Currently there are a small number of instrumented buildings in spite of the fact that our country one of those with the greatest seismic risk in the world.
That situation has limited understanding of the structural behavior of several buildings that suffered severe damage it has demonstrated the importance of having information generated in the structure, to carry out response estimations and evaluate the parameters that are used in the structural design of buildings. This way, Engineering Institute at National University (IIUNAM) and the Centro de Instrumentación y Registro Sísmico (CIRES), relaunched the seismic instrumentation of buildings in the Mexico City in 1987.
The instrumentation which registers seismic acceleration in structures has high costs including acquisition, installation, maintenance, processing and the analysis of information obtained during each earthquake as well as numerical estimates and mathematical modeling by specialists. However, modern instrumentation systems seek remote, automatic and fast performance in many of the activities of acquisition, conservation, monitoring, operation and processing, thereby diminishing their cost.
CIRES developed SADE (for Sistema Acelerométrico Digital para Estructuras in Spanish) in 2003. It is capable of registering the effect of strong earthquakes in selected points inside the structures of buildings.
When a structure is exposed to seismic accelerations, measuring the effects provides useful data to study and to evaluate its dynamic response, as well as the possible evolution of its behavior.
The SADE is composed of digital accelerometers with orthogonally fixed triaxials and biaxials sensors.
Internal view of the Digital Accelerograph System for Structures, SADE.
Measurement of acceleration and spectral magnitude by 4 points in a structure with tri-axial accelerometers with the SADE. (3 signals at the end of the graph, the functions are: sine, step and saw tooth).
This system is able to start recording when seismic accelerations overcome preset activation levels or when it registers an external trigger signal. This activation can be generated by Seismic Alert System Of Mexico (SASMEX®)), when it detects the beginning of a strong earthquake with its epicenter on the Pacific coast (Jalisco, Colima, Michoacán, Guerrero or Oaxaca).
The SADE has a precise time reference signal, generated by a global positioning system (GPS) satellite receiver which is used for dating the records obtained by the system in order to study the evolution of the phases of the movement. The sampling rate of the sensors is 100 times per second, with 16-bit precision. The eventual storage of data takes place in a Head Office of Recording and Control (CRC), where the sensors send their data over an Ethernet network.
Automatic processing of the seismic accelerations recorded in the CRC includes the calculation of power distribution at different frequencies and the determination of transfer functions belonging to the building; comparative graphs generated by the system allow the evolution of the behavior of the building to be observed. The knowledge of the evolution of the dynamic response of the building allows seismic safety conditions after strong earthquakes happen to be estimated.
SADE stores the signals measured in the sensors, makes calculations and sends reports when the acceleration measured in the selected sensors exceeds a activation level. This level is determined by means of the observation of the behavior of the structure with the excitation of normal movements of the soil of heavy vehicles and to the action of the wind on the structure; level settings are optimized by observations of the response of the building at the instant the earthquakes are logged. Accelerometers arranged in the structure transmit their information to the CRC through a digital information network and this, in turn, broadcasts the information across the Internet, to an external server to back it up.
The SADE automatically sends a results report whenever it detects an event. The data included in the report are shown in the following table. They contain information in the domains of time and frequency for each channel of the sensor; in the case of Transfer Function, the results depend on the previously determined combinations of ratios between sensors.
Centro de Instrumentación y Registro Sísmico, A. C., CIRES - MÉXICO
The SADE can be used to:
* Know and analyze the dynamic response of structure.
* Know possible changes in rigidity, if seismic forces overcome the resistance of buildings.
* Record histories of acceleration during a seismic event, as well as the distribution and level of energy associated with each frequency.
* Calculate ratio spectra between two selected points (Transfer Function) and to compare, for each seismic event, the evolution of the structural behavior.
* Automatically send the results to experts and parties responsible for structural safety, to decide whether to continue work in the property or start the evacuation to inspect the conditions of integrity of the building.
CIRES installed a Digital Accelerograph System for Structures (SADE) in 2006, with a seismic alert signal receiver in a 23 levels building located in Mexico City, in order to obtain information on the effect which strong earthquakes may cause, to assess structural dynamic response and, simultaneously, SASMEX® provides warning service. SASMEX® warning are useful to optimize the effectiveness of actions to prevent seismic risk that staff practice in the building.
Description of the System in a building
Installation of the SADE in the building consists Operation Center (CRC) connected with 11 remote stations, in three of which servo Triaxial orthogonal sensors are installed at the base of the structure, and seven Biaxial sensors for horizontal shafts are arranged according to the major axes of the structure: longitudinal, transverse and vertical. In addition, a triaxial sensor is installed in an open field, with Ethernet radio communication equipment.
The signals are measured in sensors and subsequently they are concentrated and stored in the system, which make calculations and automatically send reports of results when the measured acceleration at the default sensor exceeds a prearranged level. This level was determined by observation of structural performance in period of several weeks.
The main station of the SADE uses absolute time demarcation with reference to the GPS (Global Positioning System), in order to facilitate the identification of seismic events and their correlation with measured effects. Supervision of the system in the building is done remotely via Internet.
Since its commissioning, in 2006, the effects of is earthquakes on the structure have been measured 10 of magnitude M <6 and 5 of magnitude M> 6; the earthquake M6.3 of April 13, 2007, with epicenter in Guerrero and the earthquake of November 29, 2007, M7.4, particular stand out.
Centro de Instrumentación y Registro Sísmico, A. C., CIRES - MÉXICO
Below are the results generated from the earthquake of April 13, 2007, M6.3 which occurred in Guerrero. Figure 1 shows the graph of acceleration of the 11 points of measurement, as well as the spectral magnitude (right side) which belong to the biaxial sensors (BS1 - BS7) and triaxial sensors (TS1 - TS3) respectively.
Figure 2 shows three transfer functions in relation to the longitudinal and transverse axes corresponding to the ratios between some higher and base points. Each chart is compared with two previous results: the first one relating to activation due to environmental vibration and the second one to earthquake.
Figure 1. Acceleration and magnitude spectrum obtained automatically in a 23 story building in Mexico City during April 13th, 2007 earthquake M6.3 (Source: National Seismological Service)
Figure 2. Three transfer functions of the Longitudinal and Transversal axes obtained automatically in a building of 23 levels in Mexico City during April 13th, 2007 earthquake M6.3 (Source: National Seismological Service)