Identify the design criteria
To what requirements and what standards must the structure satisfy? Each project proposes appropriate design criteria for
- Ultimate Limit State (ULS) : resistance, stability, fatigue ...
- Serviceability Limit State (SLS) : maximum deflection, minimum eigenfrequency, maximum crack width, maximum concrete stress,...
Schematize!
Schematize the construction to a mechanical model. A mechanical model contains only the bearing structure. This is the set of beams, columns, walls and floors that transfer the loads to the foundation.
Beams and columns are only represented by their axes. Connection with the outside world or with other elements can be schematized by using boundary conditions.
The right boundary conditions internal and external
External boundary conditions, also known as supports, define the force transfer between the construction and the fixed world, for example, a pinned support or a fixed support. Supports can also be used as a substitute for structural elements that are not included in the calculation model.
An internal boundary condition defines the force transfer between the modeled structure elements themselves, for example an internal hinge. Internal boundary conditions can be present between bar elements mutually, between plates elements mutually, and between bars and plates.
Boundary conditions determine to a very large part of the distribution of forces in the model. The boundary conditions must describe as accurately as possible so that the actual behaviour of the structure is calculated. Therefore a good definition is essential!
Let software work for you!
Before calculating the structure (by hand), you should determine whether or not the construction is statically determinate. This way you can select the proper calculation method.
A structure is statically determinate if the internal forces can be calculated, based on the static equilibrium equations only. This means that there are as many equilibrium equations as there are unknown variables :
- Cantilever beams
- Continuous beams on multiple rolled supports
- Three-hinged arch
- Some types of trusses
When a structure is statically indeterminate (hyperstatic), the internal forces are not only determined by the applied forces but they are also dependent on the element’s stiffness and of the possible displacement of the supports, leading to much more complicated calculation. If a structure is not statically determinate, it is said to be statically indeterminate.
From the moment a construction is statically indeterminate or you suspect you need the calculate the structure more than once, you should use analysis software. Let the software do the hard calculations using finite element method and all kinds of matrix solvers, and treat yourself to a nice cup of tea or coffee!
Give software a try and download our free trial versions!
FEM software? Do not trust the default settings!
In Finite Element Method calculations, the model is divided into a finite number of small pieces in order to make it accessible for calculations. These "small pieces" are called a mesh. The mesh is made before the actual calculation (= elastic analysis) can start. Depending on the shape and dimensions of the model, the mesh size must be chosen properly otherwise this will impact the accuracy of the solution. Too small a mesh then a much longer solution time. Too big and the model can be too stiff and deform correctly.
The software analyzes, the engineer interprets
After the analysis, using hand calculations or software, it is (again) up to the designer to interpret correctly the obtained results. Are the results as expected and are they acceptable? If necessary, the model should be adjusted, by modifying sections or materials or adding additional elements.
Get a proper training
Are you using analysis software? Be sure to attend a training of this software. A good training will hand you the necessary skills to handle your projects more efficiently. You will learn everything about the theoretical backgrounds and the scope of application of the programs.
BuildSoft, part of StruSoft Group, organizes group trainings on regular basis in different locations. Check out our calendar for more information: http://www.buildsoft.eu/en/events