Page 373 - Design of Reinforced Masonry Structures

6.24 CHAPTER SIX

floor and the floor or the roof above. In that case, it is generally assumed that walls trans-
fer their lateral load to the floor or the roof diaphragm above and the ground floor below.
Analysis and design of vertically spanning walls is discussed in Sections 6.4 through 6.8.
The walls may also span horizontally between lateral supports such as cross-walls, pilas-
ters, buttresses, and the like. In such cases, walls transfer their lateral load to these vertical
elements which eventually transfer those loads to the foundation. Discussion on horizon-
tally spanning walls and pilasters is presented in Section 6.9.
Analysis of walls subjected to gravity loads alone or combined loads (i.e., gravity and
lateral) is very similar to that for columns subjected to similar loading conditions as dis-
cussed in Chap. 5, albeit with some differences. Typically, a wall is analyzed by considering
a 1-ft-wide strip between the supports; by contrast, a column is analyzed by considering
its entire cross section.
For a wall supporting only gravity loads, this strip is assumed as a 1-ft-wide (i.e.,
b = 12 in.) vertical column, simply-supported at the bottom and the top; its thickness (t)
being equal to the wall thickness. The gravity loads include superimposed dead and live
loads (applied at the top of the wall) plus the dead weight of the wall itself, all loads being
factored loads as required for strength design. The axial compressive strength of the wall
should be checked at the bottom of the wall because the gravity loads would have maximum
value at that point.
The lateral loads are the loads due to wind or earthquake; the more critical of the two
should be considered for design. In this context, the following should be noted:

1. The wind loads calculated based on the wind load provisions of ASCE 7-05 [6.19] are
ASD-level loads.
2. The seismic loads calculated based on seismic load provisions of ASCE 7-05 are LRFD
(or strength-level loads).
Therefore, in order to compare wind and seismic loads and select the governing lateral
load, the seismic loads should be multiplied by 0.7. The effect of lateral loads, which act
transverse to the wall strip, should be checked against the maximum flexural strength of the
vertical beam (12-in.-wide strip).

6.4.2 Dead Load of Masonry Walls
The dead weight of a wall can vary widely, depending on its width, unit weight of concrete
and grout, and whether it is solid or partially grouted. A wall may need to be grouted solid
in order to satisfy structural requirements or some fire safety requirements, or both. The
dead weight of a partially grouted wall depends on the spacing of the grouted cells in addi-
tion to the above factors. The unit weight of hollow concrete blocks depends on whether

they are made from lightweight, medium-weight or normal-weight concrete. Table A.18
provides information about the typical weight of 16-in.-long, 4-in.- and 8-in.-high units of
various modular thicknesses. The grout may be of different unit weight as well: lightweight
or normal weight.
Tables A.19 and A.20 provide information required to calculate dead load due to the
dead weight of solid or partially grouted walls built from hollow concrete masonry blocks.
The two tables are different in respect of the unit weight of grout (140 and 105 lb/ft ,
3
respectively). Each table lists dead load of three types of concrete masonry units by their
3
3
unit weights: lightweight (103 lb/ft ), medium weight (115 lb/ft ), and normal weight
(135 lb/ft ). Unit weights of walls are given in terms of per square foot of the surface area
3
measured in the plane of the wall. Dead load is tabulated for 6-, 8-, 10-, and 12-in.-wide