Engineers are constantly searching for new and more realistic methods to account for the structural behavior. Performance based strategies need to estimate the inelastic deformation and the associated damage in structures but elastic analysis cannot provide this information. Nonlinear dynamic response history analysis can provide this information, but it is a tedious procedure based on uncertainties coming from the excitation. So the scientists proposed some new design methods and rehabilitation strategies that incorporate performance based engineering concepts. It is clear that damage control should be considered as a more explicit design consideration. This goal can be reached only by consideration of some kind of non linear analysis into the seismic design methods. The most logical approach seems to be a mixture of the nonlinear static analysis (pushover analysis) and the response spectrum method.

For seismic performance evaluation of old and new structures, the static pushover analysis can be used. This type of analysis gives some output information on seismic demands imposed by the design earthquake on the structural system and its components.

The pushover analysis is a static nonlinear analysis. In this kind of analysis the structure will be loaded under permanent vertical loads and gradually increasing lateral loads that approximately simulates the earthquake forces. Then a graph of the total base shear vs. top displacement in a structure can be plotted by this analysis. By help of this plot any premature failure or weakness in the structure can be tracked. This process continues up to failure and finally the designer will be able to determine the collapse load and ductility capacity. Also the plastic rotation in the elements can be monitored, and lateral inelastic forces versus displacement response for whole of the structure can be analytically computed. By help of this kind of analysis the deficiencies of a structure can be determined and then the proper rehabilitation strategy can be selected.

Using the pushover analysis, the expected performance of structural systems can be evaluated. This can be done by derivation of the performance of the structural system by estimating its strength and deformation demands in design earthquakes by applying of static inelastic analysis, and comparing these demands to available capacities at the performance levels of interest.

The most important performance parameters that are usually evaluated are:

• global drift, interstory drift
• Inelastic element deformations (either absolute or normalized with respect to a yield value)
• Deformations between elements
• Element connection forces (for elements and connections that cannot sustain inelastic deformations).

The inelastic static pushover analysis is a method for estimation of seismic force and deformation demands. It calculates with an approximate procedure the redistribution of internal forces that no longer can be resisted within the elastic range of structural behavior of elements.

There is a critical concept in this kind of analysis and this is the “target displacement”. Performing the pushover analysis leads to estimation of the target displacement magnitude, as a representative displacement, at which seismic performance evaluation of the structure is to be valid. In fact the target displacement serves as an estimation of the global displacement of the structure is expected to experience in an expected design earthquake.

In this procedure we assume that a MDOF structure can be simulated by an equivalent SDOF system and the target displacement of the original structure can be estimated by the target displacement of the SDOF mass center. This concept is acceptable only with some limitations and only if great care is taken in incorporating in the predicted SDOF displacement demand all the important ground motion and structural response characteristics that significantly affect the maximum displacement of the MDOF structure. In this assumption the maximum MDOF displacement is controlled by a single shape factor and the higher mode effects are not considered.

The static pushover analysis procedure is becoming the dominant method implemented in the computer to evaluate the seismic performance of structures. The method assumes that the response of the structure can be checked by considering its first mode, and this mode during a monotonic increase of loading governs the motion constantly. There are some methods that are based on this methodology such as the capacity spectrum method (in ATC 40) and the nonlinear static procedure (in FEMA 273). The second procedure is used in ATC 40 by “displacement coefficient method” as an alternative method.

The Capacity Spectrum Method (CSM) approach is used to compare the structure’s capacity with the demands of earthquake ground motion on the structure. A nonlinear force displacement curve is used to represent the capacity of the structure (pushover curve). Using the coefficients that represent effective modal masses and modal participation factors, the base shear forces should be converted to equivalent spectral accelerations and the roof displacements should be converted to equivalent spectral displacements. These spectral values define the capacity spectrum. The earthquake ground motion demands can be represented by response spectra that correspond to the level of equivalent viscous damping representing the dissipated hysteretic energy. Finally both of the curves are drawn within a same graph to determine the intersection point of the two curves that expresses the performance of the structure to the design earthquake incorporated in the particular spectrum.

In this work a simplified method for nonlinear static analysis of building structures subjected to monotonically increasing horizontal loading (pushover analysis) is presented using the SAP 2000 software. Following a step by step analysis an approximate relationship between the global base shear and top displacement of the structure is determined. During the analysis the development of plastic hinges, at different stages, throughout the building are monitored. The mathematical model, the base shear- top displacement relationships and the step by step computational procedure are described. The method is applied for the analysis of an existing seven-story reinforced concrete building. The results are presented and the evaluation of the building performance is discussed. Finally an appropriate type of intervention is proposed to improve further the seismic behavior of the building.