Factors Leading to Failure of Cable Stayed Bridges
When there is failure experienced in a single structural element, there is high probability that the failure could lead to future failures of bigger intensity, including the collapse of huge structures. This preventable failures and collapse of structures have been analyzed and studied for many years, but unfortunately the research earlier done focuses more on buildings and assumes the engineering of bridges. The attention is even lower when it comes to the cable stayed bridges, regardless of the engineering guidelines required for this type of bridge. The guidelines indicate that the loss of a single cable, is not supposed to lead to entire collapse of the whole structure, but are usually assumed in many occasions. This paper therefore is intended to analyze the effects of cable loss on the cable-stayed bridge, with inclusion of the critical viewing of the large displacements. The work done is to be used by students, engineers and other interested personnel, in the urge of reducing future cases of structure failure. The effects of transverse vibrations of the cables, the sagging of cables and the structural damping of the bridges is also discussed in detail.
Table of Contents
a. Definition of the cable Stayed Bridge
b. Definition of failure and redundancy
c. Summarized history of the Cable-stayed Bridge development
2. General Topic- Main Body
a. Bridge maintenance and Inspection
b. Variations of cable bridges
i. Side-spar cable bridges
ii. Canti-lever cable bridges iii. Multiple-span cable bridges
iv. Extradosed cable bridges
c. Design Errors and Omissions
d. Quality Control and Assurance programs
e. Failure Events
i. The dynamic amplification factor
ii. Influence of cable modeling iii. Damping influence of the cables used
iv. Progressive collapse
v. Overview of failures
List of Figures
Figure 1- The structural system of the bridge in question.
Figure 2- The specific symbols that are to be used in the fault tree design diagrams (The symbols represent the events, the 'or gates', the 'basic events' and the 'and gates').
Figure 3- The tree diagram indicated the major and most common categories of failure and default (Categories include the design of the bridge, inspection, the fabrication and finally the constructions).
Definition of terms
The cable stayed bridge
This suspension bridge is designed in such a manner that the supporting cables of the bridge are directly connected to the deck, and the use of suspenders is hardly used. The bridges are an important aid to the public transportation and safety. Any nation or state should be ready to spend on infrastructure of highways with an objective to satisfy the citizens. That is the main reason why the past failures in bridge projects and structures have to be used as learning objects in pursuit of future prevention. The document is providing a framework to the engineers and designers, especially for the cable stayed bridges, a tool not for prevention of the failures, but for identifying realization of failure mechanisms, which in turn assist the designer to prevent the failures (Federal Highway Administration, 2011, p.3).
According to the Federal Highway administration (2011, p.4), failure in this context could be referred to as the incapability of any bridge, or any of its components to discontinue with the performance it served, which is its intended purpose. In the case that the bridge is still under construction, the definition will be referring to the collapse of such a structure, or the experiencing of a critical defect that is impossible to control or rather correct. Collapse occurs in the event where the structure completely fails, or a larger portion of it. Replacement is usually recommended in search scenarios. When the structure is partially deformed, or a section of it lost, then this qualifies as a critical defect and in most cases, repairs are done.
This is the level of tendency that the bridge can sustain the damages inflicted to it, and refrain from collapsing. If the bridge is non-redundant, there are high chances that the collapse of a single component leads to the collapse of the entire structure. Redundant systems have to be subjected to adverse damage of two or more components for the bridge to crumble completely.
The development of the Cable stayed bridge (Summation)
The very beginning realization of the fact that there was possibility to support the grinder of the bridge with the use of inclined stay was related to Verantius, and to Loscher, who lived in the 17th and 18th centuries. Later in the 19th century some bridges came up (Suspended bridges), and inclined cables were added to make the bridge more stiff. The current cable stayed bridges conceptualized in the latter half of 20th century, thanks to Dischinger, who made known the fact that high stability levels and stiffness would be easier to achieve if strong cables were used. The idea was combines with the technological innovations of computers to ensure controlled forces of the cables in the design of the structures. The cable stayed bridges, which are currently used in most of the continents, has served many people for over a hundred years, as its development advances due to the presence of rapid increase in innovation ideas (Caetano, 2007, p. 5).
Bridge maintenance and Inspections
All bridges are to be inspected frequently at a certain interval, in most cases the intervals should not exceed two years (according to NBIS). This is of great effect, as the owners of the bridge will be in the best position to determine any detects and act on them with immediate effect. The inspectors of the bridge should be qualified personnel, who are expected to prepare a report after the inspection process ends. However, the inspector might find it hard to use certain designs such as the pre-stressing tendons and internal pistons, which are not possible to inspect. Maintaining the bridges is also mandatory to realize more lifespan of the bridge. Slight damages on the bridge require that replacements are done and actual retrofitting. In most cases it is the deck slab and the expansion joint are prone to deterioration, hence require higher concern during the maintenance programs (Tserng, Yin & Chung, 2009, p. 237).
Variation of the cable stayed bridge
The side spar cable stayed bridge
In the side spar cable strayed bridge, the cables support hardly spans the entire roadway, but usually is Canti-levered in only a single side. The principle of the side spar is not just a bridge that is straight, but can also be in the arc design. This is because of offsetting of the tower to one side hence increasing the chances of getting a bridge that curves.
Canti-lever cable stayed bridge
An example of this bridge is the Puente Del Alamillo, which is located in Spain. Actually, this bridge is the newer version of the cable-stayed bridge, because its distribution of force does not depend just on the cantilever action, but also on the weight of the distribution and also the spar angle.
Multiple span cable stayed bridge
These are the cable strayed bridges that consist of two or more spans, and usually the loads coming from the main span are usually anchored close to the abutments (at the back) with the use of stays. The best example is the Ting Kau Bridge that includes additional stays that are to be used for stabilization of the pylons.
This bridge is characterized by the stiffness and strength of its deck hence allowing the omission of the cables (closer to the bridge tower). In this type of the cable-stayed bridges, the tower is low compared to the bridge's span.
Design errors and Omissions
When the process of design begins, there is great need to be keen to avoid any unnecessary omissions and simple errors, which have very harsh repercussions. The owner of the bridge is required to provide the right design he wishes to construct, but must also explain to the engineers and constructors the standards that are expected for the bridge. Structural details are to be analyzed to prevent bridge failure (most importantly using the design calculations). Lack of the following the design calculations, either by omitting important elements and the making of avoidable errors is one of the greatest causes of cable stayed bridges, and also the short lifespan of the bridges than expected. Designers have the option to take advantage of the QC/QA programs offered, which will help and better the working potentials of the designers, with the aim of reducing the omissions and errors.
The Quality control and assurance programs (QC/QA)
The project involved in the construction of cable bridges is supposed to be of good quality if there is any hope of a successful and durable bridge. The needs and expectations are the basis of the best projects, and must be communicated efficiently to the practitioners. There has to be teamwork of the designers, bridge owners, and the constructors for the project to be of required…