is reduced. During the winters of 1951-52, it was estimated that ice caused a 

 reduction in rate of impoundment of 40 to 50 percent at the Fort Sheridan, 

 Illinois, groin system. 



Table 7-14. Effects of ice on marine structures . 



A. Direct Results of Ice Forces on Structures. 



1. Horizontal forces. 



a. Crushing ice failure of laterally moving floating ice sheets. 



b. Bending ice failure of laterally moving floating ice sheets. 



c. Impact by large floating ice masses. 



d. Plucking forces against riprap. 



2. Vertical forces. 



a. Weight at low tide of ice frozen to structural elements. 



b. Buoyant uplift at high tide of ice masses frozen to structural 

 elements. 



c. Vertical component of ice sheet bending failure introduced by ice 

 breakers. 



d. Diaphragm bending forces during water level change of ice sheets 

 frozen to structural elements. 



e. Forces created because of superstructure icing by ice spray. 



3. Second-order effects. 



a. Motion during thaw of ice frozen to structural elements. 



b. Expansion of entrapped water within structural elements. 



c. Jamming of rubble between structural framing members. 



B. Indirect Results of Ice Forces on Structures. 



1. Impingement of floating ice sheets on moored ships. 



2. Impact forces by ships during docking which are larger than might 

 normally be expected. 



3. Abrasion and subsequent corrosion of structural elements. 



C. Low-Risk but Catastrophic Considerations. 



1. Collision by a ship caught in fast-moving, ice-covered waters. 



2. Collision by extraordinarily large ice masses of very low probability 

 of occurrence. 



D. Operational Considerations. 



1. Problems of serving offshore facilities in ice-covered waters. 



2. Unusual crane loads. 



3. Difficulty in maneuvering work boats in ice-covered waters. 



4. Limits of ice cover severity during which ships can be moored to 

 docks. 



5. Ship handling characteristics in turning basins and while docking and 

 undocking. 



6. The extreme variability of ice conditions from year to year. 



7. The necessity of developing an ice operations manual to outline the 

 operational limits for preventing the overstressing of structures. 



^ After Peyton (1968). 



7-255 



