Study On The Applied Calculation Method Based On The Model Test Of Double Wall Steel Cofferdam

With the fast development of economy in the country, due to its own advantages the double-wall steel cofferdams have been more and more widely applied in bridges' deep-water foundation. Conventional design steps were usually used to do the mechanical analysis of the components that make up the cofferdam firstly, and then to do the design according to bearing capacity of each component and to end up assembling all the components designed before into a whole cofferdam structure. However, the applicability of the three formulas in designing the double-wall steel cofferdams, which are strength calculation formula on compression-bending member, the out-of-plane and in-plane stability calculation formula of the lattice beam-columns by reference to Code for Design of Steel Structures (GB 50017-2003) and Specifications for Design of Highway Steel Bridge (JTG D64-2015), cannot be affirmed. In this context, according to the model test designed on the truss support of double-wall steel cofferdam, this article did a contrastive analysis of the relationship between the test data, the finite element data and the formula calculation, hoping to provide some important practical significance and guiding value on designing and constructing double-wall steel cofferdam. This article is not only an important project of executive study on designing and construction safety technology of double-wall steel cofferdam for Chongqing Transportation Committee, but also one of the sub-projects for researching for Chongqing local standards on designing and construction safety technology of the double-wall steel cofferdam. The main research contents in this dissertation are as follows:(1) In the beginning this article briefly introduced the application the structures and features of double-wall steel cofferdam in the country. On the basis of current materials this paper stated the present development and present application of double-wall steel cofferdams in the engineering construction of long-span bridges' deep-water foundation at home and abroad. It analyzed the strength of the horizontal brace, the applicable calculation method of its stability and its related research in the process of designing double-wall steel cofferdams.(2) The model test of double-wall steel cofferdam truss support was composed of test specimen, counterforce beam, dowel bar, load output system, counterforce foundation beam, etc. The purpose of the test is to examine the strain and transformation of the specimen. The test would continue to load until the test specimen began buckling.(3) By using the finite element analysis software, Midas FEA, to establish a truss support model of the double-wall steel cofferdam, this paper carried out a detailed stress calculation on it and extracted the stress value fyy (Axial stress) of the horizontal braces and the equivalent stress value. A comparative study between stress test values of the truss support model and the stress value of the horizontal braces in the finite element truss support model was conducive to deduce whether the finite element truss support model can correctly simulate the truss support testing model.(4) In the finite element buckling analysis, when the horizontal was under the condition of buckling load, the test extracted the axial pressure N and the maximum bending moment Mx . It was concluded that the calculated value of the out-of-plane stability is larger than the material yield strength of the horizontal lateral brace. The conclusion suggests that when horizontal lateral brace in the finite element is buckling,the out-of-plane stability calculation value contributed to the yield of the horizontal brace and that the out-of-plane stability calculation formula is suitable for horizontal braces of the truss support model.(5) This paper summarized the simplified calculation method of horizontal strut of the double-walled steel cofferdam, and took two horizontal struts to form the inverted V structure, which could stand the static water pressure of the rectangular area. To simplify the hydrostatic pressure into the concentrated load acting on the centroid of the right Angle rectangular trapezoidal, the axial stress value N could be obtained. The centroid had eccentric distance from the center point of the right Angle rectangular trapezoidal,which could help to obtain bending moments Mx and My. The horizontal bracing stress of double wall steel cofferdam could be calculated by bending bar strength formula.From the above research, the strength calculation formula of bending bar and the stability calculation formula according to Code for Design of Steel Structures and Specifications for Design of Highway Steel Bridge are applicable calculation methods for the horizontal brace of double-wall steel cofferdam and to calculate the horizontal bracing stress of double wall steel cofferdam. Since the structure designed by the method considering the axial compression is not so safe, designers should analyze the whole finite element model.