10/20/05
      

The arch is one of the simplest ways to describe 2-dimensional compression.  There are many different types of materials that can be used to create an arch, but certain materials will work much better than others. 

An arch works by transferring the load that it is supporting down into the columns and foundation.  The arch is constantly in compression because of its curved shape, and the load above the arch help keep the arch together.  There is large compressive forces acting on the arch itself and therefore it needs to be made up of a material that has a very high compressive strength. The arch is only in compression, and there is no tensile force acting on the arch.  This allows the arch to contain a material that does not necessarily have a tensile strength.   

            There are a few materials that have a very high compressive strength that work very well for these applications.  Steel, Concrete and wood are the main materials that are used in arches.  Historically, arches were created by stacking stones together in the form of an arch, and using the compression against each other to support the arch and keep the stones from falling.   

            More practical uses, such as bridge design, require a material that is going to be much more stable under dynamic and live loading.  Steel is a very good material to use for this.  It has a very high compressive strength and can be constructed without the use of formwork.  Concrete, another material used for arches, requires much more time due to the amount of framework that is involved in creating the arch.  Steel arches can be built on the ground and then raised into position by the use of large cranes.  Steel is also very ductile, which makes it a safe choice for seismic zones and areas that have very high wind loads. 

 Bayonne Bridge

           Another practical material with a high compressive strength is concrete.  Concrete has a very low tensile strength, but extremely high compressive strength.  This is well suited for an arch because they are solely in compression.  Concrete can also withstand seismic activity to an extent.  Another advantage to concrete is that there is no need for fireproofing on top of the concrete to meet fire codes.  This can be helpful when designing a large cathedral with a concrete barrel vault.  Such a large area could require a large amount of fireproofing, which could be costly.  Cast in place concrete can be more expensive than some materials, but that is only on the front end.  The use of concrete in structures can save money from an insurance standpoint.  Insurance will drop for the use of concrete in a structure due to increased safety and structural integrity, which will  help save money in the long run. 

Concrete Bridge Structure

Brick is also another material that can be used for creating arches.  It also has a high compressive strength, but is rather inefficient when trying to create a large structure.  The amount of time that it would take to create a brick archway of any massive size, would be very expensive in labor hours. 

Brick Bridge

            The following table shows different span lengths and span/depth ratio’s for certain materials.  From this table, one can see that steel is a better material to use for spanning very large areas.  This can be taken into consideration for large bridges that have to span a very wide river or area. 

             Concrete has a better span/depth ratio which than does steel.  This can be one of the main reasons that early cathedrals with barrel vaults were made of masonry.  The concrete can span areas that are more than large enough for a cathedral, and it is still able to run the length of a cathedral without have supports holding it up in the middle. 

Architectural GRAPHIC Standards CD-ROM
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