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Design of bulkheads and diaphragms

Design of flexible retaining structures: diaphragms and bulkheads. The problem of dimensioning and verifying these structures is solved by Diadim II through a finite elements model (F.E.) especially designed to evaluate the interaction between terrain and structure.

The distribution and magnitude of the loading and resisting forces exerted on the structure are, as a matter of fact, strongly influenced by the deformation of the combination terrain-structure, hence it is necessary to use an advanced numerical method that takes into account both the non-linear behaviour of the materials and the temporary phases of excavation operation.

The optimisation of the calculation algorithms and the and the user interface specifically dedicated to the problem, enable one to analyse and verify quite complex situations with high precision but within very short time. Given the terrain’s stratigraphy and the position of the aquifer uphill and downhill during the different excavation phases, the program calculates the diaphragm as an elastic beam on elasto-plastic supports.

The characteristics and deformability of the terrain are defined in terms of underground modulus that can be defined directly by the user at each stratigraphic level, or, alternatively, can be calculated automatically with different methods based on the geotechnical characterisation of the terrain.

The elastic characteristics of the structure can be explicitly defined in terms of rigidity to flexion or be automatically calculated on the base of description of the very section; as a matter of fact the program can calculate continuous diaphragms in reinforced cancrete, beam bulkheads, sheet piles and piles in reinforced concrete. The user can, furthermore, choose various fastening conditions of the top and bottom of the diaphragm.

The program enables one to apply to the structures external moments and horizontal distributed or point loads. It is possible to define multiple anchoring levels, that can tie rods (eventually pre-stressed) resistant to traction only or as rigid braces that can also absorb compression forces.

The removal of earth, downhill from the structure, can be separated, in the numerical simulation, in different phases. This is an important characteristic of the program as it enables one to take into account the deformations resulting from previous excavation phases and to re-calculate the response curve of the terrain downhill from the structure at each excavation phase.

After calculating the loads on the diaphragm the program goes over to the structural analysis phase properly said. The structure is, as a matter of fact, tested for pressure and flexion and, in case of reinforced piles the shear resistance due to reinforcement is verified.

Verification of the moment and normal stress is performed in the section under the highest stress with the Ultimate Limit State method, Eurocode 2, that takes into account the mechanical non-linearity of materials and the non-linearity of the structure.

The program determines the bending moment and the normal stress of the calculation and confronts these respectively with the ultimate bending moment before failure and ultimate normal resistive stress.

Shear Verification: this is performed with the standard method described under Eurocode 2. The program determines the maximum shear force acting on the section and that this is smaller than the bearing capacity of reinforced concrete beams (Vrd2) and of the reinforcements (Vrd3).


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