What types of structures do Civil Engineers Design?

Civil Engineering Structures

A large part of civil engineering deals with the structures which define our society. This has been true throughout the history of mankind. Today, the structures that ancient engineers have left behind help us understand the people who lived before us, how they lived, the wars that they fought and the gods that they served.

In a sense, we can say that the earliest civil engineers were the architects who designed Egypt’s pyramids and their contemporaries in other lands. Although the structures which they engineered in those days are extremely inefficient in their use of materials by today’s standards, they are still engineering marvels.

Rome’s aqueducts and roads were an engineering marvel in their day. To be able to transport water over such long distances, without the use of pumps and pipes was a true technological challenge. By understanding the physics of liquid flow, and combining it with the structural stability of the arch, those engineers were able to provide water to areas that otherwise couldn’t be farmed.

Even today, the work of civil engineers, modifies the environment around us, creates space where we can live, work and play. While the structures which we see most commonly are buildings, civil engineering deals with many other structures as well, include dams, roads, bridges, tunnels, water treatment plants and more.

Dams

The Hoover dam, the second largest dam in the United States was an engineering marvel in its day. It is sited on the Colorado River, at the border between Arizona and Nevada, creating Lake Mead, which stretches 110 miles upstream to the Grand Canyon. This dam takes advantage of the naturally existing canyon, which is an extension of the Grand Canyon to provide its foundation as well as room for the lake it created. Upon its completion in 1938, it was the tallest dam in the world. Although many have since surpassed it, it is still in the top 30.

One of the great challenges which faced the civil engineers in the creation of the Hoover Dam was in diverting the flow of the Colorado River while the dam was being created. To accomplish this, they dug four diverter tunnels through the solid rock of the canyon walls. Once completed, a cofferdam was erected upstream of the dam’s location, to divert the water through these tunnels.

At the time of its building, the Hoover Dam required more concrete than had ever been used in a single structure. Engineers on the project calculated that such a large volume of concrete would require 75 years to fully set. Since that was unacceptable, they designed a series of cooling pipes, carrying cold water throughout the dam, cooling the concrete. This allowed it to set considerably faster. At the time, this was a revolutionary method, which had never been attempted before and was invented just for the construction of the Hoover Dam.

Buildings

The world’s tallest building, the Burj Khalifa off the coast of Dubai in the United Arab Emirates is both the tallest building and the tallest man made structure in the world. At 2,723 feet tall, it created special challenges for the civil engineering team who designed it. Amongst other problems to confront, was that they were building the world’s tallest building on sand. Added to this were the high winds which upper floors of the building would be subjected to.
The secret to the Burj Khalifa’s success as a structure is a combination of its foundation and its core. The foundation consists of 192 holes, bored 1.5 meters in diameter and 50 meters deep. These pilings are all connected to a 12 foot thick raft-type foundation, which is as big as the entire footprint of the building.

Besides the foundation, the other major structural element of any skyscraper is the core. This reinforced cement structural element is in the center of the floor, and typically contains the elevator shafts and stairwells. However, its primary purpose is to support the floors and skin of the building. Burj Khalif’s “Y” shape is intended to maximize the strength of the core, by using a hexagonal core, combined with columns at the exterior points of the “Y.” These are connected together by outrigger walls on the mechanical floors, providing the same effect as buttresses on Medieval Cathedrals and castles. This combination provides excellent strength for both vertical and lateral loads.

The building is also designed with a slight twist in the “Y” as it goes up, which is designed to help combat lateral wind forces. By adding a twist, the structural engineers were able to ensure that wind would not be able to strike one face of the building throughout its entire height. That reduces the potential lateral forces on the structure. All in all, the Burj Khalif is a remarkable feat of structural engineering.

Bridges

Building bridges creates a totally different type of challenge to the civil engineer. In the case of a suspension or cable-stayed bridge, a large part of the completed bridge’s load is carried by the cables. However, those cables can’t properly support the bridge while it is under construction. Throughout the course of its construction, the sections of the bridge are cantilevered, solely supported by their own structural strength and attachment to the towers.

Successful suspension bridge designs combine various structural elements to carry their load. The purpose of the suspension cables is to transfer the weight of the bridge deck or roadbed and vehicle load to the towers, which are anchored to bedrock. Since both steel and concrete are extremely strong under compression, this allows even delicate looking towers to carry an incredible amount of load.

For the suspension cables to be able to transfer the bridge’s load to the towers, the cables need to be under tension. However, without a complete roadbed, the cables cannot be taut. This means that the roadbed must support itself, until it is joined together in the midst of the span and all the cables are in place.

To accommodate this need and provide stiffness to the roadbed, trusses are attached to the underside of the roadbed. Without sufficient truss strength to stiffen the roadbed, it would undulate like the Tacoma Narrows Bridge, which collapsed in 1940.

Construction of these bridges has to be accomplished by working from the towers outward. The distance between the towers and their distance from the bridge’s ends is very carefully calculated by the structural engineers, so that the roadbed can be built in both directions at the same time. To try and build the bridge only in one direction from the tower, without building it in the other direction, would cause the structure to become out of balance, with the towers leaning towards the side that has the weight of the roadbed attached to it. This would cause permanent damage to the columns.

Each of these structures, like every other structure which civil engineers work on, has its own design challenges. The civil engineers who work on these projects start by understanding the problems which they are faced with. The design process consists of finding solutions to those problems, then challenging those solutions, to ensure that the resulting structures will be safe.

Hi Christian!

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