EXAMPLE 6. BUILDING VALUATION & REDESIGN STUDY

EXAMPLE 6: "STUDY FOR THE VALUATION AND REDESIGN OF A BUILDING ACCORDING TO THE CANC. 59 3(a) STEP 3: ANALYTICAL ANALYSIS 3(a).1 General The main objective of the static anelastic analysis is to estimate the magnitude of the inelastic deformations that will develop in the structural elements of the structure when it is subjected to the seismic action for which the assessment or redesign is performed. These magnitudes of inelastic deformations shall be compared with the allowable values determined on the basis of the target performance level and the capacities of the members. In the static inelastic analysis, a simulation of the structure is used which takes into account inelastic load- deformation laws for the individual structural elements of the building. For concrete in particular, due to the coexistence of flexural and shear deformations, the inelastic law of bending moment - chord angle of twist (M-i) is used. The simulation is then subjected to horizontal loads distributed in a manner proportional to the inertial forces of the earthquake. The loads increase monotonically, generally until a structural element is no longer able to carry its vertical loads. During the loading process, each time a cross-section leaks (i.e. a plastic joint is formed) the simulation is modified by introducing appropriate connections and the incremental process continues. At each step of the analysis, the base shear (i.e. the sum of the horizontal loads ) and the displacement of a characteristic point of the tested structure ( control node ), which is generally taken at its top, are recorded. Thus, the base shear - peak displacement curve called the resistance curve is drawn, which is considered representative of the general seismic behaviour of a structure and is the basis for all the required checks for the satisfaction of the performance criteria. The primary resistance curve is idealised into a bilinear curve which is assumed to correspond to the force- displacement diagram of an equivalent single-stage system, the response of which is correlated with the response of the structure. The design earthquake enters the process through its imposed movement of the control node, which is called targeted movement . The targeted displacement is calculated with the help of the elastic design spectrum of the excitation by applying the so-called displacement modification method. According to this, the target displacement is obtained from the elastic displacement of the equivalent single-stage system, which is multiplied by a number of correction factors to take into account certain factors such as the difference from the peak displacement of the beam, the difference in elastic/elastic displacement, the degradation of strengths and stiffnesses due to cyclic loading and the influence of P-D effects. The last step of the Elastic Static Analysis is the check of the performance criteria, i.e. the comparison of the available strength values of the cross-sections of the structure with the required response values corresponding to a displacement of the control node equal to the target one. For fishy failure modes and behaviour the check shall be in terms of transient quantities, while for plagiform failure modes and behaviour the check shall be in terms of displacements or