Page 207 - Handbook of Materials Failure Analysis

5 Case Study 203

Table 8.10 Evaluation of the Distribution Coefficients of the Two Main
Eigenmodes
The mode “i” γ i
Mode 1 1.00
Mode 2 0.00249893

5.3.7 Calculation of the acceleration S a
The imposed acceleration “S a ” on the tank, considering its interaction with the
ground, is a data of seismic code based on the seismic zone and other parameters.
It is a fraction of the gravity acceleration. The Algerian seismic code proposes for
its determination the following relations:

T Q
8
> 1:25A 1+ 2:5η 1
> 0 T T 1
> R
> T 1
>
>
> Q
>
>
T 1 T T 2
>
ð
2:5η 1:25AÞ
>
< R
S a
2=3 (8.26)
g Q T 2
¼
>
> T 2 T 3:0s
R T
ð
> 2:5η 1:25AÞ
>
>
>
>
2=3 5=3
>
> 3 Q
T 2
>
> T > 3:0s
:
2:5η 1:25AÞ
ð
3 T R
Where, “A” is the acceleration coefficient of the zone, depending on the seismic zone
and the group use of the structure. The Algerian seismic code classes the storage
tanks as structures of group 1B (Table 8.11).
η is a correction factor of damping given by the following relation:
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

7 0:70
(8.27)
ð 2+ ξÞ
η ¼
where ξ denotes the percentage of critical damping. Its value depends on constituent
material, the type of structure, and the importance of filling (Table 8.12).
The classification of structural systems “R” is made taking into account their
reliability and their ability to dissipate the energy to the seismic action, and the
Table 8.11 Acceleration Coefficient of the Zone “A”
Zone IIa Zone IIb
Zone I (Low (Average (Average Zone III (High
Group Seismicity) Seismicity) Seismicity) Seismicity)
1A 0.15 0.25 0.30 0.40
1B 0.12 0.20 0.25 0.30
2 0.10 0.15 0.20 0.25
3 0.07 0.10 0.14 0.18