Influence of Abutment Backfill Interaction

Chapter 5 Seismic Analysis of RC Integral Bridge

5.5 Influence of Abutment Backfill Interaction

to conclude that under low intensity of ground motion, nonlinearity of soil highly influences the nonlinear behaviour of structure.

0 10 20 30 40

-2000 -1000 0 1000 2000

(b) Peak (8.475, 1642.7)

Peak (9.405, 1300.4)

Shear Force, kN

Time, s (a)

0 10 20 30 40

Peak (1259, 16.96s) Peak (966, 16.43s) nl str+lin soil no BA

Nonlinear full SSI no BA

0.1 1 10

0.0 0.2 0.4 0.6 0.8 1.0

0.1 1

_(d)

10

Normalised Fourier Transform

(c) Frequency, Hz

Fig. 5.18 Comparison of SFTHs for the nonlinear behaviour of full SSI no BA model and the nl str+lin soil no BA model at the top of 8^th pier of bridge under (a) GM# 1 and

(b) GM#2; normalized FTs of SFTHs under (c) GM#1and (d) GM#2.

5.20(a) and 5.20(b), respectively. From the Fig. 5.20(b), it is observed that in presence of abutment backfill, the overall deformation and imposed forces in the different bridge components get reduced because the abutment backfill tends to restrain both the ends of the bridge in the longitudinal direction. The backfill soil helps bridge members to sustain less amount of seismic forces through reduction in displacement at bridge substructure.

Also, the density and stiffness of the backfill soil are expected to play a major role in the response of the bridge structure. The backfill should neither be too stiff to be undeformable nor too loose such that it is unable to withstand seismic forces coming from bridge.

0 10 20 30 40

-0.6 -0.3 0.0 0.3

0.6 peak 0.58

peak 0.43

(b)

Velocity, m/s

Time, s (a)

0 10 20 30 40

peak 0.29 peak 0.19

nl str+lin soil no BA Nonlinear full SSI no BA

0.1 1 10

0.0 0.2 0.4 0.6 0.8

(d)

Fourier Amplitude, m

Frequency, Hz

(c) 0.1 1 10

Fig.5.19 Comparison of VTHs at right abutment deck joint for the nonlinear behaviour of full SSI no BA model and the nl str+lin soil no BA model under (a) GM#1 and (b)

GM#2; Fourier amplitudes of VTHs under (c) GM#1 and (d) GM#2.

(a)

(b)

Fig.5.20 (a) Undeformed shape and (b) deformed shape of full SSI with BA model after dynamic analysis under GM#1.

SFTHs are plotted at the top of 8^th pier in Fig. 5.21(a) for full SSI no/with BA models under GM#1. Peak SF at the top of 8^th pier is atleast 31% lower in full SSI with BA model as compared to the other one. From the residual shear force at the end of GM#1, it can be stated that due to abutment backfill interaction the residual force at the top of the 8^th pier is 133 kN; whereas, in full SSI no BA model, the residual SF is 709 kN. Thus, the sructure is going through significant nonlinearity at the end of dynamic motion for the full SSI no BA model. From the normalised fourier amplitude of SFTHs in Fig. 5.21(b), full SSI no BA model’s peak Fourier amplitude is higher than full SSI with BA model as it carries more seismic forces. With backfill soil, the abutments and the superstructure (deck) undergo less amount of deformation because the seismic forces dissipate through backfill and it results in significant backfill deformation by the end of dynamic analysis (Fig.5.20(b)). At a depth of 1 m below the tip of pile foundation underneath the 8^th pier,

Location #3

displacement time histories (DTHs) are shown for both the models (Fig. 5.21(c)) and in full SSI with BA model, foundation soil is more deformable than the other one. The peak acceleration at the top of 8^th pier reduces from 2.98 m/s² to 1.94 m/s²in full SSI with BA model (Fig. 5.21(d)). Hence, seismic acceleration of the bridge stricture in full SSI with BA model is less than full SSI no BA model under GM#1, which reduces the overall deck response. At location#3, free field soil has a peak displacement of 0.15 m. The full SSI with BA model and the free field soil domain have a relative displacement of 0.02 m after the instant of peak response and relative residual displacement is also 0.027 m which is insufficient to mobilize the passive pressure under dynamic loading. According to AASHTO (2012), integral abutment should be allowed to get displaced upto maximum limit of 0.091 m under cyclic loading.

In Fig. 5.22, SFTHs are monitored at the top of 8^th pier under GM#2. The abutment backfill soil influences the response of structure as well as soil in case of low intensity of input motion too. In presence of abutment backfill, the peak shear force at the top of 8^th pier reduces from 966.4kN to 805.8kN. The normalised Fourier amplitudes have been obtained by normalising the FT values with respect to peak amplitude of full SSI no BA model. In normalised fourier amplitude of SFTHs (Fig. 5.22(b)), the differences in Fourier amplitude for the frequency range of 1–4 Hz occur because full SSI no BA model carries higher seismic forces in that frequency range. In Fig. 5.22(c), ATHs are shown at left deck-abutment joint and the peaks of ATHs are 1.26 m/s² and 1.35 m/s² for full SSI with BA and full SSI no BA model, respectively. Fig. 5.22(d) shows fourier amplitude of ATHs shown in Fig. 5.22(c). The peaks of fourier amplitudes occur in the frequency range of 2.5 Hz to 3.3 Hz for the models.

0 10 20 30 40 -1500

-1000 -500 0 500 1000

(b) Peak 894.7kN

Peak 1300.4kN

Shear force, kN

Time, s

0.1 1 10

0.0 0.5 full SSI no BA 1.0

full SSI with BA

Normalised Fourier Transform

Frequency, Hz (a)

0 10 20 30 40

-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10

0.15 full SSI no BA

full SSI with BA free field soil

0.14m, 7.15s 0.17m, 7.215s 0.15m, 7.1s

Displacement, m

0 10 20 30 40

-3 -2 -1 0 1 2

3 peak 2.98

peak 1.94

Acceleration, m/s2

Fig. 5.21 Comparison of (a) SFTHs at the top of 8^th pier, (b) normalized FTs of SFTHs shown in (a), (c) DTHs of soil node in pile-foundation below the 8^th pier at location #3, and (d) ATHs at the top of 8^th pier under GM#1 for full SSI no BA model and full SSI

with BA model.

Moment-curvature response at the top and bottom cross-sections of the 8^th pier under GM#1 are shown in Figs. 5.23(a) and 5.23(b), respectively. Although in both the models, very narrow nonlinear moment-curvature loops are forming, the extent of nonlinearity in full SSI with BA model is less as compared to the level of nonlinearity in full SSI no BA model. Hence, abutment backfill is expected to play a major role in the seismic analysis of integral bridges.

0 10 20 30 40 -900

-600 -300 0

300 Peak 966.4kN

Peak 805.8kN

Shear Force, kN

Time, s

0.1 1 10

1E-3 0.01 0.1 1

(b) full SSI no BA

full SSI with BA

Normalised Fourier Transform

Frequency, Hz (a)

0 10 20 30 40

-1.5 -1.0 -0.5 0.0 0.5 1.0

1.5

Peak 1.26

Peak 1.35

Acceleration, m/s2

Time, s

0.1 1 10

0.0 0.5 1.0 1.5

(c)

Frequency, Hz

Fourier Amplitude, m/s

(d)

Fig. 5.22 Comparison of (a) SFTHs at the top of 8^th pier, (b) normalized FTs of SFTHs shown in (a) in log-log scale, (c) ATHs at 1^st pier top, (d) FTs of ATHs shown in (c)

under GM#2 for full SSI no BA model and full SSI with BA model.

-0.002 -0.001 0.000 0.001 0.002 -10.0k

-5.0k 0.0 5.0k 10.0k

residual response residual response full SSI with BA

full SSI no BA

Curvature, m^-1 (b)

Moment, kNm

(a)

-0.002 -0.001 0.000 0.001 0.002 Fig. 5.23 Comparison of moment curvature response at the (a) bottom and (b) top cross-

sections of the 8^th pier under GM#1 for full SSI no BA model and full SSI with BA model.

To obtain the material level response, the behaviour of the steel, core concrete and cover concrete fibers (locations denoted in Fig. 3.1(c)) at the top level of 8^th pier are monitored under GM#1. In full SSI no BA model, concrete fibers have not yielded. With the effect of backfill soil, further the strain demand reduces in the uniaxial response of the fibers. For Steel fiber 1 (Fig. 5.24 (a)), the response of full SSI no BA model larger strain deformation and yielding of steel; however the Steel fiber 2 (Fig. 5.24 (b)) shows linear elastic behaviour for both the models under GM#1. In full SSI with BA model, Core concrete fiber 3 and Cover concrete fiber 3 (in Figs. 5.24(c) and 5.24(d)) undergo lesser tensile deformation as compared to the behaviour of full SSI no BA model under GM#1.

Based on the comparisons of bridge response between the full SSI with BA model and full SSI no BA model, it can be stated that the presence of abutment backfill soil can dominate the overall response of the integral bridge.

-300 -150 0 150 300

(b)

full SSI with BA full SSI no BA

Axial Stress, MPa

(a)

residual response

-0.1 0.0 0.1

-9 -6 -3 0

(c) Axial Strain (%) (d)

-0.1 0.0 0.1

Fig. 5.24 Stress-strain response at top of 8^th pier cross-section for (a) Steel fiber 1, (b) Steel fiber 2, (c) Core concrete fiber 3 and (d) Cover concrete fiber 3 shown in

Fig.3.1(a), under GM#1 for full SSI no BA and full SSI with BA models.