Building a Bridge - Engineering Part 1

 I've worked with many engineers during my career. Like all professions, engineers have years of school under their belt, and before they can officially stamp a set of plans, they have to become licensed. Licensing requires years of working under those that already secured their license, and tests specific to the branch of engineering they pursue. Not all engineers pass the test, and some spend their careers performing the tasks required to design, or build, but never have the legal authority to place a stamp signifying they take full responsibility for the design. Regardless of licensing, I worked with a brilliant engineer without a license that not only designed offshore semi-submersible platforms, he sailed with them on their maiden voyage to insure the "bugs" were shaken out of the platform, and the function was as required. He was an exception, and even some with licenses were far over their heads, and to make things worse, oblivious of the fact their efforts were not only sometimes foolish, they were dangerous as well.

Engineers that design bridges are usually civil engineers. In my circumstances, most of those I worked with were employed by the state of Texas. The hierarchy of responsibility has a small board in charge of the entire state, statewide engineers, with staff in Austin, districts with a chief engineer, and subordinate engineers on staff to support the workload. Some are more than qualified, and some are (to me) functional idiots. All usually are hired as graduate engineers, and some are contractually bonded to five years of service to the state for financial help while going to college. 

With a bridge, and the basic needs understood, rough design drawings start the process. The amount of lanes, the maximum span needed, and approaches are drawn to start the process from conception to completion. This determines the final weight, how the environment will affect the structure, and since it requires access, how existing roads will be modified for use. It's a start, but there's a long way to go.

In the past, airplanes were used to take aerial photographs of certain areas. Photos were used to build a collage of photos, which were used to create larger photos that could be shrunk to a size for ease of use. Surveyed marks were placed on paving for matching photos. You probably have seen these. They are large crosses, and circles, painted on paving painted white. Today GPS is used. It's very handy, but has some inherent problems. One being that an overhead view doesn't scale correctly to actual ground measurements. Changes in elevations, and the curvature of the Earth over a large distance, add up to differing quantities during actual construction. That, and locations shown on plans don't match-up in the field. Regardless of what is used, accurate plans require field surveys to have an accurate scale for measurement. 

During the initial design survey, samples of the existing soil are taken, and bore holes are made to acquire cores for analysis of the substrate. The cores are analyzed by a soils engineer, who specializes in determining the bearing of the existing soil, and what is underneath suitable for bearing the proposed structure. Sometimes, there is available data for some of the area, but bore holes are necessary (when possible) where bearing structures will be placed. Bedrock is the best for bearing, but even bedrock needs analyzing to determine the strength. When there is no bedrock, the soil needs analyzing to determine the length, and type of piling for construction. 

In my experience, which was mostly locally, the substrate is a mixture of clay and sand. The soil is analyzed to determine what bearing a driven, or drilled pile, will provide with friction. With the right diameter, number, and length, the piles can carry a tremendous amount of weight, and provide a firm base for a bridge designed to move. While movement can be small, it's necessary to handle the dynamic load of what the bridge will be used for. On some bridges I worked on , the movement was very noticeable, and while inside a cable stay bridge, I had a few moments of vertigo as heavy traffic passed overhead. 

With the information for the subgrade, the depth of a channel -if any- is now used for determining any construction to take place under the existing water, or the necessary span to satisfy the requirements. Determining the depth may be as easy as using a grade rod, or soundings from a small boat, to a marine survey using sonar mapping. I've never seen an attempt for soil bores in a channel, but have seen soil bores as near to the channel as the drilling rig allowed. Usually, the channel is designed to be free of obstructions to allow water traffic, or to maximize the drainage. If underwater construction is required, the design of a cofferdam is a necessary step. Cofferdams can be anything from dirt piles to allow pumping to the bottom, to elaborate sheet pile configurations to contain the water at deep depths. Whether they still use caissons is out of my knowledge, but they were used successfully in the past.

I have to add that all the efforts to determine underground conditions are not always successful. Years ago, I bid, but didn't win, the emergency repairs for a small bridge. One of the interior spans fell a few feet after a one hundred year flood. Long story short, piling driven for the pile cap reached refusal before the depth required. All involved theorized the sand was much stiffer at the interior bent, so when refusal (I'll explain "refusal" later) was reached, the driving was stopped, and the pile was cut off to grade. After a high volume of water eroded the bottom to a depth below the bottom of the piling, the piling settled, and the pile cap went down. Underwater examination at the now exposed bottom found a sandstone. The piling, which compacted the few feet of sand above the sandstone, successfully supported the bridge for decades, but when the and washed away during the flood, there was nothing under the piling for a few feet. I don't know how the successful contractor performed their repair, but my bid included a drilling rig to drill into the sandstone, and cased piling.  

The soil engineer passes the information to other engineers, including the paving engineer. The paving engineer is responsible for designing a paving to handle the dynamic load of future traffic. Many things are considered, including materials for the paving, the bearing of the existing soil, and the requirement for any fill to bring the subgrade to the elevation needed for the final paving structure. 

The design of the bridge is now well on the way, but there are multiple tasks required to finally get the project to the bidding process. I'll write about that next.