Graduation of Mick van Haren

Flood fragility of a Cavity Wall

• Professor of graduation: Dr.ir. J.D. Bricker 

• Supervisors: Ir. P.A. Korswagen (TU Delft), Prof.dr.ir. J.G. Rots (TU Delft), Ir. G.W.F. Rongen (TU Delft & HKV lijn in water), and Prof.dr.ir. S.N. Jonkman (TU Delft)

In March 2021 a cavity wall section was built in the largest basin of the Flood Proof Holland facility on the TU Delft campus. The wall was constructed with calcium silicate bricks and fired clay bricks, and a weaker mortar to partly account for the virgin effect of any newly-built wall. Both inner and outer walls were connected with adequate wall ties. The dimensions of the wall (2.6m x 2.6m) were chosen to fit the steel rig already present and used for an earlier experiment. In the wall, an ordinary window frame was mounted (66cm x 145cm). 

In the basin, the wall section was subjected to several hydrostatic pressures on both sides. These experiments were performed to physically grasp the deformations corresponding to certain flood scenarios affecting cavity wall sections from ordinary terraced houses. Additionally, it was investigated how the failure of the window-wall interface would contribute to the water height inside a residence. It should be noted that, because of the fortunate absence of great flood events in The Netherlands during the second half of the twentieth century, no physical data was added to the structural understanding of flood fragility on walls. Considering that during the same period the cavity wall became standard in the construction of houses, a certain inconsistency can be recognized between the expected and observed (structural) behaviour of a modern wall during flood events.

The experimental data was collected and used to find specific characteristics, which could be used to compute an analytical model. The analytical model allowed the data to be extrapolated to find the combination of water levels that would imply real-time failure. To account for the window gap inside the wall, part of the model was featured with lower stiffness. The model gave sufficient results but also showed difficulties for small head differences. A second model was computed with DIANA FEA to investigate the non-linear behaviour. Both models showed similar results for the linear analysis but with the notion of non-linearity, the finite element model showed a failure curve that was more strict i.e. failure would happen for lower water levels. Results showed that the cavity wall in a one-way bending configuration would start to show significant cracks between 1.3 and 1.6 meters. Because of the brittleness of the masonry, this would imply failure. It was further found that the non-linear deformations would reach 4 millimeters. Considering that for these water levels the internal moments were still far from their maximum capacity, the results hint that failure occurred due to cracks that were forced to from because of the deformations.