3,060 to 6,020 pounds per square inch. Original form marks were still visible and edges were 

 stiU sharp. Three blocks had gained strength since 1932, two had remained the same, and 

 one had decreased slightly. 



Although prestressed concrete construction has been used successfully in many other 

 types of marine structures, it probably has not been used for pier decks in small-craft 

 harbors. One reason is the high cost of special prestressing beds needed for this construction. 

 If two standard width precast deck sections could be agreed upon by the construction 

 industry, one for headwalks and one for fingers, the sections could be turned out in long 

 mass-produced beds at prices competitive with other systems, especially with the use of 

 Ughtweight aggregates. A sketch of the section and details of possible seating arrangements 

 on pile supports is shown in Figure 71. 



One type of construction that uses a combination of concrete and timber in a fixed-pier 

 structure is illustrated in Figure 72. The concrete pUes (when properly treated) have a 

 reasonably long life. They can be readily replaced when worn or damaged, either piecemeal 

 or in their entirety. A timber superstructure is desirable because of the ease with which 

 attachments may be made after final construction. 



The mainwalks and firmer piers of a small-craft harbor generally have ho handrails. Public 

 safety has sometimes been an issue, but the prevailing logic seems to be that the danger of a 

 person falHng is not sufficiently great to justify the cost and inconvenience of handrails on 

 either fixed or floating dockage systems. Where small children are likely to venture out on 

 the docks unattended, locked gates can be installed at the basin perimeter or on the 

 approach pier leading from the perimeter (Fig. 73). If a fixed pier is too high above the 

 water surface during low water levels to constitute a hazard, a floating system is required. 

 This is important for easier boarding of boats. 



/. Floating Pier Structures. Where water levels do not fluctuate more than 2 feet, the 

 berthing docks and slips are almost universally of fixed construction. If the normal range is 

 between 2 and 5 feet, the use of a floating system is optional. Where water levels fluctuate 

 more than 5 feet, a floating system is mandatory. The cost of a floating system is usually 

 greater than a fixed system, but the difficulty in keeping boats properly tied and the 

 inconvenience of boarding or leaving boats during extreme low water often justify the 

 choice of a floating system. 



A successful floating dock system has the best possible combination of flotation units 

 and structural system. Numerous floats have been used, and most are described in Chancy 

 (1961), and American Society of Civil Engineers (1969). The most successful are the foams, 

 such as extruded polystyrene (Styrofoam), expanded-pellet polystyrene, and foamed 

 polyurethane. Although foam floats have been used extensively without any surface 

 protection, they attract marine growth and living organism. Large accumulations of aquatic 

 plants on the floats can be unsightly, and may foul propellers. Also, birds and sea animals 

 have been reported to tear out pieces of foam while seeking marine life burrowed on the 

 underside of the foam surface. For these reasons some external protection is now usually 



120 



