Chapter 2, Problem 2.1QPP

Classify the uncertainty associated with the following items as either aleatory or epistemic and explain your reason for your classification: average wind speed over a 30-day period, location of a certain applied load, change in strength of a soil caused by sampling method, capacity determined by a certain analysis method, magnitude of live load caused by vehicles traveling on a bridge, soil shear strength as measured by a certain method.

To determine

Whether the uncertainty associated with the given items is aleatory or epistemic.

Explanation of Solution

Introduction:

The uncertainty is categorized into two parts, that is, aleatory and epistemic. The aleatory uncertainties have inherent random nature. These can never be eliminated but can be reduced by increasing the number of samples.

The epistemic uncertainties arise due to the lack of complete knowledge of the processes or systems or due to errors made in the testing. These can be eliminated or reduced by acquiring better knowledge and improving the testing methods to reduce the errors.

The uncertainty in determining the average wind speed over a $30$ day period is aleatory as the wind speed is of random nature. Not only can it vary daily but can even vary hourly.

The uncertainty in determining the location of a certain applied load is epistemic as the location of the applied load is not random, but the variation between the calculated value and the actual value will arise due to an error in conducting the tests or due to lack of sufficient knowledge of the system.

The uncertainty in the change in the strength of soil caused by the sampling method is aleatory as the soil’s strength keeps on varying.

The uncertainty in determining the capacity by a certain analysis method is epistemic as the uncertainty arises due to the lack of knowledge of the method to be used or due to the inaccurate testing. The uncertainty can be reduced by either accurately performing the test or by improving the knowledge of the method.

The uncertainty in determining the magnitude of the live load caused by vehicles traveling on a bridge is epistemic as the uncertainty can be reduced by performing the test correctly or by improving the information or knowledge of the process.

The uncertainty in determining the soil’s shear strength by a certain method is aleatory as the shear strength of the soil is random because it depends on various factors such as the nature of the soil and thus, it cannot be determined accurately even if the test is performed correctly.

Conclusion:

The types of uncertainties associated with the given items are mentioned above.