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STATIC AND DYNAMIC ANALYSIS RESULTS
The yield point of the structure, defined based on an equivalent elasto-plastic system with reduced
stiffness, evaluated as a secant through 75% of the maximum, is adapted for this study On the other
hand, the ultimate limit state (ULS) is based on a global criterion of the attainment of 3% interstorey
drift. At this level of drift, it is generally accepted that structures would have suffered major structural
and non-structural damage. Further refinement of the limit states is not necessary since the primary
objective is to compare the different structures. Figure 3 shows the force-displacement curve for the
normal strength and the equivalent stiffness structures. It is noticeable that the initial stiffness of the
structures is the same This feature is also exhibited in the reduced stiffness structure, thus validating
the equivalence criteria used. This also confirms that imposing equivalent stiffness at the member level
will give a structure with the same overall stiffness.
10000
8000
* 6000 -
W
u
K
4000
/
/
/•
^ -~ ~
>- — 1
500 1000 1500
TOP DISPLACEMENT(mm)
N35 1-
---- NSO I
Kttnn ,
2000
FIGURE 3
FORCE-DISPLACEMENT CURVE FOR N35, N80, N100 AND N120
The level of overstrength is defined herein as the ratio of the capacity of the structure, based on the
static pushover analysis, to the code-defined design base shear. The calculated overstrength for the
different structures is given in Table 5.
TABLES
STRUCTURE OVERSTRENGTH
Model
Overstrength
N35
6.29
N80
4.51
N100
4.47
N120
4.42
N80R
4.20
NIOOR
4.67
N120R
4.93
N80RS
4.25
N100RS
4.77
N120RS
5 10
For each of the ten structures, dynamic analysis was performed with three artificial records, scaled to
the design ground acceleration (PGA) and twice the design PGA, corresponding to 0.4g and 0.8g,
respectively. As a measure of global response characteristics, the displacement and total base shear
time-history and the interstorey drift ratio from the analyses are examined. For the purposes of
discussion, NSO, N100 and N120 constitute "E" group (based on the equivalent stiffness criterion);
N80R, NIOOR and N120R constitute "R" group (based on the reduced stiffness criterion); and N80RS,
N100RS and N120RS constitute"RS" group (based on the reduced stiffness criterion with one steel
grade only).
It was observed that for buildings in the same group, subjected to the same input motion, the shape of
the displacement and base shear response was similar. This suggests that structures in the same group