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The use of modal analysis appears necessary in order to reduce both displacement demand under weak seismic events and ductility demand under strong earthquakes. Static analysis can be effective only if used with proper values of additional eccentricities. To overcome the inaccuracy of the code formulations, the authors propose a simple procedure that gives the exact values of these eccentricities and discuss the influence of the main parameters that govern the structural behavior. They also point out the difficulty in evaluating some parameters (stiffness radius of gyration, structural eccentricity) in the case of multistory buildings and discuss the validity of simplified formulations proposed to overcome this problem. The effectiveness of static analysis, applied to three-dimensional multistory structures with properly evaluated corrective eccentricities, is analyzed with reference both to regularly asymmetric multistory schemes and to an actual irregularly asymmetric structure (the main building of the Faculty of Engineering at the University of Catania, Italy).

SUMMARYThe evaluation of the dynamic response of non-classically damped linear structures requires the solution of an eigenproblem with complex eigenvalues and modal shapes. Since in practice only a small number of complex modes are needed, the complex eigenvalue problem is solved in the modal subspace in which the generalized damping matrix is not uncoupled by classical real modes. It follows that the evaluation of the structural response requires in both cases the determination of complex modes by numerical techniques, which are not as robust as techniques currently used for the solution of the real eigenvalue problem, and the use of complex algebra. In the present paper an unconditionally stable step-by-step procedure is presented for the response of non-classically damped structures in the modal subspace without using complex quantities. The method is based on the evaluation of the fundamental operator in approximated form of the numerical procedure. In addition, the method can be easily modified to incorporate the modal superposition pseudo-static correction terms.

In this paper thermal analysis was applied to determine the “Critical Stress” of concrete, different from its ultimate strength, able to produce the first damage in the structures under compressive loads. The Critical Stress can be thought as the stress able to produce the beginning of fatigue rupture within the material. Several specimens of high strength concrete were tested in order to define the incipient crack phenomena, also in internal part of the specimen not accessible by direct inspections, with the aid of infrared thermography. A finite element analysis completes the study and compares, for the same static loading conditions, the stress state with the experimental thermographic images. The final results show as the coupling of normal compressive test and the acquisition of the thermal images can be a useful aid to estimate a security stress value, indeed the Critical Stress, before the Ultimate Serviceability Limit (SLU) of the structure, defined as the maximum load condition before its failure.

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