Full scale measurements of pressure equalization on air permeable facade elements

Wind-induced pressure differences over rain screens are determined by the external pressures and the pressures inside the cavity. Minimizing this pressure difference decreases the risk of water leakage and also helps to minimize the local loads on the façade elements. Current rules to determine the wind loads on such structures are based on very little experimental evidence. Some guidance using very crude rules is given in EN 1991-1-4 [1]. A full scale measurement has been set up on a 158 m high residential building situated in Rotterdam, The Netherlands. The façade is clad with slabs of natural stone, which typically have a size of 1 m2 each. The openings between the elements are about 10 mm wide and the air cavity has a depth of (nominally) 20 mm. At a height of approximately 120 m above the ground, the façade elements have been equipped with 40 pressure transducers: 20 for the external pressure and 20 for the cavity pressure. From this experiment pressure equalization coefficients per pressure tap have been derived, which are defined as the maximum peak differential pressure coefficients divided by the maximum peak external pressure coefficients. Also wind direction dependent values have been derived. For overpressure, a pressure equalization coefficient of 0.6 has been found, which is comparable to the ratio EN 1991-
1-4 gives for the net pressure coefficient of an permeable façade (cp,net=2/3 cpe). For underpressure, values in the same order of magnitude have been found, with a maximum value of 0.8 in the middle zone. One value above 1 has been found, which lies in a corner zone (ceq=1.1). The measured pressure equalization coefficients for underpressure are higher than the net pressure coefficients in EN 1991-1-4 for permeable facades (cp,net=1/3 cpe).