SEISMIC TESTING OF NON-STRUCTURAL COMPONENTS AND ASSESSMENT OF THE PRESCRIBED RESPONSE SPECTRUM

Transcription

1 NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July -5, Anchorage, Alaska SEISMIC TESTING OF NON-STRUCTURAL COMPONENTS AND ASSESSMENT OF THE PRESCRIBED RESPONSE SPECTRUM S.M. Takhirov, A. Gilani, and L. Tedesco 3 ABSTRACT The current US building code specifies seismic certification by means of shake table tests of nonstructural components. A brief review of current status of non-structural testing in the United States by means of shake tables is presented. A key feature of the evaluation is the use of a prescribed required response spectrum. The required spectral accelerations are based on certain assumptions on spectral amplification over building s elevation. To date no study has been conducted to assess the applicability of the prescribed standard for buildings of various heights, construction material, and lateral load resisting framing. To address this issue, strong motion data from the database of recorded earthquakes was utilized. Based on these records, an empirical estimate of spectral amplification was developed which is crucial for development of required response spectra for seismic evaluation testing of non-structural components by means of shake table testing. The buildings considered in the study have permanently installed strong motion instrumentation or were studied by temporarily installed high-sensitive instrumentation used for monitoring ambient vibration. The paper employs a number of floor responses recorded in a number of buildings during low level ambient vibrations or earthquakes. The paper focuses on specifics of floor spectra recorded during () ultra-low amplitude (ambient vibration) tests, () low, and (3) high level seismic excitations. As anticipated, the spectral acceleration is largely amplified at the resonant frequencies of buildings. In many cases the floor spectra are significantly greater than the test spectra currently prescribed for use in tests of non-structural components. NEES Site Manager, PEER, University of California Berkeley, Richmond, CA 9 Senior Associate, Miyamoto International, West Sacramento, CA, Senior Manager, Ceiling Systems, USG Corporation, Chicago, IL Takhirov SM, Gilani A, and Tedesco L. Seismic testing of non-structural components and assessment of the prescribed response spectrum. Proceedings of the th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK,.

2 NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July -5, Anchorage, Alaska Seismic Testing of Non-structural Components and Assessment of the Prescribed Response Spectrum S.M. Takhirov, A. Gilani, and L. Tedesco 3 ABSTRACT The current US building code specifies seismic certification by means of shake table tests of nonstructural components. A brief review of current status of non-structural testing in the United States by means of shake tables is presented. A key feature of the evaluation is the use of a prescribed required response spectrum. The required spectral accelerations are based on certain assumptions on spectral amplification over building s elevation. To date no study has been conducted to assess the applicability of the prescribed standard for buildings of various heights, construction material, and lateral load resisting framing. To address this issue, strong motion data from the database of recorded earthquakes was utilized. Based on these records, an empirical estimate of spectral amplification was developed which is crucial for development of required response spectra for seismic evaluation testing of non-structural components by means of shake table testing. The buildings considered in the study have permanently installed strong motion instrumentation or were studied by temporarily installed high-sensitive instrumentation used for monitoring ambient vibration. The paper employs a number of floor responses recorded in a number of buildings during low level ambient vibrations or earthquakes. The paper focuses on specifics of floor spectra recorded during () ultra-low amplitude (ambient vibration) tests, () low, (3) moderate and () high levels of seismic excitations. As anticipated, the spectral acceleration is largely amplified at the resonant frequencies of buildings. In many cases the floor spectra are significantly greater than the test spectra currently prescribed for use in tests of nonstructural components. Introduction Non-structural components and equipment consist of many elements and as such are difficult to analyze numerically. To assess their seismic performance earthquake simulation testing can be used. Such technique was used by many manufacturers of non-structural components and equipment in the United States to characterize performance of their products during earthquakes. Seismic evaluation of non-structural components and equipment is based on prescribed required response spectra. This paper discusses the existing prescribed response spectra and prescribed amplification parameters and compares them to that obtained from acceleration records of real buildings. The comparison is conducted for different excitation levels varying from very low noise level excitation to very high seismic excitation that causes inelastic deformation of a NEES Site Manager, University of California Berkeley, Richmond, CA 9 Senior Associate, Miyamoto International, West Sacramento, CA, Senior Manager, Ceiling Systems, USG Corporation, Chicago, IL Takhirov SM, Gilani A, and Tedesco L. Seismic testing of non-structural components and assessment of the prescribed response spectrum. Proceedings of the th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK,.

3 building. Review of Current US-Practice on Non-Structural Testing Code requirements in the United States The current edition of the International Building Code (IBC) [] and its referenced documents address the seismic requirements for suspended ceilings. Both design and installation issues are addressed in the document. The IBC [] assigns seismic design categories (SDCs) A through F to buildings. The SDC for a structure depends on its occupancy, the underlying soil conditions, and the site-specific seismic hazard intensity. For structures located in Categories D to F, the code mandates prescriptive installation requirements, as summarized in []. For seismic force calculations, the IBC [] and the ASCE/SEI 7- [3] treat the suspended ceiling system as a nonstructural component (NSC) and specify seismic force from: (Eq.) ( R p F p / I p )W p z [. a p( )]S DS (. )S h DS where Fp is the seismic force, Wp is the unit weight of the ceiling; Ip is the importance factor; Rp, is the response modification factor, ap accounts for the response amplification; and S DS = /3F a S S designates the short-period (.-sec) spectral acceleration for the design earthquake (% probability of occurrence in 5 years). The z/h factor accounts for the amplification of the response along the building height. The code assumes an inverted triangular distribution that is capped at.. Although not explicitly stated, such amplification would not exist in the vertical direction because the building s columns are stiff along their axes and thus do not amplify the vertical component of the earthquake at the top of the columns. In the United States, the code explicitly allows earthquake simulator (shake table) testing to be performed in lieu of the analytical procedure of (Eq.). As an example, such an approach was used to assess the performance of the code-prescribed ceiling installation and the USG seismic clip []. In the United States, shake table testing must follow the requirements of a test and qualification document such as the ICC-ES AC5 [5]. The ICC-ES AC5 uses the seismic force of (Eq.) to develop the required response spectrum (RRS) with the following assumptions: components with a frequency of greater than.7 Hz are considered rigid without amplification of acceleration; and components with a frequency of less than.7 Hz are considered flexible with a maximum amplification of spectral acceleration. Thus (Eq.) can be expressed in terms of the required spectral acceleration for flexible and rigid components respectively as: (Eq.) A RIG =.S DS (+z/h). S DS and A FLX = S DS (+z/h).s DS Typical required response spectra in horizontal excitation and the code prescribed spectral amplification are presented in Figure.

4 Elevation, z/h Figure. Typical required response spectra in horizontal direction and code prescribed spectral amplification. Ultra-low excitations S S =3.g - ground S S =3.g - roof of A FLX Empiric Floor Spectral Accelerations s of building vibrations due to ambient noise are very important for expensive equipment sensitive to low level vibrations. MRI and CT scanners are representative examples of such vibration sensitive equipment and instruments. A significant engineering effort is required for vibration reduction at the site by means of advanced vibration control and engineering design to identify and control vibration sources. A capability to estimate the amount of transmitted energy and vibration amplification throughout elevation of the building is crucial for vibration reduction. This paper discusses the results of the ambient vibration study of the Berkeley Public Library []. From earthquake engineering point of view this level of vibrations can be considered as ultra-low with peak accelerations in the order of.5g or so. The study represents one of the typical cases where ultra-low vibrations of a building caused by passing traffic, contraction noise and other ambient excitations, are investigated. As is commonly done in ambient vibration studies, one level of the building (the roof of the building, in this case) was taken as a reference level. At least four velocimeters were installed on both the reference level as well as the current level under investigation. Therefore, in each set of records there are recorded velocities of specific levels of the building in reference to the roof which are differentiated to deliver accelerations at the instrument locations. At each building elevation, two out of four instruments recorded vibration in the east-west direction at the opposing sides of the building, whereas the remaining two recorded vibration in the north-south direction. The recorded accelerations for each direction were averaged to remove any torsional component of vibration. The peak accelerations obtained from the data were below.5g. s were derived by dividing the floor spectra by the spectra at the basement elevation. The building had elevations above the basement including the roof. The spectral amplifications for these elevations in soft (translational) and stiff (longitudinal) directions of the building are presented in Figure a and Figure b, respectively. One of the main goals of this ambient vibration study was to estimate the resonant frequencies of the building of A RIG

5 The buildings resonant frequency in transverse direction was lower than its frequency in the longitudinal direction. This was expected, as this difference is correlated with greater stiffness of the building in the longitudinal direction. As anticipated, the spectral amplifications in longitudinal directions were less than those of the building in the transverse direction: an amplification of 5 at the roof elevation in the transverse direction versus an amplification of about.5 in the longitudinal direction. Floor to basement spectral ratio (S f /S b ) in north-south direction Roof 3rd floor nd floor st floor Floor to basement spectral ratio (S f /S b ) in east-west direction Roof 3rd floor nd floor st floor Spectral ratio, g/g Spectral ratio, g/g Figure. at ultra-low excitation levels: a) north-south (more flexible direction), east-west (more rigid direction). Floor spectral acceleration in the same building during low and moderate level earthquake excitations This section presents a comparison between the recorded spectral amplification throughout a buildings elevation during low and moderate levels of ground excitations caused by two different earthquakes. The same building was studied to investigate the differences in amplification at these two levels of excitations. The building studied is located in Burbank, California, U.S.A. and its shaking was recorded during the 99 Northridge and the New Hall earthquakes. This is a ten-story residential building with precast concrete shear walls in lateral and transversal directions. The building design is dated back to 97. It was instrumented in 9 with accelerometers on levels in the building (CSMIP station #35). The filtered acceleration records from [7] were used in the study. The building s plan has :.9 aspect ratio with the longitudinal direction coinciding with 7 direction of the accelerometer orientation. The direction of the accelerometer orientation represents the transverse direction of the building. To avoid any beating effect associated with possible coupling between translational and torsional motions and low damping [, 9], accelerations in transverse directions were averaged. Figure 3a presents the spectral amplification at many elevations of the building in respect to the first level in longitudinal direction. The spectra amplification peaked at the first frequency of the building in this direction and it was close to at the roof level. The amplifications at the first resonance frequency of the building at the roof level are consistent for both levels of earthquake excitations, but the amplifications for lower excitations were consistently higher. A noticeably lower resonant frequency is also observed at higher level which is most likely related to some inelastic performance of the building. at moderate level was close to 7.5. Figure 3b shows big difference in ground excitation recorded for these two earthquakes at

6 the st level. The similar conclusion was made regarding the building s amplification in the transverse direction, although the amplification in this direction was much higher for low level excitation (about ) and was about the same (right below 7) for moderate level as shown in Figure a. Figure b shows st level spectra calculated for both low and moderate excitations in transverse direction. Longitudinal direction Roof@N Roof@NH th Floor@N th Floor@NH th Floor@N th Floor@NH.... Longitudinal direction N NH. Figure 3. Amplification of floor spectra relative to that of first level and spectra of st floor accelerations recorded during low New Hall (NH) and moderate Northridge (N) earthquake excitations (Burbank, CA in longitudinal direction). Transverse direction Roof@N Roof@NH th Floor@N th Floor@NH th Floor@N th Floor@NH.... Transverse direction N NH. Figure. Amplification of floor spectra relative to that of first level and spectra of st floor accelerations recorded during low New Hall (NH) and moderate Northridge (N) earthquake excitations (Burbank, CA in transverse direction). Analysis of the building s transfer function during both of these excitations showed that the first resonance frequency has slight shift toward lower frequencies. Amplifications in same building during two different moderate level earthquake excitations This section presents a comparison between the recorded spectral amplification over the

7 elevation of a building during high level ground excitations caused by two different earthquakes. The building located in Pasadena, California, U.S.A. is studied based on excitations produced by the 99 Sierra Madre and 99 Northridge earthquakes. This is a nine-story commercial building with reinforced concrete slab-column system providing moment resistance in lateral and transversal directions and beam-column system at the core of each floor. The building design is dated back to 93. It was instrumented in 99 by 5 accelerometers installed on 5 levels of the building (CSMIP station #57). The filtered acceleration records from [7] were used in the study. The building s plan has :.5 aspect ratio with the longitudinal direction coinciding with direction of the accelerometer orientation. The 9 direction of the accelerometer orientation represents the transverse direction of the building with the lowest natural frequency. To avoid any beating effect associated with possible coupling between translational and torsional motions and low damping [, 9], accelerations in transverse directions were averaged where possible. The ratios of the spectra recorded at the second, fifth and roof levels of the building in respect to the basement level in longitudinal direction are presented in Figure 5a. The spectra estimated for the basement floor are presented in Figure 5b. The spectral amplifications over elevation of a building are consistently concentrated around the first resonance frequency in this direction, which is estimated at.7 Hz. The spectra amplification curves for each level are about the same for both earthquake excitations despite the fact that the ground level acceleration spectra have completely different shapes as shown in Figure 5b. The amplification of the spectra at the second resonance frequency (estimated as.7 Hz) is less dramatic and slightly greater than. The peak acceleration at the roof level during the 99 Sierra Madre earthquake is amplified by a factor slightly less than. Since spectral accelerations are about the same at resonant frequency of the building, the spectral amplification is about the same during both earthquakes. The spectra at the roof level are amplified by more than times relative to that at the ground (basement) level in both cases. Longitudinal direction Roof@N Roof@SM 5th Floor@N 5th Floor@SM nd Floor@N nd Floor@SM... Longitudinal direction N SM. Figure 5. Amplification of floor spectra relative to that of first level and spectra of ground floor accelerations during two moderate level seismic excitations: Sierra Madre (SM) and Northridge (N) EQs (Pasadena, CA: longitudinal direction). The ratios of the spectra recorded at the second, fifth and roof levels of the building in

8 respect to the basement level in transverse direction are presented in Figure a. The spectra estimated for the basement floor are presented in Figure b. The spectral amplifications over elevation of a building are consistently concentrated around the first resonance frequency in this direction, which is estimated at. Hz. The spectra amplification curves for each level are about the same for both earthquake excitations despite the fact that the ground level acceleration spectra have completely different shapes as shown in Figure b. This can be explained by the fact that the spectral accelerations at the resonant frequency were about the same in case of both earthquakes. The amplification of the spectra at the second resonance frequency (estimated as. Hz) is less dramatic and close to. The peak acceleration had minimal amplification from basement to roof. The spectra at the roof level are amplified by more than times relative to that at the ground (basement) level in both cases. Transverse direction Roof@N Roof@SM 5th Floor@N 5th Floor@SM nd Floor@N nd Floor@SM... Transverse direction N SM. Figure Amplification of floor spectra relative to that of first level and spectra of ground floor accelerations during two moderate level seismic excitations: Sierra Madre (SM) and Northridge (N) EQs (Pasadena, CA: transverse direction). Amplifications in same building during design level earthquake excitations This section presents spectral amplification in a building subject to high level of seismic excitation which exceeds a design level earthquake. For this purpose a seven-story reinforced concrete building located in Van Nuys, California, U.S.A. was selected. The building was located approximately 7 km from the epicenter of the 99 Northridge earthquake. This is a seven-story commercial building with interior concrete column-slab frames and exterior concrete column-spandrel beam frames as a lateral force resisting system. The building design is dated back to 95 and it was instrumented in 9 by accelerometers installed on 5 levels of the building (CSMIP station #3). The filtered acceleration records from [7] were used in the study. The building s plan has :.75 aspect ratio with the longitudinal direction coinciding with 9 direction of the accelerometer orientation. The 3 direction of the accelerometer orientation represents the transverse direction of the building. The building sustained extensive damage during the Northridge earthquake []. There was extensive cracking in beams and beam to column connections. The most serious damage occurred at the top of some columns in the south frame of the fourth floor. To access spectral amplification of the building the same procedure was followed. The

9 accelerations in transverse directions were averaged to avoid noticeable beating effect associated with coupling between translational and torsional motions and low damping [, 9]. The ground spectra in both lateral and transverse directions are presented in Figure 7a. Since the spectral shapes are about the same the excitations in both directions had about the same frequency content. The roof spectral amplification is shown in Figure 7b which shows two well isolated spectral peaks associated with resonant frequencies of the building. Spectral amplifications in longitudinal and transverse directions are shown in Figure a and Figure b, respectively..5 Ground floor spectra Longitudinal Transverse.5 Roof spectra Longitudinal Transverse Figure 7. Ground floor spectra and roof spectra calculated for Van Nuys building during Northridge EQ Longitudinal direction Roof 3rd Floor nd Floor Transverse direction Roof 3rd Floor nd Floor Figure. in longitudinal and transverse directions estimated for Van Nuys building during Northridge EQ. The spectral amplifications of this building are much less than those presented earlier. At the roof level the spectral amplification is about 3. in longitudinal direction and about 3. in transverse direction. This big difference in spectral amplifications between this level and lower levels of excitation can be explained by extensive inelastic deformations of the building during the earthquake.

10 Summary of results A summary of peak floor acceleration s amplification and spectral amplification for all buildings studied is presented in Figure 9. All amplifications are presented in respect to z/h ratio depicted in Figure. The amplifications of peak floor accelerations shown in Figure 9a are well enveloped by the amplification specified by the AC5 document [5], although the latter amplification might be too conservative for high level excitations as it was noticed earlier []. Based on discussion provided above and the plots in Figure 9b, it can be concluded that the spectral amplification greatly depends on level of excitation. In general, lower excitations produce higher spectral amplifications, since the energy of seismic excitation is not dissipated by any non-linear effects in the structure. All peaks of spectral accelerations exceed the spectral amplification specified by AC5 document [5]. In addition to excitation level, other major parameters can have significant contribution to spectral amplification: design of lateral force resisting system, soil conditions, number of stories, design concept (base isolated versus fixed base), geometric shape of the building and so on... Elevation, z/h.. Elevation, z/h... AC5 High Moderate of A RIG of A RIG Figure 9. Amplification is presented as function of z/h ratio: amplification of A RIG and peaks of spectral amplifications.. AC5 Ultra-low Low Moderate High Figure. Amplifications are presented in respect to z/h ratio.

11 Conclusions The results presented here show that spectral amplification is excitation dependent. Depending on building s construction details, spectral amplification has a tendency to be higher at ultra-low excitations with a much higher amplification than that is currently anticipated by the code. With excitation level increasing, the spectral amplification has a tendency to decrease, but the cases presented here still demonstrate higher spectral amplification than the one prescribed by the code. A more detailed study is required to further investigate this issue and develop a new code requirement. The future study will be based on a statistical analysis of a large dataset of instrumented buildings throughout the world. Acknowledgments The financial support of USG Corporation to this project is kindly appreciated. References. International Code Council (ICC) (). International Building Code. ICC, Whittier, CA, California, USA.. Gilani, A.S.J., Takhirov, S.M., and Bachman, R. E. (). Current Code Requirements and Qualification Test Standard Development for Suspended Ceilings, the 9th US National and th Canadian Conference on Earthquake Engineering, Toronto, Canada, July American Society of Civil Engineers (ASCE) (). ASCE/SEI 7-: Minimum Design Loads for Buildings and Other Structures, ASCE, Reston, VA.. Gilani, A., and Takhirov, S. (). Current U.S. practice of seismic qualification of suspended ceilings by means of shake table tests. Ingegneria Sismica Journal: XXVIII-, International Code Council Evaluation Service Inc. (). Acceptance Criteria for Seismic Qualification by Shake-Table Testing of Nonstructural Components and Systems, AC5, ICC-ES. Whittier, California, USA.. Takhirov, S., Blondet, M. (997). Ambient Vibration Study of Berkeley Public Library, Report to Berkeley Public Library. Report No. UCB/SEMM-97/, Structural Engineering Mechanics and Materials, University of California, Berkeley, California, USA Boroschek, R. L. and Mahin, S. A., 99. Investigation of the seismic response of a lightly damped torsionallycoupled building, Univ. of California, Berkeley, Earthquake Engineering Research Center Report UCB/EERC- 9/, 9 pp. 9. Celebi, M., 99. Response study of a flexible building using three earthquake records, Proc. ASCE Structures Congress XII, Atlanta, Georgia,, American Society of Civil Engineers, 5.. Islam, M. S., Gupta, B., and Kunnath, S. (99). A critical review of state-of-the-art analytical tools and acceptance criterion in light of observed response of an instrumented nonductile concrete frame building. Proc., th U.S. National Conf. on Earthquake Engineering, Earthquake Engineering Research Inst., Oakland, Calif.. Taghavi, S. Miranda, E. (5) Approximate Floor Acceleration Demands in Multistory Buildings. II: Applications. Journal of Structural Engineering, ASCE, February, -.