CSM Bridge Design - AQB CTRL VM Command

Hello everybody,

I have a prestressed bridge with an important torsional component due to its curved geometry. I have set only one reinforcement layer with torsional contribution and with 4 rebars in SOFIPLUS.

When I use the CSM Bridge Design module, the created .dat file uses an AQB run where CTRL VM VAL2 1.0 is set. In the AQB Documentation we read that this command essentially makes the program consider the shift rule (Versatzmass) for the calculation of the longitudinal reinforcement due to the torsional load.

What I see in the results is that the layer with the torsional contribution remains unchanged which I find quite surprising. Furthermore when I change the CTRL VM VAL2 to 0 the same torsional layer augments substantially reaching very high quantities along the bridge while the stirrups become less.

Now I fail to understand how the consideration or not of the shift rule for the longitudinal reinforcement can have such a big impact on the one torsional layer and on the stirrups. Could somebody help me understand the inner workings of this command and which one of the VAL2 values I should use (0 or 1)? Unfortunately the AQB documentation is not very helpful in this aspect.

Kind regards,

This is based on eurocode, but the principle should apply to other codes:

The shift rule yields an increase in the longitudinal (bending) reinforcement due to shear:

  • You have the necessary reinforcement area for bending in a beam section A_bend.
  • If the section is not at the peak bending moment you also have shear, which you reinforce with shear links.
  • The calculation of the necessary shear reinforcement is based on a truss model with shear ties and concrete struts.
  • The truss model yields a tensile force in the tensile chord (usually the bottom chord)
  • This tensile force yields an increase to the longitudinal reinforcement by:
  1. Calculating it from the shear force (EC 1992-1-1 6.2.3(7))
  2. Shifting the necessary longitudinal reinforcement by d, the effective height. (EC 1992-1-1

When it comes to torsional reinforcement, the necessary links and longitudinal bars are calculated directly. So no need for shift rule. However the principle of a truss also applies, you just do it as one complete calculation.

1 Like