Analysis of reinforced concrete with FEM

Background
The FEM work bench has the capability of estimating the level of reinforcement required in a concrete structure to prevent brittle failure under tension or shear.

This is done with a simplified method (described here: ref) which uses the results from an elastic analysis. In essence it is a post processing routine for Calculix, which calculates the principal tensile stresses in the concrete and uses those to determine the minimum reinforcement required in the three coordinate directions to prevent failure. In the analysis it is assumed that the concrete material cannot carry tensile stresses, whereas the steel is utilized to its maximum capacity (i.e. reaches yield).

The required reinforcement is expressed in terms of a reinforcement ratio. This is the ratio of steel to concrete areas. For example a reinforcement ratio of 0.01 in x-direction (rx=0.01) means that the total cross sectional area of reinforcement bars running in x direction should be 1% of the concrete cross sectional area they are passing through. A hypothetical cross section of 1mx1m should in that case contain 0.01 m2 steel, which could be achieved by using 90 reinforcement bars of 12mm diameter each (steel area = 90*PI*(0.012)^2/4=0.0102 m^2). If the required reinforcement ratio over this concrete cross section is uniform then the bars could be placed at an equidistance 9x10 grid with a center-to-center distance of approximately 10cm. This is still a practical number where sufficient space between bars is left for the concrete to pass through and ensure a high quality cover. Much higher values would lead to a very dense reinforcement grid with potential quality issues, whereas much lower values could lead to large tension cracks in the cross section between the bars. A typical range in practice is from 0.002 to 0.02 (or 0.2-2%). Further guidance can be found in design codes.

If the required reinforcement ratio is not uniform over the full cross section then the cross secrion can be divided in pragmatic sub-sections with more or less uniform ratio and reinforcement applied to those cross-sections. An example will be given later on.

As a word of caution, it takes much more to design a safe and durable concrete structure than what the FEM work bench can currently provide. For example, the method does not calculate crack width (important for durability and functionality), accurate deformations (FEM results for concrete are simply linear-elastic) or take account of reinforcement anchoring requirements (causing an increase of required reinforcement ratios in anchoring zones). It also doesn’t predict concrete crushing, which could mean that the concrete fails before the reinforcement yields, leading to brittle failure of the overall structure. These and other limitations mean that the FEM concrete functionality can only be used to assess conceptual designs, whereas detailed design decisions critical to safety and performance should be left to qualified professionals.

Model geometry, loads and supports
Although the FEM concrete routine does not have any additional requirements for geometry, loads and supports, it should be borne in mind that sharp corners and a support on an edge or vertex can introduce stress concentrations that will lead to extremely high and unrealistic reinforcement ratios at or near those locations.

Material Parameters
FEM workbench has a special material object for reinforced materials, which combines a matrix material (e.g. concrete) and a reinforcement material (e.g.steel). For the analysis of reinforced concrete with FEM, the following parameters need to be specified, as a minimum: Concrete:

Young’s modulus Poisson ratio uniaxial compressive strength Friction angle (default 30 degrees)

the tensile strength of concrete and the yield strength of steel.

Application
In the remainder of this article a few practical cases will be analysed to discuss application of the method.

Simply supported beam with uniform load
Summary, picture and link.

Beam with mid-span support
Summary, picture and link.

Shear wall with uniform load
Summary, picture and link.

Shear wall with opening
Summary, picture and link.