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Factor of safety

Factor of Safety: Understanding and Implementation

Factor of safety ensures structures are designed to withstand stresses without compromising integrity. Here’s how to use them

The task of structural engineering is to ensure that every structure possesses an adequate margin of safety to tackle potential challenges such as load variations, construction imperfections, or material aging over time. Employing appropriate factors of safety in structural design is crucial to achieving this goal.

In this week’s blog post, we’ll focus on the significance of these parameters. We’ll understand what factors of safety are and how to utilize them to design structures capable of withstanding applied loads and stresses without experiencing collapses or damages.

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What are Factors of Safety?

In general, the term “factor of safety” refers to a parameter used in engineering, design, and other technical fields to evaluate the degree of safety and reliability of a system, i.e., its ability to operate safely, considering any uncertainties or variability in data or operating conditions.

In structural engineering, factors of safety are understood as multiplicative factors applied to design loads and structural resistances to ensure an adequate margin of safety. These parameters account for the extreme variability in actual load and resistance values compared to the established design values. In other words, they allow for the consideration of the actual operating conditions of a structure, which may differ from the anticipated design conditions.

What are Factors of Safety Used For?

Factors of safety are parameters adopted in structural engineering to assess the ability of a load-bearing structure to safely withstand loads and stresses. These factors reflect the ratio between the maximum load capacity (or strength) of a structural element and the maximum load (or stress) it will be subjected to during its service life.

Their main function is to ensure that the structure can withstand the anticipated loads without experiencing collapses or damages. These factors take into account the various factors that can influence structural safety, such as material imperfections, variations in design loads, and environmental conditions.

Factors of safety, therefore, enable effective risk management in structural design. They provide a safety margin that considers uncertainties and variabilities to ensure that a structure can cope with unforeseen situations or adverse conditions. This means that even in the presence of slightly higher or lower loads or resistances than the design values, the structure will remain safe and reliable.

Classification and Examples of Factor of Safety Usage

Commonly used factors of safety in structural design are divided into:

  1. factors of safety on resistance: these are applied to the design resistances of structural materials, such as concrete or steel, to decrease their value for safety. They allow for the uncertainty due to variability in material properties, production quality, and other factors that may influence the actual material strength compared to design values. To check the strength of a steel column, you start by determining the design strength of steel, which depends on the material properties and the cross-sectional area of the column. Then, the factor of safety is applied to the design strength to obtain the calculated strength. For example, if the design strength of the steel is 200,000 kN/m2, and the total factor of safety is 1.1, then the calculated strength will be 200,000 kN/m2 / 1.1 = 181,800 kN/m2;
  2. factors of safety on load: these factors are applied to partial loads considered in the structural design process. They are used to account for uncertainties in actual load values, as these may vary unpredictably during the structure’s service life. Design actions may include the self-weight of the structure, permanent loads (such as walls, floors, etc.), and variable loads (such as live load, wind load, etc.). Usually, partial factors of safety are greater than 1 to provide an adequate safety margin. For example, if the design load is 100 kN and a partial factor of safety of 1.5 is applied, the actual design load used in structural calculation will be 150 kN.
Classification of safety coefficients

Classification of safety coefficients

The Difference Between Partial and Global Factors of Safety

The coefficients described above are defined partial factors of safety, as they are applied to specific loads and resistances during the structural design process. This term reflects the fact that a single global factor of safety is not applied to the entire structure, but rather different partial factors of safety for each load and resistance considered in the design.

Since it is challenging to precisely predict material variability and loading conditions, structural engineers use partial factors of safety to introduce an appropriate safety margin for each specific aspect of the design. For example, there might be a partial factor of safety applied to permanent loads and another for accidental loads. The same principle applies to resistances. Different partial factors of safety may be used for concrete, steel, wood, etc.

Global factors of safety differ from partial ones as they consider both material strength and applied loads. They often result from the ratio between the material’s ultimate strength and the maximum expected load. These factors represent the overall safety margin of the structure and encompass all uncertainties and variabilities considered during the design. Their purpose is to ensure that the construction has sufficient overall robustness to withstand all possible loading situations without compromising integrity.

While structural design is often seen as a complex process, remember that you can always rely on the capabilities of a structural analysis software. With this tool, you effortlessly design a structural BIM model, automatically obtain structure calculations using the integrated FEM solver, and visualize calculation results and minimum safety coefficients of various structural elements directly from 3D.

Results of the calculation of safety coefficients with EdiLus

Results of the calculation of safety coefficients with EdiLus