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How Bridges Are Built Over Water?

Bridges are marvels of engineering that stand inconspicuously amongst us. We don’t think of them much even when we are passing over them. Nowhere are these structures more impressive then when the are built over water, which brings us to the question how are bridges built over water?

When the water is shallow, construction is easy. A temporary foundation is made on which piers are built to support the upper structure and the bridge is then built! It’s when the water is deep that other techniques are needed.

There are many methods to complete such as task in deep water but here we will explore the main three. These three methods of bridge building are called battered piles, cofferdams and caissons.

Battered Piles:

These are poles that are driven into the soil underneath the water. Piles are hammered into the water until the turn outward or inward at an angle. This makes the piles firm and increases their ability to carry lateral loads.

Piles are inserted in the ground using pile drivers. These are mechanical devices that may be transported to a location on a floating pile driving plant.

Battered Piles at a bridge project in Sweden

Pile drivers may also be cantilevered out over the water from piles that have been installed in advance. With the use of pile frames, pile hammers and winches, pile drivers hammer the piles into the soil until the turn outward or inward at an angle. The pile are now ready to carry lateral loads and can provide the foundation of support for the bridge.

The next step is to construct the pile caps above the piles. Once this is done, the bridge is ready to be built.

Cofferdams:

These are temporary enclosures made be driving sheet piling into the bed of a body of water to form a watertight fence. This is called the cofferdam. There is more to this bridge technique. Once the sheet piles have been inserted in the water to create a cofferdam, the water is pumped out of the enclosure.

Now, the construction workers can built the bridge as if the are working on dry land. The process then becomes relatively easy.

Cofferdam

Caissons:

There are two types of caissons, open and pneumatic.

An open caisson is a structure that is usually shaped like a box. It is open, at the top and bottom. The caisson is usually constructed on land then floated into position and sunk, so that the upper edge is above water level.

The caisson has a cutting bottom edge so that it sinks through soft silt on the bed. Inside is a series of large pipes or dredging wells. These are used to dredge up the bed material. As more material is dredged up, the caisson sinks and more sections are added to the shaft to keep it above water.

Once the caisson reaches the correct depth, concrete is laid to seal the bottom and then more concrete is poured into the caisson to form a solid post.

Steel Open Caisson

A pneumatic caisson is similar to an open caisson but it has an airtight bulkhead above the bottom edge. This is fitted with air locks. The space between the cutting edge and the bulkhead is called the working chamber. In this space, the water is removed using air pressure. Construction workers can then enter the chamber and excavate the soil.

It is important that the air pressure in the chamber be carefully monitored so the workers do not get the bends.

Pneumatic Caisson

But how do engineers pick which technique to use?

This all depends on the condition of the site and the technology available. These are important decisions to make that only exports can fully handle.

Cofferdams Powerpoint Presentation

Cofferdams are temporary enclosures to keep out water and soil so as to permit dewatering and construction of the permanent facility (structure) in the dry.

A cofferdam involves the interaction of the structure, soil, and water. The loads imposed include the hydrostatic forces of the water, as well as the dynamic forces due to currents and waves.

Because cofferdams are typically constructed under adverse conditions in a marine environment, and because significant deformations of elements may occur at various stages of construction, it is difficult to maintain close tolerances. Ample provisions must be made for deviations in dimensions so that the finished structure may be constructed according to plan.

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The different types of temporary structures

During construction, temporary structures are required. Let us assume a simple task of painting a building. How could workers go up to the upper levels to paint? Typically, workers stand on a temporary structure known as a scaffold (Fig.1).

Fig.1. The painting of a building. A scaffold is built for painters to stand on while painting the building

1. SCAFFOLDS

Scaffolds are work platforms that enable workers to do their job at high elevations. The type of work can be brickwork, painting, steel work, concreting, or window installation. Most scaffolds are made of steel pipes. In some countries bamboo is stillused for scaffolds.

1.1 Pipe Scaffolds

Pipes are used to build scaffolds. Pipes are connected using special connectors. Platforms are provided for workers to stand on. Ladders are provided for workers to go from one platform to another one (Figs.2 and 3). Pipe scaffolds cannot be used for very tall buildings. Other methods such as outrigger scaffolds are used in such situations.

Fig.2. Pipe scaffold with one platform

Fig.3. Pipe scaffold with multiple platforms. Ladders are provided to climb to higher platforms

1.2 Outrigger Scaffolds

Consider this scenario: Brickwork has to be done on the 70th story of a 70-story building. How could the workers get to the 70th story? Should they build a pipe scaffold from the ground all the way to the 70th story? That may not be very feasible. In this type of situation, outrigger scaffolds can be used. Metal beams are attached to the building. These beams are used to build a work platform (Fig.4).

In the case of outrigger scaffolds, metal beams or a metal structure are attached to the newly constructed building. This has to be done with the authorization of design engineers. This metal structure is known as outriggers. The protruding metal structure is used to build a work platform.

Fig.4. Outrigger scaffold

1.3 Modular Scaffolds

Pre-made modules are becoming common in many construction projects (Figs.5 and 6).

Fig.5. Scaffolding modules

Fig.6. Scaffolding modules are fitted together to reach high elevations

Boards: Boards are made of metal or wooden planks attached to the scaffolding for people to stand and work. Uprights also known as standards and poles are used to carry the load to base. False uprights are mainly used near entrances to the work platform. False uprights do not transfer any vertical loads to the ground. Though it may provide lateral support handrails, it does not provide any lateral supports to the scaffold system.

2. SHORING

Scaffolds are built for workers to work. Scaffolds act as work platforms for workers. On the other hand, shoring is done to support wet concrete. Once the concrete is hardened, the shoring is removed. Other than supporting wet concrete, shoring can be used to support weak columns.Let us assume that an existing column in a building is deteriorated and has to be replaced. The procedure to remove an existing column and build a new column is shown in Fig.7.

Fig.7. Provide shoring prior to removal of the column

Once proper shoring is provided and has been approved by the relevant authorities, the contractor can remove the existing load bearing column and construct a new one.

3. BRACING

Bracing is a support element provided to strengthen an existing structural element (Figs.8 and 9).

Fig.8. Deteriorated element in a structure

Fig.9. Bracing is provided for the deteriorated element

3.1 Bracing Masonry Walls

Masonry walls need to be braced during and after construction. All masonry walls have to be supported laterally. In a building, masonry walls are tied to beams, columns, and other walls.

Until the masonry wall is laterally supported, it has to be braced. In practice, bracing of masonry walls has to follow OSHA guidelines. For the exam purposes, NCEES recommends “Standard Practice for Bracing Masonry Walls Under Construction” by Mason Contractors Association of America (MCAA).

Hence, you need to know both OSHA and MCAA guidelines (Fig.10). Masonry walls are made of bricks and mortar. Until a mortar brick joint is fully developed, masonry walls have little lateral stability.

Even after the mortar is hardened, a standing masonry wall has little resistance against over-turning. As per OSHA, any wall 8 ft or taller needs to be braced.

Fig.10. Schematic diagram of a masonry wall bracing. Bracing is required tomaintain the lateral stability of a masonry wall

4. COFFERDAMS

There are instances where construction has to take place near a river, lake, or ocean. Bridge piers, harbor structures, and flood control structures are some examples. In such situations water has to be kept away from the construction area.

How can you concrete when water is pouring in? A structure has to be built to keep the water away. In such situations a temporary structure needs to be constructed to keep the water away. These temporary structures are known as cofferdams.

Cofferdams are temporary structures constructed to keep water out ofthe construction area. The majority of cofferdams are constructed in riversmainly to build bridge piers. Thanks to improvement in caisson technology, in most cases cofferdams may not be necessary anymore.