of large penetrations or out-of-roundness of cylinder hulls. These para- 

 meters will significantly influence the behavior of the structure. Other 

 parameters that need to be considered are the effect of vertical and 

 horizontal stiffeners in concrete shells and the effect of the pressure 

 gradient between the bottom and top of cylinder structures. Horizontally 

 oriented structures have the problem of being loaded into an out-of-round 

 shape. For vertically oriented structures, the end-condition effects and the 

 variable hydrostatic pressure load along the structure length complicate the 

 definition of the critical section. 



Design for Long-Term Loading . The ability to design for long-term 

 loading of pressure-resistant structures is marginally available. Con- 

 servative estimates of the maximum stress level to which concrete struc- 

 tures can be safely loaded can be made from studies on concrete column 

 members and from concrete spherical structures placed in the deep ocean. 

 A test on 18 spherical structures with 66-inch outside diameter and 

 4-inch wall thickness is still in progress after 4 years of exposure to 

 the hydrospace environment at depths ranging from 2,000 to 5,000 feet.L* 3 J 

 This study is producing results that have direct application to OTEC 

 structures. 



The ability to predict strength changes and creep behavior of con- 

 crete over long periods of exposure to the ocean environment is not well- 

 established. This deficient technological area was discussed above in 

 the Materials section. 



Design for Fatigue . The fatigue behavior of reinforced and prestressed 

 concrete in an ocean environment is not well-known. Throughout the life of 



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