半圆型结构冰压力的物理模拟试验研究

利用DUT-1非冻结合成模型冰开展1:25冰对半圆型构件的物理模拟试验研究. 首先用简单的斜面构件的物理模拟试验检验了整体试验装置和技术设备的可靠性, 然后以同样比尺进行2种冰厚在5个不同水位条件的系列试验, 获得了模型冰排在半圆型构件前的破碎堆积现象和作用于构件上的水平和垂直作用力过程.结合实测模型冰物理和力学指标, 通过优化组合, 拟合出半圆型防波堤冰荷载的试验公式. 这些公式包含了冰对半圆型结构的弯曲破坏作用力、挤压破坏作用力及爬坡和堆积作用力.

Physical Modeling Study of Ice Forces on Semi-circular Structure

Semi-circular breakwater is a kind of reinforced concrete structure that consists of an arch ring and a base slab. It is used in harbor construction and estuarine guide jetty engineering in open water areas of China. However, for building semi-circular breakwaters in the ice-infested Bohai Sea, a calculation of ice load on the structure is needed. Therefore, utilizing the DUT-1 synthetic model ice, a series of physical model tests of ice forces on the semi-circular breakwater were carried out in a wave-current flume. For knowing the credibility of test set-up, 5 inclined slope walls were tested first, and the test results of ice failure phenomena and ice forces on the structures were compared with Croasdale’s two-dimensional ice force theory concerning sloping structures. After verification from the slope walls tests, 28 group model tests of semi-circular structure with 2 model ice thicknesses and 5 water levels were done. The slope wall models and semi-circular breakwater model with 1:25 scale were made of organic glass. To simulate the friction coefficient between concrete surface and natural ice, the original glass surface was milled so the measured kinetic friction coefficient of model ice with rough surface was 0.25. A CCD kite-Camera installed above the flume and a CCD side-glance camera installed on the side of the flume recorded the model ice sheet failure, broken ice pieces climbing on the semi-circular structure, and broken ice pieces accumulating in front of the structure. For reading the images easily, the images from these two CCDs were shown and recorded on one screen. Two load sensors monitored the horizontal and vertical ice forces, respectively. Ice failure, pile-up and the characteristic ice forces were obtained based on these basic recorded data. Using the recorded ice forces, ice climb, ice accumulation, measured model ice density, flexural strength, compressive strength, elastic modulus, kinetic friction coefficient, model ice thickness and structure width, empirical formula for predicting ice force on a semi-circular breakwater was determined. The horizontal ice force formula consists of three parts: flexural failure force, crush failure force and climb as well as cumulative force. It can be expressed as F_hi=aDσ_fE^-0.25h^1.25+b₁Dσ_ch^b₂+cρ_iDhr (1-cosα). In this formula, the structure width and radius are also considered. There are special situations of flexural failure force on slope structures and crush failure force on vertical structures in the formula. The vertical ice force on the semi-circular breakwater is different from that followed Croasdales formula, because accumulation in front of the structure has a special feature; therefore the formula can express the contribution of ice flexural and broken ice pieces accumulation. The vertical ice force on the semi-circular structure is F_vi=dDσ_fE^-0.25h^1.25. The coefficient d expresses that the force of climb and accumulation in the horizontal direction increases with tangential angle in the range of 0～60°, and decreases with the angle from 60 to 90°. Comparison with typical slope and vertical situation shows that these formulas can be used for calculating design ice forces on the semi-circular structure. The first case is semi-circular breakwaters in Tianjin Harbor. In the case the design ice index are 40 cm in ice thickness, 2.2 MPa in compressive strength, 320 kPa in flexural strength and 2.1 GPa in elastic modulus.