Seismic collapse capacity of vertical inhomogeneous and P-delta sensitive frame structures (in German)

The seismic collapse capacity of a building depends both on the supporting structure and the imposed earthquake record. Most of the investigations in earthquake engineering are based on regular systems, where all structural and geometric properties are continuously distributed as specified in the design process. However, in reality many buildings exhibit some irregularities, as a result of errors in the construction work or modifications during life cycle. Consequently, it is obvious to study the effect of those irregularities on the seismic behavior of buildings subjected to earthquake excitation.

In this Master’s Thesis the seismic collapse capacity of planar multi-story generic frame structures with various imposed irregularities (in vertical direction) is investigated, performing both non-linear static analysis and incremental dynamic analysis. In particular, strength, stiffness, or stiffness and strength simultaneously, of certain elements are reduced in specified locations of the structure. At the beginning the underlying methods of analysis as well as the theoretical background is described. Then, the structural models used for the assessment of the global collapse capacity are specified. These models are vulnerable to the P-delta effect. That is, in the inelastic range of deformation the post-yield stiffness becomes negative due to gravity loads, and consequently, these structures are prone to collapse when subjected to severe earthquake excitation. Material deterioration is, however, not considered. The next session describes the computational implementation of the models and the process of analysis conducted by means of the simulation software OpenSees. The obtained results are illustrated and explained in detail. The emphasis is on the comparison of the seismic collapse capacity and its record-to-record variability of the regular system with the ones based on systems with imposed irregularities in vertical direction. This thesis ends with a summary, provides conclusions gained from the derived results, and issues to be possibly studied in further investigations are outlined.

One important finding of this thesis is that the discontinuous reduction of strength and stiffness not necessarily leads to an impairment of the structural behavior in a seismic event. Depending on the location of the imposed „damage“, the global structural behavior may in some circumstances even be enhanced.