When employing an empirical design method in Chapter 9 of the Masonry Standards Joint Committee's Building Code Requirements for Masonry Structures, how should you determine the thickness of a masonry cavity wall using the length-to-thickness ratios found in this Code? Should you use each wythe independently, add the wythes together, or use some other method?
You are probably using the 1992 version of this Code. The 1995 version of the Building Code Requirements for Masonry Structures (ACI 530-95/ASCE 5-95/TMS 402-95) does address this issue. The new MSJC Code states that the nominal thickness of the two wythes should be added together. This definition originated in American Standard Building Code Requirements for Masonry/ American Standard A41.1-1953, distributed by the American Standards Association. Much of the MSJC Code's empirical design chapter was developed using this code as a guide. Although a cavity wall will be more flexible and will have less strength than a solid masonry wall of the same thickness as the sum of the wythes of a cavity wall, masonry codes historically have made them equivalent for determining span-to-thickness ratios.