Comparative Study of Seismic Analysis of Pier Supported on Pile as per IRC:6-2017 and IRC SP:114-2018

This paper depicts the study of seismic analysis of reinforced concrete bridge piers as per provisions of Indian Road Congress (IRC) guidelines. Bridges are designed having two main structural elements named – “Superstructure” and “Substructure”. Superstructure consists of deck and supporting girder/ truss system below deck. Substructure includes Abutments, Piers, Portals and Foundations. Amongst these, Abutments/Piers are crucial part of bridge. Therefore, as per the seismic design philosophy, it is necessary to study the seismic behaviour of bridge piers. With the advancements in technology and subsequent researches in Infrastructure fields, IRC guidelines are updated and revised time-to-time. Introduction of IRC SP:114-2018 guideline for earthquake forces in bridges is an example of such developments. In this research, seismic analysis of Reinforced Cement Concrete (RCC) bridge pier is carried out as per provisions of prevailing guideline IRC:6-2017. Base shear value of IRC:6-2017 is compared with IRC SP:114-2018 which now supersedes seismic provisions of IRC:6-2017. For analysis, different span lengths of 25 m, 30 m and 36 m are used. To assess the impact of height of piers in earthquake analysis, various pier heights such as 10 m, 20 m and 30 m are assumed. The analysis is carried out as per Elastic Seismic Acceleration Method with consideration of different zones and importance of the bridge as per IRC guidelines. Effect of vertical ground motion is also considered in analysis. From analysis, it is observed that base shear and vertical forces have been increased remarkably as per IRC SP:114-2018 compared to IRC:6-2017. Vivek Gajera1, V. R. Panchal2, Vishal Vadgama3 1, 2Charotar University of Science and Technology (CHARUSAT), Changa, Gujarat, India. 3DGM Structures, Infinite Civil Solutions Pvt Ltd, Ahmedabad, Gujarat, India.


Introduction
The bridge may be defined as a structure which provides transportation facilities over physical obstacles such as valley, water body, road, railway as well as design of the bridges are dependent earthquake resistant design instead of aiming to provide earthquake proof design. Therefore, it is very important to analyse the seismic behaviour of a bridge and make its structure seismic resistant for preventing hazardous losses.
For substructure analysis, seismic forces are one of the most disastrous forces on the earth. The seismic force acts on the centre of gravity of structural components in the horizontal direction. The main function of the pier is to support the spans of the bridge and transfer the loads from superstructure to the foundation. Thus, it should be strong enough to take both vertical and horizontal loads [2]. However, piers are subjected to huge axial loads and biaxial moments in transverse and longitudinal directions.
As per history of Indian codes, it can be seen that there have been significant developments in last thirty years. However, Indian practices have not kept pace and seismic provision for bridges are highly inadequate. However, there is a serious earthquake problem in large part of the country and revisions of Indian codes are infrequent.
Generally, structural designers don't consider effect of vertical motion in seismic design of the pier. But measurement of ground motion during past earthquakes demonstrate that the vertical acceleration may exceed the horizontal acceleration. Therefore, it is necessary to analyse the effect of vertical motion in the pier [3].
After publishing of the guideline of IRC SP:114-2018, the existing provisions for seismic design in Clause 219 of IRC:6-2017 stand superseded [4,5]. Thus, it is required to find out the difference in this code.
In this study comparison of seismic analysis of bridge substructure (pier) as per IRC SP:114-2018 and IRC:6-2017 accordingly its seismic clauses are investigated. For that, various pier heights such as 10 m, 20 m, 30 m and different span lengths such as 25 m, 30 m, 36m are considered.

Validation of work
Validation of work is done with paper entitled 'Comparative Study of DDBD and FBD For Elevated Metro Bridge Substructure' published by Desai & Vyas [2]. Parametric Study of Metro Bridge pier has been considered in research paper with various spans of superstructure and different heights of piers.
Comparative results of base shear are shown in Table 2.
Base shear value is inversely proportional to height of pier. As height of pier increases, base shear value decreases. From the table, it is observed that base shear value is nearly identical to research paper calculations.

Load calculations
The  For analysis, the dead weight of superstructure and substructure, live load as well as seismic load on the pier are considered.

Results of analysis
Calculation of Base shear

Bridge crossing railway lines
If the same bridge is crossing more than two railway lines importance factor increases from 1.2 to 1.5. Accordingly, base shear & horizontal seismic coefficient (A h ) are tabulated as below in Tables 6 & 7.

Vertical component for different heights
Difference in vertical forces as per both codes are mentioned in Table 8.

Conclusions
The