Page 137 - Design of Reinforced Masonry Structures

CHAPTER 4
DESIGN OF REINFORCED

MASONRY BEAMS

4.1 INTRODUCTION

The strength design method, initially called the ultimate strength design method, was originated
by F. Stussi in 1931 [4.1] for design of reinforced concrete and prestressed concrete beams.
Gradually, it became an accepted method for designing reinforced and prestressed con-
crete structures, and incorporated in the ACI Code [4.2]. Strength design method is also
permitted by the design codes for reinforced masonry structures as incorporated in the
MSJC-08 Code [4.3] and 2009 IBC [4.4]. According to this method, load effects, such as
axial loads, and shears and moments in a beam, are determined by the methods of elastic
analysis using the factored loads. Factored loads are obtained by multiplying service loads
by code-specified load factors as specified in ASCE 7 Standard [4.5] and discussed in this
chapter. The member cross section is then designed assuming that masonry and reinforcing
steel have reached certain predefined strain values. The nominal strength of the member is
then determined based on stresses corresponding to these specified strain limits. The design
strength of a member is obtained from the nominal member strength by multiplying it by a
resistance factor (which is less than 1.0) to reflect the uncertainty involved in the prediction
of the material strength and in the analyses.
It should be recognized that masonry structures may be designed either by the allowable
stress design (ASD) method (also called the working stress method, WSD, in the past) or
the strength design method. A comprehensive discussion on design of reinforced masonry
structures by the ASD method can be found in several references [4.6–4.8].
Minimum requirements for the strength design of masonry are presented in the
MSJC-08 Code (hereinafter referred to as MSJC Code) Chap. 3 [4.3]. However, note
that masonry design by the strength method is also required to comply with the general
design requirements for masonry presented in Chap. 1 of the Code.

This chapter discusses analysis and design of reinforced masonry flexural elements
(e.g., beams and lintels) for internal forces, that is, bending and shear, based on strength
design philosophy. Discussion on the strength design and analysis of reinforced masonry
walls subjected to flexure caused by out-of-plane loads, and in-plane loads is presented in
Chaps. 6 and 7, respectively.
Philosophically speaking, the strength design approach for structural design, which is
based on limit states design concept, involves two major considerations:
1. Design for strength
2. Design for serviceability

4.1