The effect of local soil and structure height on seismic response
In the world of civil engineering, understanding seismic response and the complex interaction between soil and structures is of paramount importance. Seismic events can have a significant and often destructive impact on the structural integrity of buildings and infrastructure, making it vital for engineers to explore the complexities of the behavior of the foundation soil and the structure during earthquakes. This blog aims to investigate seismic response as well as define and explain how factors such as structure height and the type of local soil influence the interaction between soil and structures during earthquakes.
Impact of structure height
To determine the impact of structure height on seismic response, first we need to understand what the fundamental period of building vibration is. When a building is under the influence of dynamic stress, it leaves its static state and begins to vibrate. The natural vibration period, T, is the time it takes for the building, after the influence of horizontal forces, to return to its initial static position. Buildings with different masses, heights and structural properties have different natural vibration periods. The natural period of a building is a function of the mass and stiffness of the structure and is defined by the following equation:
m – mass of the structure
k – stiffness of the construction
From the provided equation, we can see that the mass is proportional, and the stiffness inversely proportional, to the fundamental vibration period of the structure. Tall structures are designed with a higher degree of flexibility, i.e., reduced stiffness, to increase their ability to absorb energy (Figure 1), making them less susceptible to damage during dynamic loads (Nandi and Hiremath, 2018). When doing designs, engineers consider the fundamental vibration periods to ensure that the building can withstand seismic forces. Properly dimensioning structural elements and using appropriate materials can help reduce the risk of damage or collapse of buildings during earthquakes.
Impact of soil types
In addition to the impact of structure height, the seismic response of a structure is significantly influenced by the characteristics of the local soil. Evidence left after major earthquakes clearly indicates that the vibration intensity at the surface is strongly controlled by sediment thickness and soil type (Verdugo and Peters, 2017). This was confirmed in the 1985 earthquake in Mexico City, where ground quaking amplifications by a factor of 20 or higher were observed on locations with deep layers of soft soils (Celebi et al., 1987; Singh et al., 1993). On the other hand, the presence of rock masses and densely compacted coherent materials beneath structures led to a significant reduction in seismic damage, as evidenced by the limited damage observed in buildings located on such foundations (Montessus de Ballore, 1911; Watanabe et al., 1960; Borcherdt, 1970; Seed et al., 1988).
To determine the impact of sediment depth and structure height on the structural integrity of the building, the Possible Damage Index, Fr, was derived, defined by the following equation (Čaušević, 2010):
Fr – Possible Damage Index of the structure
(B.S.)max – maximum shear coefficient
T – fundamental vibration period of the structure
W – weight of the structure
C – coefficient of lateral load.
Figure 2 shows that buildings with five to nine floors have the highest value of Fr when built on a relatively thin soil layer atop the bedrock. However, tall buildings, which have large values of the first natural vibration period, exhibit the highest value of Fr when built on a deposit with significant thickness, which itself has relatively large values of the fundamental vibration period. In this case, resonant phenomena occur between the building and the thick deposit layer atop the bedrock (Čaušević, 2010).
To understand this effect, it is necessary, apart from knowing the fundamental vibration period of a building, to determine the fundamental period of soil vibration, which is a function of the thickness of the foundation soil and the shear wave propagation velocity, as in the following equation:
T – fundamental period of soil vibration
Hi – thickness of the soil layer i
Vsi – shear wave propagation velocity of the soil layer i.
When a structure has a fundamental period of vibration similar to that of the foundation soil, there is a possibility of resonance. Resonance can result in increased vibration of the building (Čaušević, 2010), potentially leading to severe damage or even collapse of structures. It occurs only if the frequency of the soil vibration is approximately equal to any of the natural frequencies of the buildings in that area (Bhuskade and Sagane, 2017). Therefore, not all types of structures in a particular area are equally susceptible to the effects of dynamic loads during a specific earthquake.
In addition to the thickness of the foundation soil, the seismic response is also affected by the type of soil under the structure. The calculation of the structure’s resistance to seismic loads can be performed using a spectral seismic analysis that considers the characteristics of the local foundation soil, the spectral acceleration of the soil, and the basic period of the vibration of the structure. If we look at structures with a period between 0.25 s and 1.20 s in Figure 3, we will see a significant increase in the response of the structure to seismic forces in cases where the foundation soil is composed of soft sediments.
On the other hand, in the case of objects with periods less than 0.50 s, a significant increase in spectral acceleration is visible if the foundations are built on hard sediments or rock. In other words, high-rise structures need to be built in a place where there is solid soil of small thicknesses (Čaušević, 2010), while low-rise structures with a smaller basic period are safer if they are based on softer sediments of larger thicknesses.
It is important to consider the type and thickness of the sediment and adjust the building structure accordingly to reduce the risk of resonance and improve earthquake resistance.
Understanding the seismic response and the interaction of the structure with the foundation soil is essential for constructing earthquake-resistant buildings. Continuous research and advances in earthquake engineering are constantly improving our understanding of these phenomena, which contributes to the creation of safer and more resistant structures in earthquake-prone areas. The application of innovative approaches in the design of structures allows engineers to develop structures that, in addition to being resistant to earthquake forces, also ensure the safety of their users.
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Čaušević, M. (2010) Dinamika: potresno inženjerstvo, aerodinamika, konstrukcijske euronorme, 1. izd. Zagreb: Golden marketing – Tehnička knjiga
Bhuskade S.R., Sagane S.C. (2017) Effects of Various Parameters of Building on Natural Time Period. International Journal of Engineering Research & Technology (IJERT), vol. 6, no.4, pp. 557-561