Bridge Basics Part 2

     A bridge is basically a structure lying horizontally between two supports. Its function is to carry vertical loads. This is a simple definition of a bridge. A definition of a railroad bridge is that the load is a train.

     Seems quite simple, but the engineering process that goes into the design of each bridge must take into effect the two end supports or abutments. Whether they are strong enough to support the structure. Also, if the span between each of the abutments is strong enough to carry the loads going across it each day. These spans cannot be too long, or they will be unable to support the loads placed upon it. If the supports are strong enough to hold up the entire structure.

     Strength is used various times in the description of the railroad bridge. This factor is of extreme importance due to today’s diesel locomotive by itself weighing 100 to 200 tons. The average is 14,000 feet or 2.652 miles, with 682 wagons being pulled by eight 6000 horsepower diesel-electric locomotives controlled by a single driver.               

Train on the Montana Prairie

From the Hughes Collection

 

    

     There are basically two categories of bridges. They are a standard overpass bridges or unique-design bridges which are built over rivers, chasms or estuaries. There are also six basic forms in bridge design. They are beam, truss, arch, suspension, cantilever and cable-stay.

     The beam bridge is the simplest and most common bridge design. ¹

 

Beam

Union Pacific Railroad Bridge at Multnomah Falls

Khamar, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>,

via Wikimedia Commons

 

 

     The beam is not only the simplest, but the cheapest and most used of the bridge designs. Mainly used for short distances. Held up by the abutments or the vertical supports on each end of the beam.

     The beam or deck carries the load by bending. It’s like placing a plank between two chairs which act as the abutments. As you stand in the middle of the beam it flexes downward. When it bends downward bending which causes a horizontal compression on the top of the beam while it causes tension on the bottom. The load of the beam is then transferred to the abutments at both ends. This transfers the load on the bridge to the abutments. This is why the abutments or foundation of the bridge is so important to seek out and ensure it can hold the transfer of load. ²

     One of the most straight forward and easiest to construct. It has a single design and cost the least. Highly versatile with the convivence of being able to assemble the bridge module at a separate location.

     Some of the disadvantages are that the life of the bridge is limited to the supporting structure. After some time, the bridge will start sagging. Plus, the increasing cost of steel will affect the price of the bridge.

     Maintenance and the required painting to keep it from rusting can be tedious and time consuming.

 

Pawtuxet River Railroad Bridge, New Haven Railroad over Pawtuxet River

and Wellington Avenue, Cranston, Providence County, RI

Brewster, Robert, creator, Public domain, via Wikimedia Commons

 

Truss

 

      The second form is truss bridge. Pictured above is a single-span truss bridge. This bridge is simply a supported beam bridge. It carries loads by bending just as the beam bridge does. The advantage of the single-span truss bridge is it takes a small amount of material to be able to handle large loads.¹ The system of diagonal and triangle bars on the sides to reinforce it. They are called trusses.      

     Like the beam bridge, the load is transferred to the abutments by the compression of the beam. ²

     One of the biggest advantages of a truss designed bridge is that the engineer can construct the truss in horizontal and vertical directions, so it easily absorbs the compression and the tension. This helps compensate for the winds and the extreme weather conditions.

     Other advantages are they are easy to build, sturdier and tighter. They also are cost effective being versatile so as to move the truss and the lower deck to strengthen it or change it or change the design for the extreme weather.

    The main disadvantage of the truss bridge is with passing times the other truss bridges can only be used for light traffic. They need to be constructed for even weight distribution.

     They take up a lot of space and need the large interconnecting triangle framing that can handle heavy loads. ³

Detail of Arch Columns and Piers Buffalo Creek Bridge

Nall, Jim, creator, Public Domain, via Wikimedia Commons

 

Arch

 

     The arch bridge carries load basically by compression also. But, with the arch design there are vertical and horizontal forces at play. The arch design helps prevent vertical settling and horizontal sliding.

     The arch bridge also looks like it takes more material but in fact it takes less material than the beam bridge of the same span. ¹

     A bridge deck rests upon the arch. The load pushes down upon the metal components which makes the bridge stronger. If the arch is to wide, there is more tension and it also would be nothing less than a beam bridge.

Stone arch railroad bridge, South Keene

Keene Public Library and the Historical Society of Cheshire County

from USA, No restrictions, via Wikimedia Commons

 

     With a stone arch bridge, the load is transmitted to the center stone which is often called the keystone. It is then transmitted around the curve of the rest of the stones onto the abutments need to be rater large to support the arch bridge.

     The abutments also, when the load is transmitted to them push back upward and inward. They are reasonably simple and cheap to construct. ²

     The arch bridge can be adapted to the environment conditions. The resistance and strength of the bridge increases the life span, making it comparable to the beam bridge. There are various materials that can be used to construct it such as brick, stone, steel, plus other materials.

      The disadvantages to the arch bridge is that maintenance and construction is time consuming. You need more arches depending on the length of the bridge. For a longer life span more support is needed.

 

Suspension

 

     The forces of tension and compression on a suspension bridge are all vertical. The force, i.e. the vertical load is transferred through curved cables.

     The load or force of vertical compression is transferred to the towers. Through the tower the load, through vertical compression is absorbed into the ground and also into the anchors. The anchors also have an important job of resisting the inward and sometimes vertical pull of the cables.

 

 

Railway Suspension Bridge, Niagara Falls, U.S.A

Scan by NYPL, Public domain, via Wikimedia Commons

 

     A suspension bridge is basically an upside-down arch which is in tension with only the towers, in compression.

     The suspension bridge is different than the others we have looked at so far. It is suspended in the air by the cables. The dec needs to be heavy or stiff to help the engineers ensure the deck does not move excessively under load. ¹

Suspension bridges have been around for a while.  Prior to steel they used ropes for the cables along with wooden planks for a deck.

     Today, galvanized steel cables are used for the supporting cables are used for the supporting cables.

     The advantages of a suspension bridge is nit longer life span. Theybare pleasing to look at, almost art. Plus they can be used to going across rivers, due to their piers are not being sunk in the riverbed so the waterways do not disturb it.

     With the flexible design any of its deck sections can be replaced easily.

     The disadvantages are that high winds cause it to vibrate and swing bac and forth. This affects the life of the bridge and they struggle to support heavier load such as trains.

     Maintenance is difficult plus if only one cable gets damaged the bridge may collapse. ³

 

Cantilever

 

     The Cantilever Truss Bridge is like a suspension bridge, they generally carry heavy loads across water.

     When constructing the bridge, caissons are built first on the land and then sunk. They are a watertight structure which is used in support of the bridges abutments and towers.

     After the are sunk, the towers and anchorages are built upon them. ¹

Line art drawing of caisson.

Pearson Scott Foresman, Public domain, via Wikimedia Commons

 

     The outer spans are like diving boards. The beams are anchored down at the end. So, each cantilever are extending from each pier or abutment towards the middle. Ten the center section, short beam bridge rests upon the pair of cantilever arms.

     The load or forces ae carried by the cantilevers to the foundations by tension forces by the lower chords and the compression forces by the upper chords.

     The whole load of the bridge is centered on the suspended center span. The abutment cts as the anchor system for the beams of the cantilever bride.

 

 

A diagram of the parts of a Cantilever Truss Bridge

PennySpender1983 (talk).PennySpender1983 at en.wikipedia.Later version(s) were uploaded by User A1 at en.wikipedia., Public domain, via Wikimedia Commons

 

 

    The advantages of the cantilever bridge is it’s long life. They are the go-to bridge for areas that are flood prone and in deep rocky valleys where it is difficult to construct supporting structures.

     The disadvantages are they are extremely labor-intensive work, with a need for heavy structure. Also, maintenance is difficult.

     They are best not to be constructed in earthquake prone areas.

 

 

 

Cable Stay

 

    The cable stay Bridge has some advantages over the regular suspension bridge.1                                      

    A suspension bridge needs to be firmly anchored to the ground on both ends. This takes a lot of room and is not always possible. Due to room for the cables or appropriate bedrock to anchor them too.

     The cable stay bridge does away with this probably by needing only very strong suspension cables to hang the deck from.

     The cables are spread out diagonally from each pier to the bridge’s deck. This tends to make the bridge stronger, but bulkier than the suspension bridges. ²

     The advantages of the cable stay bridge is that it is constructed faster than most other bridges. They are also more stable with a rigid structure. They are also more cost effective.

     The disadvantages are they are ideal for only average distances. They have a high fatigue rate. Along with not being ideal for earthquake prone areas and bad climate conditions. ³