It has been known since the 1950s that under certain conditions, in the near-fault, earthquake ground motions can consist of a limited number of strong acceleration pulses. These types of ground motions have come to be referred to as ¿pulse-type¿ ground motions. These pulses are mostly associated with the normal to the fault direction, and amplify the long period coherent component of the ground motions, explicitly apparent in the velocity and the displacement time histories and the related response spectra. The identification of the near-fault velocity pulses requires further research for the definition of their characteristic parameters, mainly their period and amplitude. In this project a new procedure for the identification of the predominant pulse that is related to directivity phenomena and determination of the parameters of the velocity pulse inherent in pulse-like ground motions, will be developed and discussed. The existence of the directivity pulse in the ground motion has an important effect on the response of the structures. The aim of this study is to provide a dataset of near-source pulse-like strong motion records, suitable for seismic response analysis and ground motion studies in proximity to the seismic source.
Additional research will be developed to establish bounds on the different parameters that define the characteristics of severe long pulses, i.e., the largest incremental velocity, response spectra, displacement and energy spectra, and associated intensity measures. Such parameters will be derived from both linear and nonlinear analyses.
Finally, due to the fact that near-fault effects are still scarcely represented in design codes, a detailed discussion pertaining to the implication of near-fault ground motions on the design and assessment of structures will be presented.
The currently available research activity about near-fault demand characterization and its effect on structures needs significant improvement. The aim of the project is to contribute to the current state of the art of research in this field as follows.
Firstly, it is worth to point out that there is no consensus or generally accepted definition to discriminate whether certain wave is a ¿pulse¿ or not; this matter is a concern of this project, and efforts will be made to contribute in this direction by using the Variational Mode Decomposition method. The most significant contribution in terms of application in earthquake engineering in this regard is the recent study by Baker [1] who proposed a quantitative criterion, based on a wavelet analysis, to classify a ground motion record as pulse-like. A second important aspect that will be considered in this study is the definition of predictive relationships between the period of the pulse and other important parameters. Generally, all these relationships take into account only the contribution of the magnitude.
Several authors have pointed out that current building codes do not include the parameters necessary to take into account directivity phenomena and the impulsive motions resulting from them [2-5]. Generally, some near-fault factors can be considered inadequate to provide consistent structural protection, since they give little attention to the physical characteristics of near-fault motions [6]. It has been also stated that the evaluation of seismic demands considering inelastic response is based on recommendations derived from far-fault motions, which may lead to a significant underestimation of the actual demand [7]. Indeed, several authors [e.g., 3-5] states that even though response spectra can provide the basis for the specification of design spectra, there is a growing recognition that they do not describe adequately the seismic demands resulting by brief impulsive motions. According to the above discussion the relevance of the activity proposed in this project with the evaluation of the near-fault demand by means of energy spectra, is apparent. Energy demand reflect more in deep the presence of the pulse in the forward-directivity time histories.
Early, Anderson and Bertero [8] showed that one of the most significant aspects of near-field motions was the presence of long duration pulses. Their results showed that the non-linear response of structures to such motions was particularly sensitive to the pulse amplitude and duration. With regard to spectral shapes, Anderson and Bertero [8] pointed out that the design spectrum should be modified in the long-period region. The most important innovation in the proposed project is to consider also other definition of design spectra by including the contribution of velocity pulses directly in the design phase. As already underlined, long period pulses in near-fault records are an important factor in causing damage due to the transmission of large amounts of energy to the structures in a very short time. Under such circumstances high-energy dissipation demands usually occur. Therefore, the consideration of energy spectra seems an important way of including directly near-fault demand in performance-based seismic design.
Finally, it is worth to mention that in this study it is planned to use systems with several inelastic hysteretic models so as to be representative of steel frames and of reinforced concrete structures in flexural and shear behavior. On the other hand, it is planned to consider a wide range of the damping factor, considering that besides structural damping the presence of active dissipation devices is possible.
[1] Baker J.W. (2007) Quantitative classification of near-fault ground motions using wavelet analysis, Bull. of the Seismological Society of America, 97:1486-1501.
[2] Hall, J. F., Heaton, T. H., Halling, M. W., and Wald, D. J., Near-source ground motion and its effects on flexible buildings, Earthquake Spectra, 11(4), 1995, 569¿605.
[3] Somerville, P.G. (2000) New developments in seismic hazard estimation. Proc. 6th International Conference on Seismic Zonation (6ICSZ), Palm Springs, CA.
[4] Mollaioli F., Bruno S., Decanini L., Panza G.F. (2006) Characterization of the dynamical response of structures to damaging pulse-type near-fault ground motions, Meccanica, 41:23¿46.
[5] Bray, J.D. and Rodriguez-Marek, A., (2004) Characterization of forward-directivity ground motions in the near-fault region, Soil Dyn. Earthq. Eng. 24, 815¿828.
[6] Alavi, B., Krawinkler, H., (2004) Behavior of moment-resisting frame structures subjected to near-fault ground motions. Earthq. Eng. and Struct. Dyn., 33(6), 687¿706.
[7] MacRae, G.A., Morrow, D.W. and Roeder, C.W. (2001) Near-fault ground motion effects on simple structures, ASCE J. Struct. Eng. 127(9), 996¿1004.
[8] Anderson, J.C. and Bertero, V.V., (1987) Uncertainties in establishing design earthquakes, ASCE J. Struct. Eng. 113(8), 1709¿1724.