Figure 17. Forces on pipe-tire unit, 



In this case the pipe is 12.2 meters long (41-centimeter outside diameter 

 and 70.2-kilogram-per-meter weight in air), provides a net lift of 59.5 kilo- 

 grams per meter when totally submerged, and supports 49 truck tires. Truck 

 tires have a specific gravity of approximately 1.2 with a weight of W ta = 41 

 kilograms in air for the sizes predominantly used (i.e., 10.00-20 and 9.00-18 

 truck tires). Submerged in water this weight is reduced to approximately one- 

 sixth of W ta , or 6.8 kilograms if all air is expelled. Applying these val- 

 ues to case A (which corresponds to F = F fl = and approximately three-fourths 

 of tire material submerged) and using equation (1), it follows that the extra- 

 neous load is a small lift force of 26 kilograms, (i.e., F £ = -26 kilograms). 

 When the external load F is applied (case B) , the buoyancy force resulting 

 from air entrapped in each tire may be calculated from equation (1) to be: 



10.7(196) + 49(0 + 6.8) + (-26) = 12.2(59.5) + 49F, 



F = 34.2 kilograms per tire 



On an average, this implies that 34 liters of air is trapped in the crown 

 of each tire. It is not known at what rate this trapped air would escape 

 under static conditions; during wave action the tire crown would be alter- 

 nately vented and replenished with air. In determining the flotation require- 

 ments for the complete structure, the weight of suspended sediments that may 

 accumulate in the tire casings as well as the influence of marine growth 

 should be considered. 



b. Equilibrium of Breakwater . The load-carrying capacity of the break- 

 water must be carefully considered, particularly in areas where the weight of 

 the breakwater is likely to increase substantially with time due to deposition 

 of suspended sediments within the tire casings, biofouling, etc. In extreme 

 cases, all the tires may have to be foamed to provide adequate reserve buoy- 

 ancy, whereas at other sites the lift provided by the steel-pipe flotation 



21 



