Bridge+Use+and+Application

__1. How the Climate Effects Bridges__

Weather can affect bridges in more ways than one; small things can lead to big changes in bridges. So here are some facts about how weather effects bridges. The sun can affect the surface coating of the bridge, such as the paint. This causes the paint to deteriorate. This process doesn’t happen immediately, this process occurs over time, it takes about 5 to 10 months for this to occur. Temperature is a factor in this process because the contracting and expanding of the bridge causes small cracks. Rain/water can affect the bridge as well, because if water gets in between the cracks it can freeze and if it’s cold enough, the water will expand and cause bigger cracks. Water can affect the exterior coatings; it can seep into bridge joints, rivets, and connectors causing them to oxidize/rust over time. Temperature change can affect the bridge by causing the bridge to expand and contrast which will weaken the structure over time. Some ask the question: “Why do bridges freeze before roads?” This occurs because bridges don’t have any protection from the cold; what I mean by this is that roads have the soil as insulation from the cold; bridges don’t have anything to protect them so they freeze quicker. Also since many bridges are above freeze points like lakes and rivers, which can make freeze time quicker. __ 2.Geography: Topography and Span __



Beam bridges are normally used to span short distances and small valleys or roadways. They are supported from below; therefore they need solid ground to secure the supports to. These bridges cannot be put in marshes or swamps because the ground is very soft. They usually not built over very steep inclines. Arch bridges are supported at the sides, so they can go over a little bit wider gaps but seeing that they don’t need supports on the bottom, they can go over very steep valleys. They only need solid ground on either end so they can be built on marshy land. The roadways of suspension bridges are supported from above by cables. They are usually built over large lakes or seas. This is because the floors of these obstacles are usually solid ground, which is needed to secure the tower that holds the cables up.

For bridging small gaps such as rivers, roadways, or small valleys you wouldn’t use suspension or arch bridges. This is mainly because of cost, but it is also in order to have a taller gap from the deck to the obstacle. In a beam bridge, that provides passage over another roadway, would be better than an arch bridge because there is a taller opening for large vehicles. You would use an arch bridge as opposed to the other two types in a situation where you can’t have supports in the middle. For example, when there is a very steep valley, it would not be very practical to use a beam bridge or a suspension bridge. This is because you can’t have supports down the middle and in the case of the suspension bridge, the gap would be too short. You would use a suspension bridge instead of an arch or beam bridge when there is a large gap. Here it wouldn’t be possible to have multiple arches or segments in a beam bridge.

Today, span of bridges are very vast and support great weights. A new span may be built next to an old one and moved sideways into place to reduce interruption of traffic.There are different bridges with different spans like the arch bridge, suspension bridge, and the beam bridge. Arch bridges are structures in which each span forms an arch. The arch bridge is one of the oldest types of bridges. The spans range up to about 1700 ft. Then the suspension bridges have the longest spans in the world and are used to cross great distances. These types of bridges are used to cross deep water channels, cannons or gorges, where construction of supporting piers can be difficult. Some suspension bridges have a main span longer then 4000 ft. A cantilever is a beam that is supported only at one end and that extends beyond its support, like a diving board. Sometimes two cantilever arms meet at mid-span. Usually they do not meet but are connected by a light suspended span.

__3. Material Usage__:

Many different materials have been used over the many years that bridges have been in use. Many primitive bridges were made out of timber and vines woven into ropes, the ancient Romans used strong natural stone to make their signature arch bridges across their vast empire. More modern day bridges can be made out of concrete or reinforced concrete. Stronger materials for modern day bridges are things like steel or iron. Here are some more in-depth explanations of the materials.

__Timber and Vines__: More primitive bridges were made out of timber and woven vines. The bridges made from these materials were mainly suspension bridges called rope bridges. These bridges wouldn’t have lasted long as the wood and the vines would rot and cause the wood to break underfoot and the vines to snap when the load moved across it. These timber and vine bridges also where not very safe as the bridge had a possibility to flip over as a load was crossing it and also weren’t very stable as the vines would twist. This material usage is fairly cheap as you only have to have a forest near for the vines and the timber. This material is useful for places where it is too expensive to make bridges out of other materials.

__Natural Stone__: Another type of material is natural stone; it is used for stronger types of bridges that will also last longer than bridges made from timber and vines. Natural stone is very strong when compression forces are acting on it making it ideal for using stone to make roman arch bridges which use compression to hold it up. Stone bridges could also be built with mortar to strengthen the bridges stability and in general to help to just hold the bridge together although not all stone bridges were made with mortar it is helpful. To make a bridge with this type of materials could be somewhat expensive (especially if made with mortar). The time it takes to gather the materials can be quite long as the quarry you would gather the stone from could be far away causing long travel times and the time it take to stack the stone in to the correct shape.

__Concrete/Reinforced Concrete__: For more modern day bridges can be made out of concrete. Concrete has a high compressive strength but it is only recommended if you are going to have a bridge with only compressive forces, as most bridges have more than just compression forces, concrete reinforced with steel bars going through it (called reinforced concrete) is better for those types of bridges. With this material the concrete will crack but the steel bars hold the concrete together and the crack will remain very small and it will cause no damage to the bridges stability.

__Iron/Steel __: Steel is used mainly for bridges that have a longer span. Many of today’s bridges are built with steel. Steel is so favorable because it can be bent into the shapes that are needed and because of just snapping when a force is applied it can bend a considerable amount before breaking. This characteristic can be useful when there are natural disasters causing the bridge to shake and move but not crumple under the forces acting on it. Some of the bridges made of steel also have a copper coating on the outside so that when the copper reacts with the outside conditions and then stop as the patina protects the other copper and the steel underneath from corrosion.