Cable supported bridges perform better during earthquakes compared to other types of bridges. There are seismic weak points in cable supported bridges which are likely to become source of damages when the structure experiences an earthquake.
It is thought by most bridge engineers that, existing cable supported bridges have not experienced severe earthquakes, so it is advantageous to investigate weak points. Because neither suitable strengthening technique is developed nor much stronger bridge against earthquake can be designed unless seismic weak points of cable supported bridges is investigated.
The seismic vulnerability in cable supported bridges will be discussed in the following sections.
Fig.1: Cable Supported Bridge
Performance of Cable Supported Bridges During Earthquakes
Performance of following types of cable supported bridges during earthquakes is discussed:
- Suspension bridges
- Cable stayed bridges
- Bridges during construction
Performance of Suspension Bridges during Earthquakes
Seismic vulnerabilities in suspension bridges are in the different bridge components which will be explained in the following sections. Figure-2 illustrates parts of suspension bridge components.
Fig.2: Different Parts of Cable Suspension Bridge
Suspension Bridge Towers during Earthquakes
Towers in the suspension bridge are the first and initial component on which loads are applied and frequently towers are made of steels. The weak point of the towers during earthquake is the tower plate cells.
The plate cell of towers is not strong that is why it might buckle. The buckling of the tower shaft, which primarily supports loads, will risk the integrity of the entire structure.
The cause of the failure is due to not only large tower displacement but also because P-delta effect is considerably large as well.
During earthquake, foundation uplift could occur and consequently the tower moves forward and back and/or side to side. If one side of the base of the shaft disconnect from the pier, the load moves away from its original position.
Thus, compression load on the tower shaft will substantially increase and eventually the tower shaft will buckle. Figure-3 shows different parts of bridge suspension bridge among which towers are shown.
A practical measure to prevent this kind of failure is the placement of vertical stiffeners to the non-compact plates of the tower cells.
The steel tower shaft is carried by concrete pedestals. The concrete pedestals are subjected to local high flexure and quite large shaking. These may initiate tension cracks and possibly concrete cover spalling. An appropriate measure to tackle this problem is to employ prestressed strands.
Fig.3: Suspension Bridge Parts
Fig.4: Suspension Bridge
Suspension Systems during Earthquakes
The strongest part of suspension bridge components in terms of resistance against earthquakes is the suspension system which consists of saddles, cable band, suspenders, and main cable.
This interesting capacity to withstand earthquake shocking may be due to large safety factor provision while cables were designed, cable flexibility absorb considerable shocking.
The major weak point of suspension system during earthquake is the slip of connection between cable saddles and tower of the Bridge. This earthquake vulnerability comes up due to huge deflection and great cable angle between main span and side spans.
Fig.5: Cable Saddles
Fig.6: Cable Saddles
Fig.7: Main Span and Side Span of Suspension Bridge
Stiffening Girders during Earthquakes
Stiffening girders are the weakest parts of the suspension bridges. The main purpose of stiffening girders is to withstand live load and wind load. Therefore, they will suffer considerable deterioration if an earthquake that surpasses the wind load occurs.
In those bridges that are constructed for quite a long time, stiffening girders are built in form of trusses. Lateral braces and their connections experience most of damages caused by earthquake. Therefore, strengthening of lateral braces and connections is assumed to be suitable seismic retrofit.
Fig.8: Suspension Bridge Parts
Fig.9: Golden gate bridge stiffening girders, approximately fifty percent of top lateral braces and connections need to be strengthened
Foundations during Earthquakes
Liquefaction of the soil on which the foundation is fixed is the most dangerous cause of damages while an earthquake occurs. So, it is substantially significant to deal with soils which are prone to liquefaction. For that purpose, there are several techniques that can be employed for example stone column, densification, and displacement piles.
Wind Locks and Expansion Joints
Expansion joints and wind connections are the weakest element is the suspension bridges. Stiffening girders are displaced side to side while the bridge experience earthquake shocking. Consequently, suspenders suffer large deflections because stiffening girders are connected to suspenders. So, if the suspenders deflection is larger than the ability of expansion joints and wind connections, then they will be damaged. This seismic weak point may be tackled by increasing the strength of wind connections and expansion joints.
Performance of Cable stayed Bridges during Earthquakes
Cable stayed bridges are not distinctly different from suspension bridges. They share similar span property like both are long and flexible. Cable stayed bridges and suspension bridges are nearly composed of similar components and hence they have similar earthquake weak points for instance Tower buckling and soil liquefaction.
Like suspension systems in suspension bridges, stays are the strongest part of cable stayed bridge during earthquakes. It should be said that, the resistance of towers in cable stayed bridges against seismic forces are greater than that of suspension bridges.
This is because of the cables which are work like bracings. If concrete is employed in the construction of piers and towers, they would be much weaker and more susceptible to earthquake damages.
Fig.10: Cable Stayed Bridge Components
Fig.11: Cable Stayed Bridge Components
Fig.12: Rio – Antirrio Cable Stayed Bridge in Greece
Earthquake Performance of Bridges during Construction
Obviously, bridges under construction are weaker and vulnerable to earthquake and suffer greater damages compare with the situation where it is completed. This is because, partially completed bridge are easily influenced by moderate or even smaller earthquake seismic forces. Therefore, measures to protect the structure during construction from earthquake forces need to be considered. Due to the fact that the occurrence of both earthquake shocking and movements due to wind simultaneously is rare, therefore in most situations, the measure which is considered to avoid the effect of wind vibraion while the structure is constructed is enough solution to avoid detrimental effects of seismic forces.
Fig.13: Cable Stayed Bridge Under Construction
Fig.14: Bridge Under Construction