Page 482 - Design of Reinforced Masonry Structures

7.44 CHAPTER SEVEN

The value of fundamental period, T, appearing in Eqs. (7.51) and (7.52) should be
determined from structural properties and deformational characteristics of the seismic
force–resisting system by a properly substantiated analysis. Although this analysis can-
not be performed until after the structure has been designed and member properties are
completely known, its value must be known a priori to use Eqs. (7.51) or (7.52). To obvi-
ate this difficulty, an approximate value of the fundamental period, T , is determined from
a
Eq. (7.59):
x
T = C h () (7.59)
a t n
where h = height of the structure from the base to the highest level (level n)
n
C , x = coefficients listed in Table 7.4
t
The height h to be used in Eq. (7.59) need not include heights of penthouses and
n
other rooftop-supported structures weighing less than 25 percent of the weight at the roof
level.

TABLE 7.4 Values of Approximate Period Parameters C t and x [7.13]
Structure type C t x
Moment-resisting frame systems of steel in which the frames resist 0.028 0.8
100 percent of the required seismic force and are not enclosed or (metric 0.0724)
adjoined by more rigid components that will prevent the frames
from deflecting where subjected to seismic forces.
Moment-resisting frame systems of reinforced concrete in which the 0.016 0.9
frames resist 100 percent of the required seismic force and are not (metric 0.0466)
enclosed or adjoined by more rigid components that will prevent
the frames from deflecting where subjected to seismic forces.
Eccentrically braced steel frames and buckling restrained braced 0.03 0.75
frames (metric 0.0731)
All other structural systems 0.02 0.75
(metric 0.0488)

The approximation involved in Eq. (7.59) for the value of fundamental period should be
evident from the fact that it does not take into account the nature of building configuration
(e.g., length-to-width ratio, squat versus slender structure, flexible roof versus rigid roof,
etc.), particularly in the “all other structural systems” category.
Alternatively, for structures not exceeding 12 stories in height with each story being at
least 10 ft (3 m) high, and the seismic force–resisting system consisting entirely of steel or

concrete moment-resisting frames, the approximate fundamental period can be determined
from Eq. (7.60):
T = 0.1N (7.60)
a
where N = number of stories
ASCE 7-05 makes an exception for concrete or masonry shear wall structures for which
T can be determined from Eq. (7.61):
a
.
T = 0 0019 h (7.61)
a
C w n