Page 138 - Design of Reinforced Masonry Structures

4.2 CHAPTER FOUR

Design for strength includes considerations for the structure or structural member to
carry maximum anticipated loads on the structure. For a beam, it means designing for
moment and shear caused by factored loads. Design for serviceability deals with functional
and performance aspects of a structure or structural member under service loads, such as
deflections, fatigue, and vibration control. Deflection of reinforced masonry beams is dis-
cussed at the end of this chapter.
The readers should note that discussion on analysis and design of reinforced masonry
presented in this and other chapters of this book are pertinent to both reinforced concrete
masonry and reinforced clay masonry (brick masonry).

4.2 HISTORICAL DEVELOPMENT

The genesis of the provisions for the strength design of reinforced masonry as specified
in the MSJC Code can be traced to Technical Report No. 4115 titled “Strength Design of
One-to-Four Story Concrete Masonry Buildings” [4.9] published by the ICBO in February
1984. This report had gone through several independent reviews before and after its pub-
lication. Following these reviews, the report was renewed in February 1985 for two more
years. Again, this report was reviewed by the newly formed ICBO Evaluation Services,
Inc., Whittier, California, and renewed in September 1985 for two more years.
Design provisions based on strength design philosophy were first introduced in the
1985-UBC [4.10] in which Chap. 24 was completely revised from its earlier code ver-
sions. A strength design approach for slender wall was introduced in Section 2411 of
1985-UBC after it was approved by The Masonry Society (TMS) and the Structural
Engineers Association of California (SEAOC). This was followed by the adoption of the
strength design criteria for shear walls, which were introduced in Section 2412 of the
1988-UBC [4.11]. Codification of strength design approach also made it possible for engi-
neers to design slender walls with h/t ratio in excess of 25 (briefly discussed in Chap. 6 of
this book). In the 1997-UBC [4.12], provisions for strength design of reinforced masonry
were covered in Chap. 21.
This chapter presents a discussion of the strength design philosophy as applicable to
design of reinforced masonry beams. This is followed by several examples, which illustrate
application of strength design philosophy and the provisions of MSJC Code [4.3] and 2009
IBC [4.4] to analysis and design of reinforced masonry structures.

4.3 STRENGTH DESIGN PHILOSOPHY

4.3.1 Basic Concepts

The “strength design” philosophy is based on the simple premise that a structure should
be safe under the maximum loads that it may be required to carry during its service life.
This principle can be restated by saying that the “strength” of a structure should be greater
than or at least be equal to the maximum anticipated loads (also referred to as the “ultimate
loads”) that it may be called upon to resist. Mathematically, this principle can be stated as
Design strength ≥ required strength (4.1)
The term “strength” can be defined as the resistance that a structure (or a structural
member) can offer. The strength of a structural member depends on the strength of the
material (reinforced masonry in our case) from which it is constructed. The latter is
determined from tests, which are always subject to some degree of uncertainty associ-
ated with them. We would therefore require that the design strength (or the maximum