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Positive moment at joist support

The “Positive moment at joist support” warning may appear after the analysis of the job in CYPECAD when accessing the “Joists” menu, clicking on “Errors” and selecting the joists coloured in red:

The warning also appears in the Final design report, where the group and coordinates of the joists with this condition are indicated. In this case, the following is indicated “Joists (X1, Y1) – (X2, Y2), … have a positive moment at the support. Pin the edge and re-analyse or study the continuity of the bottom reinforcement.

This warning implies that the sign of the bending moment at one end of the indicated joists is positive. The user must examine the distribution of forces of the different elements in the structure to determine whether this condition should be interpreted as a design error and if so, make the necessary modifications and re-analyse the job.

Possible solutions to this warning include the following. The rational design of a structure involves the delivery of structural elements with a lower stiffness, such as joists, to elements with greater stiffness, such as beams. If the opposite situation is found in the model, i.e. joists with greater stiffness than the beams they are supported by, the positive moment warning may appear for these joists.

The following image shows the distribution of forces for the previous structure, indicating the positive moment at the supporting point of the joist on the central beam:

In this case, the solution is to stiffen the central support beam by using a larger section:

When re-analysing, the deformation of the central beam is smaller and the distribution of forces on the joists of the side panels varies, thus modifying the sign of the moment of the joist support.

The example shown can be downloaded using the following link.