SEISMIC INSTRUMENTATION OF BUILDINGSBy Carlos M. Villaraza. FASEP INTRODUCTION Technology on building instrumentation for seismic monitoring has improved tremendously in the past decade. The purpose of the Guidelines and Implementing Rules on Earthquake Recording instrumentation for Buildings proposed by ASEP is to provide information on the specifications and uses of earthquake recording instruments for buildings as provided in Section 105.2 of the National Structural Code of the Philippines 2010 Volume 1, Sixth Edition (NSCP 2010). Installation of earthquake recording instruments was first required in the National Structural Code of the Philippines 1992, Fourth Edition, wherein structural engineers were only interested in the strength design capacity on the buildings based on seismic parameters provided in the Uniform Building Code (UBC) of the United States, referral code of the NSCP. Structural code developers started to recognize the importance of not only strength but serviceability and performance as well. The experiences from the 1994 Northridge Earthquake in the US and the 1995 Kobe Earthquake in Japan gave credence to these considerations. DPWH therefore deemed it necessary to improve our understanding of the building response based on real seismic event from local earthquake generators by enforcing placement of earthquake recording instrumentation for buildings as the NSCP provision was reiterated in 2001, Fifth Edition, as well as in the latest 2010, Sixth Edition. The NSCP 2010 states that "Unless waived by the building official, every building in Seismic Zone 4 over fifty (50) meters in height shall be provided with not less than three (3) approved recording accelerographs. The accelerographs shall be interconnected for common start and common timing." The Philippines needs to have its own earthquake baseline data for validating the seismic design parameters used during and future structural design of buildings, in order to support earthquake disaster mitigation efforts. Hence, the waiver stated in the NSCP 2010 is temporarily suspended until such time that considerable sets of adequate earthquake records have been obtained for various specified types of buildings and relevant provisions in the NSCP have been amended. However, for the purposes of the Earthquake Recording Instrumentation for Buildings, the Department of Public Works and Highways' has identified buildings in Table 1 to be necessarily installed by the said seismic monitoring system. With GPS capability C. or prototypes or large. Three (3) accelerographs at Ground floor/Lowest basement. earthquake recording instruments shall include intensity meter. however. Provincial / City / Municipal Halls and Buildings Ground Floor Level 3. and Floor below Roof 2. 1. Computerized Analyses: Using special purpose public-domain or private software. Hospitals. quasi-static. structures are analyzed for prescribed loads determined either by code provisions or postulated site-specific ground motions. Accelerograph for recording waveform and transformed to FFT B. such as a shaking table or hydraulically powered and electronically controlled loading systems. Laboratory Testing: Subsystems. or dynamic loading. 3. EVALUATION OF SEISMIC BEHAVIOR AND PERFORMANCE OF STRUCTURAL SYSTEMS There are three main approaches to evaluate seismic behavior and performance of structural systems. scaled models of complete systems are tested under static.TABLE 1. Laboratory testing has also contributed substantially to understanding of dynamic soil properties and the interaction phenomenon between the soil and structure. Send data to data center of the government Note: For Provincial / City / Municipal Halls. by . This approach does not necessarily demand a time-dependent testing scheme. schools and other buildings fifty (50) meters high and above Either of the following: 1. EARTHQUAKE RECORDING INSTRUMENTATION REQUIREMENTS TYPE AND HEIGHT OF BUILDING LOCATION REQUIREMENTS GOVERNMENT BUILDINGS A. 2. Hospitals with 50-bed capacity or more. testing of structural systems under controlled simulated environments is desirable. components. Middle floor. Natural Laboratory of the Earth: The third main approach to evaluate the behavior and performance of structural systems is to use the natural laboratory of the Earth. and schools with twenty (20) classrooms or more but not less than three (3) storeys Ground Floor / Lowest Basement 2. One (1) accelerograph at Ground floor/Lowest basement interphased with two (2) triaxial accelerometers at middle floor and floor below roof 1. to develop integrated networks which measure the seismic source. strong ground motions as well as moderate-level motions would be experienced frequently. 4. 3. 7. a well-instrumented structure for which a complete set of recordings has been obtained should provide useful information to: 1. correlate the damage with inelastic behavior. the goal being to improve the models. follow the spreading nonlinear behavior throughout the structure as the response increases and determine the effect of this non-linear behavior on the frequency and damping. significant progress in improved designs can be achieved. determine the importance of non-linear behavior on the overall and local response of the structure. check the appropriateness of the dynamic model (both lumped-mass and finite element) in the elastic range. Integral to the “natural laboratory” approach is the instrumentation of selected structures so that their responses can be recorded during future earthquakes. determine the ground-motion parameters that correlate well with building response damage. make recommendations eventually to improve seismic codes (Çelebi and others. and the structural response processes. 5. the transmittal of ground motion. 2.observing and studying the performance (and possibly the damage to structures) following earthquakes. in optimum test areas. it is essential that integrated arrays of instrumentation be planned and installed to assess thoroughly the relation of ground motion that starts at a source and transmitted through various soils to a substructure and finally to a superstructure. As a result of this understanding. . More specifically. For such design studies a natural laboratory would be a seismically prone area that offers a variety of structural systems. In addition. OBJECTIVES FOR SEISMIC INSTRUMENTATION OF STRUCTURES The main objective of seismic instrumentation program for structural systems is to improve understanding of the behavior and potential for damage of structures under the dynamic loads of earthquakes. facilitate decisions to retrofit/strengthen the structural system as well as securing the contents within the structures. The nearby free-field and ground-level time history should be known in order to quantify the interaction of soil and structure. The direction for seismologists and engineers working together is clear. An instrumentation program should provide enough information to reconstruct the response of the structure in enough detail to compare with the response predicted by mathematical models and those observed in laboratories. and 6. By determining why specific designs lack earthquake resistance and then by using extensive laboratory testing of modified designs. Thus. 1987). design and construction practices can be modified so that future earthquake damage is minimized. the data should make it possible to explain the reasons for any damage to the structure. at least three vertical accelerometers are required at the basement level. a minimum of three accelerographs be placed in every building over fifteen stories high. is not necessarily a useful first stage for the instrumentation being discussed. Figure 1.Code versus Extensive Instrumentation The NSCP 2010. This type of instrumentation scheme is called the ideal extensive instrumentation scheme herein and is illustrated in Figure 1b. If vertical motion and rocking are expected to be significant and need to be recorded. . As an example. for seismic ZONE 4. Experiences from past earthquakes show that the NSCP minimum guidelines do not ensure sufficient data to perform meaningful model verifications. a minimum of 12 accelerometers would be necessary to record these modes. NSCP-Code type instrumentation is illustrated in Figure 1a. Typical Instrumentation Schemes The NSCP-type instrumentation. three horizontal accelerometers are required to define the horizontal motion of a floor (two translations and torsion). because it is designed for monitoring. recommends. Rojahn and Matthiesen (1977) concluded that the predominant response of a high-rise building can be described by the participation of the first four modes of each of the three sets of modes (two translations and torsion). therefore. The purpose of this requirement by the NSCP was to monitor rather than to analyze. Like the superstructure. Placing sensors at critical locations of the foundation to capture all its relevant motions will at a minimum facilitate study of its behavior. modal damping) using recorded responses of structures (Ljung. or they obtain the motion at foundation level by convoluting the motion through assumed or determined layers of strata to base rock and deconvoluting the motion back to foundation level. . so measurements taken at other sites can be misleading. Downhole data are especially scarce. although a few such arrays have been developed. Actual measurements are useful for an engineer when deciding if a building is reacting according to the building model or demonstrating a weakness. To confirm these processes requires downhole instrumentation near or directly beneath a structure. Instrumentation needs of a structure have been addressed by Rojahn and Matthiesen (1977). Engineers use free-field motions as input motion at the foundation level. These downhole arrays will serve to yield data on: the characteristics of ground motion at bedrock at a defined distance from a source and the amplification of seismic waves in layered strata.Figures 1c and 1d illustrate typical special purpose instrumentations. more information is required to interpret the motion of the foundation substructure relative to the ground on which it rests. These methods have evolved into single-input single-output and multiinput multi-output versions that enable construction of modal shapes. Within the last decade plus. USEFULLNESS OF THE INSTRUMENTATION RECORDS Structural Engineers Soil structures vary depending on location. Furthermore. 1987). This is easily accommodated along the instrumentation scheme of the superstructure. high-precision record synchronization must be available within a structure if the response time histories are to be used together to reconstruct the overall behavior of the structure. Hart and Rojahn (1979) and Çelebi and others (1987). Diaphragm effects are best captured by adding sensors at the center of the diaphragm as well as the edges (Figure 1c). the foundation system needs to be instrumented to study its response. system identification techniques have made it possible to identify structural characteristics (modal frequencies. Performance of base-isolated systems and effectiveness of the isolators are best captured by measuring tri-axial motions at top and bottom of the isolators as well as the rest of the superstructure (Figure 1d). However. 4.com Phone: (02)658 2773 FAX: (02)658 3660 CP: 09189224970 . seismic design parameter studies and seismic risk analyses.0. Civil Engineering from the University of Santo Tomas – Manila (1976). *About the Author Carlos M. th National Structural Code of the Philippines. Email: cmvillaraza@yahoo. related to such scenarios as: The movement exceeded the building limits – get everyone out immediately. He has been involved in the review of the earthquake provisions of the National Structural Code of the Philippines since 1987 and Chairman of the NSCP 2010 General Requirements and Loads & Actions. The movement was in between these levels – get an engineer to physically inspect the building REFERENCES 1. 6 Edition. A graduate of B. Measuring real-time data on movement. a member of the APEC Informal Network on earthquake provisions for the Harmonization of Building Codes in the APEC Region and the review of ISO provisions covering lateral loads and general loading conditions. Seismic Instrumentation of Buildings: Special GSA/USGS PROJECT (2002). strain and other measurements of the health of a building or infrastructure may be related to the question: ‘Does the structure react the way the building model predicted?’ An actual measurement of the stress or strain experienced by a building or infrastructure during an earthquake or other significant event may be related to such questions as: “Has this building been exposed to more stress than it was designed for? Is it safe to occupy? Is this dam at risk? Building and Infrastructure Owners and Managers It is useful to have real data from the site available immediately to base decisions on. Guideline for ANSS Seismic Monitoring of Engineered Civil Systems – Version 1. Celebi. al. Volume 1. 2. 3. Villaraza is a Structural-Earthquake Engineering Consultant. A Quantitative Basis for Building Instrumentation Specifications. M. He has over forty years of extensive experience in structural engineering design. D Skolnik et. An instant measurement (internet or text) can assist in determining building safety for occupants. The movement was within the calculated building tolerances – business as usual. he received his Post Graduate Diploma in Earthquake from the International Institute for Seismology and Earthquake Engineering in Japan (1986).NSF CMMI Research and Innovation Conference 2009 (Hawaii).S.