This paper goes in depth about a variety of tests used to prevent bridge collapses. These tests will prevent a bridge collapse which can cause death to innocent victims. In order to ensure that a bridge is safe for public use, it must pass at least one of the tests mentioned in this paper. Failure to pass one of these tests may result in having to modify one of the many pieces that make up a bridge. The tests mentioned in this paper are Monotonic Load Test, Stress Spectrum Test, and the Carbon Fibre Reinforced Polymers (CFRP) Failure Test. Each of these tests are performed in a different way, and all test different things such as stress and load. The tests mentioned above are explored in detail in this paper.Bridges are very important for society to communicate and interact with one another. Bridge collapsing usually occurs as a result of a mathematical error done by a civil engineer. Usually, bridges are designed and constructed very efficiently and carefully as well. Designing a bridge itself includes a variety of precautions as well as tests that need to be performed in order to ensure safety and security. Most bridges are made out of steel, because of its strength and durability [1]. Although steel is used widely in the world of civil engineering, it does not always ensure durability. There have been many instances in which still bridges have collapsed, which might include aging, poor planning or even poor construction [1]. As shown in Figure 1, the different causes of a bridge collapse.
Bridges withstand a lot of weight as well as pressure which may also result in bridge failure or collapse [1-2]. Some examples of bridges that have recently collapsed due to design defect and overload are the Tacoma Narrows Bridge (1940) and the Memorial Bridge in Latchford, Canada (2003) [1]. Another key factor to bridge collapse, is the lack of maintaining them and inspection from time to time to ensure that the structure is still safe for use. A great example of this is the Silver Bridge in 1967 which caused many fatalities. Professional engineers have created standards in order to prevent such cases from happening. They have created a variety of computerized tests that depict the amount of load the bridge can withstand, as well as other tests to make sure the structure is safe [1]. The failure of bridges is something very serious that needs to be dealt with immediately. Civil engineers need to perform as many of these tests as possible in order to ensure the safety and durability of these bridges.
2.1 ACI Standards .There are two standards of how to conduct the Monotonic Load Test that the American Concrete Institute (ACI) decides. The first standard is the ACI 318 which is called the “Building Code Requirements for Structural Concrete” and it is conducted in a 24 hour span [3]. The other standard is the ACI 437 and is called the “Code Requirements for Load Testing of Existing Concrete Structures” [3].
2.2 Monotonic Load Test in ACI 318.In the Monotonic Load Test by ACI 318 the concrete structure is applied with a test load magnitude (TLM) which consists of the applied test load (ATL) [3]. The TLM is defined with these three equations:“Where D is the dead load due to self-weight of the concrete structural system and any superimposed dead loads, L is the live load due to use and occupancy of the building, Lr is the roof live load produced during maintenance by workers, equipment, and materials or during the life of the structure by movable objects such as planters and people, S is the snow load, and R is the rain load” [3]. The structure is applied with the ATL in four increments for 24 hours, then the load is unloaded and left standing for another 24 hours. The results are then measured and the structure is said to pass the test if it satisfies these equations:
In these equations, ∆l is the deflection of the structure recorded during the load in 24 hours. “lt is the free span of the member under load test, h is the overall height of the member, and ∆r is the residual deflection measured following load removal” [3]. In the case that the structure does not comply with these equations, then the test can be repeated to see if it passes the following equation where ∆r2 is the difference in deflection from the second test and the first and ∆2 is the maximum deflection in the second test [3].
2.3 Monotonic Load Test in ACI 437
In ACI 437, the Monotonic Load Test has some difference in between the acceptance and the magnitude of the load applied. However, the procedure of the test is the same. This standard is used when only a section of the structure is being tested, while in the standard ACI 318, the whole structure is tested [3]. In the standard ACI 437 TLM is applied using different equations. These equations are:
In which “Dw is the load due to self-weight of the concrete structural system and Ds is the superimposed dead load” [3]. The structure passes the test if it passes the following equations:
If the structure doesn’t pass one of these equations, then the test can be repeated.
2.4 Stress Spectrum Test
The Stress Spectrum test is most frequently used in the assessment of steel bridges. Steel bridges are most frequently used for the passage of vehicles, which receive a variety of different magnitudes daily. “When the strain time histories are available, the stress time histories are obtained through simply multiplying the measured strain data by the elasticity modulus of steel in assumption of elastic strain” [2]. The Rainflow Cycle Technique, The Standard Typhoon Stress Spectrum and the Derivation of Standard Daily Stress Spectrum are enormous parts of completing this test. The Rainflow Cycle Technique [4] is used to “seek the peaks and valleys in the stress time history and then extract stress ranges and the number of cycles for each specific stress range” [2]. Along with the results retrieved from the Rainflow Cycle technique, the Daily Stress spectrum can be retrieved. Usually, the daily stress spectra indicates more or less the same amount of stress during similar “normal” traffic patterns as well as wind conditions [2,4]. The Standard Typhoon Stress Spectrum is a much more general form of that takes into account both traffic and wind conditions as well. Typhoon effects can be seen when combining the results of the two standard stress spectra [2,4].
2.4 Carbon Fibre Reinforced Polymers (CFRP) Failure Test
Fiber Reinforced Polymer (FRP) [5] is a very useful material to strengthen steel and concrete [5-6]. However, FRP was first shown to the Florida Department of Transportation (FDOT) as a replacement to reinforced concrete (RC) because of its durability and because it is very unlikely to corrode [6]. During this test, a steel beam with carbon fibre reinforced polymers (CFRP) is applied with a load until it breaks. The results are that the steel beam does one of this failures: debonding, delamination, splitting, and rupture [5]. In debonding, the the CFRP separates from the beam, causing the beam to be fragile [5]. Delamination is like debonding, but the CFRP separates in the “longitudinal direction” [5]. In addition, splitting is when the beam cracks making it weak and easy to collapse. Finally, rupture happens when the stress in the CFRP reaches its maximum and it snaps [5]. In addition to these types of failures, deformation can happen when the steel beam changes shape like bending.
III. Conclusion
Bridges are one of the main sources of communication, dating back to the beginning of civilization. Once bridges were built, it was much easier for civilians to communicate and maneuver around. Safety should be the number one priority in building bridges, ensuring that no one will get hurt. There is a series of tests performed on bridges once they are built in order to ensure safety for all. The tests all test different things such as stress which is how much pressure the bridge receives everyday. For example, a bridge meant for people to cross most likely receives less pressure than a bridge that is meant for vehicles to cross. In order to prevent failures from happening, the tests should be done always before opening the bridge to the public to not cause fatal accidents.