Bridges Brian Brenner Aaron Levine Cindy Tran
Some Uses of Bridges • Walkways • Highways/Roads • Railways • Pipelines • Connecting lands • Crossing rivers and canyons
Types of Bridges • Arch • Truss • Cantilever • Cable-Stayed • Suspension
Questions and Answers Brian BrennerProfessor of Civil EngineeringTufts University
How do you start engineering a bridge, in other words, what comes first? • There is a defined process that begins with conceptual engineering, moves on to preliminary and final design, and then moves into the construction phase. The first step, once an owner decides the need for a bridge and gets funding, is the conceptual design. At this time, the various types of bridges and options are reviewed and decided upon. This is also the time when environmental work for the bridge is done.
What do you take in consideration while choosing the best type of bridge for the area? • This is evaluated in a “type study” during the conceptual design phase. Factors for deciding include cost, structural requirements (the bridge has to be strong enough to span), constructability (it must be possible and practical to build the bridge), environmental requirements (in general, the bridge must be shown to not make the environmental worse overall in such categories as air quality, water quality, noise and many other categories), aesthetic requirements (the bridge must look nice), traffic requirements (how many lanes for cars, how many railroad tracks) and maintenance requirements (the bridge must be capable of being maintained over its life).
How do you know how strong to make the bridge in accordance to the area’s wind, weather and other factors acting on the bridge? • All of the loading requirements are specified in the bridge design code maintained by AASHTO. Bridge weight, types of trucks to use, wind loads, earthquake loads, and other loads are all described in the bridge design code. The structural engineer will prepare a mathematical model of the proposed bridge, applying loads to the model. The model will test all of the parts of the bridge to make sure that each part is strong enough to withstand loads. The process includes “factors of safety”- each step of the way is conservative to make sure the bridge is strong enough. For example, the loads are assumed to be much greater than they are. The strength of the materials are assumed to be much weaker than they are. The ratio of provided strength to required strength is the factor of safety.
What process do you have to go through to get a bridge plan approved? • For each part of the process, described previously, plans (pictures of the bridge) and reports are submitted to the bridge owners for review. The owners will comment on the submittal, and then the engineer makes required changes and moves on to the next phase. When the final submittal is reviewed and approved, the owner hires a contractor to build the bridge.
How does cost play a roll in building bridges? • Bridge owners require the least expensive bridge that gets the job done. Cost is essential.
How do you test the bridge to make sure its safe for use after opening it? • Bridges are usually not physically tested after opening. But they are inspected every two years to make sure that the parts haven’t corroded or require repair or replacement.
What makes a bridge stay up? • Forces • Compression – a pushing or squeezing force • Tension – a pulling or stretching force • Action and Reaction • Bridge pushes ground down • Ground pushes bridge up
Arch Bridges • Keystone – the wedge-shaped stone of an arch that locks its parts together • Abutments – the structures that support the ends of the bridge
Arch Bridges • Works by Compression
Cable-Stayed Bridges • Piers – the vertical supporting structures • Cables – thick steel ropes from which the decking is suspended • Decking – the supported roadway on a bridge
Cable-Stayed Bridges • Works by Tension AND Compression
Suspension Bridges • Similar to Cable-Stayed • Different construction method
Suspension Bridges • Works by Tension and Compression
Other Types Truss Southern Pacific Railroad Bridge, Tempe, AZ Cantilever Firth of Forth-Forth Rail Bridge, Edinburgh, Scotland