Dutch Buildings Flood Fragility

Abstract

To pragmatically answer the question of whether masonry walls can withstand a hydrostatic water pressure, a clay-brick wall was built and tested at Flood Proof Holland. The specimen was exposed to various hydrostatic loads, also in combination with debris impact loads, with the goal of providing experimental data for the calibration of structural models and later development of fragility models to study and assess (economic) damage and life loss from potential Dutch floods.
The 2.7x2.7 m2 wall, 100 mm thick, was constructed on a steel rig and subjected to a water level difference of up to 125 cm, where the level, at what was considered the back of the wall, was kept low (0 cm). The wall, which was restrained on all four boundaries by being glued at the bottom and top steel beams and constrained against the lateral steel columns of the rig, was subjected to various combinations of water level in front and behind it with the most unfavourable combinations leading to minor cracking. The out-of-plane deformation, perpendicular to the face of the wall, reached approximately 2 mm for the hydrostatic pressure produced by 95 cm of water. The masonry structure, which was also subjected to a vertical overburden by the steel rig mimicking the vertical stress produced by gravity loads on buildings, deformed in two-way bending, meaning that the horizontal supports also prevented the wall from moving.
Non-linear, finite element models calibrated against the experimental results offered additional insight into the behaviour of the wall. It was observed that the wall behaved linear-elastically up to a water level of about 90 cm. At higher water levels, the deformation of wall increased significantly which is associated, according to the models, to the initiation of cracking in the wall and the redistribution of the bending stresses from the vertical (stiffer yet weaker) direction to the stronger horizontal direction. At 125 cm, the wall deformation reached up to 5 mm. From hereon, damage in the wall would have progressed, further reducing its stiffness up until collapse at approximately 150 cm of water depth. This latter value could not be achieved experimentally but is inferred from the trend of the experimental results and the understanding provided from the non-linear models. The models explored comprised analytical and numerical models with linear-elastic and non-linear material models, in one- or two-way bending. The behaviour of the wall up until the initiation of cracking could be predicted well with the simpler, one-way bending analytical or FE linear-elastic models, but more complex, two-way bending models were required to output the same fidelity at higher water levels, with only the non-linear, two-way bending FE model capable of representing the experimental behaviour at the highest water levels.
These calibrated models were also used to predict that, if the wall had not been constrained on its sides and thus only at the top and bottom, more alike walls in buildings will long end walls, it would have failed much earlier, at a water depth of about 90 cm depending on the vertical overburden applied.
These values for water depth consider that no water was present behind the wall. During the test, water did not infiltrate through the wall, with the highest water pressures leading only to wetness at the back of the wall. In reality, the inside of a building may be subjected to a water level because of water entering the building through windows or doors. The lower differences of water level between the front (outside) and back of the masonry are also associated with lower out-of-plane displacements and lower internal bending moments in the wall. However, in terms of hydraulic head, if the head was located higher and closer to the middle of the wall, it was observed to be more detrimental than an identical head where the inside of the building is dry.
The masonry wall was also subjected to a combination of hydrostatic pressure and impact of debris. Two types of debris were considered: a floating tree log representing soft debris, and a suspended steel cube, mimicking hard debris. Impacts of the soft debris of up to 1.8 m/s (90 Joules) did not lead to any additional deformations of the wall, while collisions with the steel cube (up to 180 J) in combination with water depth of about 90 cm, did generate accumulated out-of-plane deformations, visible cracking in a diagonal pattern with leakage of water through the cracks, and ultimately local failure of the masonry.
The experiments conducted herein prove that traditional, single-wythe masonry walls in regular buildings can safely withstand water depths of up to 90 cm when the inside of the building is dry. This corresponds roughly to a hydraulic head of 90 cm when the level inside is low. At larger water depths damage in the form of cracks is expected and the boundaries of the wall and its overburden become relevant in determining whether the wall has sufficient capacity to avert collapse. For a square wall, restrained on all four boundaries, the maximum water level was determined to be about 150 cm; but, wider walls or walls without lateral supports are much more vulnerable having fewer possibilities to redistribute stresses, especially to the horizontal direction and thus exhibiting more brittle failure after the onset of cracking.
Future work should hence focus on testing the difference between one-way and two-way bending in masonry walls, assessing the effect of leakage through walls with openings, and observing the influence of cavity walls where two walls are built in front of each other with a space in-between, which are common in Dutch buildings, and where water outside and inside the building may exert pressure against the empty cavity. The rate at which such a cavity is filled with water would also provide additional insight into the flood resilience of dutch buildings.