A difficulty with concrete decks built over foam cores is the occasional formation of a 

 hazardous ice sheet over the decks in cold weather. HoUow concrete floats are not as 

 susceptible to this problem, probably because the trapped air conducted heat from the 

 warmer water to the deck slab and kept it above freezing temperature. The foam acts as an 

 insulator preventing this heat transfer. No low-cost solution has been found for this problem 

 that may be faced in certain regions. 



Lightweight floating docks tend to be "bouncy" and, for this reason are often rejected in 

 favor of the heavier types. One thin shelled float deliberately leaves a pocket of unfilled 

 space below the foam core. After launching, these pockets fill with water through small 

 holes punched in the bottom of the shells. The trapped water moves with the float, adding 

 measurably to its inertia without increasing the load on the supporting foam. The result is 

 less bounciness with no increase in the deadweight of the floating components before 

 launching (Fig. 77). 



g. Vertical Loading and Deck Levels. Minimum deck loading criteria for fixed structures 

 are usually specified by a building and safety agency of the area from whom a construction 

 permit must be obtained. Without specific design requirements, fixed structures built over 

 the water should be designed for a deck loading of not less than 50 pounds per square foot 

 for fingers and 100 pounds per square foot for main walks and building floors. Where 

 vehicles are to be allowed on main walks, the design loading should be increased 

 accordingly. The structural integrity of any floating system requires careful analysis to 

 ensure its capability of supporting the design loads as well as resisting windloads, currents, 

 and impact stresses. Floating sUp systems are normally designed with flotation adequate to 

 support the dead load plus a live load of 20 pounds per square foot of deck space. The 

 height at which the deck rides above the water surface under dead loading only is 

 determined to some extent by the sizes and types of boats to be berthed. The height usually 

 ranges from 15 to 20 inches, but the level selected must be constant throughout the system. 

 In this respect, gangways often add considerable extra dead load in the landing area of the 

 floating system, requiring enough additional flotation to maintain this prescribed level. 

 Some agencies specify the allowable deck level limits for dead loading plus a further 

 requirement that the system will not setfle more than 8 or 9 inches under fuU hve loading. 

 This effectively places a lower Umit on the percentage of the deck area that must have 

 flotation. Figure 78 shows the minimum percentage of deck area requiring flotation for 

 various minimum freeboard requirements with various dead and Uve loads and the resulting 

 submergence of the floats under the assumptions noted, provided the floats are square cut 

 (vertical sides). 



If the design dead load deck level is other than 18 inches, subtract the number of inches 

 the design deck lies above 18 inches from (or add the number of inches the design deck Ues 

 below 18 inches to) the required minimum ^(j)+i,\ and enter the left side of the graph at 

 that point (Fig. 78). The correct values of P, Sj) and ?>(j)+i) will then be given by the 

 procedure shown in the following example problems: 



128 



