of light-frame construction fell into the category of having rigid diaphragms even though designers have been assuming flexible diaphragms. Revisions have been made to the 2006 IBC that will allow many structures—which were classified as having rigid diaphragms under the 2003 IBC—to be classified as having flexible diaphragms.
First it is important to take a look at the provisions of the new section in the American Society of Civil Engineer’s 2005 Minimum Design Loads for Buildings and Other Structures (ASCE 7-05):
"184.108.40.206 Flexible Diaphragm Condition. Diaphragms constructed of untopped steel decking or wood structural panels are permitted to be idealized as flexible in structures in which the vertical elements are steel or composite steel and concrete braced frames, or concrete, masonry, steel, or composite shear walls. Diaphragms of wood structural panels or untopped steel decks in one- and two-family residential buildings of light-frame construction shall also be permitted to be idealized as flexible."
The 2006 IBC modifies this ASCE 7-05 definition by adding a new set of criteria at the end of ASCE 7-05 Section 220.127.116.11 for idealizing a diaphragm as flexible in light-frame construction. The flow chart in Figure 1 is provided to illustrate the use of these diaphragm provisions.
Answers to FAQ’s:
Q: Why does the 2006 IBC modify ASCE 7-05 Section 18.104.22.168?
A: In ASCE 7-05, diaphragms constructed of wood structural panels or untopped steel decking are permitted to be idealized as flexible in light-frame construction only when such construction is utilized for a one- or two-family residential building. However, it was realized that the nature of the construction, rather than the occupancy of a building provides a more appropriate basis for selecting an idealization. The modification sets forth the following four conditions that must be met for other occupancies:
- Only nonstructural toppings no greater than 1-1/2 inches thick are allowed over wood structural panel diaphragms.
- Each line of vertical elements of the lateral-force resisting system is within the allowable drift limits.
- Vertical lateral load resisting elements are wood or steel sheet shear walls.
- Portions of cantilevering wood diaphragms are designed in accordance with IBC Section 2305.2.5.
These conditions are in part based on research that was part of the CUREE Caltech Woodframe project, which showed that for regular, light-framed, wood diaphragm buildings, better building performance (lower drift and lower repair cost) resulted when flexible diaphragm force distribution was used. It is expected that these new provisions will allow many more wood framed structures to have the diaphragms idealized as flexible for design purposes. More information is contained in CUREE W30 Recommendations for Earthquake Resistance in the Design of Construction of Woodframe Buildings available through www.curee.org
S.K. Ghosh Associates, Inc., is a structural, seismic, and code consulting firm located in Palatine, Ill., and Laguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at firstname.lastname@example.org and email@example.com, respectively.