Insurance claims personnel are often asked to consider whether damage to a building is related to excessive wind forces or whether a structure is deficient in its capacity to resist wind. Extending or denying wind damage related coverage is linked to the expected wind loading on a structure. Wind velocity data is often obtained from weather station reports obtained from instrumentation in open country. This data may or may not be reflective of the actual wind speed at the claimant's loss site. A wind speed of 70 mph may not be sufficient to cause damage to the claimant's properly designed and maintained structure, yet a wind speed of 90 mph may be damaging. A condition at the loss site that can cause higher wind speeds than that reflected by open country wind speed data is called the Venturi effect.
Named after physicist Giovanni Venturi (1746-1822), the Venturi effect refers to the increase in velocity of a fluid as it travels through a restricted area. Figure 1 below depicts a fluid (air, water, and so on) traveling at an initial velocity. As the fluid reaches the restricted area (throat, Venturi), the velocity increases based on the principle of continuity. As a refresher, the principle of continuity states that mass flow through these areas must remain constant. As the fluid exits the restricted area, the velocity returns to a more nominal value.
Figure 2 below is a view of building showing how the Venturi effect can cause excessive wind loading to other structures. In Figure 2, the wind velocity profile is shown to the right of the structure. As the wind moves from right to left, a restricted area is encountered, and the wind velocity increases (Venturi effect). This causes an increased wind velocity that can damage a building in its path. Even though the open country wind velocity may not be of sufficient magnitude to damage buildings, those structures subjected to the Venturi effect may be damaged because of the increased wind velocity.
Figure 3 shows another way the Venturi effect can damage a structure. The up slope near the sign tends to restrict the air flow, causing an increased velocity which can be damaging, despite the existence of nominal open country wind velocities.
Figure 4 shows a sign support column that failed during strong winds. The arrow shows the imprint of the sign as it fell downwind. Wind speed at the local airport was approximately 80 miles per hour, which should not have been sufficient to cause the failure of the sign support column. However, the up slope near the sign was sufficient to accelerate the wind speed to damaging levels, causing failure of the sign column.
Figure 5 shows denting to a grain bin as a result of wind speed acceleration because of its proximity to another bin and the Venturi effect. It should be noted that wind speed data in open country such as that recorded at an airport may not be an indicator of the severity of the wind load on a structure subjected to the Venturi effect. This suggests that a scene inspection is in order to rule in or out damage caused by excessive wind speed brought on by the Venturi effect.
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