Structural engineers are generally involved in the most "visible" of the civil engineering projects. The structural engineer: "arranges and proportions structures and their parts so that they will satisfactorily support the loads to which they may feasibly be subjected." (McCormac, Structural Steel Design, 1981, Harper Row)
Coursework for structural engineers builds on fundamentals from Statics (CVEN 2121), Dynamics (CVEN 3111), and Mechanics of Materials (CVEN 3161). It is important to understand forces - where they go and how much can be supported. In addition, the properties of the building materials that will resist and transmit these forces are important.
Specific structural engineering courses include:
(1) reinforced concrete - its behavior, methods used in design, and the codes and
specifications governing practical design
slabs on grade, retaining walls, bridges, tanks (wastewater
treatment facilities), buildings (domes, church, parking ramp)
(2) Steel - beams, bridges, roof trusses, multistory buildings
(3) Timber - beams, columns, trusses
The structural engineer can be involved in the
following:
studies of structural forms that can be used
determining loading conditions and calculating stresses, deflections,
etc.
layout of structures
design of parts, by proportioning the elements in the structure (like
beams & columns)
preparation of design drawings
The goal of the engineer is to design a structure that can safely meet necessary requirements, can be practically fabricated and built, has reasonable cost, and is aesthetically pleasing. In addition, the impact of the structure on the surrounding environment should be considered.
Bridges
One of the most popular civil engineering structures are bridges. The designer must select a type and span length appropriate to the site and use, consider the effects of wind and weather on the super-structure, consider the effect of tides and currents on underwater foundations (scour), and must select construction materials. Finally, the design must also consider aesthetics; a well-designed bridge can be one of the most beautiful man-made structures. The five major bridge-types are: beam, arch, suspension, cantilever, and cable- stayed.
The beam bridge is the most simplistic: merely a slab across supports. The arch bridge is somewhat more elegant. It was first used in Mesopotamia, and later widely used by the Romans. The Roman bridge was a perfect semi-circle of stone held together by its own weight (no cementing agent was used between the blocks). A Chinese, Li Chun, is believed to be the first to obtain wider spans by flattening the arch. More modern arch bridges are frequently made of steel or reinforced conrete.
Suspension bridges have the advantage of the longest span length. The Akashi- Kaikyo bridge in Japan is 2.42 miles long, with a 1.2 mile "span" between supports. Opened in 1998, it took 10 years, 200,000 tons of steel, and enough wire to circle the earth 7 times to construct the bridge. The bridge has been designed to withstand at 8.5 magnitude earthquake (Richter scale) and 179 mph winds (see http://www.hsba.go.jp/e- index.htm). By comparison, the longest suspension bridge in the U.S. is 4,260 ft long (0.8 miles), the Verrazano-Narrows bridge in New York. Other famous suspension bridges include the Golden Gate and Brooklyn bridges. In a suspension bridge, towers are located at each end of the main span, the deck stretches between the towers and each shore, cables run from one shore, over each tower, and to the opposite shore, suspenders hang from the cable to the deck, and solid achorages of steel and reinforced concrete hold the cables in place at each end. Anchorages may be as large as 213 ft long x 118 ft wide, for example, on the 0.9-mile long Humber Bridge in England.
Cantilever bridges support loads underneath by piers or towers and on-top by a frame-work. There is generally girding visible both above and below the deck. The bridges are generally quite stiff and are optimally suited for sustaining heavy train traffic. Finally, cable-stayed bridges are built by cables which directly connect the deck to the towers. They are optimally suited for medium-length bridges since large cable anchors at each end are not needed. Span lengths are generally less than 0.3 miles, although the newest bridges can span up to 0.6 miles. Originally pioneered in Germany, these bridges have since been used in the U.S. The most famous of these is the 4.1-mile long Sunshine Skyway Bridge across Tampa Bay, Florida.
Skyscrapers
Skyscrapers are another impressive achievement of structural engineers. Currently, the world's tallest buildings are the "Petronas Twin Towers" in Kuala Lumpur, Malaysia. These buildings are 1483 ft tall, with 88 stories, and were completed in 1997. With the completion of these towers, the Sears Tower in Chicago, Illinois, dropped to the third tallest structure. The Sears Tower was completed in 1974, and is 1450 ft tall with 110 stories. It has the highest "occupiable" level of any building in the world. The 4th tallest buidling is the Jin Mao Tower in Shanghai, China, at 1379 ft with 88 stories to be completed in 1998. The 5th highest structure is the One World Trade Center in New York at 1368 ft and 110 stories that was completed in 1972. Most of the tallest buildings in the U.S. were built between 1930 to 1933 (Chrysler Building, Empire State Building, RCA Building) and 1965 to 1977 (John Hancock Center, John Hancock Tower, Citicorp Center).
There was a slow down in skyscraper construction in the 1980s, but interest in reaching for the sky has recently enjoyed a resurgence. Currently, there are a number of buildings of around 1300 to 1500 ft currently being built in Asia, including one in Shanghai, Taiwan, and Indonesia; most are slated for completion between 2000 and 2005. There are currently 3 "megastructure" concepts over 1 miles tall envisioned for construction in Tokyo, Japan. The tallest of these is the X-Seed 4000, proposed to be 800 stories and 13,123 ft tall; this is followed by the Try 2004 and Aeropolis 2001 buildings at 400 and 500 stories each and approximately 6570 ft tall.
There are a number of good websites on these and other tall
buildings, including:
In Canada
Tallest 500
Pictures
More Pictures
Petronas
Twin Towers
The first drive to construct tall buildings began in Chicago in the late 1800s. Prior to the construction of these buildings, engineers developed the use of the steel-I-beam and reinforced concrete. The steel could be used as a strong skeleton with both compression and tensile strength, while being fairly light-weight itself. In addition, the invention of the elevators in the 1850s made it practical for people to use tall buildings. Innovations in foundation work and fireproofing also contributed to the ability to build taller structures. Early buildings often had a lot of sway at the top, sometimes deflecting as much as a few feet in a strong wind and being significant enough to make occupants of upper floors seasick! Diagonal wind braces were refined in the 1920s and 30s, and in the 1960s tubular designs came into use. The Sears Tower has bundled tubes to resist the wind.
One of the most famous tall buildings is the Empire State Building in New York, which was the tallest building in the world for over 40 years. It was completed ahead of schedule in 1931, after taking only 18 months to build the 19 stories. The building has a 57,000-ton steel frame. A testament to its strength is its withstanding a crash by a U.S. Air Force B-25 bomber into the 72nd and 73rd floors in 1945.
Dams
Structural engineers also contribute to the design and construction of hydro-electric dams. Most dams are made of concrete, and are heavy to resist the pressure of the water they contain. The Hoover Dam on the Colorado River was constructed from 1931 to 1936, using more than 6.6 million tons of concrete to create the quarter-mile long and 70 story tall arch-gravity dam. Without a chilled water cooling system to aid the curing of the concrete, what took 2-yrs for the concrete to set would have taken over 100 yrs!
The Itaipu Dam on the Parana River between Paraguay and Brazil is nearly 5-miles wide and was constructed between 1971 and 1984. The dam contains 28 million tons of concrete.
The latest and greatest of the dam-building efforts is currently underway in China. The Three Gorges Dam will be 607 ft tall and 1.2-miles long when completed, creating a 370 mile long reservoir 525 ft deep (max) and ave. 3,500 ft wide on the Yangtze River. Construction began in 1994 and is scheduled to take 20 years, making it one of the largest and most expensive single construction projects in history (cost estimates for the project range from $24.5B to $75B). While technically feasible to construct, there is concern over the myriad of problems that will and may be created - 1.9 million people will be displaced and 4500 villages and 153 towns underwater by the created reservoir; sedimentation could quickly reduce the efficiency of the dam; potential failure in an earthquake triggered by the weight of the reservoir water on an existing active fault line. This is one example of a project that while technically feasible, potentially should not be built due to out-weighing factors in other areas. My geologist friend once said: "given enough concrete, civil engineers think they can do anything." However, geotechnical, environmental, and water resources issues may outweigh the dreams of structural engineers to build ever bigger projects.
FAILURES
When structural engineers do their jobs right, little attention is called to the difficulty of their achievements. However, failures generally attract widespread attention and frequently result in the loss of life. Some of the "most notable" structural failures are:
1. Tacoma Narrows bridge - this is the video shown in most physics classes. Insufficient consideration of wind forces and the harmonic of the bridge caused it to undulate spectacularly (up to 5 ft deflections) before complete failure.
2. Hyatt Regency Hotel Pedestrian Walkway (1981)
On July 17, 1981, two suspended walkways in the atrium of the Hyatt Regency hotel in Kansas City, Missouri, collapsed suddenly. This resulted in 114 deaths, with an additional 185 people injured. The hotel had been in service 1 year before the collapse. The walkways were suspended by tension rods from the atrium roof structure, such that the second floor walkway was suspended directly below the fourth floor walkway.
Later analysis to determine the cause of the failure found a deficient connection where the steel suspension rods were connected to box beams that supported the 4th floor walkway. These connections failed, causing the 4th and 2nd floor walkways (64 tons) to collapse to the floor of the atrium. As it turned out, the connections as originally drawn in the design plans were not capable of supporting the gravity load required by the building code. However, this deficiency was compounded by a change made to the detail during construction which doubled the load on the connection and made failure inevitable. Who was responsible? It seems that both the structural engineer who designed the original connection and the fabricator who changed the plans during construction. It is uncertain as to whether the original connection as designed was buildable. Upon review by the engineering board, the structural engineer was ruled to be responsible for the overall structural integrity, including the performance of connections.
3. Earthquake-Caused failure of 2 Oakland Bridges (1989)
On Oct 17, 1989, the 7.1 Richter-magnitude Loma Prieta earthquake occurred. The epicenter of the earthquake was located 60 miles (100 km) south of the San Francisco- Oakland Bay Bridge. Although earthquakes are unpredictable natural disasters, California currently requires that all buildings be designed to withstand earthquakes. The SF-Oakland bridge was a two deck bridge, one part a twin suspension structure and the second part a series of simple span trusses and a long cantilever truss. The seismic design of the bridge was based on coefficient of 0.1g, in accordance with the required design standards at the time the bridge was built in 1936. However, during the 1989 earthquake it was estimated that the maximum longitudinal acceleration suffered by the eastern portion of the bridge was 0.22g, which caused 24 of the 2.5-cm diameter bolts used to achor the fixed shoes to the tower columns to shear off. Due to the bolt breakages, a 15 m (50 ft) span of the upper deck collapsed onto the lower deck; 1 motorist was killed.
In addition to the collapse of the SF-Oakland bridge, the Cypress Viaduct also collapsed during the earthquake. The Cypress Viaduct was the first continuous double- deck freeway structure in California, constructed between 1955 and 1957. The bridge was designed to lower seismic standards compared to today, so some retrofitting was being done in 1989 at the time the earthquake hit. During the quake, the upper deck collapsed onto the lower one, trapping and crushing vehicles below.
The lessons learned from these two collapses were a confirmation of the need to design for realistic earthquake-generated forces and displacements; it also illustrated the penalty of delaying seismic strengthening when it is determined that old structures are not up to current standards.