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CHAPTER VII 



MARINE STRUCTURES 



The following discussion considers specific problems of the design of marine sub- 

 structures of simple types, such as wharves and piers, with particular reference to the 

 use of timber as affected by marine borers, and to the use of substitutes for timber, 

 such as reinforced concrete. While the recommended practice applies to all marine 

 structures, no attempt is made to cover large special projects, such as bridge piers, 

 which must by the nature of their requirements be built of materials immune to 

 borer attack. Structural theory is considered only insofar as it affects the particular 

 phases of the subject under discussion. 



Economic Life 



One of the most important phases entering into the design of general marine 

 structures is the determination of probable economic life. The economic life of a 

 structure is here defined as the duration of the predominant need for the facilities 

 furnished by that structure, on that site, without essential structural alteration or 

 replacement by a structure of different design. Substructure materials cannot be 

 judged and selected properly unless this element is considered and understood. The 

 actual life of a structure is apt to be neither the economic life nor the (potential) 

 physical life, when these do not coincide, but a compromise, whose position between 

 the two is determined by the relative weight of such factors as unrealized physical 

 life, and replacement or reconstruction cost, against difference in service value or 

 earning power between the old structure and one better designed to serve a changed 

 or changing economic need. Obviously, the most desirable consummation is that 

 economic life and physical life be as nearly coincident as possible. 



A majority of the harbors in the United States for example, are in the course of 

 development, changes are constantly taking place and will doubtless continue to take 

 place because of new and unforeseen conditions. Even if future conditions could be 

 foreseen it would in most cases be impossible at the present to finance and create the 

 higher degree of development of the future. Improvement must be gradually acquired; 

 changes must be expected to continue; and with such change there will usually be a 

 fairly definite limit to the economic life of structures. In many European harbors, on 

 the other hand, facilities are relatively fixed and permanent and changes are made with 

 relative difticulty and at great expense. Permanent construction is desirable, inasmuch 

 as it reduces maintenance; but permanency of a substructure material is not necessarily 

 the most significant measure of its value. Construction of high physical permanence 

 should not be used for structures of limited economic life — which justifies the use of 

 materials having a correspondingly limited physical life. 



In this connection the factor of obsolescence is of considerable importance. Both 

 the continual improvement in ships and methods of handling cargoes by sea and the 

 great increases in rolling loads in land transportation, which often occur within the 

 space of a few years, as well as changes in the character and volume of commerce, 

 unite to render obsolescent existing structures, and to require changes in general 

 harbor facilities, where the two kinds of transportation meet. Where harbors can 

 advantageously provide for shipping by building quays along the shore line, ships can 



