Earthquake Engineering Research - HKU Libraries - The University ...

474
quantities of interest are: (1) absolute acceleration which determines the forces acting on the shear
walls; (2) the maximum relative displacement of the superstructure (in case of free sliding); and (3) the
residual relative displacement which will indicate the position of the superstructure at the end of the
ground motion. The following range of values of different parameters has been estimated, that would
cover a wide variety of multistory masonry buildings:
TABLE 3.1
DATA FOR COMPUTING SEISMIC RESPONSE
Time Period
(TP) sec.
0.40
0.50
0.60
0.70
0.80
Mass Ratio
(9)
6 to 8
8 to 10
10 to 12
12 to 14
14 to 16
Damping
Coefficient
(f)
0.05 to 0.15
0.05 to 0.15
0.05 to 0.15
0.05 to 0.15
0.05 to 0.15
Coefficient
of friction
(M.)
0.10 to 0.25
0.10 to 0.25
0.10 to 0.25
0.10 to 0.25
0,10 to 0.25
Restricted
Sliding base
range (A) mm
2 to 130
3 to 140
2 to 150
9 to 145
2 to 165
It is assumed that a coefficient of friction less than 0.10 in sliding will be difficult to obtain in actual
building construction, and for a value greater than 0.25, no sliding motion may occur in most real
earthquakes and the system may act just like a fixed-base structure. For four-story building, top three
and half portion of the total mass of the building is assumed to be lumped as the top mass in the
mathematical model. In the same way, as the story of the building increases, the mass ratio increases.
In estimating the mass ratio, it is assumed that mass in each story is same.
4. DISCUSSION OF RESULTS
The influence of various parameters on maximum response of the free sliding (without stopper),
restricted base sliding (with stopper) and fixed base multistory masonry buildings subjected to Koyna
and El Centra shocks is presented in the present investigation through representative Figures 4.1 to 4.6.
A comparison of the absolute acceleration response in the restricted base sliding has been made with
that of the free sliding and fixed base buildings. The results of this study are discussed in the following
paragraphs.
Figs. 4.3 to 4.6 shows the typical acceleration response of the masonry buildings subjected to Koyna
and El Centra earthquakes for different parameters. In these figures, the upper dashed line shows the
maximum acceleration response in the fixed base structure, whereas the bottom dashed lines show the
maximum acceleration developed in the building with free sliding base system during the seismic
ground motion at different mass ratios. The firm lines show the maximum acceleration response
computed in the buildings with the restricted base sliding system during the earthquakes at different
mass ratios.
4.1 Influence of Viscous Damping
The representative acceleration response curves show that at a particular mass ratio, time period and
coefficient of friction in the case of restricted base sliding buildings, an increase in viscous damping
(from 5% to 15%) decreases the acceleration response (Figs. 4.1 and 4.2). This is perhaps because of
increasing energy dissipation in the internal friction of the system as the damping coefficient increases.
In view of this well-established trend, the acceleration response for other cases has not been studied.