The process of evaluating and designing the retrofit of existing buildings differs from the conventional structural design of new buildings. The current state-of-the-art analysis and design approach for the seismic evaluation of existing buildings is founded on a performance-based philosophy. There are two parts to a performance-based analysis and design.
First, there is the establishment of a performance objective. This answers the question for the designer and the owner, “What degree of damage to the building am I willing to tolerate in the event of an earthquake?” It is not economically feasible to design all buildings to a performance objective that limits all damage or allows the building to remain fully operational and allow immediate occupancy following an earthquake. Therefore, performance objectives exist that allow a certain degree of damage to occur while still protecting life safety and preventing building collapse.
Second, there is the establishment of the seismic demand used in the analysis of the building. Statistical analysis is used to determine the probability of the maximum considered earthquake (MCE) occurring at the building site at any given time. The MCE demand level varies based on the time frame considered and the probability that there will be ground motion at the site that exceeds the MCE (i.e. 5% probability of exceedance in 50 years). Together with these two variables the mean return period of an earthquake can be established (i.e. it can be expected that an earthquake of ‘X’ magnitude, or the MCE, will occur approximately at least every 975 years).
There are various performance objectives and seismic demand levels that may be considered. Any given combination of performance objective and seismic demand level will result in a varied stringency of analysis and design. Combining a strict performance objective (i.e. operational post-earthquake) with an earthquake of relatively long return period (2500 years) will likely result in a robust, yet potentially expensive, design.
In conventional structural analysis and design, the seismic demand used for the design of the seismic force resisting system is reduced by a system-wide Response Modification Factor, R. This coefficient is established based on the ductility of the lateral system selected for design. The R-Factor is intended to act as a representation of the ability of the lateral system to dissipate energy as it flexes, bends, and undergoes inelastic deformation under seismic load.
In the evaluation of existing buildings, the concept of reducing the demand to account for ductility in a system is captured by using component specific m-factors. Rather than reducing the seismic demand, m-factors are applied to scale up the strength or capacity of individual structural elements that experience ductile or “Deformation Controlled” failure. These m-factors vary by component and allow the design professional to apply a uniform seismic demand to the system while modifying the strength of each individual element of the system according to its ductility. This philosophy is ideal for seismic retrofits that require the introduction of an entirely new lateral system or the strengthening of only a few discrete components.