An analytical study of the effect of non-uniform excitation on the seismic response of Sadr Bridge

Document Type : Research Article

Authors

1 Ph.D candidate , Department of Civil Engineering, Kharazmi University, Tehran, Iran

2 Professor , Department of Civil Engineering, Kharazmi University, Tehran, Iran

3 Associate Professor, Department of Geotechnical Earthquake Engineering, International Institute of Earthquake Engineering and Seismology, Tehran, Iran

Abstract

1. Introduction

Large-dimensional structures, such as long-span bridges, receive different ground motions at different supports in earthquake events. Seismic wave propagation and local site conditions cause spatial variation of ground motion. It may result in pounding or even collapse of adjacent bridge decks owing to the out-of-phase response. In addition, dynamic Soil-Structure Interaction (SSI) resulting from the interaction of the bridge with the surrounding soil also affects the dynamic bridge response. In most bridges, shallow foundations are not appropriate, because they do not provide the required capacity or may experience excessive settlements or deformations. In such structures, pile groups are used as foundation systems. Pile foundations have to be designed to support not only vertical loads, but also lateral loads due to the earthquake, wind and vehicle impact loads. Therefore, soil-pile interaction is added to above factors in dynamic behavior of long-span bridges. From the above reasons, it is very important to consider both Soil-Pile-Structure interaction (SPSI) and Spatially Varying Earthquake Ground Motions (SVEGM) effects in evaluation of the seismic response of long-span bridges.
Material and methods
This paper presents a study about the spatial variability effects of ground motion and Soil-Pile-Structure Interaction (SPSI) on the dynamic response of a long bridge. Two decks of the considered bridge with length d1 = 100 m and d2 = 150 m are supported by four isolation bearings connected to three elastic piers standing on the pile foundations. The structure of the bridge continues from both sides. The decks are considered as lumped mass model with the total mass of m1 = 1.2´106 kg and m2 = 1.8´106 kg. All of the bearings have the same dynamic properties with an effective stiffness Kb1 and equivalent viscous damping Cb1 for the left span, and Kb2 and Cb2    for the right span. The concrete piers with heights of h1 = 14 m, h2 = 16 m and h3 = 15 m are modelled as elastic columns with lumped mass m3 = m4 = m5 = 2´105 kg at the top of each pier. The lateral stiffness of the piers are      Kp3 = 2´108 N/m, Kp4 = 108 N/m and Kp5 = 3´108 N/m. To simplify the analysis, a constant damping ratio of 5% is used for bearings and piers. The most widely used model to perform the analysis of piles under lateral loads, consists in modeling the pile as a series of beam elements and representing the soil as a group of unconnected-concentrated springs perpendicular to the pile that is known as Discrete Winkler Model. The Spatially Varying Earthquake Ground Motion (SVEGM) is simulated by SIMQKE-II record generator. Target response spectrum and power spectral density function used in the simulation are determined depending on the January 17, 1994, Northridge earthquake. To evaluate the effect of SPSI, the soil surrounding the pile foundation is modelled by frequency-independent springs and dashpots in the horizontal and rotational directions. The effect of soil-pile mass is considered by lumped-mass soil-pile model. A new analytical model is proposed to study the effect of both SVEGM and SPSI on dynamic response of long bridges.

2. Results and Discussion

The results indicate that considering the effect of non-uniform excitation and soil-structure interaction can increase the relative displacement of the deck in the longitudinal and transverse directions by 275% and 176%.
Also, considering the interaction effect, on average, shows a reduction of 67% and 75% of the base shear and moment considering non-uniform excitation, shows an increase of 37% and 29%, respectively.
 
3. Conclusion

The main conclusions drawn from this study can be written as:

1. Based on the results obtained from the proposed analytical model, SVEGM affects the seismic behavior of long-span bridges. Influence of SVEGM on decks displacements and maximum shear forces in piers is more significant in softer soil types. It means soil condition as an important factor affects the dynamic response of long bridges.
2. The importance of the SPSI effect on the dynamic response of the bridge is also investigated in comparison with fixed-base case. It is observed that the results obtained from the SPSI case are usually amplified in comparison with the fixed-base case. This effect is more significant in softer soil types. It means that the variation of the soil conditions where the bridge supports are located on, has important effect on the bridge dynamic response. Since the proposed model is very similar to real soil-pile-structure systems, suggested equation derived from it, can be used to simulate the seismic behavior of long-span bridges.
3. If the effects of SPSI and SVEGM are considered simultaneously, it should be noticed that the results will not be the same as what obtained from the addition of the response determined from these effects separately. The effects of SPSI and SVEGM amplify each other especially in softer soil conditions. It is also observed that considering the effect of SPSI with respect to the SVEGM can change the dynamic response of long-span bridges in comparison with the cases in which one of these factors or none of them is considered.
4. In general, the recommendation of fixed-base case with uniform ground excitation in dynamic design regulations of bridges is valid only if SPSI and SVEGM effects are negligible. These assumption can be used in seismic design of bridges on very stiff soil conditions and not very long bridges. Otherwise, this recommendation leads to usually underestimating the dynamic response or even bridge structure damage.

Keywords

Main Subjects

References

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History

  • Receive Date: 02 August 2018
  • Revise Date: 03 September 2021
  • Accept Date: 11 September 2021

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