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Possible cases of structural lability (weak structures)

TonyC 2016-08-22 18:28:27

I'm opening this thread to list a number of possible modeling errors that can be the cause of possible structural lability in a structural calculation.

The possible causes of instability may be due to:

1) Errors in changing the internal constraints of the rods. A very common mistake is the junction of cantilevers (click the link to see an example of incorrect modeling: Internal constraints overhang). For the cantilever beams the internal constraint of the beam must be rigidly attached at both ends (S;S;S;S;S;S).
To be sure that the lability is due to the incorrect modification of internal constraints, it is recommended to restore the constraint "solidarity between start and end nodes" in all rods of the model and retry the calculation.

2) Error in modeling of stairs. Another very common mistake is the failure to connect the stairs elements to the structural elements of the floor, or the non-overlapping ramp slab in the case of a "L", "C", plan configuration etc. (Click the link to see an example of incorrect modeling: Stairs, modelling error). To be sure that lability is due to this reason it is advisable to eliminate any generated structural models for the stair objects and relaunch the calculation.
In the case of "L" or "C" plan configurations, it is appropriate that the ramp landings overlap as shown in the picture attached: Stairs, correct modelling.

3) Incorrect modeling of connecting beams between footings/plinths on poles. A possible modeling error is shown in the picture attached: Connecting beams. The connecting beams must be entered as "Elevation" beams and designed to unite the centers of their respective footings that need to be connected. For further details, see the following discussion:

4) Support of a beam made of two ring beams. (See attached picture: Support beam on a ring beam). The "ring beam" is not a structural element and is therefore not a suitable element for a beam to rest upon (which instead is a structural element). In this case you should replace the "Ring Beam" with a structural Beam element.

5) No terrain allocated to foundation beams. This means analyzing a structure without any connection with the ground. Just a simple visual check to see if the foundation structure is properly defined. In fact, the program colors the "foundation beams" with a darker shade in comparison to the elevation beams (super structure). See attached picture here: Foundation beams.

6) Beams that rest on other beams (see attached picture here: Wrong beam to beam support). A simple check is to see if the "load bearing" beam is divided at the intersection with the stairs beam (see attached picture here: Beam resting on a correct beam type)

The above is only a practical example. It is not excluded that there may be other situations not covered in this article but that can however lead to an instability error in calculating the stresses.