George A. Papagiannopoulos

A seismic design method for reinforced concrete moment resisting frames using modal strength reduction factors

A performance-based seismic design method for plane reinforced concrete (R/C) moment-resisting frames (MRF) is proposed. The method is a force-based seismic design one, utilizing not a single strength reduction factor as all modern codes do, but different such factors for each of the first significant modes of the frame. These modal strength reduction factors incorporate dynamic characteristics of the structure, different performance targets, and different soil types. Thus, the proposed method can automatically satisfy deformation demands at all performance levels without requiring deformation checks at the end of the design process, as it is the case with code-based design methods. Empirical expressions for those modal strength reduction factors as functions of the period, deformation/damage and soil types, which can be used directly in conjunction with the conventional elastic pseudo-acceleration design spectra with 5% damping for seismic design of R/C MRFs, are provided. These expressions have been obtained through extensive parametric studies involving non-linear dynamic analyses of 38 frames under 100 seismic motions. The method is illustrated by numerical examples which demonstrate its advantages over code-based seismic design methods.

Earthquake design for controlled structures

An alternative design philosophy, for structures equipped with control devices, capable to resist an expected earthquake while remaining in the elastic range, is described. The idea is that a portion of the earthquake loading is undertaken by the control system and the remaining by the structure which is designed to resist elastically. The earthquake forces assuming elastic behavior (elastic forces) and elastoplastic behavior (design forces) are first calculated according to the codes. The required control forces are calculated as the difference from elastic to design forces. The maximum value of capacity of control devices is then compared to the required control force. If the capacity of the control devices is larger than the required control force then the control devices are accepted and installed in the structure and the structure is designed according to the design forces. If the capacity is smaller than the required control force then a scale factor, ?, reducing the elastic forces to new design forces is calculated. The structure is redesigned and devices are installed. The proposed procedure ensures that the structure behaves elastically (without damage) for the expected earthquake at no additional cost, excluding that of buying and installing the control devices.

Seismic Design of Plane Steel Frames Using Modal Strength Reduction (Behaviour) Factors

A new method for the seismic design of plane steel moment resisting frames is developed. The method is based on the determination of the maximum seismic structural response through spectrum analysis utilizing different values of the strength reduction (behaviour) factor for the first few significant modes. These modal strength reduction factors are derived with the aid of (a) design equations that provide equivalent modal damping ratios for steel moment resisting plane frames as functions of period, allowable interstorey drift and damage levels and (b) damping reduction factors that modify elastic acceleration spectra for high levels of damping. One can use these modal reduction factors in conjunction with a design spectrum and modal synthesis tools to calculate the design forces of a steel framed structure. By performing seismic designs of a steel frame it is demonstrated that the use of modal strength reduction factors instead of a single value for that factor for all modes, as it is the case with current seismic codes, leads to more accurate results in a more rational way.