Understanding the Role of Z/2 in Seismic Design Coefficient Calculation per IS 1893:2016

Seismic design is a crucial aspect of building engineering, ensuring structures are resilient against earthquakes. The Iranian Standard Code (IS 1893:2016) plays a significant role in seismic design, particularly in defining the design horizontal seismic coefficient. This article delves into the detailed explanation of why 'Z/2' is incorporated into the formula as it pertains to integral calculations for seismic design coefficient.

Introduction to Seismic Design and IS 1893:2016

Seismic design is the comprehensive process of designing and building structures to withstand the dynamic loadings associated with earthquakes. It is essential to ensure that the structures do not suffer excessive damage, collapse, or pose a threat to human life during seismic events.

The Indian Standard Code of Practice for Earthquake Resistant Design of Buildings (Chapter 16 of IS 1893:2016) provides guidelines for the design and construction of structures to resist ground shaking produced by earthquakes. This code is widely recognized and codifies the principles and methods for ensuring seismic safety in construction.

Understanding the Seismic Design Coefficient

A fundamental concept in seismic design is the Seismic Design Coefficient (Ku), which is the ratio of the ground motion to the effective weight of the structure. The seismic design coefficient is calculated and utilized to determine the seismic forces to be applied in the design and analysis of structures.

IS 1893:2016 specifies the formula for the design horizontal seismic coefficient as:

Formula Explained: Z/2 x I/R x Sa/g

Z: Zone Factor is a crucial parameter that varies with the geographical location of the site. It is determined on the basis of hazard analysis for a particular zone for the Maximum Considered Earthquake (MCE) I/R: Intensity/Radius Factor is a dimensionless factor that depends on the ratio of the peak ground acceleration (PGA) in the considered intensity zone to the radius of the site. This factor accounts for the near-surface effects on seismic ground motion. SA/g: The acceleration due to gravity is a constant, which is typically set at 9.81 m/s2

According to IS 1893:2016, the formula used to determine the design horizontal seismic coefficient is:

"Ku Z/2 x I/R x SA/g"

This formula uses Z/2 and not simply Z, which is a concept that requires explanation to understand its significance in seismic design.

The Role of Z/2 in Seismic Design Coefficient Calculation

The use of Z/2 in the formula is a unique feature of IS 1893:2016 and reflects several key considerations:

Hazard Analysis: The zone factor Z is determined based on the hazard assessment for a given area. IS 1893:2016 emphasizes the importance of this assessment, taking into account the historical seismic activity and the probabilities of future events in each zone. However, when considering the actual seismic loads, it is common practice to multiply the hazard factor by a factor of 0.5 to account for uncertainty and to make the design more conservative. Conservative Design: Seismic design aims to be conservative to ensure safety. By considering only half the value of Z, the design ensures that structures are capable of withstanding slightly higher seismic events than might be predicted by the hazard analysis alone. Residual Plastic Drift: The use of Z/2 allows for some residual plastic drift in the building structure. This accounts for the inherent plasticity of materials and the possibility of structural imperfections during construction and material aging.

This approach provides a safer margin for error, accounting for uncertainties in both the structure's behavior and the seismic data. The quasi-Static PGD distribution, which is a key concept in seismic design, ensures that the building can safely dissipate energy during an earthquake.

Conclusion

Understanding the role of Z/2 in the seismic design coefficient as per IS 1893:2016 is crucial for engineers and architects working on seismic-resistant structures. This factor reflects a cautious approach to design, ensuring that structures are robust against seismic forces beyond the expected maximum considered earthquake.

This article on 'Why is Z/2 taken in IS 1893:2016 for design horizontal seismic coefficient' seeks to clarify the significance of this critical factor in the context of seismic design. Its inclusion in the design formula provides a conservative yet practical approach to ensuring the safety and resilience of structures in regions prone to earthquakes.