In the construction and design of bridges, understanding the forces that act upon the structure is critical. This post delves into the complexities of loading and seismic design parameters that play a pivotal role in shaping the resilience and integrity of bridge designs, particularly in seismically active areas. The case study of a typical continuous bridge highlighted in our series serves as a perfect backdrop to explore these essential aspects of bridge engineering.
For a background on this study review Post #1 here.
Static Loading Considerations
Before diving into the seismic aspects, it's crucial to establish the static loads that the bridge must withstand under normal conditions. These loads include the self-weight of the bridge, live loads (such as vehicles and pedestrians), and superimposed dead loads (SIDL), which encompass non-structural elements like the road surface and barriers.
Superimposed Dead Load (SIDL): This includes the weight of crash barriers, assumed at 10kN/m per side, and the wearing coat, contributing an additional 22 kN/m^2.
Live Load: For simplicity, the live load is assumed as 15kN/m per lane, with the bridge accommodating three lanes, totaling a live load of 45kN/m.
In our approach we have conservatively included live load as a direct equivalent static load for seismic calculations. Generally, codes may require a reduced portion of live load to be considered in seismic design to account for the bridge's mass during seismic events, recognizing the possibility of having some live load on the structure at the time of an earthquake.
Seismic Design Parameters
The seismic design of bridges requires a detailed understanding of the potential ground motion and how it interacts with the structure. This involves defining seismic zones, soil types, and the bridge's importance factor, all of which influence the design seismic forces.
Seismic Zone: The case study assumes a location in Seismic Zone V, the highest seismic risk category, with a zone factor of 0.36, indicating the percentage of gravity acceleration (g) that the seismic design forces could reach.
Importance Factor: Reflecting the critical nature of bridges in infrastructure, an importance factor of 1.5 is applied, enhancing the design forces to ensure greater safety margins.
Soil Type: The bridge is assumed to be founded on medium soil, which affects the seismic response by modifying the ground motion characteristics.
The seismic parameters are critical in defining how the bridge will respond to seismic events and together influence the design spectrum. The spectrum, in turn, dictates the seismic forces that the structure must be designed to resist. The design spectrum is a graphical representation that shows the maximum expected ground acceleration as a function of the structure's natural period. It is crucial for determining the seismic loads on different parts of the bridge.
Generating Seismic Spectra
The seismic analysis requires the generation of site-specific response spectra, which represent the variation of seismic force, acceleration, or displacement with the natural frequency of the structure. The study employs spectra derived from IRC:SP:114, tailored for the unique seismic and soil conditions of the site. This approach ensures that the seismic forces calculated are closely matched to the expected ground motions. Figure 1 (Figure 2 in the original paper) illustrates the spectra for the Elastic Response Spectrum Method, a critical tool in defining the seismic demands on the structure.
Substructure Loading and Analysis
The substructure, comprising piers and abutments, plays a vital role in transferring the loads from the superstructure to the foundation. The analysis incorporates a detailed examination of vertical loads for each pier and abutment, considering the combined effects of static and seismic forces. Table-1 summarizes the relevant geometric and section parameters as well as loads, providing a clear basis for the design and analysis of the substructure components under seismic conditions.
Conclusion
Understanding the loading and seismic design parameters is essential for the safe design of bridges in seismically active regions. By carefully considering these factors, engineers can ensure that the structure is capable of withstanding both everyday loads and the extraordinary forces unleashed during an earthquake. The next post in our series will further explore the seismic analysis of the bridge, focusing on how these loads and design parameters influence the choice of bearings and the overall seismic performance of the structure.
This series is brought to you by Varun Garg, based on a paper he co-authored with Mr. Rajiv Ahuja for the Structural Engineering Digest, Quarterly Journal for the Indian Association of Structural Engineers in March 2021. The paper can be downloaded by clicking the link below.