Actively bent grid shells are curved structural surfaces made of flexible members. The application is mainly in roof structures. The members, spanning between two supports on the perimeter, are connected at their intersections by joints, and assembled in an initially straight grid. By lifting at a sufficient number of points the straight grid is subsequently transformed into a bent shape and new geometry is preserved by fixing the supports. Subsequently, the curved surface is braced with a third layer of flexible members. According to this specific principle of construction, actively bent grid shells can be built within a short period of time.
ThinkShell, a research team at École des Ponts ParisTech, has developed a number of actively bent Glass Fiber Reinforced (GFRP) grid shells over the last ten years. Due to mechanical properties such as lightweight, high elastic limit strain, and high stiffness, Carbon Fiber Reinforced Polymer (CFRP) becomes an interesting material choice for construction of long-span actively bent grid shells.

In the actively bent members of the grid shells considerable amount of permanent stresses is present. The main source of stress is caused by the transformation of the straight grid into a bent shape during erection. Fiber Reinforced Polymer (FRP) is sensitive to creep behavior, which may lead to possible collapse mechanism due to creep-rupture or creep-buckling as a result of reduced stiffness of the material on a long-term. Limited knowledge is available of the time-dependent long-term creep behavior of CFRP, in the field of structural design of buildings. The main objective of this research was to define stress limits related to time-dependent long-term creep behavior of CFRP in members of actively bent grid shells, using the Stepped Isostress Method (SSM).

The most important conclusion drawn from the experiments using the SSM is that, in actively bent grid shell design, it is crucially important to stay below the sustained stress limits to avoid creep-rupture, as the CFRP fails in a brittle manner. From the SSM analysis using test data from bending experiments it was concluded that if the permanent load in the CFRP is 55% of the ultimate load, creep-rupture will happen in 40 days. This result does not seem realistic as the sustained stress limit from CUR96 corresponds to a service life of 50 years. Out of the four steps in processing of raw test data using the SSM, the rescaling step is the most critical step. There is strong dependence between the rescaling step and the horizontal shifting step. To determine the rescaling values and horizontal shifting for the bending experiments, a corresponding conventional creep tests as a reference test is proposed.
The SSM represents a promising method for accelerated creep testing and corresponding test data analysis for CFRP in members of actively bent grid shells. Sustained stress limits derived from the SSM may be used as more realistic, i.e. less conservative stress limit values to avoid creep-rupture, compared to the values defined in CUR96, for the specific design parameters.