2001 ASD Supplements - unprotected PDF - American Wood Council

SW-22OTHER CONSIDERATIONS5.3 SubdiaphragmsAfter the San Fernando earthquake of 1971, a numberof code changes were introduced to the UniformBuilding Code that require continuous cross-ties. Theseand other code changes that require minimum attachmentof concrete and masonry walls to wood diaphragms wereproposed and enacted to improve the seismic performanceof large flat-roofed structures using flexible structural-usepanel diaphragms.Since the enactment of these code changes, thesubdiaphragm (also known as the mini-diaphragm) concepthas been recognized and extensively used to providea method of meeting the wall attachment and continuouscross-tie code requirements while minimizing the numberand length of ties required to achieve continuitybetween chords. A formal definition of a subdiaphragmcan be found in the 2000 International Building Code,“SUBDIAPHRAGM portion of a larger wood diaphragmdesigned to anchor and transfer local forces to primarydiaphragm struts and the main diaphragm.”In practice, the subdiaphragm approach is used toconcentrate and transfer local lateral forces to main structuralmembers that support the roof vertical loads. Thesubdiaphragm approach is often an economical solutionto code required cross-ties for the following reasons:• Main structural members are already present• Main structural members generally span the full lengthand width of the buildings with few connectors.• Main structural members are large enough to easily accommodateloads.• Main structural members are large enough to allow“room” for requisite connections.In general, the bigger the roof, the greater the savingsthat can be made by using subdiaphragms.Each subdiaphragm must meet all applicable diaphragmrequirements provided in the applicable buildingcode. As such, each subdiaphragm must have chords, continuoustension ties, and sufficient sheathing thickness andattachment to transfer the shear stresses generated withinthe diaphragm sheathing by the subdiaphragm. In addition,building codes may contain aspect ratios that arespecific to subdiaphragms.The subdiaphragm is actually the same structure asthe main roof diaphragm, thus the subdiaphragm utilizesthe same roof sheathing to transfer shear stresses as themain diaphragm. As such, sheathing nailing and thicknessrequirements of the roof diaphragm may not besufficient for the subdiaphragm requirements. In this case,the subdiaphragm requirements would control and dictatethe roof sheathing and fastening requirements in thesubdiaphragm locations. Fortunately, the portion of themain diaphragm that is utilized as a subdiaphragm is achoice left to the designer; thus the dimensions of thesubdiaphragm can be chosen to minimize potentialdiscontinuities in sheathing thicknesses or nail schedules.Similarly, the roof diaphragm requirements may be morestringent than those for the subdiaphragm. For more informationon the subdiaphragm concept and a designexample, see APA Form EWS Z350 - Lateral ConnectionDetails for Low-Slope Roofs.APA – The Engineered Wood Association