Analysis and modelling of the seismic behaviour of high ... - Ingegneria

4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS
For these conditions, S = 1.0 and the reference periods TB, TC, and TD are
respectively equal to 0.05 s, 0.25 s, and 1.2 s. The period T1 for the structure was
calculated with the formula: T1 = 0.05·H 0.75 , with H = 7.5 m (total structure height),
giving T1 = 0.22 s. The value of Sd (0.22 s) = 0.167.
The seismic weight at each level includes the total gravity loads and the following
fraction of the imposed live load: 48% at the first level (φ= 0.8, ψ2i = 0.6) and 60%
at the second level ((φ= 1.0, ψ2i = 0.6). The resulting total seismic weight, W,
including the weight of the structural members is given in Table 4.1. In this
calculation, the weight of a 3.5 m and a 1.75 m long column segments were
applied respectively at the first and second levels. Assuming rigid diaphragm
response, the seismic weight was divided equally between the 5 moment resisting
frames, resulting in W = 552 kN/frame. The parameter in Eq. (4.2) is equal to
0.85 because T1 < 2 TC. Substituting these values, the base shear, Fb, is equal to
78.2 kN per frame. In Table 4.1, the base shear force was distributed at each level
using Eq. (4.1) and assuming mi = Wi/g.
Level
z
(m)
wd
(kPa)
ψewL
(kPa)
TOTAL W
(KN)
W / Frame
(kN)
F / Frame
(kN)
2 7.0 3.94 3.00 1398 280 52.6
1 3.5 3.94 2.40 1361 272 25.6
2759 552 78.2
Table 4.1. Calculation of the seismic weights and seismic loads (torsion excluded)
The design was performed only for one of the interior moment resisting frames.
Accidental torsion was included in the design and the most critical frames are
those located at 3.0 m from the center of the structure. For these frames, an
amplification factor, , equal to 1.15 (= 1.0 + 0.6·3.0/12.0) was used. Therefore, the
final seismic loads were: 60.5 kN and 29.4 kN at levels 1 and 2 with a total base
shear of 89.9 kN. In the design of the frames, these loads were combined with the
total gravity dead load and 60% of the imposed live loads.
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