Issues in Instruments Selection for Structural Performance Evaluation Tests

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Specific instruments are required for structural performance evaluation. Issues while selection of instruments for these structural evaluation is discussed. When a specific structural concrete is design, necessary and required factor of safety are provided and theoretically the structure will be safe. However, there are situations which theoretical consideration may not suitable to provide certainty about the safety of the structure. Structural performance assessment might be needed due to doubted flaws in construction. Occasionally design of the structure, damages and deterioration of structures, and structural performance assessment is conducted for uncommon features of the structure. The performance of the structure is assessed through non-destructive, partially destructive, and destructive tests. There are number of problems that can arise in instrument selection for structural performance test and these issues will be explained in the following sections.

Issues in Instruments Selection for structural Performance Evaluation

• Total range and working range of the selected instrument
• Precision of the selected instrument
• Accuracy of the selected instrument

Total Range and Working Range of the Selected Instrument

The total range of the equipment, which is used for testing performance of the structure, is the largest and smallest values that the instrument can read. The equipment working range is the highest and lowest reading of structural response that is obtained during the test. The total range of the device used for structural performance test must not be surpassed to avoid error in the data acquisition for instance some devices provide incorrect measurement when the measured value is close to the maximum range of the device. Achieving measurements which is smaller than the minimum range of the device need to be prevented because this leads to provide a result which is not precise. That is why it is considerably significant to consider this issue when the equipment is selected for testing of the structural performance.

Precision of the Selected Instrument

It is should be observed that the precision is an absolute value or a number which is not changing. Therefore, it is possible that the results provided by an equipment is precise but it could be inaccurate. When equipment provides the same error in each result, then it is considered to be precise.

Accuracy of the Selected Instrument

The difference between the reading of the instrument, which is used to test the structural performance, and the true value that is being measured is the instrument accuracy. The accuracy of an instrument is the total of number of errors and effects of the equipment. The total response of the equipment consists of five different parts which includes instrumental error, response because of the load, response caused by non-load effects on the structure, for example, temperature fluctuations may affect deflection and strain, errors caused by external influences on the instrument, errors because of instrument mounting effects. It is possible that all components do not present in certain device. The effect of each of the above instrument response components can be different and one may create larger effect, therefore all of them should be considered otherwise serious errors may arise. Not only do all instruments have specific error quantity related to its measurements but also the data acquisition system itself may lead to extra errors. So, to pinpoint the source of error caused by instrumentation and data acquisition, it is substantially important to take resolution, sensitivity, linearity, repeatability, hysteresis and backlash into consideration.

Sensitivity of the Equipment

It is the ratio of instrument output to its input and is specified be the most appropriate straight line. The sensitivity influence data acquisition system not the instrument accuracy. Figure 1 show the sensitivity of an instrument and it is determined by suitable straight line that is drawn though a scattered data points.

Fig.1: Sensitivity Definition for Equipment

Instrument Linearity

An instrument is linear when its output has a linear relationship with the response. And the linearity error as shown in Figure-2 is the measure of how much the output is distant from the best fit straight line.

Fig.2: Linearity and Linearity Error in Instruments

Resolution

It is one of the components of instrument accuracy and it is smallest value of a response that can be measured. For example, the resolution of digital gauge is the smallest value that is showed by the gauge, and one single tick mark is the smallest value in mechanical gauge and in this case the resolution greatly depend on the individual that read the response or measurement.

Instrument Repeatability

It is the instrument ability to provide the same output for the same response over several trials when the response is continually declining or rising.

Hysteresis

It is the variation in the reading of the specific point based on whether the response was declined or increased at the time of achieving the reading. Figure 3 explains the hysteresis.

Fig.3: Hysteresis

Backlash

It is the measure of variation in response needed to get a change when the reading of the device is reversed. For example, in case of loading and unloading as shown in Figure-4.

Fig.4: Backlash

How to Reduce or Eliminate Instrument Accuracy Issues?

Sensitivity and resolution problems, they can be prevented by selecting suitable device depending on the wise and sensible estimation of the values or quantities which will be measured. Therefore, it requires to establish a plan for the test in advance. Thus, repeatability, hysteresis, and backlash errors compose the total instrument errors. Repeatability can be computed by providing inputs to the device through a range and the largest error due to repeatability is the distance between two points of the two different trails on the condition that both must be corresponding to the same input. The error caused by linearity may be specified by creating input versus output graph. The error estimation can be improved by graphing both directions. For example, loading and unloading graph and for each part of the graph at least three points should be taken. Eventually, the error will be the greatest distance from any point to the best fit straight line as can be observed from Figure-2. The error due to hysteresis and backlash can be found by generating a single graph for hysteresis and backlash and similar to linearity, at least three points should be considered for the graph. The error will be the distance between input point to corresponding output point. Errors that related to the mounting of the device involves arbitrary environmental vibration of the mounting, creep of adhesive materials such as glue or epoxy which is employed to maintain the mounting bracket at its position, mechanical mounting slip. These errors can be prevented be taking necessary measures for instance install the device properly, select appropriate mounting device, and protect the device from influences of wind and vibrations. Noise and drift are examples of the external influences on the device which might cause error. The former arbitrarily increase and decrease in the input of the equipment whereas the latter is the situation in which the reading of the device is not the same and changing despite the fact that the response has not changed, and temperature can be the cause of this kind of error.