When building things, we want to make sure the materials we use or the structure we build can withstand the loads we are going to apply to them. For example, we need to use a screwdriver that is capable of handling the torque needed to tighten a screw, or else the screwdriver would break, or the screw would not get tightened adequately. On a larger scale, we want to construct a building that will be able to carry the intended number of persons and objects (e.g., furniture, equipment, appliances) that will occupy it. The objects that constitute the sum of weight that the building has to carry are called loads.
Loads that vary in the amount of force over time, or ones that change position, are called live loads. On the other hand, loads that are not mobile or maintain their influence over a given area for long periods are known as (you guessed it) dead loads. These loads (if considered to affect a particular area) are also called stresses. For more detailed knowledge about stress, you might want to check out and try our stress calculator. But for now, we'll stick with calling these forces "loads".
However, there are other loads that can influence a structure. Examples of these loads are earthquake loads, wind loads, and even loads due to snow or rain. These are things we need to consider when building a safe structure. To arrive at the strength that the structure needs to support to be reliable (i.e., it would not fail when any unexpected additional loads are present), engineers came up with a set of numerical constants that they multiply the structure's design load by to check for these things. These constants are called the factor of safety.
The usefulness of a factor of safety is to extend the target capacity of a structure so that the engineers can appropriately determine which materials to use in its construction. To help you visualize this, imagine you are about to cross a small creek. To do this, you have to make a simple bridge out of a single wooden plank. A couple of wooden planks are available to use: there are thin ones that are light to carry and position over the creek, but they seem too flimsy to let you cross safely. Then there are the larger ones that are very sturdy. However, they are too heavy even fashion a bridge out of.
Luckily for our expedition there are medium planks that seem strong enough to carry you, but are still light enough to place over the creek. You tried to position one and started walking over it. But, before you reached the middle of the plank, you heard a creak! You hurriedly walk back to safety before it breaks. You go back to the medium planks and choose one that is a little bit larger and heavier. Trying again, you made it safely across the creek. Had you known the strengths of these planks (their maximum strengths) and compared it to your weight (design load), you would have easily known which plank to choose. Choosing the plank with a maximum capacity that exceeds your weight and then some is like applying a factor of safety.
You can just simply use our factor of safety calculator by typing in your known values for the maximum strength and design load. It will instantly show you its factor of safety. Alternatively, we can calculate the factor of safety of a building (or that plank) by dividing the maximum strength of the structure by the intended design load. We can also express this statement in the equation shown below:
Factor of Safety = maximum strength / design load
For a structure to be considered safe, its factor of safety must be greater than 1. A factor of safety that is equal to 1 means that the structure's maximum strength or capacity is equal to its determined design load. This means that the structure would fail if any additional load was applied.
On the other hand, if the factor of safety is less than 1, it means that the structure could fail at anytime, even before reaching the design load. Its maximum strength just can't support the load it should carry. This calls for a recalculation of the design to make the structure stronger, then.
We can also rearrange the factor of safety equation to determine the appropriate material for part of a structure. By transposing the design load variable together with the factor of safety variable, we'll arrive to an equation that looks like this:
Maximum strength = factor of safety * design load
This means that in order for the member of the structure to be reliable, its maximum strength should be equal to the product of its safety factor and the load it is intended to carry. To ensure this, we can choose from tables of materials strengths, such as this book the [Structural Steel Members],(https://www.aisc.org/globalassets/aisc/publications/historic-shape-references/structural-steel-shapes--information-and-tables-for-engineers-and-designers-and-other-data-pertaining-to-structural-steel-1926.pdf) to find material with specifications and cross-section that would result in a value equal to or greater than our calculated maximum strength.
You can also use our factor of safety calculator to determine the maximum strength by leaving it blank and typing in the values for the factor of safety and the design load.
We can use factors of safety when designing objects as simple as a screwdriver (as we discussed earlier), to much-complicated machines like a turbocharger compressor. The reason for this is so that these machines and objects are made safe for us, and serve us their purpose without easily fail in times of trouble.
Here are some examples of where factors of safety are used:
· designing a building with safety in mind
· designing other structural objects such as water tanks, and even transmitter towers
· designing tools and other mechanical objects that have parts that would require some safety factor because they will be subject to loads
· designing automobile parts
· developing and designing other devices or gadgets that would be subject to loads