Question

how do you like to learn the bridge engineering (300 words)

Answer 1

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:

• Dead loads include the weight of the bridge itself plus any other permanent object affixed to the bridge, such as toll booths, highway signs, guardrails, gates or a concrete road surface.
• Live loads are temporary loads that act on a bridge, such as cars, trucks, trains or pedestrians.
• Environmental loads are temporary loads that act on a bridge and that are due to weather or other environmental influences, such as wind from hurricanes, tornadoes or high gusts; snow; and earthquakes. Rainwater collecting might also be a factor if proper drainage is not provided.

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.

Load Combinations

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.

The Uniform Building Code (UBC), the building code standard adopted by many states, defines five different load combinations. With this method, the load combination that produces the highest load or most critical effect is used for design planning. The five UBC load combinations are:

1. Dead Load + Live Load + Snow Load
2. Dead Load + Live Load + Wind Load (or Earthquake Load)
3. Dead Load + Live Load + Wind Load + (Snow Load ÷ 2)
4. Dead Load + Live Load + Snow Load + (Wind Load ÷ 2)
5. Dead Load + Live Load + Snow Load + Earthquake Load

The American Society of Civil Engineers (ASCE) defines six different load combinations. As with the UBC method, the load combination that produces the highest load or most critical effect is used for design planning. However, the load calculations for ASCE are more complex than the UBC ones. For the purposes of this lesson and the associated activity Load It Up!, we will use the five UBC load combinations.

Determination of Member Size

Figure 1. Force acting on a column.

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After an engineer determines the highest or most critical load combination, s/he determines the size of the members. A bridge member is any individual main piece of the bridge structure, such as columns (piers) or beams (girders). Column and beam sizes are calculated independently.

To solve for the size of a column, engineers perform calculations using strengths of materials that have been pre-determined through testing. The Figure 1 sketch shows a load acting on a column. This force represents the highest or most critical load combination from above. This load acts on the cross-sectional area of the column.

The stress due to this load is σ = Force ÷ Area. In Figure 1, the area is unknown and hence the stress is unknown. Therefore, the use of the tensile and compressive strength of the material is used to size the member and the equation becomes Force = Fy x Area, where force is the highest or most critical load combination. Fy can be the tensile strength or compressive strength of the material. For common building steel, this value is typically 50,000 lb/in². For concrete, this value is typically in the range of 3,500 lb/in² to 5,000 lb/in² for compression. Typically, engineers assume that the tensile strength of concrete is zero. Therefore, solving for the Area, Area = Force ÷ Fy. Keeping the units consistent is important: Force is measured in pounds (lbs) and Fy in pounds per square inches (lb/in²). The area is easily solved for and is measured in square inches (in²).

Figure 2. Force acting on a beam.

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To solve for the size of a beam, engineers perform more calculations. The sketch in Figure 2 shows a beam with a load acting on it. This load is the highest or most critical load combination acting on the top of the beam at mid-span. Compressive forces usually act on the top of the beam and tensile forces act on the bottom of the beam due to this particular loading. For this example, the equation for calculating the area becomes a bit more complicated than for the size of a column. With a single load acting at the mid-span of a beam, the equation is Force x Length ÷ 4 = F_y x Z_x. As before, force equals the highest or most critical load combination pounds (lbs). Length is the total length of the beam that is usually known. Usually, units of length are given in feet (ft) and often converted to inches. F_y is the tensile strength or compressive strength of the material as described above. Z_x is a coefficient that involves the dimensions of the cross-sectional area of the member. Therefore, Z_x = (Force x Length) ÷ (F_y x 4), where Z_x has units of cubed inches (in³).