Seismic response analysis of long-span and asymmetrical suspension bridges subjected to near-fault ground motion

Highlights

• The seismic responses of long-span and asymmetrical suspension bridges subjected to near-fault ground motions are investigated systematically and comparatively.

• The influence of velocity pulse effect, site effect, and structure-soil interaction on the seismic responses of interests in the suspension bridge seismic analysis is considered and analyzed.

• Recommendations for the seismic design of long-span and asymmetrical suspension bridges subjected to near-fault ground motions.

Abstract

The objective of this paper is to investigate the seismic responses of long-span and asymmetrical suspension bridges subjected to four intensity level (Small, moderate, huge, and super earthquakes) of near-fault ground motions. A typical suspension bridge located in Yunnan province of China is selected herein to study the dynamic response of long-span and asymmetrical suspension bridges. And the corresponding finite element model based on the platform of OpenSEES is established to consider the influence of velocity pulse effect, site effect, and structure-soil interaction on the seismic responses of interests e.g., tower, girder, and pile, etc., in the suspension bridge seismic analysis. Besides the near-field and far-field ground motion records are employed from the data base in Pacific Earthquake Engineering Research Center of the United States for comparison analysis. Finally numerical analysis results have suggested that the influence of near-fault effect on the response of long-span and asymmetrical suspension bridges and the different dissipation capacity of nonlinear viscous damper in various intensity levels should be paid more attention to in the seismic design of this type bridges.

Introduction

Affected by irregular terrain and site construction conditions e.g., high mountain and deep valley in southwestern regions of China many long-span and asymmetrical suspension bridges have been constructed or are being built in recent years [1]. Compared to traditional multi-tower suspension bridges [2] and self-anchored suspension bridges [3], not only the ground-anchored and single-tower suspension bridges have unique and beautiful construction form but also own less number of towers and strong topographic adaptability. Nevertheless the long-span and asymmetrical suspension bridges possess more complex dynamical response under ground motion excitation than that of traditional suspension bridges. And them of which suffer from the serious bending-torsion coupling vibration damage and fail nearly to perform potential reparability [4].

As known the southwestern regions of China where these long-span and asymmetrical suspension bridges are located usually are actively seismic zones [5], e.g., the 2008 Wenchuan Earthquake (occurred on May 12, 2008 in the Sichuan province with magnitude M = 8.0), the 2013 Lushan Earthquake (on April 20, 2013 in the Sichuan province with magnitude M = 7.0), and the 2019 Yibin Earthquake (on June 17, 2019 in the Sichuan province with magnitude M = 6.0). As an important traffic lifeline channel, the bridge's role in mountain rescue and disaster relief is unquestionable. For a special bridge such as long-span and asymmetrical suspension bridge, it is likely to cross a complex fault area, once damaged under the action of pulse-type ground motion, its repairability is extremely poor, which will seriously affect the rescue and disaster relief capabilities of the mountainous area. Hence it is of great importance to consider the near-fault effect on the response of bridge structures in the seismic design. As a typical pulsed destructive ground motion the near-fault ground motion has the nature of long period and high amplitude impulses in the velocity time history of ground motion due to the impact of sliding impulse effect and directionality effect [6]. Owing to long-span and asymmetrical suspension bridges have lower stiffness and greater structural flexibility, the low-frequency resonance effect of near-fault ground motion is more prominent. Therefore, it is very important to study the seismic response of long-span and asymmetrical suspension bridges subjected to near-fault ground motion.

Some attempts have been made at investigating the dynamic response of long-span and asymmetrical suspension bridges subjected to near-fault ground motion. For example rooted on the seismic hazard analysis of existing bridges subjected to near-fault ground motion [7], [8], [9], that is main reason of bridge damage in near-fault region that the excessive internal force and displacement response caused by the effect of near-fault ground motion result in the entire or local failure of bridge structures. Adanur [10] and Cavdar O. [11] investigated the characteristics of nonlinear seismic response of a sea-crossing suspension bridge under near-fault and far-field ground motions. The results showed that the velocity pulse of near-fault ground motion has a significant amplification effect on the displacement response of the suspension bridge and the internal force response such as bending moment, shear force, and axial force, because the multi-tower suspension bridge system has insufficient rigidity and greater structural flexibility and the problem of low frequency resonance effect of near-fault ground motion. Cavdar O [12] conducted a sensitivity analysis on the random response of a long-span suspension bridge under near-fault ground motion. The large amplitude spectral accelerations of near-field earthquake ground motion can excite the long-period response modes of many suspension bridges. And the pulsed ground motion can significantly amplify the random response of suspension bridge structures. Zheng Qinfei et al. [13] conducted an experimental study on the collision response of a single tower self-anchored suspension bridge subjected to near-fault ground motion. The result revealed the closer the pulse period of ground motions is to the mode period of long-span suspension bridge, the more intense the collision effect and the more significant amplifying effect on the deformation of both side pier and main tower. Hence the near-fault ground motion can be responsible significantly for the responses of suspension bridges located in immediate vicinity of fault.

Moreover, current seismic design specifications for bridge structures still adopt the principle of three-level fortification especially in China [14], and usually the impact of “rare earthquakes” on buildings are also ignored by designers. Because of the complexity and high randomness of earthquakes, rare earthquakes [15] (also known as “mega-earthquakes” and “super-earthquakes”) are not impossible to occur, and the probability of occurrence is relatively small compared with rare earthquakes and frequent earthquakes. However in the past 50 years, mega-earthquakes that exceeded the magnitude of large earthquakes in all devastating earthquakes have occurred many times around the world. For example, the Wenchuan earthquake (M 8.0) in 2008 [16], the Chile earthquake (M 8.8) in 2010 [17], the North Sumatra earthquake (M 8.5) in 2012 [18] and the Nepal earthquake (M 8.1) in 2015 [19], this high-intensity earthquake caused many bridge structures to collide between two adjacent segments of bridge and even collapse, and as well brought the casualties and economic losses. After the Wenchuan earthquake, many experts and scholars in China pointed out that the impact of extremely rare earthquakes (or “mega-earthquakes”) should be considered in the structural anti-seismic fortification system in China. Obviously, it is very necessary to perform the seismic performance demand analysis of long-span suspension bridges in high-intensity seismic zone based on four-level fortification. Owing to highly coupling mode effect of these suspension bridges with greater flexibility the response of structures including single-tower suspension bridge subjected to strong ground motion can govern the design of structures. Besides in 2015 China promulgated and implemented the fifth generation seismic zoning map (China Earthquake Parameter Zoning Map: GB 18306-2015) [20], it is particularly remarkable that the fourth-grade fortification standard-rare earthquake (huge earthquake) was proposed with an annual exceeding probability of 10⁻⁴ (exceeding probability of 0.5% in 50-year). Now the four-level seismic fortification criterion has been widely concerned by many scholars all over the world [21], [22]. Nevertheless there are many differences of norms between China and in America in the aspects of definition on the level of four-level fortification. The consensus among them failed to be achieved. Firstly Lu Dagang et al. elaborated the risk-oriented theory of Chinese four-level fortification criterion strategy. Secondly the analytical method to determine the risk-oriented seismic fortification level is proposed and the PGA value of the ground motion is also determined according to the four-level fortification principle of the earthquake environment of China. However, long-span and asymmetrical suspension bridges in the near-fault region have not been specially studied for their seismic performance according to the concept of four-level fortification [3], [12].

Thanks to the increasing near-fault seismic records the seismic response of structures, e.g., high-rise structures, long-span cable-stayed bridges, suspension bridges and other flexible structures, in the vicinity of near fault has been studied gradually in recent years [23]. Near-fault ground motion with more abundant low-frequency components is significantly different from far-field ground motion. The velocity large pulse effect of long period and short duration is prominent characteristic which results in the damage or even collapse of structures [24]. Because of the particular nature of near-fault ground motion, the dynamic response of bridge structure has been highly examined by scholars all over the world. For example M Minavand [25] studied the seismic response of concrete box girder arch bridge in the condition of isolation and non-isolation cases under near-fault ground motion. The results showed that the seismic demand of pier and bridge deck under near-fault earthquake is greater than that under far-field earthquake. HB Ma [26] established a probabilistic seismic demand model for near-fault earthquake to study the seismic response of conventional bridge structures. The near-fault ground motion can cause more serious damage to conventional bridges. Lifeng Xin [27] explored the seismic performance of long-span concrete-filled steel tube arch bridges under near-fault earthquake. The strain index was used as the criterion to identify the potential dangerous areas of concrete-filled steel tube arch bridges. It was pointed out that the seismic demand mainly depends on the pulse period and pulse amplitude. Seyed Ardakani and Saiidi [28] proposed an empirical simple method to estimate lateral residual displacement based on the experimental data from shake table test of six bridge columns, the results is reliable compared to that from the Japanese codes. Alhan [29] examined the dynamic response of a high-damping rubber bearing structure under near-fault ground motion. For accurate seismic performance evaluation it may be important to consider the stiffening of high damping rubber bearings (HDRBs) in the seismic analysis of the structures equipped with HDRBs that are located in near-fault regions. Except the symmetrical and regular bridge structures few attempts are made at investigating the seismic response analysis of long-span and asymmetrical suspension bridges subjected to near-fault ground motion. Meanwhile to the best of author’s knowledge under combined influence of velocity impulse effect of near-fault ground motion and site-soil effect the response mechanism of key components in long-span and asymmetrical suspension bridges, e.g., pylons, suspender, and girder, is rarely studied recently.

Following the aforementioned extensive review the objective of this paper is to investigate the seismic responses of long-span and asymmetrical suspension bridges subjected to four intensity level (Small, moderate, huge, and super earthquakes) of near-fault ground motions for providing in-depth insights into the vibration mechanism of this type of bridge. A realistic long-span and asymmetrical suspension bridge is employed herein. Additionally under the consideration of the soil-pile-structure interaction based on the dynamical p-y model the seismic response characteristics of considered long-span and asymmetrical suspension bridge is analyzed comparatively under near-fault impulse ground motion and ordinary ground motion. Aside from the consideration of ground motions in the four-level fortification levels the influence of velocity pulse effect and site-soil effect on seismic response of interesting components e.g., pylon, suspender, and girder in mid-span is further discussed hereon. And the energy dissipation capacity of viscous damper under near-fault impulse ground motion or ordinary ground motion is analyzed. Finally several important conclusions are drawn to provide the guidelines to the seismic design of long-span and asymmetrical suspension bridges.

To investigate the dynamic response of long-span and asymmetrical suspension bridges subjected to near-fault ground motion an asymmetrical long-span and single-tower suspension bridge (shown in Fig. 1) with main span of 780 m located in Yunnan province of China is employed hereon. The used suspension bridge has the rise-span ratio 1/11 of main cable and owns streamlined flat steel box girder with width of 31.4 m. The main tower near Yuxi shore with height of 156 m adopts reinforced concrete portal frame structure whose column body is rectangular hollow box section. In addition the rectangular hollow box section is utilized in cross beam of main tower. The group pile foundation laid under main tower is adopted to ensure adequate stability and strength of foundation. The pile cap with thickness of 7 m and dimension of 21.6 m × 21.6 m connects firmly sixteen cast-in-place piles arranged by 4 m × 4 m with diameter of 2.5 m. The anchorages next to Yuxi and Chuxiong shores are situated under and above highway tunnel respectively (seen in Fig. 1). For guaranteeing the stability of structure under wind load the wind resistance measure is equipped such as wind nozzle, transverse wind-resistant support, vertical support and longitudinal damper at the tower, and one expansion joint at each end of the stiffening girder. Among them, the layout of the constraint system in the finite element model is shown in Fig. 2.