Effects of Fling-Step and Forward Directivity on the Seismic Response of Buildings

Earthquake Spectra, Vol. 22, No. 2, pp. 367-390, May 2006

Erol Kalkan, U.S. Geological Survey, Menlo Park, ekalkan@usgs.gov

Sashi K. Kunnath, University of California, Davis, skkunnath@ucdavis.edu

Printable article (1.5 Mb)

Downloadable Twenty-five Processed Fling, Forward Directivity and Far-Fault Records

Fling-Step | Forward-Directivity | Far-fault

Plain English Summary  

Near-fault ground motions are often characterized by coherent long-period velocity pulses that may result in sudden and extreme deformation demands in structural components. This study investigates the consequences of well-known characteristics of pulse-type motions on the seismic response of moment-frame steel buildings. The severity of inelastic demands was evaluated for four, six and thirteen-story existing steel buildings subjected to near-fault ground motions with fling-step and forward directivity, and compared to their response to far-fault ground motions. Additionally, idealized pulses are utilized in a separate evaluation study to gain further insight into the effects of high amplitude pulses on structural demands. Simple input pulses were also synthesized to simulate artificial fling-step effects on ground motions originally having forward directivity. Findings from the analytical simulations reveal that median maximum demands as well as the dispersion in the peak values for the three buildings were higher for near-fault records than far-fault motions. The arrival of the velocity pulse in a near-fault record causes the structure to dissipate considerable input energy in relatively few plastic cycles whereas cumulative effects from increased cyclic demands are more pronounced in far-fault ground motions. For pulse-type input, the maximum demand is a function of the ratio of the pulse period to the fundamental period of the structure. More significantly, records with fling effects were found to excite systems primarily in their fundamental mode while waveforms with forward directivity in the absence of fling caused higher modes to be activated. It is also concluded that the acceleration and velocity spectra, when examined collectively, can be utilized to reasonably assess the damage potential of near-fault records.

 

Resumen Espanol

Los registros sísmicos cercanos a la falla a menudo se caracterizan por pulsos de velocidades altas, que pueden dar lugar a demandas de deformación repentinas y extremas de los elementos estructurales. En este estudio se investigan las consecuencias de las características conocidas de los movimientos de tipo pulso en la respuesta sísmica de edificios estructurados en base a marcos de acero. La severidad de las demandas inelásticas son evaluadas en edificios existentes, con estructura resistente de acero, de cuatro, seis y trece pisos, sometidos a movimientos sísmicos cercanos a la falla con “fling-step” y “forward directivity”. Las respuestas obtenidas de estos edificios son comparadas con sus respuestas a movimientos sísmicos lejanos a la falla. Además, para lograr una mayor comprensión de los efectos de pulsos de gran amplitud en las demandas estructurales, se utilizan pulsos idealizados en un estudio de evaluación paralelo. Asimismo, pulsos simplificados son sintetizados para simular efectos “fling-step” artificiales en registros sísmicos que originalmente tienen ‘forward directivity’. Los resultados de las simulaciones revelan que la demanda máxima media, así como la dispersión en los valores máximos de los tres edificios, son más altos para los registros cercanos a la falla que para aquellos registros lejanos a la falla. La llegada de la velocidad del pulso en un registro de falla cercana provoca que la estructura disipe una considerable cantidad de la energía de entrada en relativamente pocos ciclos inelásticos, mientras que los efectos acumulativos del aumento de la demanda en los ciclos es mayor para los registros cercanos a la falla. Para inputs de tipo pulso, la demanda máxima es función del cociente entre el período del pulso y el periodo fundamental de la estructura. Más significativamente, se encontró que los registros con efecto “fling” excitan al sistema principalmente en su modo fundamental, mientras que las ondas con ‘forward directivity’ en ausencia de efecto “fling” causan mayor excitación en los modos superiores. También se concluyó que los espectros de aceleración y velocidad, cuando se examinan en conjunto, pueden ser utilizados para evaluar en forma razonable el daño potencial provocado por registros sísmicos cercanos a la falla.

Typical velocity and displacement time histories of (left) Far-fault, (middle) Near-fault (forward directivity), and (right) Near-fault (fling-step) ground motions

 

Ground Motions Selection and Processing

The ground motion database compiled for NTH analyses constitutes a representative number of far-fault and near-fault ground motions from a variety of tectonic environments. In this paper, a total of 21 records were selected to cover a range of frequency content, duration, and amplitude. Near-fault records were chosen so as to consider the presence of both forward directivity and fling-step effects. Hence, the assembled database can be investigated in three sub datasets. The first set contains seven ordinary far-fault ground motions recorded within 80km of the causative fault plane from earthquakes in the magnitude (Mw) range of 6.4 to 7.5 at soil or stiff soil sites. The second set includes seven near-fault ground motions characterized with forward directivity effect. These ground motions were populated from the SAC steel project (Somerville et al. 1997b) and the CDMG Strong Motion Instrumentation Program (Somerville 1998), and contains records either taken from soil or stiff soil sites. These records are from earthquakes having a magnitude (M) range of 6.7 to 7.1, and recorded at closest fault distance of 0.0 to 15 km. In the final set, a total of seven near-fault ground motions characterized with fling-step displacement were collected. They were recorded from 1999 (M7.4) Kocaeli (Turkey) and 1999 (M7.6) Chi-Chi (Taiwan) earthquakes at distances of 2.2 to 13.8 km. Pertinent information on the ground motion data sets including faulting mechanism, site classification of stations and peak ground acceleration (PGA), peak ground velocity (PGV) and peak ground displacement (PGD) of records are presented in Table below. Also shown in this table is the fling displacement of near-fault records.


Note that raw acceleration data was used for fling records of Kocaeli and Chi-Chi earthquakes, since conventional data processing procedures eliminate or distort the original waveforms through filtering. Utilized in this study is a data processing technique proposed in Iwan et al. (1985) and refined in Iwan and Chen (1994) to recover the long period components from near-fault accelerograms, and the process has been extensively elaborated in Boore (2001) and Boore et al. (2002) during the correction of 1999 Chi-Chi and Hector Mine earthquake records, respectively. In this study, the pre-event mean was removed using the zero-order correction described in Graizer (1989) and Boore (2001) prior to the application of the baseline correction. The major concern in baseline correction is the selection of appropriate corner periods to establish the segmental polynomial fits to satisfy two requirements: First, true tectonic deformation should be represented in the displacement time history. Second, the final velocities should oscillate around zero reference after the end of the time-series. The residual displacement due to fling-step were computed based on the GPS measurements for Chi-Chi earthquake ground motions, this information was retrieved from the study of Boore (2001). Since such reported information is not available for Kocaeli earthquake stations, the baseline corrections of the Kocaeli records were only performed considering the requirement of the zero-velocity crossing at the end of the time-history. The purpose of applying such a correction procedure in this study is to get consistency in fling records reflecting the true permanent ground displacement, thus to investigate consequences of static offset in the displacement time history on structural response.

 

Simple Mathematical Models for Near-fault Ground Motions Pulses  

Near-fault ground motions are often characterized by coherent long-period velocity pulses that may result in sudden and extreme deformation demands in structural components. This study investigates the consequences of well-known characteristics of pulse-type motions on the seismic response of moment-frame steel buildings. The severity of inelastic demands was evaluated for four, six and thirteen-story existing steel buildings subjected to near-fault ground motions with fling-step and forward directivity, and compared to their response to far-fault ground motions. Additionally, idealized pulses are utilized in a separate evaluation study to gain further insight into the effects of high amplitude pulses on structural demands. Simple input pulses were also synthesized to simulate artificial fling-step effects on ground motions originally having forward directivity. Findings from the analytical simulations reveal that median maximum demands as well as the dispersion in the peak values for the three buildings were higher for near-fault records than far-fault motions. The arrival of the velocity pulse in a near-fault record causes the structure to dissipate considerable input energy in relatively few plastic cycles whereas cumulative effects from increased cyclic demands are more pronounced in far-fault ground motions. For pulse-type input, the maximum demand is a function of the ratio of the pulse period to the fundamental period of the structure. More significantly, records with fling effects were found to excite systems primarily in their fundamental mode while waveforms with forward directivity in the absence of fling caused higher modes to be activated. It is also concluded that the acceleration and velocity spectra, when examined collectively, can be utilized to reasonably assess the damage potential of near-fault records.

Idealized sinusoidal pulses: (Left) Fling-step (Type-A), (Right) Forward directivity (Type-B)

Related Publications

Kalkan E. and Kunnath S.K. “Relevance of Absolute and Relative Energy Content in Seismic Evaluation of Structures”, Advances in Structural Engineering, Vol. 11, No 1, pp. 17-34, Feb. 2008.

Kalkan E. and Kunnath S.K. Effective Cyclic Energy as a Measure of Seismic Demand, Journal of Earthquake Engineering, Vol. 11, no. 5, Sept. 2007.

Kalkan E. and Kunnath S.K. Adaptive Modal Combination Procedure for Nonlinear Static Analysis of Building Structures, Journal of Structural Engineering, ASCE, (AWARD WINNER PAPER: ASCE Raymond Reese Research Price in Structural Engineering - 2008), Vol. 132, no. 11, pp. 1721-1732, Nov. 2006.

Kalkan E. and Kunnath S.K. Assessment of Current Nonlinear Static Procedures for Seismic Evaluation of Buildings, Engineering Structures, Vol. 29, pp. 305-316, 2007.

Acknowledgment

We wish to thank Carolina Magna for Spanish translation.