Factors leading to failure of cable-stayed bridges

Failure of Cable Stayed Bridge

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

1. Introduction

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

3. Conclusion

4. References

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).

Introduction

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).

Failures

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.

Redundancy

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.

Extradosed bridge

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 tree diagram for the cable stayed bridges

The general frame of the fault tree diagram comprises of four major components, the operation (design), the construction, fabrication and lastly the inspection categories. In most cases the design categories indicates and appreciates the fact that the collapse of fatal defects of the cable-stayed bridge may occur during the servicing of the bridge. The inspection category relates to the issues dealing with the frequency of bridge inspection, due to designs, which hardly consider the inspection components of the bridge. Failure of the bridge could also be because of the process of construction, especially of the girder of the bridge (usually made of steel). The delay in construction could also lead to failures. The fabrication of the steel is a process that is to be given consideration, as it may also lead to the collapse of the bridge. An illustration of the cable stayed bridge diagram is shown in figure 3 for more elaborations.

The Cable Stayed Bridge FailureFigure 3: The cable stayed bridge fault tree diagram

Design/Operations

Construction

Inspection

Fabrication

Failure events of the cable stayed bridge

Dynamic Amplification Factor (DAF)

The analysis of the nonlinear dynamic or the approaches of quasi-statistics are used in the calculations of the response of sudden losses of cables in the bridge. For most systems, the dynamic amplification factor is usually 2.0. The value has been recommended by PTI. However, the complexity of the system matters. In the cable stayed bridge case, other values other than the 2.0 are possible and expected. The recommendations for the PTI clearly stipulate that the value could be limited to 1.5. The DAF is most likely calculated to the variables of all elements of the structure (Walther, 1999, p. 131).

The influence of cable modeling

If there is loss of a cable in sudden circumstances, the act always leads to the immediate redistribution of the load to the other structural components, which are in most cases adjacent. The next cables undergo great displacement in the end nodes. Because the cables will always sag, there is a non-linear relationship between the corresponding forces of the cables and the forces of the displacements. The displacements of the cables will also experience vibrations that cause drastic cable vibrations. To avoid failures, the inclined cables of the bridges have to be stiff in horizontal paths relating to the upper nodes, and in vertical direction in the case of lower nodes, so as to be able to sustain or rather absorb the forces in case of large displacements (Wolff & Starossek, 2009, p.19).

The damping influence of the cables

Most of the case scenarios where there are enormous displacements of huge cables are caused by the low damping values of the specified cables and the effects of corrosion of the cables. The use of secondary cables is advised, which are used to connect the other main cables, with the aim of enhancing stability and stiffness (Emmerich, 2001, p. 126). The span of the Kessock Bridge in Scotland, which was about 240 meters in length, experienced massive oscillations just after it had been opened, due to the winds that were at high speeds of about 22 m/s. After the incident, there was fresh installation of dampers (the tuned mass type) as the solution. Years later greater and more adverse oscillations occurred. After analyzing of the structure, then it was evident that the dampers used had undergone stronger vibrations than they could hold. There is therefore need to ensure proper installation of the required quality of dampers (Emmerich, 2001, P. 127).

Progressive collapse of the bridges

Non-linear dynamics are used in most cases to track the progress of a collapsing structure. The behavior of a grinder and the cables is always assumed as elastoplastic, in case the bridge is undergoing progressive collapse, then the stress caused by the cables will be higher compared to its initial state. Nonetheless, there is the possibility of the slacking. Cable sagging and frequent vibrations also lead to the progressive collapse of the bridges. In most cases though, especially in the cable stayed bridges, the failing of a single cable hardly leads to progressive collapse. The only effect is plastification and raptures at the points of anchorage in the cables. Most of the bridges tolerate the loss of cables. In fact only the loss of three or more cables might result to the compete collapsing of the bridge (Wolff & Starossek, 2008, p. 5). The progression is characterized mainly by the factors responsible for the girders stability, the lack of lead bracings to maximize deflections and the high stress effects affecting longitudinal girders. The event of one has the effects on the other, making it a progressive sequence.

Overview of failures

In reality, there is dependent classification of failure or vulnerabilities. Identification of the vulnerabilities using the redundant approach has been approved to be the best criteria for the analysis of possibilities that might have led to the failure of the cable-stayed bridge. A good instance is the case of the state of New York, where procedures for screening vulnerabilities have been established. The vulnerabilities include the details of the concrete, the issues of collision, overloading the bridge, hydraulics and many others.

Extreme failures in most cases are caused when two vulnerabilities occur at the same time. Mostly issues concerned with construction, design and maintenance are some of the elements that have led to disasters, with regards to the preceding incidences. The elements mentioned before are crucial in determining the lifespan of the bridge. Choice of the components is mandatory for acquiring the best structures because of the nature of the bridge such as one constructed with many structural equipments-decks, towers and stays (Ren, 1999, p. 31).

The deficiencies that occur during the process of engineering and management are what have led to the most of the failures experienced. Currently, most of the programs overseeing the safety of bridges are recommending competence in the engineers, especially when designing the bridge and during the construction phases too. The issue of economy is also important, and the management concerned should lay strategies that are economic friendly. Management of the structuring process is therefore to be analyzed and prioritized in the project, for fruitful endings. Safety of the bridges is important, as the failures have led to the inconveniences and death of many people like the case of earthquakes (the high magnitude earthquake in Turkey-August 17), where numerous deaths occurred (Walter, Amar & Craig, 1999, p.28).

Conclusion (Recommendations and Opinions)

Most of the structures including facilities like the bridges and even buildings have to be constructed in such a manner that will allow then to function and operate for many years. In fact, many of these structures are built to function between fifty to one hundred years, without incurring any additional costs in repairs (Ang, 2011, p.4). However, there has to be funds allocated aside for inspection and maintenance of the bridge, so that any structural issues cropping up are dealt with in their early stages. The personnel I charge of controlling the construction of such structures should be qualified and approved to avoid mediocre management. Just like the human body, there is need for a monitoring system for the cable-stayed bridges to ascertain the health conditions of the bridge. This is to be conducted depending on the nature of the defects the bridge experiences, for instance the HMS for the other bridges (like the suspension bridge) will be different from that of the cable-stayed bridge. After the monitor and the analyzing, there are recommendations to the best remedies such as retrofitting (Holnicki-Szulc & Rodellar, 1999, p.385). There is future hope for long life-span bridges, if the management is enhanced, the right equipment used and the best engineers contacted and utilized.