The combination of ground anchor technology and bridges greatly improves the seismic effect

The combination of ground anchor technology and bridges greatly improves the seismic effect:

Earthquakes are ranked first among all natural disasters because of their suddenness and destructive power.

When the earthquake disaster comes, the damage of the bridge in the earthquake area means the interruption of the lifeline, which directly hinders the progress of disaster relief operations, increases the loss of life and property and indirect economic losses, and brings difficulties to the recovery and reconstruction after the disaster.

What damage will an earthquake cause to bridges?

The time and place of earthquake occurrence are unpredictable, and have the characteristics of short duration and violent energy release. Bridge earthquake hazards mainly include the following four aspects:

(1) Superstructure crash: The phenomenon of falling beams caused by the failure of supporting connectors or the failure of substructures often occurs in destructive earthquakes, most of which occur in the direction of the bridge (referring to the direction of the central axis of the bridge).

2) Damage of support connectors: Bridge supports, expansion joints, shear keys, supporting connectors, etc., are considered to be weak links in the bridge structural system with relatively weak seismic performance.

The failure form of the support is mainly manifested as the displacement of the support, the pulling out of the anchor bolt, the shearing, the shearing of the active support, and the destruction of the structure of the support itself.

(3) Abutment and pier damage: If the pier column is damaged, the ability of the bridge to withstand earthquakes will be weakened and collapse will occur.

The seismic damage of the abutment is more common in earthquakes, due to the loss of bearing capacity of the foundation, etc. caused by the abutment slippage, collision damage between the platform and the superstructure, and abutment tilt.

A new bridge seismic technology that provides excellent seismic performance

More than 1 million people have died in 1,800 earthquakes of magnitude 5 or greater recorded globally since 2000. Bridges are the most vulnerable part of the transport network when earthquakes strike, hampering emergency response, search and rescue missions, and the delivery of aid, increasing the number of potential deaths.

While engineers have designed structures that can withstand destructive natural forces such as extreme typhoons, catastrophic earthquakes such as the 2010 Haiti earthquake (more than 310,000 deaths) or the 2011 Tohoku University earthquake in Japan (more than 20,000 deaths) remain a challenge.

To mitigate the effects of such a large earthquake, a team of researchers at the University of Technology Sydney (UTS) has developed an application that uses ground anchors as the primary seismic resistance system to ultimately protect bridges from catastrophic earthquakes.

Despite the implementation of strict design codes worldwide and technological advances in seismic design and structural protection, more needs to be done to reduce mortality and financial losses.

Of particular importance is the fact that rapid urbanization has created higher population concentrations in seismically active areas such as Japan and Indonesia, with a recorded population of 230,000 in the country following an earthquake in 2004.

Associate Professor Fatahi and his team have developed an advanced three-dimensional computer model that can simulate and assess the seismic resilience of anchored bridges that suffered the world's largest catastrophic earthquake.

Its research found that combining ground anchor technology with bridges, the use of multiple high-strength steel strands anchored into the formation can provide excellent bridge seismic performance. It is well known that in the case of consolidation of pier beams, the movement of the superstructure of the bridge seriously damages the bridge and even collapses after the formation of plastic hinges in the curved area. Bridges that are not consolidated will result in falling beams.

Other types of bridges, such as inclined bridges, will produce rotation and separation of the bridge superstructure, causing the bridge superstructure to break away from the supports and cause some damage to the abutment. As happened in the 2010 Chile earthquake. In addition, the displacement of the superstructure of the bridge will bring greater shear force and bending moment to the substructure of the bridge (including the piers, pile foundations and supports). This results in the need to increase the cross-section of the corresponding location to accommodate the needs of larger earthquakes.

Currently, bridge engineers use viscous dampers, cable restraints, and costly shape memory alloys to reduce seismic displacement of bridge superstructures. These systems limit the displacement of the superstructure by transferring considerable axial forces to piers or abutments, resulting in increased seismic requirements, geometry, and costs.

The design principle of the new structure discussed in this article is to effectively anchor the superstructure in the hard soil layer behind the abutment through multiple ground anchors, and the seismic force will be transmitted to the soil layer through the steel strand ground anchor system, which can effectively limit the back and forth sliding of the bridge superstructure

In "Isolated Segmented Cantilever Bridge Protection", published in the journal Soil Dynamics and Seismic Engineering, the same bridge and the same seismic input were used to compare the effects of viscous damper constraints and ground anchor constraints.

By establishing a complex three-dimensional numerical model, this paper comprehensively considers the interaction between structure and soil, plastic hinge formation and material nonlinearity. The nonlinear time history analysis of the bridge used seismic signals from the 1994 Northridge earthquake, the 1971 San Fernando earthquake, the 1995 Kobe earthquake, and the 1999 Chi-Chi earthquake. These seismic signals have caused great damage to the structure.

The ground anchor model takes into account the free length and anchor length of the anchor. The free-length part is simulated by a cable unit, and the anchor-length part is simulated using more complex connections to illustrate the interaction of nonlinear grouting with soil. The sliding action of the grouting body and the rock formation is simulated using a nonlinear plastic spring. The results used to evaluate include the longitudinal displacement of the superstructure and the bending moment of the piers.

The results show that after using the ground anchor technology, the superstructure of the bridge generated a longitudinal displacement of 105 mm and 95 mm in the Northridge earthquake and the Kobe earthquake, respectively. In contrast, under the same earthquake, the longitudinal displacement of bridges using viscous dampers was 2019mm and 1600mm, respectively. In addition, under the Kobe seismic signal, the viscosity damper scheme used 90% of the bending resistance of the bridge, while the ground anchor scheme used only 10%.

In addition to the structural advantages of the ground anchor, the researchers also noticed that the viscous damper was at risk of leaking silicone contents, which could cause the viscous damper to fail completely. Therefore, the component needs to be checked regularly.

In order to test the effect of ground anchor on the normal performance of the bridge, the effects due to shrinkage, creeping and prestress were also analyzed. The construction phase analysis considers three phases: the initial construction phase, one year after completion, and 30 years after completion.

In the construction phase analysis, it was found that the ground anchor has enough stiffness to suppress the seismic impact of the bridge superstructure, while maintaining its own flexibility, and there will be no constraint damage problem. Due to the low initial construction cost of ground anchors, ground anchor restraint systems are highly cost-effective. Ground anchor technology is readily available and inexpensive compared to systems that require specialized manufacturing, such as viscous dampers. Moreover, due to the presence of the anchoring system, the cross-sectional size of the substructure is greatly reduced, the cost is significantly reduced, and the seismic demand is reduced. Similarly, in contrast to viscous dampers, ground anchor systems are maintenance-free and do not require frequent and continuous checks to maintain their effectiveness. These benefits suggest that ground anchoring systems should be seen as an effective tool for bridge engineers around the world, especially in countries affected by severe earthquakes.

The UTS team is currently conducting a new study to assess the effectiveness of using ground anchors in diagonal bridges to constrain the bridge superstructure. The research team found that by placing the ground anchor at an angle in the bridge superstructure, the rotation angle caused by the impact of the bridge superstructure can be counteracted by the moment generated by the ground. These findings will further strengthen the position of ground anchoring systems as a powerful tool that can be used to significantly enhance the seismic behavior of bridges that are vulnerable to earthquake damage.