Earthquakes are significant challenges to the safety and stability of building structures. This hazard, necessities sound engineering considerations and structural consulting practices from all Civil Engineers and structural consultants so that the buildings overcome these earthquake load hazards. In India, the Indian Standard IS 1893 2016, provides a comprehensive guideline for analysing and designing earthquake loads on buildings and infrastructure. Foremost to IS 1893 is the idea of the Response Reduction Factor (R), which acts an important role in designing building structures that can resist seismic forces while also allowing for some amount of flexibility and ductility.
The Response Reduction Factor is primarily a numerical parameter that accounts for the ability of the building structure to dissipate the gained energy during an earthquake. It reflects the inherent strength and ductility of the structural building materials used, as well as the overall seismic design philosophy adopted in the building construction process. Assessing how to effectively apply this factor in force calculation is paramount for practicing structural engineers motivated in creating resilient structures in earthquake-prone regions.
In this blog, we will discuss the usage of the Response Reduction Factor as envisaged in IS 1893, its significance in final design force, and how it can be effectively applied to enhance the seismic performance of buildings at the same time meeting economy.
Before diving in to the Response Reduction, let us discuss Earthquake in general. We know that Earthquake is ground motion and when this occurs, Inertia induces forces in the building. I have discussed this Earthquake phenomenon in previous blogs.
We have the understanding that Earthquake proofing is not practical due to economic reasons, Architectural and functional reasons. Based on experience and seismic studies, we have a seismic design philosophy and IS 1893 suggests how to calculate this seismic design force.
The total Base shear Vb is calculated from the formula Ah x W where Ah is the acceleration and W is the Dead load and Live load in the building in certain proportions. While we know W quite accurately, we have no much conclusive idea on what can be the actual Ah ie, the ground acceleration that can occur during earthquake. IS 1893 relates ties this acceleration to various site and design parameters. The formula is Ah = Z Sa/g I * 1/R
Let us now emphasise the importance of R value or the Response Reduction factor and the 3 reasons it is in the denominator to reduce the considered design force.
General Understanding of Response Reduction Factor
Generally, Engineers remembers R value as 3 or 5 depending on if ductile detailing is provided or not for the project. While that is correct, it is important to understand why this is a reduction factor and why it is 3 generally and 5 when ductile detailing is adopted. If it was ductility alone, the minimum value need not had to be 3. It could have been 1.
The idea of this blog is to explore the significance of Response reduction Factor R in IS 1893 2016. To discuss what all parameters decides the value of R and for what reasons it influences the Response reduction factor. Primarily, there are 3 points that are related to R value. They are Ductility, Over strength and Redundancy. These are discussed in the below sections in detail.
Significance of Ductility
Ground movement is Earthquake and this is a form of energy. The building which is on the ground gains this energy during acceleration of the ground due to earthquake. This energy gained by the building gets dissipated in different ways. It will be dissipated in the form of building oscillation, heat, sound and so on. Even crack formation is a form of energy release.
When ductility is adopted i.e. when we follow ductile detailing as per the provisions of IS 13920, the rebars deforms / elongates and dissipate the energy. This means that the chances of crack formation reduce. We only need seismic resistance and meet the design philosophy. Code do not expect you to exceed the performance. Therefore, it allows a reduction in considered force by allowing a higher R value of 5 when ductile detailing is adopted. In a way, it is like meeting the expected performance criteria as desired in the seismic design philosophy either by normal detailing in which case, the R value is kept lower at 3 which means considered design force is more. R value is kept 5 when ductility is adopted which means the considered force is less but same performance is achieved by considering more force in design. However, it is important to note that ductile detailing is mandatory in seismic Zone III and above.
Over Strength Factor
If a building is engineered, it generally possesses more strength than design strength that we calculate while designing the structure. The over strength factor depends on many parameters, which may not be very evident all the time. It depends on the load safety factors to an extent. Most building structures also have reserve strength it it. Building structures located in lower seismic zones exhibit different reserve strength values from those in higher seismic zones. This is primarily because of varying gravity loads to seismic loads ratio. This results in zone-dependent values for the over strength factor. The construction practices, material strength and the material partial safety factors also finally decides the actual strength factor.
Redundancy
The role of this redundancy factor is to consider the improved reliability of seismic framing systems that use multiple lines of vertical seismic framing in each principal direction of a building structure.
A redundant Earthquake framing system could be composed of multiple vertical lines of frames, each of it designed and structurally detailed to transfer seismic-induced inertial forces to the foundation of the building structure. In a way these accounts alternate load paths in the building structure that allows a reduction of Response reduction factor.
Summary of Earthquake load as per IS 1893
While some of the IS code provisions and the formula and the equations used to calculate the parameters appear to be a direct empirical calculation, there will be a logic and a reason behind choice of each value. While some of it will be explained in the code, some may not be. It is better to have an explanatory code for every code clause. Engineers also can look in to the additional references mentioned by the code committee in the code from where we can get more information.
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