Types of Bridges
There are many types of bridges and the key structural components of bridges are the beam, the arch, the truss, and the suspension bridges. The beam bridge and the truss bridge were introduced above, the arch bridge will be discussed using the example of a stone bridge, and suspension bridges employ the use of cables and are more commonly referred to as cable bridges.
I would like to focus on four bridges. Three of which are material bridges and the fourth a design bridge. The timber bridge, the stone bridge, and the steel highway bridge are material bridges and the drawbridge is a bridge of design.
There are many considerations to make when designing a bridge. First of all is function. Is the bridge needed for travelers to cross water using vehicles? Will boats need to be accommodated if the bridge is over water? Is a foot bridge needed for people to walk over a road from a parking garage to a mall or business plaza? And then there is location. Is the bridge to be built in a small city or a crowded one? Is the bridge along the coast or in the mountains? Location and function will determined the materials used. Determining the materials is important because of maintenance and the anticipated duration of the bridge. If the load (heaviness of traffic) is great then perhaps a concrete bridge would be a better selection than a timber bridge to meet the needs and its function. The size of bridge, past practices, and new ideas interplay. New technology and new products are continuously tested, manipulated, and tried. But, in the mind of all the planning is cost and public opinion. Without the public's approval a bridge may never be built. Likewise, if there are not enough resources a bridge can be put on hold or an inexpensive one will take its place. In today's world, we also have a new consideration which is emerging as a primary concern and that is of the environment. How the bridge and the materials used affect the environment is emerging as a significant concern in an ever changing climate, a more polluted and congested world, and the diminishing of useable space.
The most important consideration for timber bridges is wood preservatives to keep the wood from rotting. The preservative must be compatible with the wood used be it a softwood or a hardwood. A softwood may be harder than a hardwood. Softwoods and hardwoods are classified by their leaves or needles. Softwoods have needles and hardwoods leaves. The Forest Products Laboratory under the U.S. Department of Agriculture is continuously doing research on how wood performs and addressing the question of how wood will stand up to natural disasters. We are familiar with sawn lumber processed in saw mills. But, since the mid-1940s, the U.S. has been using glue-laminated timber called glulum which can be made from either hard or soft woods just as the sawn lumber. One of the advantages is that glulum timber can be produced in a variety of shapes and sizes. We are familiar with it in furniture that is made from processed woods.
Forty to fifty years ago timber bridges were common. But, with heavier traffic and greater volumes of traffic more durable materials and design are more desirable and trusted. Engineers site that timber is short-lived due to rotting, has high maintenance due to inspection and preservation, and is not strong enough for today's needs.
Solutions to engineers concerns have been proposed in 1989 by the Timber Bridge Initiative (TBI). The initiative provided for a training program to promote the benefits of the "modern" timber bridge. Solutions include using composites of steel and wood to reduce bridge movement, water-shedding joints and water proofing to address deterioration, and a stressed-type timber to reduce water movement between boards.
All in all, it still seems that the acceptable places for timber bridges today are in rural areas where engineers are unnecessary. These bridges would be bridges for light traffic, foot bridges, railroads, and the esthetic covered bridges, and, they would be regulated by the county instead.
When we think of a draw bridge we envision a castle with a moat and the story that went along with it. And, this is usually our first encounter with a drawbridge. It is called the bascule. We have come a long way since then primarily because our needs have changed, and, so, the function of the draw bridge. We no longer need to keep people out but to help them get across water. This would not be so complicated except we must also let ships and boats pass under it.
There are several types of draw bridges. One is the vertical lift span that that works like an elevator. The span (a section of road) rises and then lowers. Another is the retractable span that slides back and forth. A section of road moves under the bridge to let ships pass and then slides back. A third draw bridge is the swing bridge. This bridge rotates horizontally on vertical pivots and swings to the side.
However necessary, a drawbridge is costly both to operate and to maintain its moving parts. Its one drawback is that it stops the flow of traffic for a time or for more time than people care to wait.
Early bridges were made of stone. The process is called stone masonry. The advantage of stone bridges over timber bridges, in their time, was that they can bear heavy loads. And, when stone bridges were formed in an arch they could possibly bear unlimited weight. This was desirable when legions of armies marched over stone bridges in the Roman era or when tanks traversed over stone bridges in more modern times.
Stone bridges are more expensive to construct than the timber bridges and were built when the needs and the function of the bridge met. Brownstone, granite, brick, or field stone (stones found when clearing out land) were used. Design, esthetics, and cost dictated the type of stone used and how elaborate of a design the arch would be.
The only maintenance of stone bridges would be due to cracking. And, if neglected over time dirt would be allowed to settle in and the bridge could sprout growth in the form of moss, plants, bushes, or even trees.
In comparison to these smaller bridges, the steel bridge highway can be a massive project that can take a team of engineers to design it and another team of engineers to oversee its construction. More attention is given to weather conditions and the earth in its construction. Wind velocities during construction are a concern. The bridge must be stable at each point of construction. Expected wind velocities must be determined for the strength of the bridge to hold in place and not sway or crumble under the pressure or gusts of wind especially at shore points where wind can be brutal. Perhaps, not much attention was spent on natural disasters as hurricanes, tornados, earthquakes, or tsunamis years ago when the likelihood of such events could not materialize. But, today, it is a different story. Natural disasters are occurring in areas unexpectently. Determining design, materials, and the likelihood of natural disasters are becoming crucial to new bridge design.
The earth presents its own problems. The earth gives off its own pressures and surcharges. Studies of type or types of soil present in the area where the bridge is to be constructed dictates equipment and materials used. It also determines if support equipment will be needed during excavation. The condition of the soil may require the use of lagging walls and/or piles to support the construction of bridge walls. Code and specifications are more numerous and may be more complicated. All engineers must be in constant communication so that design and construction needs are met.