Concrete crib structures

Abstract

The Slipway Launch Method for concrete crib construction pioneered in Canada is established on a firm engineering basis with the help of time-sequence dynamic analysis developed by the author. The time-sequence diagrams enable the launch paths of the crib to be traced position by position. The linear and angular velocities, as well as accelerations computed for each position, enable the interacting forces between the moving crib and the supporting launch structures to be evaluated. Also, the degree of dip of the front end can be evaluated and, if necessary, the launch height can be adjusted. If a submarine launch is adopted, the dynamic analysis will indicate the maximum hydrostatic pressure for which the water-resistant timber deck has to be designed. It will also indicate whether there is a danger of the crib cartwheeling as it leaves the launchway.After launching, the critical rate of build-up by slip-forming in order to avoid overstressing of the green concrete is evaluated. The floating stability during build-up by alternately ballasting with water and adding wall concrete is checked stage by stage. Finally, design for the permanent conditions using Janssen pressures for bin-action of the fill-material in the crib-pockets is presented. Keywords: concrete cribs, launching, dynamic analysis, time-sequence, submarine launch, slip-forming, floating stability, bin-action.