Page 141 - Design of Reinforced Masonry Structures

DESIGN OF REINFORCED MASONRY BEAMS 4.5

Load factor assigned to a particular kind of load can be different depending on whether
the load is to be used singly or in combination with other loads for design purposes. For
example, a load factor of 1.4 is assigned to dead load when considered acting alone, but
a load factor of 1.2 is assigned to it when considered acting in conjunction with the live
load. Furthermore, when dead and live loads are considered acting together, the live load is
assigned a load factor of 1.6, much higher than the 1.2 factor assigned to the dead load. The
reason for assigning a much higher value of load factor to the live load is the higher degree
of uncertainty associated in estimating the live load than the dead load.
When load factors are assigned to various loads in a load combination, consideration is
given to the probability of the simultaneous occurrence of those loads. Also, in deciding a
load combination, it is very important to recognize whether the effects of one type of load-
ing would offset the effects due to the other, the two effects being of the opposite types. For
example, dead load, which is a gravity load, acts on a structure as a stabilizing force, which
opposes the effects produced by the lateral loads (such as wind or seismic) which act as
overturning forces and tend to destabilize the structure. Thus, the effects of these two types
of forces are of the opposite types. In such cases, a conservative approach is taken, and the
load factor assigned to the stabilizing force (dead load in this case) may be less than 1.0.
Typically, when lateral loads are combined with the gravity loads, a load factor of 0.9 is
assigned to the dead load. This is because the dead load reduces the effects of lateral loads,
and that it might have been overestimated.
Due consideration must be given to various load combinations to determine the most
critical design condition. Loads to be considered for a specific combination should be
determined according to the applicable standards and codes such as ASCE 7 [4.5] or 2009
IBC [4.4]. A comprehensive discussion on the analysis of structural loads on buildings is
presented in Ref. 4.12. See Chap. 7 for a brief discussion on this topic.

4.4 ASSUMPTIONS IN STRENGTH
DESIGN PHILOSOPHY

Analytical model adopted for the strength design of reinforced masonry is similar to that
used for design of reinforced concrete structures. Assumptions in strength design of rein-
forced masonry parallel those for design of reinforced concrete specified in ACI 318 [4.2].
Accordingly, readers familiar with the strength design of reinforced concrete would find it
easier to understand the strength design of reinforced masonry.
At the very outset, it should be noted that the reinforced masonry beams are constructed,
typically, from hollow concrete or clay units which, after placement of tension and shear
reinforcing bars, are grouted solid. Solid grouting of beams is essential to ensure that
masonry and reinforcement act in unison.

Provisions for strength design of reinforced masonry are stated in MSJC Code [4.3]
and incorporated in 2009 IBC [4.4]. The assumptions for the strength design of reinforced
masonry as specified in the MSJC Code can be summarized as follows (see Fig. 4.1):
1. Masonry, grout, and reinforcement act compositely in resisting applied loads by main-
taining strain continuity between these three constituents of a masonry structure.
2. Nominal strength of reinforced masonry cross sections for combined flexure and axial
loads shall be based on applicable conditions of equilibrium and compatibility of
strains.
3. Strain in reinforcement and masonry shall be assumed to be directly proportional to the
distance from the neutral axis, that is, strains vary linearly from zero at the neutral axis
to maximum at the extreme fibers of the beam.