In an integral abutment bridge, bearings may also be absent at the locations of the piers. 7.2 (a) Schematic diagram of back-built abutment foundation in plan, (b) vertical section A-A (h1 = height of abutment pile; h2 = height of casing pile) and (c) spring-dashpot model of the abutment and its back-built foundation in medium stiff clay.
Introduction
- Introduction
- Seismic Soil Structure Interaction
- Major Concern and the Motivation of the Study
- Objectives of the Study
- Outline of the Thesis
In an integral bridge with pile foundations, the seismic load on the piles under the abutments and piers can increase significantly with the total length of the bridge. Finally, the objectives of this research and the organization of the dissertation are presented.
Literature Review
Introduction
Characteristics of IAB
- Length
- Skew angle
- Loadings on IAB
In addition to the primary loading effects (dead load and live load), integral bridges are subjected to secondary loading effects due to (a) creep and shrinkage, (b) thermal gradients, (c) Abutment-Backfill Interaction (ABI) and (d) Soil stacking interaction (SPI) (Arockiasamy et al., 2004). The bending moment resulting from thermal expansion can significantly affect the axial capacity of the pile (Abendroth et al., 1989).
Abutment Behaviour
- Foundation type
- Abutment backfill interaction
Active earth pressure tends to be mobilized by a very small lateral displacement as the abutment moves away from the backfill (Barker et al., 1991). The use of batter piles for skewed IABs is not suggested in construction practice (Hassiotis et al., 2006).
Soil Pile Interaction
- Computational approach
So, preprocessing and output visualization in 3D finite element (FE) analysis of the entire bridge-soil system can be quite tedious and time-consuming (Elgamal et al., 2008). In inertial interaction, the masses of the structure and foundation are considered, which play a major role in the dynamic response.
Field Studies
Zero-thickness interface elements can be used to model soil-pile-abutment-backfill interactions to allow slip at these interfaces (Zhang et al., 2008). However, unlike 2D models, 3D models can account for skew effects (Deng et al., 2015) and eccentric loading effects.
Code Provisions
Apart from temperature variation for general bridge movement control, no specific guidelines for IAB are mentioned in the Australian Bridge Design Code (Austroads, 2004). A case study to perform static (service load, creep and shrinkage) and dynamic (displacement based) analysis with IAB design is given in Wood et al.
Retrofitting of Pile Foundation
Originally, in this retrofitting technique, sheet steel piles were installed around the existing pile cap to the design target depth. Instead of sheet piles, precast AB piles can be embedded in the grout surrounding the pile or pile cap to the target construction depth to increase the lateral load capacity of the existing pile foundation (Wang, 2015).
Summary
To mitigate the stress and forces in the overall integral system "fill-fill-layer-foundation" during earthquake shaking, rubber-soil mixtures or "mechanically stabilized" geosynthetic reinforced soil can be used as fill. The best results in their behavior and design should be obtained by monitoring projects completed in the past.
Gap Areas
The performance of possible retrofitting techniques for RC pile foundation should be investigated through extensive field studies and numerical modeling. The possibility of pile foundation of an integral bridge sustaining large displacement and forces during SSI should be investigated.
Scope of Present Work
Modelling
Introduction
In this chapter, the details of the material modelling, namely for concrete, steel reinforcement and soil, are discussed.
Modelling
- Modelling of bridge structure
- Modelling of soil and foundation
- Modelling of spring-dashpot system
In the present study, the bridge has been analyzed for loading in the longitudinal direction or in the direction of traffic. The lateral boundary conditions for the soil domain have been implemented by tied degrees of freedom (TDOF) (Elgamal et al., 2008; Kontoe et al., 2007) at the two lateral ends of the domain. The base input motion has been given in the horizontal direction to study the horizontal response of the soil structure system.
To include soil-pile interaction, the spring pots were modeled to represent the near-field and far-field effects of the soil domain.
Site Resposne Analysis and Selection of Ground Motions
Introduction
Site Response Analysis (SRA)
DEEPSOIL v5.0 software was used to compare the OpenSees response for 1D linear and nonlinear time domain analyses. Variations of the hyperbolic model are adopted to represent the soil backbone curve along with extended Masing discharge-recharge rules (Masing, 1926) to model hysteretic behavior (Hashash et al., 2016) in DEEPSOIL. But in NSRA, the amplification of seismic waves depends on the nonlinearity of the soil material at that specific depth of the soil column.
The response of free field soil column for ELSRA is mainly different from NSRA under 0.2 g PGA of Ricker pulse.
Selection of Ground Motions
Seismic Analysis of RC Integral Bridge
Introduction
Analysis
It can thus be stated that the response from the FB_SD no BA model is more amplified from the foundation to the deck level compared to the full SSI no. Under GM#1, the SFTHs at the top of the 8th pier in FB_SD with BA model are shown in Fig. Residual shear force is higher for the nonlinear behavior of the full SSI no BA compared to that in the FB_SD no BA model due to nonlinearity in the continuum soil domain.
The response of the FB_SD no BA model is higher compared to the full SSI no BA model because API overestimates the soil stiffness for foundation soil.
Linear and Nonlinear Time History Analyses
- Structural response
- Response of pier sections
- Response of soil
- Response of full SSI no BA model across depth
Complementary Investigation of Material Properties in Full SSI No BA Model
- Linear structural behaviour
- Nonlinear structural behaviour
Fig.5.16 Comparison of SFTHs at the top of the 8th bridge pillar for linear behavior of full SSI no BA model and lin str+nl bottom no BA model under (a) GM# 1, (b) GM#2 and. The observed peak velocities are 0.71 m/s and 0.40 m/s for the linear behavior of the full SSI no BA model and the lin str+nl ground no BA model, respectively. However, also under GM#2, the shear response of the nl str+lin soil-no-BA model exceeds that obtained for nonlinear behavior of the full SSI-no-BA model (Fig.
Due to the ground nonlinearity, the peak velocity response of the bridge decreases significantly under both ground motions for the nonlinear behavior of the full SSI no BA model.
Influence of Abutment Backfill Interaction
Fig.5.20 (a) Undeformed shape and (b) deformed shape of full SSI with BA model after dynamic analysis according to GM#1. Thus, the structure is undergoing a significant nonlinearity at the end of the dynamic motion for the full SSI no BA model. 5.21(b), the full SSI without the peak of the BA model The Fourier amplitude is higher than the full SSI with the BA model as it carries more seismic forces.
5.24(c) and 5.24(d)) undergo smaller tensile deformation compared to the behavior of a full SSI no BA model under GM#1.
Introduction of Nonlinear Spring-Dashpot Model
- Comparison between full SSI no BA and FB_SD no BA models
- Comparison between full SSI with BA and FB_SD with BA models
The peak acceleration at the bottom of the pier in FB_SD with the BA model is slightly higher than that obtained for the full SSI with the BA model. 5.29(d) for FB_SD with the BA model is at a lower frequency than the frequency for the peak FT of the full SSI with the BA model. Thus, in FB_SD with the BA model, the piers developed significant nonlinearity compared to the full SSI with the BA model.
This further proves that the FB_SD model with BA shows higher extent of nonlinear behavior and compared to the full SSI model with BA.
Mean Maximum Response
Residual shear force is also the lowest for the nonlinear behavior of the full SSI with BA model, but the residual displacement in full SSI with BA model is more than nonlinear full SSI no BA model. Deck acceleration and displacement are lower in lin str+nl soil no BA model compared to the non-linear behavior of full SSI no BA model and nl str+lin soil no BA model. Residual displacement is the lowest in ie str+lin soil no BA model compared to the values obtained in the non-linear behavior of all other models.
Under NTHA, it is observed that the shear force and bending moment response in piers for FB_SD no BA model is higher than that obtained for full SSI no BA model.
Discussion
The full SSI with BA model has the lowest bridge structure response in NTHA. Compared with the nonlinear behavior of the full SSI no BA model and the full SSI with BA model, the shear forces and bending moments in the pillars are higher for the FB_SD no BA model and FB_SD with BA model, because the soil stiffness of the foundation is higher with the API force-displacement curves. The structural response is significantly amplified at the deck level for the FB_SD no BA model and results in higher forces and moments at the pier and deck intersections.
To study bridge design, abutment fill modeling is also a key parameter to examine bridge response.
Study on Overall Length of Integral Bridge
- Introduction
- Modelling
- Structure and Foundation Properties
- Properties of Soil links
- Analysis
- Results
- Response for medium stiff clay soil
- Response for loose sand
- Comparison of response in loose sand and medium clay soil
- Response of bridge in dense sand and soft clay
- Summary
The length of piles and support piles are varied in medium firm clay and loose sand depending on the soil properties (Prakash and Sharma, 1990). Displacement time histories (DTH) at the top and bottom of the abutments are monitored under GM#1 in Figs. The overall deck displacement of a bridge model in loose sand is less compared to the deck displacement of the same model in medium firm clay soil.
Under GM#1, it is observed that the residual SF at the top of the third pier of the 9-span bridge is insignificant as that of the 7-span bridge (Fig.
Retrofitting of Pile Foundation…
- Introduction
- Retrofitting of RC Pile Foundation
- Modelling of encasement method
- Encasement method at pier pile foundation
- Analysis
- Results
- Comparison between M1 and M2 models
- Comparison between M2 and M3 models
- Discussion
So the length of the new encapsulated piles is considered to be 4 m for the pile foundation. 7.6, the moment-rotation response in the 3rd pier is shown for a cross section located at a depth of 1.2 m from the top of the pier. For M3 models in different types of foundation soil, the response of the overall bridge is observed to be significantly reduced, especially in medium stiff clay and dense sand.
The displacement response of M3 model in loose sand and soft clay is very similar to that of the M2 model.
Conclusions and Future Scope of Work
Conclusions
The simplified modeling of soil-structure interaction through a nonlinear jump-dashpot system tends to overestimate the bridge-foundation response compared to the integrated bridge-foundation response obtained by modeling soil as a continuum. In the presence of backfill soil, the jump-dashpot modeling approach leads to higher nonlinear response of integrated bridge foundation system compared to the response obtained using the continuum soil model. With an increase in overall length, seismic demand on an integral bridge with RC pile foundation increases due to the monolithic effect of the bridge structure-deck-pier system.
From the damping point of view, the SSI models release more seismic energy than the FB_SD models in the presence of the continuous soil domain.
New Contributions of the Present Study
Retrofitting pile foundations in an integral bridge by adding new piles encased in jet grout has been shown to be more effective in reducing the overall response of the bridge to medium-stiff clay soils.
Scope of Future Work
On the performance of super-long integrated abutment bridges-parametric analysis and design optimization (PhD thesis). The New York State Department of Transportation's Experience with Integrated Structure Bridges. 1992), Dynamic pile-soil-pile interaction. Detailed investigation of integrated abutment bridges and performance of bridge joints in traditional bridges (PhD thesis).
A1 (a) Orientation of bridge deck and abutment in plan and (b) weak and strong axis bending of an integral abutment.
