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Determining the column safety factor in GenCol

The safety factor (SF) gives an indication of the ratio of the column capacity to the applied loading. A safety factor is determined for each load case at three locations within the column: top, middle and bottom. The interaction diagram below has been annotated to show how the SF is determined.

The interaction diagram displays an envelope of the section’s capacity for a certain load case. If the applied load case, falls outside of the section’s envelope, it will result in a SF<1, and therefore, an unsafe design option. However, if the applied load case falls within the envelope, it results in a SF>1 and ultimately, a safe design.

The Safety factor is calculated as follow:

  • SF = B/A
    • Where A and B are calculated by using Pythagoras.

It follows that:

  • SF < 1: Loading exceeds the capacity of the column and results in an unsafe design.
  • SF > 1: Loading is less than the capacity of the column and results in a safe design.

The calculated safety factors, at each of the locations within the column, are given within the output table, as can be seen below:

The critical load case, is the load case with the smallest safety factor.

Design codes state that a safety factor of more than 1 is required, to ensure your design is safe. When your safety factor is smaller than 1, you will have to increase your column size, or increase the amount of reinforcing, to receive a resultant safety factor larger than 1, at all three locations within the column.