II. MATERIAL REQUIREMENTS FOR COASTAL STRUCTURES 



1 . General . 



Primary considerations in the selection of a material for coastal 

 structures are availability, strength, durability, life of the material as 

 compared to desired life of the structure, costs, and ease of maintenance 

 as compared to maintenance costs. The selection of materials for coastal 

 use must also consider the structural stability and flexibility. Also, 

 depending on location, the impact of the environment on the material may be 

 a determining factor. 



2. Structural Properties . 



a. Specific Gravity . This property must be considered if the material 

 is to be placed in the water, on the ocean bottom, or on dryland, or is to 

 be used as a floating structure. Rock and earth as well as concrete of 

 high-specific gravity are a must for submerged structures, while floating 

 structures must be of low-specific gravity materials such as wood and most 

 synthetics. Heavy materials are used for floating structures only if the 

 design provides for adequate buoyancy. 



b. Material Strength . Material strength in tension, compression, and 

 flexure may determine the size and stability of a structure. Most metals 

 are high in tensile strength while unreinforced concrete is low in tensile 

 but high in compressive strength. Rock and soils may be of high strength 

 as individual pieces or particles but may require high strength adhesives 

 to create an acceptable coastal structure. 



c. Resistance to Cyclical and Impact Loading . Resistance to cyclical 

 and impact loading, such as waves or coastal storm conditions, may require 

 consideration of material flexibility within its elastic limit or the 

 flexible limits of an adhesive used in the structure. For example, cement 

 used in making concrete has virtually no flexibility while asphalt as a 

 binder can be quite flexible. 



d. Resistance to Seismic Forces . These forces, which can be both 

 horizontal and vertical, may result in excessive structural stresses. 

 Seismic forces may require a structure to be stiff and rigid, such as 

 concrete or steel, or constructed of nonhomogeneous materials such that 

 seismic forces can be relieved in planned isolated areas of the structure. 



e. Material Flexibility . This property includes the ability to bend 

 without breaking or to be adaptable to change in configuration. Wood has 

 some degree of flexibility; rubber and certain synthetic materials have 

 flexibility to a high degree. While structural steel is usually considered 

 a stiff material, steel shapes such as cable, wire rope and rods are highly 

 flexible. 



f . Structural Size. The structural size may determine the material of 

 the structure . Large structures such as breakwaters are usually composed 

 of many pieces of one or more materials that may not be bound together to 

 create a homogeneous mass, or composed of sections of the same material 



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