Susan M. Frey, P.E., S.E. June 2006
Designers of structural masonry, like designers of steel and concrete structures, must decide when and why to use load factor design (strength design) methods or allowable stress design (ASD or working stress design) methods. Based upon recent data from a variety of college course example problems on masonry building elementdesign, this paper examines both design methods. The summary of comparisons and choices might prove useful to the general design community. The designs involved the 1997 Uniform Building Code (UBC), the 2003 International Code (IBC), and the 2002 and 2005 Masonry Standards Joint Committee (MSJC) “Building Code Committee for Masonry Structures, ACI 530/ ASCE 5/TMS 402.”
All designmethods addressed in this paper focus on criteria for reinforced, rather than un-reinforced, masonry design. Many west coast designers have recently moved from the UBC masonry design requirements to the IBC/ MSJC design codes, although designers of structures in California and parts of Hawaii continue to use UBC. The UBC has included strength design for several code cycles, starting with the factoredload method for slender out-ofplane wall design in the 1991 edition and adding strength design of all other elements in the 1997 edition. The 1997 UBC changed the actual presentation of the loads (gravity, wind, seismic) as well as their factors, but not the general design approach for the masonry material strength design. The current designer’s learning curves are twofold: UBC designers aremoving to the use of the MSJC design code, and former MSJC designers now have the option of using strength design, starting with the 2002 MSJC edition. The 1999 MSJC edition, as well as earlier versions, provided only an ASD methodology. The discussion that follows provides some opinions, findings, and recommendations based on work in consulting engineering, academia, and seminar presentations. The1997 UBC is still provided for two reasons: (1) several states and other areas are still using it as the adopted code, and (2) some code requirements that provided good practice are present in that code, but not in the current MSJC codes, and are still suggested as guidelines for the IBC/MSJC code designs. One’s choice of design method may of course follow one’s passions! Some designers believe thatstrength design is more accurate, produces a more economical design, or is “a modern approach” to design, compared with ASD methods, which they consider more conservative or “an antiquated approach.” Masonry design courses at the collegiate level usually require a prerequisite course in concrete design, in which load-factored, strength design is the only method presented. Newly graduatedengineers are comfortable with factored loads and material reduction factors. However, ASD relates very directly to the materials and components, produces economical designs in most cases, is reasonably fast to do by hand as well as with a spread sheet, revisits an understanding of basic static stresses relating to linear stress and strain relationships in the member, and collates the unfactored loads forserviceability checks and foundation designs.
Masonry: Design Method Choices Page 1
In addition, the 2002 MSJC, and to some extent the 2005 MSJC, as these codes transition into strength design, have some strength design requirements that preclude a reasonable member designed with factored load cases. These criteria, which relate to detailing and maximum reinforcement ratios, are beingmodified in upcoming code changes and updates, based on user input. One more code cycle should resolve most of these issues; the 2005 MSJC Code has already addressed some of the issues. In the meantime, however, ASD for certain members produces a reasonable design, whereas strength design might not. In the near future, it is anticipated that the codes will be updated so that similar designs can be...