Page 212 - Handbook of Materials Failure Analysis

208 CHAPTER 8 Seismic risk of RC water storage elevated tanks

Table 8.18 Evaluation of the Projecting Dimensions Ratio
The external diameter of the vessel (D) 6.20 m
The external diameter of the supporting system (d) 4.00 m
The ratio D d=D 0.35

it may be necessary to add bracing elements positioned to rebalance the distribution
of rigid elements. Analysis of seismic damage, clearly shows that regular and sym-
metrical structures perform better than those whose geometric forms and the distri-
bution of resistive elements are complex, even when the seismic rules were correctly
applied. The calculations, as prescribed by these seismic codes, represent seismic
stresses to structures of a fairly realistic way for regular structures whose seismic
response is simpler than for irregular structures [15]. The symmetry and regularity
in plan have the function of providing better resistance to earthquakes. Structures
should preferably have, on one hand, simple forms and on the other hand, distribution
as regular as possible of masses and stiffness as in plan and elevation. The aim is to
ensure the best possible distribution of forces through the structure, so as to involve
all the resistant elements to the absorption and dissipation of the energy developed by
the seismic action. Taking inspiration of the seismic building rules [16], applied to
buildings, we can consider the following criteria:
• The tank should have a symmetrical configuration in both directions, both for the
distribution of rigidities than those of the masses. The inspected tank presents a
perfectly symmetrical configuration in both directions, both for the distribution
of rigidities than for the masses. This leads us to say that the center of mass
coincides with the center of torsion (rigidities).This criterion is satisfied.
• The tank form should be compact. The dimensions of the projecting parts shall
not exceed 25% of the total tank size in the corresponding direction in accordance
with this relation: D d < 0:25 (see Figure 8.8). In our case, the ratio of the
D
projecting dimensions is greater than 25% (see Table 8.18).This criterion is not
satisfied (Figure 8.8).
• The geometric slenderness ratio λ g of the supporting system, which defines the
ratio between the buckling length and the minimum gyration radius of the cross-
section of the support, should be between the following values: 3 λ g 5. In our
case, this criterion is satisfied (see Table 8.19).
In conclusion, the regularity in plan is partially verified. This parameter is classified
as class B.

5.6 REGULARITY IN ELEVATION
The principle of regularity in elevation is to avoid discontinuities or excessively fast
changes in the distribution of inertias, stiffness, and masses. Taking inspiration from
the seismic building rules [16], applied to buildings, we can consider the following
criteria: