Proper structural design in civil engineering helps ensure that public works projects don’t end in tragedy. It also ensures the structure will last at least as long as the planners expect. Sometimes, though, even civil construction engineers with many successful projects under their belts get things wrong.
Imagine driving on a bridge that’s rolling in front of you with troughs so deep you can’t see the vehicle ahead. That’s how one motorist described driving on the forever-famous Tacoma Narrows Bridge shortly before its spectacular collapse, which was captured on film.
Clark Eldridge did the original Tacoma Narrows Bridge design, but the cost was too high. The federal government got involved because the bridge was a strategic part of military base plans in that area. And so another well-known civil structural engineer with major bridge projects to his credit was brought in.
Leon Moisseiff had designed the Golden Gate Bridge, the George Washington and Manhattan bridges in New York City, and the Ben Franklin Bridge in Philadelphia. In fact, he designed so many successful bridge projects that his name graces an annual award for published papers on improving engineered structures.
Unfortunately, Moisseiff and Eldridge came up against a determined enemy: wind.
Key Principles a Civil Structural Engineer Abides By
Civil engineering design establishes the overall concept for the structure. Then, the civil structural engineer does structural analysis to see if the proposed structure, and its materials, will withstand the expected loads and forces.
#1. Structures Must Bear Loads Efficiently
Environmental loads, live loads and dead loads make up the major classes of loads that structural engineers analyze. Not long after the Tacoma Narrows Bridge collapse, engineers discovered its design had missed a key environmental load factor.
Environmental Loads
Environmental loads include wind, water, snow, ice, earthquakes and anything else that can be thrown at a structure by its surroundings. In 1940, engineers typically used trusses, with many openings in them allowing air passage, to support bridge decks.
To save money, and create a more beautiful bridge, Moisseiff specified a series of girders with solid metal plates filling the gaps between them.
That set the stage for lateral winds to work against the sides of the bridge, creating low-pressure points alternating in strength from side to side. The bridge deck twisted back and forth. That action, coupled with the rolling undulations of the deck, turned out to be too much for the bridge to handle. At 11:02 a.m. on November 7, 1940, a 600 foot-long piece of the midsection dropped 195 feet into the water.
Dead Loads
Dead loads include all elements of a structure that account for its weight. Dead loads are static or permanent. When environmental factors cause dead loads to move beyond their tolerances, you’ve got a serious problem. The cables and deck of the Tacoma Narrows Bridge became two dead loads moving beyond their specifications.
Live Loads
Live loads account for all other loads that aren’t part of the structure. They are a major contributor to structural failures because they are intermittent and subject to change. The main live loads on bridges are the vehicles traveling on the deck.
#2. Structural Elements Must Remain Stationary Despite Forces
In the Tacoma Narrows Bridge collapse, the force of the lateral wind load became so great that it started twisting the bridge, back and forth. It turns out that wind has played havoc with many bridges over the years, including the Tay Rail Bridge in Scotland in 1879.
Besides poorly built spans and poor maintenance, no one took wind loads into consideration when designing the Tay Rail Bridge. Accounting for wind loads was not common in the 1800s.
Seventy-five people died on Dec. 28, 1879, when the center of the bridge collapsed sending a train 90 feet down into the water. The wind that evening was gusting to 90 mph. A board of inquiry blamed the bridge designer Sir Thomas Bouch who died within a year of the tragedy.
All loads exert force on a structure. Even dead loads have forces. For instance, stem walls put weight, force, on footers. The structural parts must work together to counteract live, dead and environmental forces.
#3. Smart Structural Design Means Smart Material Choices
Materials are the building blocks used by a civil structural engineer. However, there is an immense variation in material strength and longevity. What’s more, the structural design must factor in each material’s weight and how it applies forces to other materials once put in place.
A civil structural engineer spends a great deal of time analyzing material specifications to find the smartest materials for the job. Sometimes they even have to invent new materials by combining others. For example, a standard truss might not carry the planned load, so the engineer will design a hybrid truss to do the job.
Structural Design as a Career
If you like to figure out how to put things together and you enjoy physics and math, you could make structural design and civil engineering a career.
Structural Design Definition
When you practice sound structural design, you balance the weight and cost of materials against their designed purpose. The materials must hold their shapes so the structure can hold its shape.
The materials also must be able to withstand the forces we expect them to encounter during their lifetimes as parts of the structure. Another critical aspect is how you join materials together—chosen methods have to be as fit for the structure’s lifetime as the materials.
Civil engineering design is exacting and carries great responsibilities, so what can a civil structural engineer expect to make?
Civil Structural Engineer Salary
Folks who go into civil structural engineering can expect to earn between $60,000 and $90,000 per year. The pay varies a lot between areas. For example, if you work in New York, the average pay is $86,074. When working in North Carolina, on the other hand, the average is $63,127. Skill level, experience, and which civil engineering field you work in also affect what you can earn.
Job Outlook for a Civil Structural Engineer
The job outlook for structural engineers is strong and expected to continue that way through 2028. The demand will grow by about 6% per year, making it on track to slightly outpace the average of all other occupations.
Conclusion
A civil structural engineer puts their reputation on the line each time they certify a design for any civil engineering project. Novel, unexpected conditions undermined the designers of the Tacoma Narrows Bridge and the Tay Rail Bridge. These cautionary tales show how demanding civil structural engineers’ jobs are and highlight how important it is to consider all forces when designing civil structures.
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