Experimental Facilities for Earthquake Engineering
The laboratory has some state-of-the-art facilities related to experiments in the specialisation of earthquake engineering. These are :
Ambient Vibration Survey System
Ambient Vibration Survey (AVS) provides an inexpensive means of obtaining dynamic characteristics, namely natural periods and mode shapes, of existing structures. AVS is very useful to assess the accuracy of analytical model of real life complex structures (e.g., buildings and bridges) for which (a) structural drawings may be incomplete, (b) building material properties may not be known, (c) the extent of deterioration with time is undetermined and (d) soil-structure interaction is poorly understood.
AVS method recognizes that all structures are continually subjected to low level random vibrations, e.g., due to wind, wave and micro-tremors, and this input is broad-banded and relatively flat within the dominant natural frequencies of the structure. AVS provides a fast, non-destructive, effective and inexpensive means of obtaining in-situ response of existing structures. Most studies require less than three days of site work and the results can be provided in a report form within three weeks of field work.
The AVS system at hand consists of force balance accelerometers (±1g) , velocitymeters (340 Volts/m/sec), 8-channel signal conditioner for accelerometers and velocitymeters (on-line 220V AC or internal 12V DC battery power), a two-channel spectrum analyser, a six-colour pen-plotter and two two-channel strip chart recorders.
Free and Forced Vibration Survey System
Model Testing using Impact Hammer
The Laboratory has acquired a modally tuned impact hammer for modal analysis under free vibrations. The hammer consists of an integral quartz force sensor on the striking end of the hammer head. The important specifications of the hammer are:
Frequency Range: 80 kHz
Hammer Mass: 0.14 kg
Hammer Length: 20.3 cm
Head Diameter: 1.5 cm
Head Tip Diameter: 0.63 cm
Accelerometer Range: ±500 g/±50 g
Eccentric Mass Shaker System
For obtaining the dynamic characteristics (e.g., modal natural periods, mode shapes and associated modal damping ratios) of structures with large mass or stiffness, forced vibration testing is of immense value. When modal natural periods associated with second or higher modes are to be determined, the ambient vibration is usually too weak to distinguish the signal from the noise. Larger level of shaking may be required to record the higher mode response.
A 22 kN peak-force eccentric mass shaker has been developed in the laboratory to generate harmonic excitation up to 10 Hz. The shaker is driven by a closed-loop feed-back control mechanism to maintain the desired frequency.
Small Shaking Table
For the purpose of studying small size models or for shaking structures that are not too stiff or too heavy, low levels of excitation may be sufficient to obtain responses that are larger than the prevailing ambient noise level. The laboratory has acquired an electro-dynamic shaker of 133 N peak-force rating with a frequency range up to 200 Hz.
Cyclic Hysteretic Loading System
The laboratory is equipped with a range of electronically-controlled linear variable differential transducers (LVDTs) in addition to the traditional devices, like strain gauges for measuring linear strains and dial gauges to record displacements.
For dynamic measurements, Velocity Meters (induction type) and Accelerometers (both force balance and capacitive types) are used.
Data Acquisition System
The lab has acquired the System 500 from M/S Measurements Group Inc., USA, for data acquisition from strain gauges, LVDTs, Load Cells and Transducers. In the system, we can configure from 5 to 1200 channels, at any time, and in the present system we have 40 channels including 25 channels for strain gauges and 15 channels for LVDTs and Transducers.
The following other features are included in the system:
Built-in bridge completion for 120-, 350- and 100-ohm strain gauges.
Scanning and recording intervals as short as 0.02 seconds for up to 1200 inputs.
Stable, accurate, low-noise signal conditioning.
16 bit successive approximation A/P converter
One touch auto balance.
Collaboration for strain gauge inputs.
Thermal output compensation.
Online display (Graphic/numeric) for channel outputs.
The system is complete with a Windows based software (WINS000/Strain Smart) designed for experimental Stress Analysis.