Page 197 - Handbook of Materials Failure Analysis

5 Case Study 193

Table 8.3 Classification of Tanks According to Their Vulnerability Index I V [11]

• The green level, 0 I V 33: The examined tank is not vulnerable. It presents a
good behavior to the earthquake and it doesn’t require special repairs.
• The orange level 1: 33 I V 90: The tank has an average vulnerability to
earthquake. It needs monitoring.
• The orange level 2: 90 I V 133: The tank has fairly high earthquake
vulnerability.
• The red level: 133 I V 199: The tank has high earthquake vulnerability.
Therefore, the tank must be decommissioned immediately.

5 CASE STUDY
As a practical application, we illustrate the method of vulnerability index in a water
3
storage elevated tank of 100 m capacity and located in average seismicity zone in
Algeria. This tank shown in Figure 8.1 of frustoconical form has been made of rein-
forced concrete in 1986 and has been investigated and analyzed 28 years later. Sev-
eral visits were made to the archives of the Hydraulic Administration of Tizi Ouzou.
We were able to find the formwork drawings of our tank dating from 1986. No note
of calculation has been found. A detailed review of these execution plans (see
Figure 8.2) found in very good condition after 28 years of archiving have enabled
us to evaluate the seismic load in Section 5.3. The geometrical characteristics of
the tank are given in Table 8.4.

5.1 THE BRACING SYSTEM
The bracing system of an elevated tank is constituted by the set of resistive elements
ensuring its rigidity and stability to the horizontal forces caused by the earthquake. It
constitutes one of the most important aspects of seismic design. The classification of
this parameter is defined taking into account the capacity of energy dissipation to the
seismic action and deformation. For this, there is currently interest to equip tanks
with a regular structural mesh in order to avoid twisting of bracing elements. The
tank studied is installed on a supporting system of pilings which is a structure that
has a low rigidity distribution of mass and stiffness that changes from abrupt manner
(see Figures 8.1 and 8.2). The parameter is classified as class C.

5.2 QUALITIES OF THE BRACING SYSTEM
Forthisparameter,classificationisdoneaccordingtothequalitycontrolofmaterialsand
quality control of execution. Systematic tests on the materials used must be made by the
company on site during construction, in order to control the concrete strength. This task