The two design approaches (allowable-stress design and strength design) can each have the same result, and also the same level of safety (measured in terms of probability of failure under service loads). In allowablestress design, the probability of failure is controlled directly by the factor of safety. In strength design the probability of failure depends on the quotient of the load factor and the capacity reduction factor (φ factor). Most modern codes are based on strength design, because it gives a more uniform factor of safety against collapse.
Strength design: In strength design, design actions (axial forces, shears, and moments) are computed using service loads, and are then increased by load factors. The factored design actions are then compared with nominal member strengths, decreased by strength-reduction factors.
Service actions × LF ≤ φ × nominal capacity
Allowable-stress design: In allowable-stress design, stresses corresponding to service loads are compared with allowable stresses. The allowable stresses are material strengths, reduced by a factor of safety. Factors of safety for masonry typically range from 2.5 to 4.
Stresses from service loads ≤ failure stresses / safety factor
Empirical design: Empirical design (design carried out based on aspect ratios, dimensional limits, and approximate gravity-load stresses on gross areas) is addressed by Chap. 5 of the MSJC Code (2008a), and is permitted for masonry structures and elements in very limited circumstances. It is not discussed further in this book.
Veneer design: According to Chap. 6 of the MSJC Code (2008a), masonry veneer is designed prescriptively through control of connector type and spacing. Design of veneer is not discussed further in this book.
Glass-block masonry design: Design of glass unit masonry is addressed by Chap. 7 of the MSJC Code (2008a). It is not discussed further in this book.
AAC masonry design: Design of autoclaved aerated concrete (AAC) masonry is addressed by App. A of the MSJC Code (2008a). It is covered in detail in Chap. 14 of this book.
