I'm starting this series of posts because the years I spent in construction were a valuable education, and from personal experience, many are interested in the construction of a bridge. It's a fascinating, tedious process, with many involved. The process is multi-faceted, with many involved never even seeing the finished bridge; much less driving over the structure.
The process starts with the design. The design is never contemplated, unless there is a specific need. That's where engineers become involved, and regardless of opinion, they have a huge responsibility to get it right. Errors are not only expensive, they can be dangerous.
It all starts with a survey. Property surveys are first. If the right-of-way is not secured, property owners are notified of the future construction, and the negotiations begin. The surveyor determines all corners, who is affected, officials contact the property owners, and an equitable value is determined for the land needed. This can be a daunting task, since many are happy with the metes and bounds of their land, have concerns about how the bridge will affect their lives, and may start a process of using the courts to stop the project. The time required may be years, and numerous legal challenges needing addressing. If all is taken care of, and the land is now the property of those wanting the bridge built, the next process is to make a detailed survey to determine elevations, obstructions, any utilities, and surrounding areas.
A detailed survey is a time consuming process. Utility owners are contacted for locations of their utilities, adjoining property is examined for problems that may develop, such as height problems, noise, or vibration issues during the construction. Vibration? Yes. Some industrial facilities have vibration devices that prevent damage, or releases. If they are close to where heavy equipment, or pile driving, is in the future, the effects have to be considered. Shutting down an operation that makes millions each day is not something to be taken lightly.
The start of the survey is determining known property bounds, marking corners, and then making a detailed survey of the land. Elevations need to be known, any obstructions noted, clearances known, access known, and finally, what's underground.
Generally, the elevations are determined by creating a grid with a determined length for stationing, and offsets to baselines. The controlling baseline is usually the centerline of the proposed structure, which may already have stationing from previous projects. The stationing in the U.S. is on one hundred foot increments, and feet are broken into tenths of feet. There are no inches, since they are not only cumbersome to deal with, using a decimal system makes calculations much easier. In the construction phase, if inches are required, the decimal equivalents can be calculated by multiplying, or dividing by twelve. Those in bridge construction become accustomed to the calculations, and do so without a calculator for even inches. Fractions are seldom used, since such precision isn't usually required.
What's used for surveying? In the past, it was with transits and chains. A transit is a telescope, with a compass, and the telescope can be tilted 180 degrees for determining angles, or back sights. The compass is so well made, a magnifying glass is needed for the smaller increments of the azimuth angles. A chain is a strip of metal, with babbit at every foot. The babbit is punched, when the chain is stretched to a determined tension, at a certain temperature for the material of the chain. The chain is stretched horizontally from one known point, which is under the transit when beginning. The point is either a witness monument, steel rod, or wood stake with a nail driven at the correct point. Since the chain might be pulled at an angle, the transit operator sights the target, notes the degrees to the target, and calculations are performed with trigonometry to determine the actual horizontal distance. The height of the target held by the rodman is known, and the height of the transit telescope (HI) is known. The calculations use these corrections for accurate results. Since the actual elevation under the transit is a known elevation, calculations determine the elevation of the point sighted. It's time consuming, tedious, and not performed much any longer. New surveys use GPS. total stations, and computer software. This eliminates much of the time required, and errors in handwritten calculations. That, and many instruments don't require a second person to hold a rod. The savings can be tremendous over time, and the process can be much safer than wandering through the brush with a machete to clear a line of sight. Old timers have horror tales about snakes, alligators and other critters that they encountered during a survey.
After the survey is completed, the data is used in a CAD (computer aided drafting) program to start the drawings required for the project. It's a good start, but there's a lot of engineering yet to be performed. That I'll save for another post.
Thanks
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