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What is permissible stress design?

Permissible stress design (PSD) is a traditional approach used in civil engineering for the design of structures such as buildings, bridges, and dams. It involves calculating the maximum allowable stress that a material can withstand without failure and then designing the structure based on this permissible stress. The PSD approach is simple and straightforward and has been used for many years in the construction industry.



The PSD approach involves calculating the maximum stress that a material can withstand, which is known as the allowable stress or the permissible stress. This stress is calculated based on the strength of the material and its properties, such as elasticity, ductility, and toughness. The allowable stress is then used to determine the maximum load that the structure can carry without failure.


In the PSD approach, engineers determine the loads that the structure will experience, such as dead load, live load, and wind load. They then design the structure based on the allowable stress of the material used for construction. The allowable stress is determined by dividing the ultimate strength of the material by a factor of safety, which is typically between 2 and 4.

One of the advantages of the PSD approach is that it is simple and easy to use. It allows engineers to quickly design structures based on the allowable stress of the material used for construction. The approach also provides a high degree of confidence in the structure's safety since it is based on the strength of the material.


However, there are several limitations to the PSD approach. One limitation is that it does not take into account the variability in the strength of the material. Materials can have a wide range of strengths, and the PSD approach assumes that the material has a uniform strength throughout. This can lead to overdesign or underdesign of the structure, depending on the variability of the material strength.


Another limitation of the PSD approach is that it does not consider the loadings that the structure will experience over its lifetime. The approach assumes that the loads are constant and do not change over time. However, loads on structures can vary over time, leading to fatigue and failure of the structure.


In recent years, the PSD approach has been replaced by limit state design (LSD) in many countries. The LSD approach is based on the concept of defining limit states, which are conditions beyond which a structure is deemed to be unfit for its intended use. The LSD approach considers both the ultimate limit state and the serviceability limit state of the structure, leading to safer and more reliable designs. It is still used however, for many aspects of temporary works.


In conclusion, the permissible stress design approach has been a traditional approach used in civil engineering for many years. It involves calculating the maximum allowable stress that a material can withstand without failure and designing the structure based on this permissible stress. The approach is simple and easy to use, but it has several limitations, such as not considering the variability in material strength and the loadings that the structure will experience over time. With the advancement of technology and research, the LSD approach has become a more reliable and safer approach for the design of structures.

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