Post Tensioned Concrete

By Fiona Wright 

Post-tensioning is a process of reinforcing and prestressing concrete. Stresses are brought into the concrete using bars or strands of steel while it is being constructed. The strands of steel are typically used in horizontal construction such as foundations and beams. Steel bars are used for vertical purposes such as columns and walls.
Typical steel strands used in post-tensioning have a tensile strength of 270,000 pounds/in². In comparison, a typical non-prestressed piece of reinforcing has a tensile strength of 60,000 psi. Strands typically have a diameter of 1/2 in., and are stressed to a force of 33,000 pounds. The stresses brought into the concrete offset the expected external loads the concrete will be subjected to.
In post-tensioning the steel is held in a duct, which stops the steel and concrete from binding after the concrete solidifies. The steel can then be stressed after the concrete sets. This is usually done in two stages. The first prestress applied is up to 50% of the final force and this is done when the compressive strength reaches 12-15N/mm².
The second stage is when the final stressing occurs and this is done when the concrete meets its design strength. The strands and bars are tensioned by stretching them with a hydraulic jack. They are then fixed in place with an anchoring component, which holds the force in them for the life of the building. Post-tensioning allows the engineer to have all the benefits of using prestressed concrete while keeping the freedom of constructing on site.
Concrete is very strong in compression and weak in tension. Steel however is very strong in tension. By combining the two materials we create a component, which is strong in both tension and compression. More benefits can be found by using the steel to place the concrete into compression. This increases it's tensile strength, which allows for longer spans and thinner sections. It also helps to stop the formation of shrinkage cracks, which in turn stops moisture passing through the material.
The more the concrete is compressed, the less likely it is that cracking will occur. Using post-tensioned concrete allows for flexible column spacing and structural depth.

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19th Century History Of Suspension Bridges

By Paul Nerrad 

Many of the most famous bridges in the world are suspension bridges. The Golden Gate Bridge in San Francisco being one of them. Since the early 19th century this type of bridge has been in existence. The defining feature is that a suspension bridge has the deck hung below suspension cables on suspenders that are vertical. The history of suspension bridges dates back to early drawings found in 1595 in the book Machinae Novae by Fauso Veranzio a Croatian engineer.

In 1801 in Pennsylvania the first suspension bridge was constructed by James Finley. This type of bridge is known as a chain bridge. In England the Dryburgh Abbey Bridge was constructed in the same fashion in 1817 with a 137 meter span, the Union Bridge was built in 1820 and the Manai Suspension Bridge in 1826. The Clifton Suspension Bridge was completed in 1864 with a 214 meter central span and is one of the longest of this type of bridge.

The wire cable suspension bridge, the Footbridge at Falls of Schuylkill, was built in 1816 following the collapse of the chain suspension bridge built by James Finley. It spanned 124 meters and has a 0.45 wide deck. The first large wire cable bridge in the United States was the Wire Bridge in Philadelphia, Pennsylvania designed by Charles Ellet, Jr. And completed in 1842 with a span of 109 meters.

The structural forces of a suspension bridge are compression in the pillars with the tension in the cables. All the force is vertically downward and is stabilized by the cables. Towers suspend the cables which hold up the deck and the weight is transferred by the cables to the towers and then to the ground.
The advantages of a suspension bridge over any other type of bridge is that it can be made of materials such as wood and wire rope. Spans can be longer than in other type of bridge and less material is required to build them. A waterway can remain unobstructed as construction can be done from above. They are better able to withstand an earthquake. Conversely they are stiffer in high winds.

An underspanned suspension bridge is where the main cable hangs under the deck and is anchored to the ground. The deck is less stable and very few bridges have been built in this manner. An example is the Pont des Bergues designed by Guillaume Henri Dufour in 1834.
In older bridges the main suspension cable was often made from linked or chain bars. In modern bridges cables are made from strands of wire giving it greater reliability. If one strand in a hundred breaks it poses less of a threat than one link or one chain which would close the entire bridge. Wire strands are easier to lift from mid-air when constructing rather than lifting heavy chains or links.

Most modern suspension bridges have an open truss structure supporting the roadbed. The Tacoma Narrows Bridge collapsed in 1940 due to its use of plate girders. However with newly developed technology plate structures are being reintroduced as they do not have the danger of vortex shedding found in the Tacoma Narrows Bridge. There are three types of forces on all bridges; the dead load, the dynamic load and the live load. The dead load is the weight of the bridge. Gravity can force a bridge to collapse. A dynamic load refers to environmental conditions such as weather, earthquakes and gusts of wind. Live loads are the traffic that moves across the bridge and the changes in temperatures.

This article was prepared by Paul Nerrad on behalf of Crafty Design www.logcabintoys.com. Crafty Design is located in Surrey, British Columbia, Canada and specializes in Canyon Suspension Bridge Kits.

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How A Bridge Takes Form Over The Course Of Construction

By Carolyne Roehm 

Modern urban bridge-building is the result of an elegant blending of art and science, and the finished product is a welcome complement to its host city's skyline in addition to being a crucial part of its infrastructure. Types of bridges will differ based on the mechanisms by which they are supported, but there are still some common steps that must take place over the course of construction, regardless of its design.
The Bridge Foundation
The first thing that has to be plotted out and positioned into place are the load-bearing pillars that will form the foundation. These can take a number of forms and sizes, but are usually comprised of a heavy, concrete base with a cage of steel bars running throughout it for additional reinforcement. Into this, the steel pillars that will form the support towers for the entire bridge are mounted. On some projects, these pillars can rise hundreds of feet into the air, and support millions of pounds of weight without buckling.
The placement of these concrete is an incredibly important aspect of construction, and can make a tremendous difference in the overall integrity of the structure. The bedrock beneath the water must be stable enough to keep from shifting over time. When crossing points are chosen for a bridge installation over a long body of water, the firmness and mineral composition of this underlying sediment is one of the factors that go into the selection process.
Since most of the mass of this initial founding will be underwater, there is a special technique for pouring the concrete into place. A steel tube is put into place that's the diameter of the support pillar, and water is pumped out of the center with electric motors. The steel cage that will act as the reinforcement is then inserted, and buried to a depth of between 30 and 60 feet. At this point, the concrete can be poured, and the foundation will be complete.
Suspension and Framework
The framing of the bridge, and the suspension cable system that will help to support its weight are typically built together, section by section. The flatbed framework that the actual road will lay on top of is too heavy to support itself, otherwise. Over the course of construction, every time a 50 to 100 foot extension of steel girders has been bolted into place, the suspension cable for that section that will permanently connect it to the closest support pillar is installed.
This process is repeated as many times as is necessary to span the distance, and sections are put into position evenly on each side of the support pillars, which allows for equalized weight distribution throughout the entire process. In this sense, the suspension bridge's skeletal structure will take form from the pillars outward, and meet at roughly the halfway point in between each one.
Paving and Utilities
At this stage, the new suspension bridge is ready to be turned into a seamlessly integrated segment of the state's roadway infrastructure, and that means adding pavement. The framework that has been previously laid provides a solid surface that the asphalt can be poured onto, and crowned in much the same way that a road on solid ground would be.
Just like a stretch of interstate highway, the bridge will require lighting. Electrical components and wiring are run throughout the hollow sections of the frame, which protects them from both the elements and from the possibility of being vandalized. This includes a particularly large set of beacon lights that will be mounted on top of the suspension towers.
Final Preparations
The bridge is essentially ready to be open to public use once all of this is completed. Ladders and elevated walkways that will allow the state's department of transportation personnel, and other engineers to access any part of the structure in order to perform periodic inspections are installed, and the mayor of the town will host an opening ceremony. A marvel of engineering is ready to serve the community.

Carolyne Roehm, a leading insurance specialist, has written numerous articles about insurance and related products. She has more than 15 years of experience. Ms. Roehm currently is a blogging expert for many insurance related websites, magazines and forums. Carolyne's most recent blog postings related to builder's risk can be read at Canon Insurance. Click here to see.

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