Engineers make assumptions all the time. All these assumptions are backed up by logic and theory, so instead of pursuing exact values, in civil engineering often times we are okay with close enough values, as long as the results are within acceptable limits and we can justify it with relevant proven theory. Doing so, saves a great deal of time and effort, which makes developing theory or designing and solving problems possible in reasonable amount of time and with reasonable effort. Let’s randomly list some examples below. These are structural examples but the same kind of approach applies to any branch in civil engineering and other engineering disciplines as well. For example:

• We might assume a certain load applied on a column is constant although it may vary to some extent which helps simplify structural analysis.

• We might assume that zero deformations in a structural member if it is small enough, so that certain terms in mathematical equations can be eliminated, which would greatly simplify the math.

• We might assume that a member is perfectly rigid if the deformation of that member is negligibly small. This is of great help when making structural analysis, not only manually but also when making computer aided analysis that uses finite element method, which greatly reduces the calculation load.

• We might assume that a member deforms in a perfectly elastic manner, meaning, it will recover all of its deformation upon removal of the load if loads are small enough. But in reality there may remain a negligible amount of permanent (plastic) deformation even at a zone that we considered elastic zone.

• We might assume that an impact load which is small enough can be treated as static load for simplicity, although it is dynamic in nature.

• We might assume that the sine of a very, very small (infinitesimal) angle is equal to the angle itself, which would considerably simplify the equations to solve mathematically, such as when performing structural stability (geometrical stability) analysis.

• We can solve a great variety of structural problems with energy equations. For example we can solve problems by assuming energy entering into the system is equal to the work the system does, although a certain amount of energy is always lost due to friction.

• In the image below, you see a simplified, typical shape of a stress strain diagram of concrete where “σ” represents stress and “ε” represents strain, as obtained by compressing it. The red and green lines represent two possible assumptions of modulus of Elasticity of the concrete member in question, which are obtained using different methods. Again, these are our assumptions. They are made to make a reasonable guess for E. As you can see below, the slope of the curve is not constant, it varies, which means E is also not a constant value (because E=σ/ε), but based on extensive research and past experience, engineers come up with reasonable estimates, to obtain a simplified, constant value for E, which is very important parameter when we insert concrete in structural analysis calculations.

Simplifications, assumptions are inseparable part of any engineering field. As long as we are satisfied that making an assumption does not cause us to sacrifice anything of significant value and we are still within acceptable limits as backed up by theory, we can make assumptions. This is how the theories are developed, codes are written, designs are made. Without them, developing engineering theories and applying them to real life would be very difficult and probably in most cases impossible. After all, engineering is applied science and mathematics, for solving problems practically in real world.

Author: A. TUTER

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