In Situ Dynamic Characteristics of Reinforced Concrete Shear Wall Buildings

Abstract

Structural engineers routinely need to make assumptions about the dynamic properties (natural periods, mode shapes, and damping) of a building to simulate its response to dynamic loads, such as strong winds or earthquake ground motions. However, the assumed properties may significantly differ from those of the actual building, once it is constructed, due to differences between the idealized model and in situ conditions. The main objectives of this study are to evaluate how common models and assumptions used to predict the dynamic properties of buildings compare to those measured in actual buildings, and to develop improved prediction models.To this end, ambient vibration measurements were performed in 39 buildings on the island of Montréal and the dynamic properties of up to six vibration modes were identified, for each of these buildings, using the enhanced frequency domain decomposition method. Though the initial goal was to obtain a representative sample of different types of buildings, 27 of these 39 buildings turned out to be reinforced concrete buildings with shear walls providing the main resistance to lateral loads. Hence, the scope of this study was narrowed to the dynamic properties of reinforced concrete shear wall (RCSW) buildings. The measured dynamic properties of this subset of 27 buildings were then used to evaluate different models, proposed in building codes and in the literature, to predict the natural periods and damping characteristics of these types of buildings. Finally, simple models to predict the natural periods of torsion and second translation modes were proposed based on regression analyses. These models agree very well with those that have been proposed in the literature. Again, equations corresponding to different probability levels (mean, mean minus one standard deviation, and mean plus one standard deviation) were produced as a measure of uncertainty.This study should help engineers select realistic values of the dynamic properties of RCSW buildings for structural analysis and design, and should ultimately improve engineers' ability to predict the dynamic response of these buildings. Further, the proposed models could lead to improved recommendations in building codes.iii This research could not have been possible without the help of many building owners and managers, who allowed our team to perform vibration measurements in their buildings. I would also like to thank the numerous graduate and undergraduate students who were involved in these tests for both their participation and the stimulating discussions that often ensued:

Ambient Vibration Measurements of Dynamic Properties of School Buildings in Montreal, Quebec

“…The problem lies in the fact that the number of buildings studied was very limited and later studies, where more data were available, showed a poor fit. The original buildings studied were also all located in California, and it is questionable if buildings located elsewhere would follow a similar trend (Gilles, 2011). This has been recognized by research, and alternative simplified expressions have been derived based on experimental periods measured in buildings in different regions of the world.…”

“…Following the same procedure as in Gilles (2011), multiple linear regressions were performed on the data sets for C2 and C3 buildings to explore how expressions of the type fit the measured data, where is the measured fundamental period and is the height of the building above ground, in meters. Three cases were considered based on the allowed values for the exponent .…”

Measured natural periods of concrete shear wall buildings: insights for the design of Canadian buildings

“…Further, a significant inconsistency can be observed in eq. [5] when some shear walls do not extend to the top of a building (Gilles 2011). …”

“…Below is a brief description of the data collection and analysis procedures, which are described in more detail in Gilles (2011) and . For each building, velocities resulting from ambient excitations were recorded at several locations, distributed spatially along the height and plan dimensions, using two Lennartz LE-3D/5s tri-axial seismometers (Lennartz Electronic 2011).…”

Dynamic characteristics of light-frame wood buildings

“…The code period formula used to calculate the building's fundamental period has been evaluated by several researchers (Goel and Chopra 1998;Lee et al, 2000;Morales, 2000;Gilles, 2011) for its applicability to concrete shear wall buildings. Only one study proposed an alternative period equation for wood based buildings (Camelo, 2003), which presents a clear need to evaluate the code period equation, using a comprehensive data base of natural periods from real buildings.…”