5.2Bridge Engineering

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Analysis and design

Unlike buildings whose design is led by architects, bridges are usually designed by engineers. This follows from the importance of the engineering requirements; namely spanning the obstacle and having the durability to survive, with minimal maintenance, in an aggressive outdoor environment.

 Bridges are first analysed; the bending moment and shear force distributions are calculated due to the applied loads. For this, the finite element method is the most popular. The analysis can be one, two or three-dimensional. For the majority of bridges, a two-dimensional plate model (often with stiffening beams) is sufficient or an upstand finite element model. On completion of the analysis, the bridge is designed to resist the applied bending moments and shear forces, section sizes are selected with sufficient capacity to resist the stresses. Many bridges are made of prestressed concrete which has good durability properties, either by pre-tensioning of beams prior to installation or post-tensioning on site.

In most countries, bridges, like other structures, are designed according to Load and Resistance Factor Design (LRFD) principles. In simple terms, this means that the load is factored up by a factor greater than unity, while the resistance or capacity of the structure is factored down, by a factor less than unity. The effect of the factored load (stress, bending moment) should be less than the factored resistance to that effect. Both of these factors allow for uncertainty and are greater when the uncertainty is greater.

Bridge maintenance

Bridge maintenance consisting of a combination of structural health monitoring and testing. This is regulated in country-specific engineer standards and includes an ongoing monitoring every three to six months, a simple test or inspection every two to three years and a major inspection every six to ten years. In Europe, the cost of maintenance is considerable and is higher in some countries than spending on new bridges. The lifetime of welded steel bridges can be significantly extended by aftertreatment of the weld transitions. This results in a potential high benefit, using existing bridges far beyond the planned lifetime.

Bridge health monitoring

There are several methods used to monitor the condition of large structures like bridges. Many long-span bridges are now routinely monitored with a range of sensors. Many types of sensors are used, including strain transducers, accelerometers, tiltmeters, and GPS. Accelerometers have the advantage that they are inertial, i.e., they do not require a reference point to measure from. This is often a problem for distance or deflection measurement, especially if the bridge is over water.

An option for structural-integrity monitoring is "non-contact monitoring", which uses the Doppler effect (Doppler shift). A laser beam from a Laser Doppler Vibrometer is directed at the point of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface. The advantage of this method is that the setup time for the equipment is faster and, unlike an accelerometer, this makes measurements possible on multiple structures in as short a time as possible. Additionally, this method can measure specific points on a bridge that might be difficult to access. However, vibrometers are relatively expensive and have the disadvantage that a reference point is needed to measure from.

Bridge failures

The failure of bridges is of special concern for structural engineers in trying to learn lessons vital to bridge design, construction and maintenance. The failure of bridges first assumed national interest during the Victorian era when many new designs were being built, often using new materials.

In the United States, the National Bridge Inventory tracks the structural evaluations of all bridges, including designations such as "structurally deficient" and "functionally obsolete".

The history of bridge construction

Video from Interesting Engineering

https://www.youtube.com/watch?v=UoTkgqfJw5o

The Making of Tintagel bridge

Video from English Heritage

https://www.youtube.com/watch?v=YCHjpbyuuLw&feature=emb_logo

Bridge Engineering Basics

Video from Boat of Knowledge Ohio University

https://www.youtube.com/watch?v=SbCVRr5eANA

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References

 O'Brien, Eugene J.; Keogh, Damien L.; O'Connor, Alan J. (October 6, 2014). Bridge deck analysis (Second ed.). Boca Raton. ISBN 9781482227246. OCLC 892094185.

O’Brien, E.J; Keogh, D.L (December 1998). "Upstand finite element analysis of slab bridges". Computers & Structures. 69(6): 671–683. doi:10.1016/S0045-7949(98)00148-5. hdl:10197/4054

Žnidarič, Aleš; Pakrashi, Vikram; O'Brien, Eugene; O'Connor, Alan (December 2011). "A review of road structure data in six European countries". Proceedings of the Institution of Civil Engineers - Urban Design and Planning. 164 (4): 225–232. doi:10.1680/udap.900054. hdl:10197/4877

 "The new Minnesota smart bridge" (PDF). mnme.com. Archived from the original (PDF) on August 23, 2012. Retrieved January 30, 2012.

"Basic Principles of Vibrometry". polytec.com. Archived from the original on June 10, 2012. Retrieved January 25, 2012.

Omer; et al. (2018). "Performance evaluation of bridges using virtual reality". Proceedings of the 6th European Conference on Computational Mechanics (ECCM 6) & 7th European Conference on Computational Fluid Dynamics (ECFD 7), Glasgow, Scotland.

 Yang, Y.-B.; Lin, C.W.; Yau, J.D. (May 2004). "Extracting bridge frequencies from the dynamic response of a passing vehicle". Journal of Sound and Vibration. 272 (3–5): 471–493. Bibcode:2004JSV...272..471Y. doi:10.1016/S0022-460X(03)00378-X.

Yang, Y. B.; Yang, Judy P. (February 2018). "State-of-the-Art Review on Modal Identification and Damage Detection of Bridges by Moving Test Vehicles". International Journal of Structural Stability and Dynamics. 18 (2): 1850025. doi:10.1142/S0219455418500256. ISSN 0219-4554.

 Malekjafarian, Abdollah; McGetrick, Patrick J.; OBrien, Eugene J. (2015). "A Review of Indirect Bridge Monitoring Using Passing Vehicles". Shock and Vibration. 2015: 1–16. doi:10.1155/2015/286139. ISSN 1070-9622.

https://en.wikipedia.org/wiki/Bridge#Analysis_and_design