When we are designing beams under bending and shear it is necessary to consider the additional longitudinal force of tension caused by the VEd shear. This additional tensile force is caused by the horizontal component required to equilibrate the compressive force of the concrete strut.
If the horizontal forces that appear in the cross section, which we can see in the figure below, are solved:
The component of the force on the compressed strut is:
It is assumed that half of this force will support the reinforcement located at the zone in tension of the beam and, therefore, the additional force to be applied in that zone will be:
To resist this additional force, it will be necessary to add, in any cross section, a longitudinal reinforcement apart from the one that is already required to resist the bending.
The following condition must always be satisfied:
After this brief introduction, we will now see how STATIK takes this effect into account.
It all begins with the selection, in the analysis parameters, of the option ‘Include shear and torsion in the calculation of the longitudinal reinforcement (shift rule)’.
But that is not the only option to consider. If we go into the member attributes dialog box, on the sixth tab we will see the option of giving the attribute ‘fan’ to the begin and the end of the member.
What is this option?
As we can see in the following image:
The ‘fan’ drawing represents the zone in which the stress field (compressive) ‘travels’ directly to the support.
We assume this zone is located inside the length between the cross sections situated one effective depth from the support. In other words, picking this option, we will be telling the program that the support is an intermediate support of a continuous beam.
With all of the above, we conclude that this option must be picked every time we have an intermediate support of a continuous beam.
When it is an ‘end’ support, the program will automatically detect that it is that way and choosing this option or not will not have any effect.
If we do this, in the zone close to the intermediate supports of a continuous beam, the total reinforcement will not be increased by the presence of shear (in the image above in which the necessary reinforcement appears, the As curve (MNV) will have a horizontal part at the height of the value of necessary reinforcement considering only the ones of the moment and the axial force: As (MN), that is, the diagram is not shifted in that zone, which is what we intend.