Columns. Negative or reduced axial forces

It may occur that, for example, when using the option: Columns > Edit reinforcement or Envelopes > Forces in columns and shear walls, the axial force in the columns may be negative. This may be due to a number of reasons:

  • An external fixity beam has been introduced that touches the column. Bear in mind that if an element of this type touches a column, the vertical movement of the support is restricted at this point and, therefore, negative axial forces can arise in floors below due to the structure ‘hanging’ from this point.
  • Horizontal pressures due to wind or seismic loads are present. It is common knowledge that when these loadcases are introduced, it may occur that some columns are in tension. It may also occur that the worst case combinations (the ones which require the most column reinforcement), are those which arise when wind or seismic loads are acting.
  • Column that starts on a beam. If columns that start on beams have been introduced, it may occur that negative axial forces may arise when the column ‘hangs’ itself from the higher floors due to their stiffness being greater. It should be checked that more unfavourable situations do not develop during the construction process with respect to the final situation, for which the structure is being designed. This matter has been mentioned because depending on how the shoring of the floors has been executed, the forces in the columns (especially those not starting at foundation level) and beams can vary with respect to the final situation. In other words, columns which are going to be in tension, can be in compression during the construction process, and the beams reaching them can behave in a completely different manner.
  • Combination of long and short spans. Due to a specific span combination, it may occur that the slab deflects in the opposite direction to which gravity is acting. This pulls the columns below it upwards, to the point where negative axial forces are obtained.

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