We know that bridges play an important part in our daily lives. We know they are essential components of cities and the roadways between populations of people. Some bridges are simple and straightforward; others are amazingly complex. What are some bridges that you know that might be called simple bridges? (Possible answers: Log over a creek, bridges over streams.) What are some bridges you know that might be considered more complicated? (Possible answers: Golden Gate Bridge, other large bridges, bridges that carry both highway traffic and train traffic.) What makes some bridges simple and other complex? (Possible answers: Their size, multiple purposes, environmental conditions, environmental forces, material maintenance requirements, etc.)
It is not easy to create a bridge the size of the Sky Gate Bridge. Have you ever wondered how engineers actually go about designing an entire bridge? Bridges are often designed one piece at a time. Each pier (columns) and girder (beams) has to meet certain criteria for the success of the whole bridge. Structural engineers go through several steps before even coming up with ideas for their final designs.
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For designing safe bridge structures, the engineering design process includes the following steps: 1) developing a complete understanding of the problem, 2) determining potential bridge loads, 3) combining these loads to determine the highest potential load, and 4) computing mathematical relationships to determine the how much of a particular material is needed to resist the highest load.
One of the most important steps in the design process is to understand the problem. Otherwise, the hard work of the design might turn out to be a waste. In designing a bridge, for instance, if the engineering design team does not understand the purpose of the bridge, then their design could be completely irrelevant to solving the problem. If they are told to design a bridge to cross a river, without knowing more, they could design the bridge for a train. But, if the bridge was supposed to be for only pedestrians and bicyclists, it would likely be grossly over-designed and unnecessarily expensive (or vice versa). So, for a design to be suitable, efficient and economical, the design team must first fully understand the problem before taking any action.
Determining the potential loads or forces that are anticipated to act on a bridge is related to the bridge location and purpose. Engineers consider three main types of loads: dead loads, live loads and environmental loads:
Values for these loads are dependent on the use and location of the bridge. Examples: The columns and beams of a multi-level bridge designed for trains, vehicles and pedestrians should be able to withstand the combined load all three bridge uses at the same time. The snow load anticipated for a bridge in Colorado would be much higher than that one in Georgia. A bridge in South Carolina should be designed to withstand earthquake loads and hurricane wind loads, while the same bridge in Nebraska should be designed for tornado wind loads.
During bridge design, combining the loads for a particular bridge is an important step. Engineers use several methods to accomplish this task. The two most popular methods are the UBC and ASCE methods.
Every beam shape has its own cross sectional area calculations. Most beams actually have rectangular cross sections in reinforced concrete buildings, but the best cross-section design is an I-shaped beam for one direction of bending (up and down). For two directions of movement, a box, or hollow rectangular beam, works well (see Figure 3).
What are examples of load types? (Possible answers: Vehicles, people, snow, rain, wind, the weight of the bridge and its railings and signs, etc.) Why would the loads make a difference in how an engineer designed a bridge? (Answer: Engineers must figure out all of the loads that might affect bridges before they design them.) If you were an engineer, how would you go about designing a bridge to make sure it was safe? (Discussion points: First, fully understand the problem to be solved with the bridge, its requirements and purpose. Then figure out all the possible types of loads [forces] that the bridge might need to withstand. Then calculate the highest possible load the bridge might have to withstand at one time. Then figure out the amount of construction material required that can resist that projected load.)
Pairs Drawing: Divide the class into teams of three students each. Have each engineering team sketch a bridge to carry a train across a river that is 100-meters wide. Have them describe the type of bridge and where the compressive and tensile forces are acting on it.
Complete the Design/Presentation: Have student teams return to their bridge design from the pre-lesson assessment and think about the potential loads on their bridge, given the just-discussed engineering design process steps. Have them draw in the loads and the direction that they would act on the bridge. What do they think the highest load combination would be (how many of these loads could actually happen at the same time)? Then, ask for one or two engineering teams to volunteer to present the details of their bridge design to the class.
Human Bridge: Have students use themselves as the raw construction material to create a bridge that spans the classroom and is strong enough that a cat could walk across it. Encourage them to be creative and design it however they want, with the requirement that each person must be in direct contact with another class member. How many places can you identify tension and compression? How would you change the design if the human bridge had to be strong enough for a child to walk across it? What other loads might act upon your bridge?
Have the class participate in the yearly West Point Bridge Design Contest. Access excellent and free downloadable bridge design software and other educational resources at the US Military Academy at West Point website: bridgecontest.usma.edu/
Students take a hands-on look at the design of bridge piers (columns). They determine the maximum possible load for that scenario, and calculate the cross-sectional area of a column designed to support that load.
Students learn about the variety of materials used by engineers in the design and construction of modern bridges. They also find out about the material properties important to bridge construction and consider the advantages and disadvantages of steel and concrete as common bridge-building materials ...
This pier design example is based on AASHTO LRFD Bridge Design Specifications (through 2002 interims). The design methods presented throughout the example are meant to be the most widely used in general bridge engineering practice.
Refer to Design Step 1 for introductory information about this design example. Additional information is presented about the design assumptions, methodology, and criteria for the entire bridge, including the pier.
For the pier in this design example, the maximum live load effects in the pier cap, column and footing are based on either one, two or three lanes loaded (whichever results in the worst force effect). Figure 8-4 illustrates the lane positions when three lanes are loaded.
The positioning shown in Figure 8-4 is arrived at by first determining the number of design lanes, which is the integer part of the ratio of the clear roadway width divided by 12 feet per lane. Then the lane loading, which occupies ten feet of the lane, and the HL-93 truck loading, which has a six-foot wheel spacing and a two-foot clearance to the edge of the lane, are positioned within each lane to maximize the force effects in each of the respective pier components.
The total braking force is computed based on the number of design lanes in the same direction. It is assumed in this example that this bridge is likely to become one-directional in the future. Therefore, any and all design lanes may be used to compute the governing braking force. Also, braking forces are not increased for dynamic load allowance. The calculation of the braking force for a single traffic lane follows:
It is assumed in this design example that the structure is located in Seismic Zone I with an acceleration coefficient of 0.02. For Seismic Zone I, a seismic analysis is not required. However, the Specifications require a minimum design force for the check of the superstructure to substructure connection. Also, at locations of expansion bearings, a minimum bridge seat must be provided.
Since the bearings at the pier are fixed both longitudinally and transversely, minimum bridge seat requirements for seismic loads are not applicable. Also, since the bearing design is carried out in Design Step 6, the calculations for the check of the connection will not be shown here. Therefore, the earthquake provisions as identified in the above paragraph will have no impact on the overall pier design and will not be discussed further.
For the purpose of this design example, a total force of 20 kips will be assumed. This force acts in the longitudinal direction of the bridge (either back or ahead station) and is equally divided among the bearings. Also, the forces at each bearing from this load will be applied at the top of the bearing (i.e., five inches above the pier cap).
The first step within this design step will be to summarize the loads acting on the pier at the bearing locations. This is done in Tables 8-4 through 8-15 shown below. Tables 8-4 through 8-8 summarize the vertical loads, Tables 8-9 through 8-12 summarize the horizontal longitudinal loads, and Tables 8-13 through 8-15 summarize the horizontal transverse loads. These loads along with the pier self-weight loads, which are shown after the tables, need to be factored and combined to obtain total design forces to be resisted in the pier cap, column and footing.
In addition to all the loads tabulated above, the pier self-weight must be considered when determining the final design forces. Additionally for the footing and pile designs, the weight of the earth on top of the footing must be considered. These loads were previously calculated and are shown below: 589ccfa754
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