# Common structural modelling errors and lability in a structural calculation: possible causes and solutions

## Structural Calculations with the finite elements method: here are the most common structural modelling errors that can cause lability in a structural calculation

When designing a project, structural calculation problems are common. The lability present at the moment when structural calculation is launched may be related to several factors.

Lability of a structure is due to the lack of appropriate constraints for the structure under examination.

Here are a few examples, created with the Edilus structural calculation software of possible modelling errors that may be the cause of lability in a structural calculation.

• #### Erroneous changes to the inner constraints

A very common mistake is the disconnection of cantilevered beams. In this case, the internal constraint at the initial and/or final end on the cantilever element causes lability.

Internal constraints regarding the structural model

For cantilever beams, the internal connection of the beam must be solid for both extremes. If there are internal disconnection for some reason, there may be a lability.

Regarding EdiLus software, to be able to ensure that the lability is due to poor modification of internal constraints, we recommend restoring the initial and final node constraint to all model rods, and subsequently retry the calculation.

• #### Error in staircase modelling

Another common mistake is the lack of connections of the staircase to the floor structural elements or failure to overlap the staircase ramp in the case of ‘L’, ‘C’ shaped configurations, etc.

From this example we can see how the architectural design looks correct but in reality the structural model highlights the lack of lability (disconnected rod).

Wrong staircase modelling

One possible solution to the problem might be to eliminate the generation of the structure of all the stairs and then try again the calculation.

In case of “L” or “C” plan view configuration it is more appropriate if the ramp landings would overlap.

EdiLus, for instance, allows to add the starting and arrival landing. In this case the rods that schematize the ramps are well connected among themselves and to the structure.

Correct staircase modelling

• #### Wrong modelling of beams connecting plinths or plinths on poles

In the example below the connection joist is represented with a plinth-parallel and it is not connected to the structure.

plynths on piles connection

The connecting beams should be inserted as ‘Super-Structure‘ beams and designed in order to join the centers of the respective plinths that you want to connect.

Proper modelling of connecting beams presupposes that the nodes at the base of the columns, where the plinths are applied, are connected by the same beams. This way it is possible to hold differential yields and avoid plinth displacements.

You can get a proper modelling by designing a ‘super-structure’ beam, between the two plinths, connected to the base of the pillars.

“Super-structure” beam connection

• #### Supporting a beam between two ring beams

In EdiLus software, the ‘cfc joist.’ is not a structural element and is not a suitable element where to base a beam (which is a structural element). One possible solution would be to replace the “ring beam” with a structural element such as a beam.

beam on a ring beam (non calculated joist)

• #### Not assigned terrain to the foundation beams

In the following example a structure no connection to the ground is represented.
With EdiLus, the foundation beams (sub-structure) are highlighted with a darker shade as compared to the super-structure ones.

Foundation beams colour coding

• #### Beams not supported by other beams

In this case the roof design looks correct but the structural model highlights a lability condition (rods are not connected).

non connecting rods

A possible solution consists in verifying that the ‘supporting/load-bearing’ beam would divide with the supported beam (the beam should split-up) at the intersection point.

Avoid making those critical structural calculation mistakes! Download the 30 day free trial of Edilus – the structural calculation software

Tags: