7.6 ICE FORCES 



Ice forms are classified by terms that indicate manner of formation 

 or effects produced. Usual classifications include: sheet ice, shale, 



slush, frazil ice, anchor ice, and agglomerate ice. (Striegl, 1952, 

 Zumberg and Wilson, 1953, and Peyton, 1968.) 



There are many ways ice can affect marine structures. In Alaska, 

 great care must be exercised in predicting the different ways in which 

 ice can exert forces on structures and restrict operations. Most 

 situations in which ice affects marine structures are outlined in Table 

 7-12. 



The amount of expansion of fresh water in cooling from 39°F. to 

 32°F. is 0.0132 percent. In changing from water at 32°F. to ice at 

 32°F. the amount of expansion is approximately 9.05 percent, or 685 

 times as great. A change of ice structure to denser form takes place 

 when, with a temperature lower than -8°F. , it is subjected to pressures 

 greater than about 30,000 pounds per square inch. Excessive pressure, 

 with temperatures above -8°F., causes the ice to melt. With the temper- 

 ature below -8°F. , the change to a denser form at high pressure results 

 in shrinkage which relieves pressure. Thus, the probable maximum pres- 

 sure that can be produced by water freezing in an enclosed space is 

 30,000 pounds per square inch. 



Designs for dams include allowances for ice pressures of as much 

 as 45,000 to 50,000 pounds per linear foot. The crushing strength of 

 ice is about 400 pounds per square inch. Thrust per linear foot for 

 various thicknesses of ice is about 28,800 pounds for 6 inches, 57,600 

 pounds for 12 inches, etc. Structures subject to blows from floating 

 ice should be capable of resisting 10 to 12 tons per square foot (139 

 to 167 Ibs./sq.in.) on the area exposed to the greatest thickness of 

 floating ice. 



Ice also expands when warmed from temperatures below freezing to a 

 temperature of 32°F. without melting. Assuming a lake surface free of 

 snow with an average coefficient of expansion of ice between -20°F. and 

 32°F. equaling 0.0000284, the total expansion of a sheet of ice a mile 

 long for a rise in temperature of 50°F. would be 3.75 feet. 



Normally, shore structures are subject to wave forces comparable in 

 magnitude to the maximum probable pressure that might be developed by an 

 ice sheet. As the maximum wave forces and ice thrust cannot occur at the 

 same time, usually no special allowance is made for overturning stability 

 to resist ice thrust. However, where heavy ice, either in the form of a 

 solid ice sheet or floating ice fields may occur, adequate precautions 

 must be taken to ensure that the structure is secure against sliding on 

 its base. Ice breakers may be required in sheltered water where wave 

 action does not require a heavy structure. 



Floating ice fields when driven by a strong wind or current may exert 

 great pressure on structures by piling up on them in large ice packs. 



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