Page 387 - Design of Reinforced Masonry Structures

6.38 CHAPTER SIX

Parapet
2' – 8"

F P2
20'

F P1
Direction & seismic loads

2
FIGURE E6.1B Seismic forces on the wall: Seismic load on the wall = 42 lb/ft ,
2
and seismic load on parapet = 101 lb/ft .

6.6 ANALYSIS OF MASONRY WALLS FOR
OUT-OF-PLANE LOADS

6.6.1 Assumptions in Analysis

For analytical purposes, a wall may be visualized as a vertically oriented slab; multiple
walls may be visualized as a series of slabs oriented vertically. It is common design practice
to assume simple support conditions at locations of wall supports over the height of build-
ing. That is, a wall is analyzed as an element simply supported at the floors, or at the floor
and the roof level (i.e., pinned-end conditions). This is a reasonable assumption considering
the fact that the connections between the walls and their support elements (floors or roof)
usually do not posses sufficient stiffness or strength to transfer the wall moments into the
support elements to justify a rigid connection. Furthermore, because both wind and earth-
quake are dynamic phenomena, the assumption of pinned supports is consistent with the
modal response of the walls subjected to these lateral loads [6.22].
MSJC-08 presents two separate procedures for analysis of reinforced masonry walls:
one for the allowable stress design (ASD) and the other for strength design. Analysis and

design of reinforced walls subjected to out-of-plane loads based on ASD philosophy is
discussed in Ref. 6.24 and is not repeated here. The following discussion presents analysis
and design of walls based on the strength design principles.
As the wall approaches ultimate load conditions, the lateral force on the wall (wind or
seismic) causes an out-of-plane deflection, d . As a result of this deflection, both factored
u
gravity loads supported on the wall (from floor or the roof), P , and the factored dead
uf
weight of the wall, P , act eccentrically with respect to the midheight of the deflected wall,
uw
causing a moment in the wall. The values of these moments are maximum at the midheight
of the wall where the moment due to the lateral loads is also maximum. Furthermore, the
load from the roof or the floor, P , might also be acting eccentrically with respect to the
uf
centerline of undeflected wall. All these loads and forces are assumed to be at the strength