Closed-form expressions for transfer functions of rectangular foundations

RECTANGULAR FOUNDATIONS

Veletsos et al. (1997) developed closed-form expressions for the lateral and torsional transfer functions of rigid, rectangular foundations, resting on an elastic half-space, based on integral

expressions formulated by Luco and Wong (1986). Figure 2-8 shows the coordinate system, used by Veletsos et al. (1997) with some modifications. It should be mentioned that notations, used by Veletsos et al. (1997), were modified to be consistent with notations, used in this dissertations.

Figure 2-8. coordinate system, used by Veletsos et al. (1997) for developing a closed-form solution for the transfer function of rectangular foundations [redrawn from (Veletsos et al., 1997)].

The time variation of the free field motion is formulated in the frequency domain using a space- invariant, local Power Spectral Density (PSD) function, SFFM. The variation of the motion between two arbitrary points, defined by the position vectors 𝑟⃗₁ and 𝑟⃗₂, was expressed using a Cross PSD function, 𝑆(𝑟⃗1, 𝑟⃗2 𝜔), as shown in Equation (2-1). In this equation, the wave passage effect is represented by the quantity Ψ, as shown in Equation (2-2), and the term Г is referred to as the incoherence function, as shown in Equation (2-4).

parameter, the distance between the two arbitrary points, and the frequency of the motion. The maximum value of the incoherence function is one and happens when the position vectors are equal. The products ГSFFM and ΨSFFFM represent the components of the ground motion variability caused by the stochastic incoherence and the wave passage effects, respectively (Veletsos et al., 1997). 𝑆(𝑟⃗₁, 𝑟⃗₂ 𝜔) = Γ(𝑟⃗₁, 𝑟⃗₂ 𝜔) Ψ(𝑟⃗₁, 𝑟⃗₂ 𝜔)𝑆_𝐹𝐹𝑀(𝜔) (2-1) Ψ(𝑟⃗₁, 𝑟⃗₂ 𝜔) = 𝑒[−𝑖𝜔(𝑦1𝑐−𝑦𝑦 2)] (2-2) 𝑐_𝑦 = 𝑉̅𝑠,𝑟 sin 𝛼_𝑦 (2-3) Γ(𝑟⃗₁, 𝑟⃗₂ 𝜔) = 𝑒−((𝜔 𝑉̅⁄ 𝑠,𝑟)[𝜅𝑥2(𝑥1−𝑥2)2+𝜅𝑦2(𝑦1−𝑦2)2]) 2 _(2-4)

where y1 and y2 are the components of 𝑟⃗₁ and 𝑟⃗₂ in the direction of wave propagation, 𝑉̅_𝑠,𝑟 is the average, strain-reduced, overburden-corrected, near-surface, shear wave velocity of the soil beneath the foundation; αyis an angle between the propagation direction of the motion and the vertical axes; 𝜅_𝑥 and 𝜅_𝑦 are the incoherence parameter in the directions x and y-axes; 𝜔 is the circular frequency; and i is the square root of -1.

Veletsos et al. (1997) used the averaging technique, developed by Scanlan (1976), to calculate the lateral and torsional components of FIM, while variations of the ground motion beneath a rectangular foundation are expressed according to PSD function, shown in Equation (2-1). Veletsos et al. (1997) defined three terms, which are 𝑆_𝐿𝐿, 𝑆_𝑆𝑆, and 𝑆_𝐿𝑆; where 𝑆_𝐿𝐿 is the PSD function for the lateral or horizontal components of the foundation displacement, 𝑆_𝑆𝑆 is the PSD

function for the displacement component developed by the torsional component of the foundation motion along the side of the foundation, and 𝑆_𝐿𝑆 is the cross PSD function for the two motion components of the foundation input motion. These three terms are calculated according to the averaging technique as shown in Equations (2-5) to (2-7) (Veletsos et al., 1997).

𝑆𝐿𝐿 = 1 𝐴2∫ ∫ 𝑆(𝑟⃗1, 𝑟⃗2 𝜔) 𝑑𝐴1𝑑𝐴2 (2-5) 𝑆_𝑆𝑆 = 𝑏2 𝐼_Ψ2∫ ∫ 𝑦1𝑦2𝑆(𝑟⃗1, 𝑟⃗2 𝜔) 𝑑𝐴1𝑑𝐴2 (2-6) 𝑆𝑆𝑆 = 𝑏 𝐼Ψ𝐴∫ ∫ 𝑦2𝑆(𝑟⃗1, 𝑟⃗2 𝜔) 𝑑𝐴1𝑑𝐴2 (2-7)

Where 𝐴 equals 4𝑎𝑏; 𝑎 and 𝑏 are half-width and half-length of the foundation; 𝑑𝐴1and 𝑑𝐴2 are elemental areas; 𝐼_Ψ is the polar second moment of area of the foundation about a vertical centroidal axis, 𝐼_Ψ = 1₃𝐴(𝑎2_{+ 𝑏}2_{); 𝑠(𝑡) equals 𝑏ψ(𝑡); ψ(𝑡) shows the rotational time history of the} foundation around the vertical axis.

Veletsos et al. (1997) used Equations (2-5) to (2-7) to developed closed-form integral expressions for lateral and torsional (rocking) kinematic transfer functions for rectangular surface foundations subjected to inclined and incoherent seismic motions. These equations are presented in Appendix Chapter 12, Section 12.2 to make this dissertation a self-sufficient source.

As discussed previously, Veletsos et al. (1997) developed closed-form solutions for lateral and torsional transfer functions for rectangular foundations by considering the wave passage effect and incoherence of motions. As shown in Figure 2-7, Luco and Wong (1986) numerically solved their

same, although they are not equal. Therefore, in this research, experimental rocking transfer functions were matched with the analytical torsional transfer function, developed by Veletsos et al. (1997), to estimate the incoherence parameter for the rocking motion.

Seismic motions in this research were generated using a one-directional shake table. As a result, the wave passage effect would be negligible, since the seismic motions approximately propagated vertically from the bottom of the soil layer to the soil surface. Figure 2-9 shows the lateral and torsional transfer functions of a typical square surface foundation, subjected to vertically incident incoherent waves. The lateral and torsional transfer functions are shown in the figure in terms of √𝑆𝐿𝐿/𝑆𝐹𝐹𝑀 and √𝑆𝑆𝑆/𝑆𝐹𝐹𝑀, respectively. The transfer functions are plotted against a frequency parameter, 𝑑_𝑦 = 𝜅𝜔𝑏/𝑉̅_𝑟; where b is the half of the foundation width. The frequency parameter represents a combined effect of the incoherence parameter and the frequency of the motion. This parameter is mainly defined to plot kinematic transfer functions for square foundations with different widths, while they are excited with motions with various incoherence parameters, on the same figures.

Figure 2-9 shows that as 𝑑𝑦 increases while the lateral transfer function monotonically decreases from one, the torsional transfer function increases from zero to a peak value and then monotonically decreases. It should be emphasized that the peak in the torsional transfer function is not unique to only this transfer function. As showed in Figure 2-6 and Figure 2-7, the transfer function between the lateral component of the free field motion and the lateral component of the foundation input motion monotonically decreases as the frequency increases. But, the transfer function between the lateral component of the free field motion and the transverse (also the vertical) component of the foundation input motion increases from zero to a peak value and then monotonically decreases. Furthermore, transfer functions between the lateral, transverse, and

vertical components of the free field motion and the torsional and rocking components of the foundation input motion increase from zero to a peak value then decrease. Therefore, when the direction of the component of the foundation input motion and the free field motion is the same, the transfer function does not have a peak; however, when the direction is not the same, the transfer function has a peak, due to the interaction of between the two directions. The following summary discusses the effect of different parameters on the frequency corresponding to the peak value, 𝑓∗_, in the torsional transfer function.

• For a square foundation, as the width of foundation, or the incoherence parameter increase, the 𝑓∗ _{value decreases. However, when the shear wave velocity of the soil increases, the} 𝑓∗ _{value increases (Luco & Wong, 1986; Veletsos et al., 1997).}

• For a rectangular foundation, as the length of the foundation increase respect to the width of the foundation, the 𝑓∗ _{value decreases (Luco & Wong, 1986; Veletsos et al., 1997).}

• For a rectangular foundation, as the incoherence parameter in the direction of the length of the foundation increases relatively to the incoherence parameter in the direction of the width of the foundation, the 𝑓∗ _{value decreases (Luco & Wong, 1986; Veletsos et al.,} 1997).

• For a rectangular foundation, as the wave passage effect increases, the 𝑓∗ _value decreases(Luco & Wong, 1986; Veletsos et al., 1997).

• For a circular foundation, as the ration of the foundation or the incoherence parameter increase, the 𝑓∗ _{value decreases. However, when the shear wave velocity of the soil} increases, the 𝑓∗ _{value increases (Mita & Luco, 1986; Veletsos & Prasad, 1989).}

Figure 2-9. Transfer functions of foundation input motion for square foundations, excited by vertically incident incoherent waves: (a) lateral transfer function; (b) torsional (rocking) transfer function (Veletsos et al., 1997).

2.7.TRANSFER FUNCTION CALCULATION

Theoretically, the transfer function of two motions is the ratio of the Fourier transform of an output motion time history, y(t), to the Fourier transform of an input motion, x(t). In frequencies in which amplitude of the Fourier transform of x(t) is zero or close to zero, the transfer function would have unrealistic spikes (Lalanne, 2010). Therefore in practice, transfer functions are conventionally calculated based on smoothed power and cross-power density functions (Zerva & Zervas, 2002; Kim & Stewart, 2003; Ghayoomi & Dashti, 2015; Borghei & Ghayoomi, 2018b). The smoothed power and cross-power density functions describe smoothed amplitudes of the input and output motions in the frequency domain. Lalanne (2010) describes in detail these functions and methods which can be used to estimate these functions. Mikami et al. (2008) recommended keeping the smoothing level of power and cross-spectral density functions constant to consistently compare

transfer functions estimated from different events. They showed that the smoothing level could be quantified by an effective frequency bandwidth, Be, which is approximately equal to 1/DW, where DW is the time duration of each window in the full signal. In this study, Be is selected as 0.2 Hz to be consistent with the value used by previous researches in this subject (Kim & Stewart, 2003; Mikami et al., 2008). Windows were generated using Hanning function with 50% overlaps

between windows. Welch’s method was used to estimate smoothed power and cross-power spectral density functions, while amplitudes of transfer functions were estimated based on Equation (2-8).

𝐻(𝑓) = |𝑆𝑥𝑦(𝑓) 𝑆𝑥𝑥(𝑓)|

(2-8)

where Sxx(f) is a smoothed power spectral density function of x(t); and Sxy(f) is a smoothed cross- power spectral density function of x(t) and y(t).

The coherence function,γcoh2(f), Equation (2-9), can be used to quantify the noise level of the input and output motions and the linearity between them. In the absence of noise, when the relation between the two motions is linear, the coherence function is one. The γcoh2(f) would be between zero and one, in conditions when noise is present in the measured motions, the relation between the motion is non-linear, or the response is due to motions other than x(t) (Lalanne, 2010).

𝛾𝑐𝑜ℎ2 (𝑓) =

|𝑆_𝑥𝑦(𝑓)|2 𝑆𝑥𝑥(𝑓) 𝑆𝑦𝑦(𝑓)

(2-9)

2.8.CENTRIFUGE MODELING

2.8.1. Introduction to centrifuge modeling

In this research, small-scale physical models were tested in the geotechnical centrifuge, representing target prototype structures, that were placed on dry and unsaturated soil layers. The purpose of these tests was to study soil-structure interaction effects in different degrees of saturation. By spinning a specimen in a centrifuge with target speed, a suitable centripetal acceleration field can be applied to the specimen. In general, an increased gravitational field in the centrifuge would lead to proportionally smaller model dimensions according to similitude laws between the prototype and the physical model (Wood, 2004).

2.8.2. Scaling factors in centrifuge modeling

Scaling factors developed based on nondimensional similitude laws are used to convert data from prototype to model scale and vice-versa. The scaling factors are commonly defined as the ratios of various parameters in the prototype to the ones in the model. Several researchers have studied the scaling factors for centrifuge modeling, and these scaling factors are readily available in the literature, and some of them are shown in Table 2-1 (Wood, 2004; Iai et al., 2005; Garnier et al., 2007; Towhata, 2008).

Table 2-1. List of scaling factors for centrifuge experiments (Wood, 2004; Iai et al., 2005; Towhata, 2008).

Quantity Scaling factors (prototype/model) for a centrifuge test in η-g level Length η Density 1 Acceleration 1/η Stiffness 1 Stress 1 Axial Force η2 Strain 1 Displacement η

Pore fluid viscosity* _{1 [1/η]}

Pore fluid density 1

Soil permeability* _{1/η [1]}

Hydraulic gradient 1

Frequency (dynamic time) 1/η

Time (diffusion) * _η2 _[η]

Time (creep) 1

Time (dynamic) η

Velocity 1

Shear wave velocity 1

*_{: [If the viscosity of the model pore fluid is increased to force the scaling factors for diffusion} time and dynamic time to be equal.]

2.8.3. Substitute pore fluid

Three time-dependent phenomena mainly control the behavior of soils, including creep, consolidation (diffusion time), and dynamic motion propagation (dynamic time). The time scaling factor for each of these phenomena is different in centrifuge modeling. The scaling factor for creep time is unity, as shown in Table 2-1 (Wood, 2004). Since creep does not considerably occur for the experiments performed in this research, it is not further discussed in this section.

The difference between the scaling factor for dynamic time and diffusion time is essential when considering generation and dissipation of pore water pressure during a seismic event. A well-

acceleration, these two scaling factors can be forced to be equal. It should be emphasized that while the viscosity of the pore fluid is increased, other properties of the fluid, for example, the density of the fluid should not be significantly altered (Wood, 2004).

Researchers have successfully tested various substitute fluids to be used instead of water in dynamic centrifuge experiments (Stewart, et al., 1998; Dewoolkar et al., 1999). For instance, Dewoolkar et al. (1999) by conducting triaxial compression tests, permeability tests, and seismic centrifuge experiments have shown that a fluid made by dissolving powdered methylcellulose

(“metolose”) in water is an acceptable substitute pore fluid for seismic centrifuge experiments on

saturated sands. (D. P. Stewart et al., 1998; Dewoolkar et al., 1999)

Metolose was used in this research as the substitute pore fluid. Shin-Etsu Chemical Company commercially produces the material under the trade name METOLOSE, grade 90SH-100. Metolose powder with a specific concentration was dissolved in warm water to produce fluid with viscosity. The preparation of the substitute fluids is explained in detail in Chapter 3, Section 0.