Answer:
Downwardly curved, synclastic roof geometry is numerically stable for downward loads. The loads in the direction of the arc ultimately increase the tensile load in the membrane, and thus stabilize the structure.
The loads against the arc shape of the roof reduce the tensile load in the membrane. If the tensile stress of the membrane cable approaches zero in this case, numerical difficulties arise due to the sole membrane tension resistance of the element used. In this case, the solution process reacts with a poor convergence, instability messages, or messages about a positive definite Jacobian matrix.
