Page 382 - Design of Reinforced Masonry Structures

WALLS UNDER GRAVITY AND TRANSVERSE LOADS 6.33

a = component amplification factor (varies from 1.0 to 2.5) as listed in ASCE 7-05
p
Table 13.5-1 or Table 13.6-1, as appropriate.
S = short-period design earthquake spectral acceleration parameter.
DS
W = component operating weight.
p
R = component response modification factor (varies from 1.0 to 12) as listed in
p
ASCE 7-05 Table 13.5-1 or Table 13.6-1, as appropriate.
I = component importance factor.
p
z = height in structure of the point of attachment of the component as measured
from the base. For items at or below the base, z = 0. The value of z/h need not
exceed 1.0.
h = average roof height of the structure as measured from the base.
Eq. (6.7) was developed based on building acceleration data from research that studied
the amplification of ground acceleration over the building height of instrumented buildings.
The term (1 + 2z/h) represents the amplification factor, which takes a value of unity for
components anchored at the ground level (z = 0) and a value of 3 for components anchored
at the roof level (at z = h). The response modification factor R represents the wall over-
p
strength and ductility or energy-absorbing capability of a component. Note that R = 2.5 for
p
reinforced masonry walls and 1.5 for un-reinforced masonry walls.
ASCE 7-05 Section 13.5.2 requires application of Eq. (6.7) for the design of parapets.
But in determining design wall forces on a wall with a parapet or the design force for
anchorage, Eq. (6.7) [ASCE 7-05 Eq. (13.3-1)] is not applied to the parapet. Instead, design
forces determined per ASCE 7-05 Section 12.11.1 [Eq. (6.3) above] are applied to the entire
wall including the parapet. Design forces calculated from Eq. (6.7) are much greater than
those calculated from Eq. (6.3).
It is instructive to understand the philosophy that forms the basis of Eq. (6.7). The multi-
pliers to the component weight W in Eqs. (6.7) through (6.9) may be thought of as seismic
p
response coefficients for nonstructural elements [similar to C , seismic response coefficient
s
for the seismic force-resisting systems (SFRS) as used in ASCE 7-05 Eq. (12.8-1)], which are
independent of their position over the height of the structure. These equations recognize the
unique dynamic and structural characteristics of components as compared to those of struc-
tures. Components typically lack desirable attributes of structures, such as ductility, toughness,
and redundancy, which permit greatly reduced seismic lateral forces for their design. This is
reflected in the lower values of R [in the denominator of Eq. (6.6)] as compared to the R-values
p
for the structures (SFRS in ASCE 7-05 Table 12.14-1). In addition, the components may exhibit
unique dynamic amplification characteristics; the term a in Eq. (6.7) reflects this attribute of
p
components. The cumulative effects of parameters R and a in Eq. (6.7) is to require greater
p
p
forces for the component integrity and connection to the supporting structure as a percentage
of the component mass than are typically calculated for the main SFRS. This, in essence, is the
philosophy underlying Eq. (6.7). A discussion on this topic can be found in Ref. 6.23.

Equations (6.7) through (6.9) would be used in this chapter to calculate the seismic
lateral force normal to the parapets for which R = 2.5. For parapets, two values of a are
p
p
assigned as follows, depending on whether they are unbraced or braced:
1. a = 2.5 if the parapet is unbraced or braced to the structural frame below its center of
p
mass.
2. ap = 1.0 if the parapet is braced to the structural frame above its center of mass.
Table 6.6 lists values of a and R for some of the architectural elements.
p
p
ASCE 7-05 Section 13.3 mandates additional requirements for applying F in design.
p
It requires that F be considered as acting independently in two orthogonal horizontal direc-
p
tions in combinations with the appropriate service loads associated with the component