Design Procedure by Strut-Tie Model Method
The strut-tie model design of structural concrete is begun by developing a strut-tie model for a given design situation. However, as there is not a unique strut-tie model associated with a particular problem, an iterative process shown below is necessary. The iterative process has been one of major disturbing factors for the strut-tie model method to be used in practice. Special purpose graphics programs developed for the strut-tie model method can simplify the iterative process greatly.
Step 1: Define the boundaries of D-region, and then evaluate the forces acting on the boundaries of the region. For the region requiring principal stress trajectories, decide the boundary and loading conditions for a finite element analysis. Set up the initial design conditions for the design of structural concrete with D-regions.Step 2: Sketch a strut-tie model for the region. Step 3: Solve for the cross-sectional forces of struts and ties, and determine the required cross-sectional areas of struts and ties. The areas must be determined so as to satisfy the strength conditions of struts and ties. Return to Step 2 with adjustment of initial design conditions if the required cross-sectional area of a strut is greater than the available area of the strut which is determined based on the shape of sketched strut-tie model and design conditions of the region or structural concrete. Step 4: Verify the strength of nodal zones. If one of the nodal zones fails, reexamine the strength by changing the anchorage type of reinforcing bars and/or the size of bearing (loading) plate, or return to Step 2 with adjustment of initial design conditions. Step 5: Determine the required cross-sectional areas of steel ties within the limits of effective strength of steel ties. The specifications on the minimum reinforcement ratios, placement of reinforcing bars, and anchorage must be satisfied.
