Effect of seismic sequences on the response of RC buildings located in soft soil sites

Transcription

1 Effect of seismic sequences on the response of RC buildings located in soft soil sites *Jorge Ruiz-García 1), Marco V. Marín, and Amador Terán-Gilmore 2) 1) Universidad Michoacana de San Nicolás de Hidalgo, Morelia, México 2) Universidad Autónoma Metropolitana, México D.F., México 1) ABSTRACT On September 19 and 2, 1985, two strong subduction interface earthquakes struck Mexico City leading to a very large stock of damaged, or even collapsed, reinforced concrete (RC) building structures located in soft soil sites of the former lakebed zone. Therefore, the aim of this study is to gain further understanding on the effects of soft-soil seismic sequences on the seismic response of RC framed-buildings. This investigation employed artificial sequences since only two real sequences were gathered during the 1985 earthquakes. The nonlinear response, in terms of peak and residual lateral displacement demands, of four RC buildings having 4, 8, 12, and 16 stories is evaluated under the set of artificial sequences. Results clearly shown that the relationship of the dominant period of the aftershock to the dominant period of the mainshock as well as the damaged period of the frame to the dominant period of the aftershock have a significant impact in the building response. 1. INTRODUCTION Man-made structures located in seismic regions are not exposed to a single seismic event, but also to a seismic sequence consisting of foreshocks, the mainshock (i.e. the seismic event with the largest earthquake magnitude), and aftershocks. Under some circumstances, aftershocks could trigger larger peak lateral displacement demands and/or larger permanent displacements than those experienced during the mainshock. As a consequence, aftershocks could increase the structural damage or, even, drive the structure without major damage to demolition due to excessive permanent displacements. A clear example of this scenario was observed after the September 19, 1985 Michoacan earthquake (Mw=8.) and the following aftershock on September 2 (Mw=7.6) that struck Mexico City (Rosenblueth 1986). In addition, it is well documented that medium-rise buildings located in the old bed-lake zone of Mexico City, mainly reinforced concrete (RC) buildings with frame-based structural system, having between 1) Professor of Civil Engineering 2) Professor of Civil Engineering 114

2 8 and 16 stories suffered moderate-to-severe structural damage as a consequence of the mainshock (Rosenblueth 1986, Meli 1989). Since a strong aftershock shook the city the following day, many buildings increased their state of damage, or suffered excessive permanent displacements. Thus, several dozen damaged RC buildings had to be demolished after the earthquakes because of the technical difficulties to straighten and to repair buildings with large permanent drifts and the threat of future aftershocks (Rosenblueth 1986). However, in spite of the 1985 experience, any study was conducted up to date to investigate the effect of aftershocks, or seismic sequences, in the building s response and to caution practicing engineers about this likely seismic scenario. The objective of this study is to present the results of an analytical investigation whose main goal was to further understand the effects of seismic sequences in the response of existing framed-buildings built on very soft soil conditions. For this purpose, two sets of artificial seismic sequences were generated in this study. Although it has been shown that artificial sequences could lead to a very different response than that from the real sequence (Ruiz-García 211, Ruiz-García 212, Goda 213), it was found that only two real mainshock-aftershock acceleration time-histories were available from the Mexican Strong Motion Database (SMIS 1999). Therefore, this investigation employed a set of artificial seismic sequences that tried to represent the seismic environment of the lake-bed zone of Mexico City. In addition, six RC framed buildings designed with the 1997 Mexico City Seismic Regulations were considered in this investigation to assess the seismic performance of existing RC frames designed after the 1985 Michoacan earthquake. However, the influence of the soil-structure interaction in the seismic response was not considered in this study. Besides that, although the reported investigation provides further information about the effects of seismic sequences on existing structures, it should be noted that proposing an evaluation procedure to take into account the effect of aftershocks is beyond the scope of the research reported in this paper. 2. REINFORCED CONCRETE BUILDINGS AND SEISMIC SEQUENCES CONSIDERED IN THIS STUDY 2.1 Design and modeling of building frame models Four regular three-bay reinforced concrete buildings for standard occupancy having different number of stories (N=4, 8, 12, and 16) designed according to the 1997 edition of the Mexico City Seismic Provisions and previously analyzed by Terán-Gilmore (1994) were considered in this investigation. In all buildings, the first-story has 4.5m while the remaining stories have 3.m. Fig. 1 shows the typical plan view of the study-case buildings. It was assumed that the buildings were located in soft soil sites of Mexico City. Elastic design spectrum was reduced by a response modification factor equal to 4, which requires designers to provide ductile detailing to the elements. A force-based design, which is customarily in Mexican design practice, was employed for preliminary sizing of structural elements, but final sizing and detailing was determined to satisfy a 115

3 lateral drift requirement of %. Detailed description of the design process of the buildings can be found in Terán-Gilmore (1994). Analyzed frames Fig. 1 Plan view (units in meters) of existing RC buildings considered in this study The buildings were analyzed using the nonlinear dynamic analysis computer program RUAUMOKO (Carr 29). Only half of the building was modeled due to symmetry in the building s plan. An exterior and interior frame were modeled as twodimensional centerline models, as shown in Fig. 1, assuming fixed columns which imply that soil-structure interaction was neglected. Both frames were attached through rigid frame elements to experience the same lateral deformation at each floor. Beams and columns were modeled as frame elements which concentrate their inelastic response in plastic hinges located at their ends. Flexural moment capacity for beams and columns was determined using nominal yield strength capacity of steel equal to 42 kg/cm 2. Additional strength and stiffness due to floor slab contribution in beams was taken into account according to the recommendations of Pantazopolou and French (21). Moment-curvature curves considering cracked sections were obtained for each beam, while axial load-flexural bending interaction diagrams were considered in the columns. Since beams and columns are expected to behave predominantly in flexure (i.e. flexure dominant behavior), a stiffness-degrading Takeda-type moment-curvature relationship with strain-hardening ratio equal to % was considered to model the hysteretic behavior of the beams and columns. This hysteretic behavior was assumed since the RC elements are expected to be provided with adequate steel reinforcement detailing that preclude strength degradation and pinching due to high shear stresses, slippage of steel bars or other phenomena.two levels of unloading stiffness degradation, controlled by the parameter in the RUAUMOKO library (Carr 29), were considered in this study: low ( =) and high ( =). Rayleigh damping equal to 5% of critical was assigned to the first and second modes for the 4-story frame, while this damping was applied to the first and fourth modes for the 8-, 12-, and 16-story frames. During the analysis, local P-delta effects were included (i.e. large displacement analysis). Main 116

4 dynamic and mechanical properties of each frame obtained from conventional modal analysis and nonlinear static (pushover) analysis are summarized in Table 1. Table 1. Fundamental period of vibration, T 1, yield strength coefficient, drift, y, and normalized modal participation factor, model MODEL T 1 (s) C y C y, roof yield 1 1, roof obtained for each frame y (%) 1 1, roof C- 4N C- 8N C-12N C-16N SET OF SEISMIC SEQUENCES CONSIDERED IN THIS STUDY This study focused its attention on the effects of aftershocks on the inter-story drift demands induced to existing RC frames located in soft soil sites. For this purpose, real (i.e. as-recorded) mainshock-aftershock acceleration time histories are needed for performing nonlinear dynamic analyses and subsequent statistical studies. However, it was found that only two real mainshock-aftershock acceleration time-histories recorded at Central de Abastos (CDAF) station during the September 19 and 2, 1985 earthquakes were available from the Mexican Database of Strong Motions (SMIS 1999). Fig. 2 illustrates the recorded acceleration time-histories, while Table 2 reports relevant ground motion features of the sequences (e.g. the peak ground acceleration, PGA, and predominant period, T g, of each ground motion). (cm/s 2 ) /19/1985 9/2/1985 a) Time [s] (cm/s 2 ) Time [s] Fig. 2 time histories of the mainshock-aftershock sequences recorded during the 1985 Michoacan earthquakes at CDAF station: a) NE component, b) N9E component b) 117

5 The predominant period of the ground motion was defined as the period at which the maximum ordinate of a five percent damped relative velocity spectrum occurs (Miranda 1993). For the soft soil deposits of Mexico City, T g has been found to be closely related with the dominant period of the soil deposit computed from one-dimensional elastic models assuming that the response of the soil deposit is dominated by vertically propagating shear waves in a layered deposit and the second mode of vibration is approximately one-third of the fundamental period of vibration (Reinoso 1999). It should be noted that the peak ground velocity (PGV) of the aftershock ground motion is around 35% of the PGV of the mainshock ground motion and that T g of the mainshock is longer than the T g of the corresponding aftershock in the NS component. Table 2. List of earthquake ground motions employed to derive the artificial seismic sequences considered in this investigation Date M s Station Name Station ID Comp PGA T g (cm/s²) (s) 19/9/ Central de Abastos CDAF NE /9/ Central de Abastos CDAF N9E /9/ Central de Abastos CDAF NE /9/ Central de Abastos CDAF N9E SET A 25/4/ Villa del mar 29 EW /4/ Villa del mar 29 NS /4/ Jamaica 43 NS /4/ Rodolfo Menéndez 48 EW /4/ P.C.C. Superficie 25 EW /4/ Córdova 56 EW /4/ Liverpool 58 EW /4/ Roma-B RB EW SET B 24/1/ U. Colonia IMSS 44 EW /4/ U. Colonia IMSS 44 EW /4/ San Simón 53 EW /4/ Roma RO EW /12/ Roma RO EW /9/ Roma RO EW /12/ SCT SC EW /9/ SCT SC EW

6 Since two seismic sequences are insufficient for developing conclusions about the effects of aftershocks, artificial seismic sequences that represent the ground motion features of the real sequences should be employed. Two approaches have been commonly employed in the absence of real sequences: 1) back-to back, or repeated, approach, and 2) randomized approach. The first approach consists on repeating the real mainshock, at scaled or identical amplitude, as an artificial aftershock, which assumes that the ground motion features such as frequency content and strong motion duration of the mainshock and aftershock(s) are the same. The second approach consists on ensemble a set of real mainshocks, and generating artificial sequences by selecting a mainshock and simulating the remaining aftershocks by repeating the mainshock waveformat repeatedly, at reduced or identical amplitude, with no change in spectral content as an artificial aftershock. Recent studies have demonstrated that the first approach is unrealistic and it leads to a totally different response as compared with real sequences (Ruiz-García 211, Ruiz-García 212, Goda 212). Therefore, the randomized approach was employed in this investigation. For generating artificial seismic sequences, a first set of 8 acceleration time histories recorded at the lake bed zone of Mexico city were selected, with 4 records having T g values around 3. sec and other 4 having T g values close to 2.3 sec. However, only the records having T g s around 3.s were employed as mainshock ground motions, which means that the dominant period of the mainshock is close or longer than that of the aftershock similar to what was observed from the real sequences. Peak ground velocity (PGV) was selected as ground motion intensity measure since it is highly correlated with energy demands imposed to the structures located in very soft soil sites (Terán-Gilmore 24). Thus, each earthquake record employed as mainshock was scaled to reach the peak ground velocity registered at the well-known East-West component of the Secretaria de Comunicaciones y Transportes (SCT) station during the September 19, 1985 earthquake. In addition, the 8 records were scaled to reach 35%, 7% and 1% of the PGV of the mainshock ground motions when employed as aftershock earthquake ground motion. Therefore, 28 artificial sequences were generated randomly for each V A /V M ratio (i.e., 5,, and, where V A and V M are the PGV of the aftershock and the mainshock, respectively) since no repetition of the mainshock waveformat was allowed in the set. For instance, Fig. 4 shows two typical artificial seismic sequences with their three levels of V A /V M ratio. For V A /V M ratio equal to one, it should be noted that although both the mainshock and the aftershock were scaled to reach the same PGV, the peak ground acceleration of the aftershock could be larger than that of the mainshock as shown in the right-bottom plot. Additionally, it is interesting to examine the seismic response of the study-case frames under seismic sequences with different frequency content. Therefore, a second set of artificial sequences was generated in this study that includes 8 acceleration timehistories recorded during 5 historical earthquakes, as listed in Table 2, which were also scaled to reach the PGV of the record captured at SCT station. Thus, 8 artificial seismic sequences were generated by selecting one mainshock ground motion and employing the remaining 7 maishocks ground motions as the following aftershocks ground motions. This process was repeated to generate a total of 56 artificial seismic sequences. In should be noted that under this randomized approach, the PGA and/or 119

7 the T g of the aftershock ground motion could be larger than those/that of the mainshock ground motions. In particular, a subset of 28 artificial seismic sequences has PGA of the mainshock larger than that of the corresponding aftershocks, while 6 out of this 28 have also predominant period of the mainshock longer than that of the aftershock. (cm/s 2 ) T g =2.96s T g =2.96s V A /V M =5 (cm/s 2 ) T g =2.96s T g =2.3s V A /V M =5 (cm/s2) V A /V M = (cm/s 2 ) V A /V M = -3 3 (cm/s 2 ) V A /V M = (cm/s 2 ) V A /V M = -3 Fig. 4 Examples of artificial seismic sequences included in set A considered in this study A comparison between the spectral velocity spectra obtained from each real sequence and the corresponding mean spectrum from sets A and B is shown in Figs. 5a and 5b, respectively. It should be noted that set A of artificial sequences resembles, on average, the frequency content of the real sequences, while set B has smaller frequency content than both real sequences. S v (cm/s) NE N9E mean set A a) T(s) S v (cm) mean M mean set B T(s) Fig. 5 Comparison of spectral velocity response spectra computed from the mainshock-aftershock sequences recorded during the 1985 Michoacan earthquakes at CDAF station and the : a) NE component, b) N9E component b) 111

8 4. RESPONSE UNDER SEISMIC SEQUENCES 4.1 Response under set A At a first stage, the influence of stiffness degradation in the response of the studycase frames under the mainshocks and set A of artificial seismic sequences corresponding to three V A /V M ratios was examined in this study. Figs. 6 and 7 show the height-wise distribution of median interstory drift ratio (IDR) for each frame with lowand high-level of member s stiffness degradation, respectively. From the figures, it can be seen that for velocity ratio equal to 5, the influence of member s unloading stiffness degradation is negligible. However, the influence of stiffness degradation becomes more important as the relative intensity of the aftershock with respect to the mainshock increases (i.e. for V A /V M ratios equal to and ), mainly for the 8- and 16-story frames. It is interesting to note that the effect of the aftershocks is negligible for frames with low-stiffness degradation until the aftershocks are scaled to reach the same peak ground velocity of the mainshocks. Therefore, there is evidence that stiffness degradation has an important role in the response of RC frames subjected to seismic sequences in soft soil sites. C-4N (α=) M M+A_35% M+A_7% M+A_1% C-8N (α=) C-12N (α=) Fig. 6 Heightwise distribution of IDR for all frames having low member s stiffness degradation under set A of seismic sequences C-16N (α=) C-4N (α=) M M+A_35% M+A_7% M+A_1% C-8 N (α=) C-12N (α=) Fig. 7 Heightwise distribution of IDR for all frames having high member s stiffness degradation under set A of seismic sequences C-16N (α=)

9 The estimation of residual displacement demands is important for seismic assessment since the decision of repairing or demolishing a structure should be based on the technical difficulties to straighten leaned structures after earthquake excitation, which was the case after the 1985 earthquakes (Rosenblueth 1986). Therefore, the distribution along the height of lateral residual displacement demands for the four RC frames including low-and-high member s stiffness degradation, respectively, is illustrated in Figs. 8 and 9. In order to provide a context to the following results, it should be mentioned that FEMA 356 (2) recommended seismic provisions for the assessment and rehabilitation of existing buildings in the U.S. specify limiting values on residual drift demands linked to system performance levels. For instance, RIDR max should not exceed 1% for Life Safety and 4% for Collapse Prevention performance levels. In addition, a recent field investigation in Japan highlighted that a residual interstory drift of about % is perceptible for building occupants and a residual inter-story drift of about % could cause human discomfort (McCormick 28) From the figures, it can be seen that frames with high-unloading stiffness deterioration have the benefit effect of constraining permanent displacements even under the strong aftershocks (i.e. artificial aftershocks scaled to reach 7% or 1% of the peak ground velocity recorded in SCT station), while frames including structural members with high energy dissipation capacity (i.e. with low unloading stiffness deterioration) would lead to larger inter-story residual drift demands. Unlike peak inter-story drift demands, it should be noted that lateral residual drift demands do not follow a clear trend as the intensity of aftershocks increases. For example, median residual drift demands triggered by low intensity aftershocks (e.g. scaled to 35% of the PGV of SCT station) are larger than those induced by high-intensity aftershocks (e.g. scaled to 35% of the PGV of SCT station) in the lower stories of frame C-12N. However, this is a result of the large record-to-record variability inherent in the estimation of residual drift demands as discussed in (Bojorquez 213). C-4N ( = ) M M+A_35% M+A_7% M+A_1% 5 5 R C-8N ( = ) 5 5 R C-12N ( = ) 5 5 R C-16N ( = ) 5 5 R Fig. 8 Heightwise distribution of median RIDR for all frames having low member s unloading stiffness degradation under seismic sequences 1112

10 C-4N ( = ) M M+A_35% M+A_7% M+A_1% 5 5 R C-8N ( = ) 5 5 R Fig. 9 Heightwise distribution of median RIDR for all frames having high member s unloading stiffness degradation under seismic sequences Primary goal in this investigation is to identify relevant ground motion features in the seismic sequences that could be more influential in the seismic response of RC buildings located in soft soil sites. The artificial seismic sequences described in earlier sections have two main ground motion relationships: 1) the amplitude, measured by the ratio of peak ground velocity of the aftershock with respect to the peak ground velocity of the mainshock, V A /V M, which is also a measure of the relative intensity of the aftershock with respect to the intensity of the mainshock, and 2) the frequency content, measured by the ratio of the dominant period of the aftershock to the dominant period of the mainshock, T g,a /T g,m. For set A of artificial seismic sequences, V A /V M ratios were defined as 5,, and, while values around 6 and ratios were established for T g,a /T g,m ratios. For instance, the distribution along the height of interstory drift ratio, IDR, computed for the C-4N model is shown in Figs. 1 and 11 for the two T g,a /T g,m ratios. As can be expected, as the intensity of the aftershocks increase, the IDR also increases mainly in the bottom stories due to damage concentration. However, it should be noted that the T g,a /T g,m ratio has a strong influence in the amplitude of IDR, since sequences having T g,a /T g,m ratios around 6 (i.e. dominant periods of the aftershock are shorter than those of the mainshocks) trigger larger interstory drifts than those derived from sequences with T g,a /T g,m around one. This observation could be explained since the damaged period of vibration of analyzed frame (i.e. period of vibration of the frame at the end of the mainshock, T d, which was estimated from a simple Fast Fourier Transform analysis of the inter-story acceleration response) becomes closer to the dominant period of the aftershock when dominant periods of the aftershocks are around 2.3s than when they are approximately 3.s. This influence is more pronounced as the intensity of the aftershock increases. After examining these figures, it should also be noted that the influence of T g,a /T g,m is more important in the building response that the relative intensity of the aftershock with respect to the mainshock since the increment of IDR from V A /V M equal to to is small when the period ratio is around one as opposed when the period ratio is approximately 6 (e.g. there is an increment of median IDR of 44% in the first-story when T g,a /T g,m is around 6, while the increment is about 23% when the period ratio is around one). C-16N ( = ) 5 5 R C-16N ( = ) 5 5 R 1113

11 V A /V M =5 V A /V M = Individual Individual Mean Mean Median Median Fig. 1 Heightwise distribution of IDR for the C-4N frame under seismic sequences with T g,a /T g,m around and with three different ratios of peak ground velocity of aftershockto-mainshock V A /V M =5 individual mean Median V A /V M = individual Fig. 11 Heightwise distribution of IDR for the C-4N frame under seismic sequences with T g,a /T g,m around 6 and with three different ratios of peak ground velocity of aftershock-to-mainshock 3.2 Response under set B In the second stage of this investigation, all study-case RC frames were subjected to the set B of artificial seismic sequences. Recall that in this set was included the earthquake record gathered at the well-known SCT station during the 1985 Michoacan earthquake and all artificial aftershocks have the same PGV of the SCT record. Figs. 12 and 13 show the distribution over the height of median IDR for each frame with lowand high-level of member s stiffness degradation, respectively. Again, it can be seen that the artificial sequences lead to larger peak inter-story drift demands, but the amplitude depends on the level of member s unloading stiffness degradation (i.e. high mean Median V A /V M = Individual Mean Median V A /V M = individual mean Median

12 member s unloading stiffness degradation trigger larger peak interstory drift demands than those triggered when low member s unloading stiffness degradation is assumed). C-4N (α=) M M+A_1% C-8N (α=) C-12N (α=) C-16 N (α=) Fig. 12 Heightwise distribution of IDR for all frames having low member s unloading stiffness degradation under set B of seismic sequences C-4N (α=) M M+A_1% C-8N (α=) C-12N (α=) C-16N (α=) Fig. 13 Heightwise distribution of IDR for all frames having high member s unloading stiffness degradation under set B of seismic sequences Similarly to the seismic response under set A, a close look at the response under set B revealed that the T g,a /T g,m ratio plays an important role in the response, which is even more important than the intensity relationship between the mainshock and the aftershock. For instance, Fig. 14 shows the height-wise distribution of inter-story drift triggered in the C-8N frame model (T 1 =1.32s) when subjected to 3 sub-sets of the set B generated from the maishocks M2, M7 and M8 listed in Table 2. The response under the corresponding mainshock is also shown in red line. To help in providing an explanation of the effect of the aftershocks, a simple Fast Fourier Transform analysis (FFT) of the inter-story acceleration response due to the mainshock was employed to obtain an estimate of the damaged period of vibration of frame (i.e. period of vibration at the end of the mainshock excitation). Under the first subset with mainshock M2, six out of seven sequences triggered larger inters-story drifts. For example, one of the largest responses are recorded when the M8 acts as aftershock, having T g,a /T g,m =1.61, although the peak ground acceleration (PGA) of the artificial aftershock is smaller than that of the mainshock as shown in Fig. 15a. This could be explained since the FFT analysis revealed that the mainshock slightly elongated the frame s first mode period of vibration and, thus, the frame s period of vibration at the end of the mainshock will 1115

13 become closer to the aftershock s dominant period (note that most of the T g,a /T g,m ratios in this subset are larger than one). It is interesting to note that when M8 is employed as mainshock, the artificial sequences did not increase the interstory drifts even though most of the artificial aftershocks have PGA larger than that of the mainshock as shown in Fig. 15c. This could be explained since mainshock M8 induced high nonlinear response in the structure which lead to increase the frame s period of vibration significantly (until about 2.s using the FFT analyses) and, thus, the relationship between the damaged period of vibration of the frames and the aftershock s dominant period is larger than one (i.e. the period of vibration of the building tends to move beyond the dominant period of the aftershocks) since all artificial sequences in the subset have T g,a /T g,m ratios smaller than one. Finally, the response under subset with mainshock M7 showed that interstory drift only increased when M8 is employed as artificial aftershock. Again, this could be explained since the estimated period of vibration of the frame at the end of mainshock M7 was about 1.9s and, thus, since the only aftershock with longer dominant period was M8, with T g,a /T g,m =9 as illustrated in Fig. 15b, the drift response increased as shown in Fig. 14. It should be noted that other aftershocks in this subset have larger PGA than that of M7, but they have T g,a /T g,m ratios shorter than one (i.e. damaged period of vibration is longer than aftershock s dominant period). The aforementioned observations provide further explanation why RC buildings with periods of vibration from s to 1.5s in the lake-bed zone of Mexico City (with soil dominant periods around 2s) that suffered structural damaged after the September 19, 1985 earthquake could increase their state of damage during the September 2 earthquake, although the latter seismic event had smaller intensity. M2+A M7+A M8+A M2 M7 M8 Mean Mean Fig. 14 Heightwise distribution of IDR for the C-8N frame under three subsets of artificial seismic sequences generated from the mainshocks M2, M7 and M8 listed in Table

14 3 T g =1.28s T g =2.6s 3 T g =1.89s T g =2.6s (cm/s²) M2+M8 a) (cm/s²) M7+M8 b) 3 T g =2.6s T g =1.89s (cm/s²) M8+M7 Fig. 15 Examples of artificial seismic sequences included in set B considered in this study Finally, Figs. 16 and 17 shows the distribution along the height of residual interstory drift demand corresponding to the family of study-case frames which takes into account low and high member s unloading stiffness, respectively. Although residual drift demands do not show a clear pattern, it can be seen that seismic sequences could increase their amplitude. However, only frames C-4N and C-8N might experience interstory residual drift demands that could lead to human discomfort. c) C-4N ( =) M M+A_1% 5 5 R C-8N ( =) 5 5 R C-12N ( =) 5 5 R C=16N ( =) 5 5 R Fig. 16 Heightwise distribution of median RIDR for all frames having low member s unloading stiffness degradation under set B of artificial seismic sequences. 1117

15 . C-4N ( =) M M+A_1% 5 5 R Fig.17 Heightwise distribution of median RIDR for all frames having high member s unloading stiffness degradation under set B of artificial seismic sequences. 5. CONCLUSIONS C-8N ( =) 5 5 R C-16N ( =) 5 5 R The objective of the research reported in this paper was to effect of column base flexibility on the residual drift demands of modern low-rise steel moment-resisting frames, particularly in the amplitude and height-wise distribution of residual interstory drift demands. The following conclusions are drawn from this ongoing investigation: Two strong earthquakes struck Mexico City on September 19 and 2, 1985 causing large damage to, mainly, RC buildings. In spite of these historical earthquake events, any investigation was conducted up to date to understand the effect of seismic sequences in the response of buildings located in soft soil sites. Perhaps, this lack of research was based on the insufficiency of enough ground motion recordings of mainshock-aftershock sequences. Therefore, this paper has summarized the results of an analytical study aimed at providing further understanding on the influence of seismic sequences on drift demands in regular existing RC moment-resisting frame buildings located in soft soil sites. This investigation focused on investigating whether aftershocks could increase peak (transient) and (residual) permanent drift demands in frame models with different number of stories. Due to the insufficiency of real seismic sequences gathered in soft soil sites, two sets of artificial mainshock-aftershock sequences were generated using a randomized approach using real mainshock records gathered in soft soil sites. Under set A of artificial sequences, it was shown that interstory drift demands (IDR) in the study-case RC frames tends to increase as the ratio of the peak ground velocity of the aftershock with respect to the mainshock, V A /V M, increases. The effect of stiffness degradation is important since frame models with member s low unloading stiffness degradation led to smaller drift demands than their counterparts having members high unloading stiffness degradation under the artificial seismic sequences. On the contrary, low unloading stiffness degradation drives, in general, larger residual interstory drift demands. An important observation derived from sets A and B is that the building response strongly depends on the ratio of dominant period of the aftershock to the dominant period of the mainshock, T g,a /T g,m, and, most importantly, in the ratio of C-12N ( =) 5 5 R 1118

16 the damaged period of vibration (i.e. frame s period of vibration at the end of the mainshock) to the dominant period of the aftershock T d /T g,a. It was shown that when the latter ratio is farther than one, interstory drift demands are not increased by the aftershock, although the peak ground acceleration of the aftershock could be larger than that of the mainshock. ACKNOWLEDGEMENTS The authors would like to express their gratitude to the National Council for Science and Technology (CONACYT) in México for the financial support provided to develop the research reported in this paper through the project CB While the first author acknowledges to Universidad Michoacana de San Nicolás de Hidalgo, the third author wishes to thank to Universidad Autonoma Metropolitana-Azcapotzalco. REFERENCES Bojórquez, E. and Ruiz-García J. (213), Residual drift demands in moment-resisting steel frames subjected to narrow-band earthquake ground motions, Earthq. Eng. Struct. Dyn., in press. Carr, AJ. (29), RUAUMOKO-Inelastic Dynamic Analysis Program. User s manual, Dept. of Civil Engineering, University of Canterbury, Christchurch, New Zealand, Federal Emergency Management Agency (2), Prestandard and Commentary for the Seismic Rehabilitation of Buildings, Report FEMA 356, Washington, D.C. Goda, K., and Taylor, C. (212), Effects of aftershocks on peak ductility demand due to strong ground motion records from shallow crustal earthquakes, Earthq. Eng. Struct. Dyn., 41, McCormick, J., Aburano, H., Ikenaga, M., Nakashima, M. (28), Permissible residual deformation levels for building structures considering both safety and human elements, Proceedings of the 14th World Conference on Earthquake Engineering, Beijing, China, Paper No Meli, R., Avila, JA. (1989), The Mexico earthquake of September 19, 1985-Analysis of building response, Earthq. Spectra, 5 (1), Miranda, E. (1993), Evaluation of site-dependent inelastic seismic design spectra, J. of Struct. Eng., ASCE, 119(5), Pantazopoulou, S. J. and French, C.W. (21), Slab participation in practical earthquake design of reinforced concrete frames, ACI Struct. J., 98(4), Reinoso, E. and Ordaz, M. (1999), Spectral ratios for Mexico City from free-field recordings, Earth. Spectra, 15(2), Rosenblueth, E. and Meli R. (1986), The 1985 Mexico earthquake: causes and effects in Mexico City, Concrete International, ACI, 8(5), Ruiz-García, J. and Negrete-Manriquez, J. (211), Evaluation of drift demands in existing steel frames under as-recorded far-field and near-fault mainshock aftershock seismic sequences, Eng. Struct., 33, Ruiz-García, J. (212), Mainshock-aftershock ground motion features and their influence in building s seismic response, J. of Earth. Eng., 16(5),

17 Sociedad Mexicana de Ingeniería Sísmica (1999), Base Mexicana de Datos de Sismos Fuertes, Catálogo de Acelerogramas , Sociedad Mexicana de Ingeniería Sísmica, A.C. Terán-Gilmore A. (24), On the use of spectra to establish damage control in regular frames during global predesign, Earth. Spectra, 3,