The Influence of Wind Tunnel Testing on the Design of Curtain Walls for Tall Buildings

Abstract

There is a myriad of constraints to be addressed in the design of the tall building curtain wall, and the response to each contributes to its appearance and performance. Wind, its forces and flows, defined through wind tunnel testing is one such constraint. This paper outlines the impact wind has on the curtain wall design process as it relates to resisting wind on the one hand and harnessing it to benefit on the other. As to other constraints, seismic design procedures deal with displacements on the order of 1/50 of the floor-to-floor height on occasion resulting in 75mm or greater vertical joints near the building corners. Energy conservation limits on heat gain/loss translate to average vision areas of approximately 50% of curtain wall area, the remainder being opaque glass, metal, stone or some combination thereof. Building maintenance equipment explains the presence of vertical rails or the pattern of tie back buttons. Concern for the formation of ice that might join the wind flow and possibly cause injury to people and property below is among the reasons for the oft-seen flush curtain walls. And control of vertical flame spread often translates into rated spandrel construction of approximately 1 meter in height. The visual impact of responding to wind is more subtle. Structurally, wind forces are reflected in the mechanical properties of the curtain wall framing members and therefore their sightlines. Since deflection is generally the governing factor, moment of inertia, I, is the property of interest. The distribution of I values is derived from the magnitude and location of extreme wind forces, usually suctions. Given the mechanical relationship of force to I to framing member dimensions, the variation in extreme suctions has little noticeable impact on the sightlines of vertical framing members or mullions. In single skin curtain walls, where the exterior plane offers the main resistance to water penetration, assessment of wind pressures contribute to the vertical dimension, or dam height, of horizontal framing members containing weep holes. The wider application of rain screen/pressure equalization (RS/PE) principles in the design of the high rise curtain wall has called for greater focus on the “size” of pressures and suctions as well as their respective magnitudes. Unlike the wall system described above, the RS/PE wall is composed of an inner as well as outer surface separated by an air chamber. The objective of this system is to minimize water penetration in joints in the outer surface by allowing the chamber pressure to equal the exterior pressure through a series of protected openings. The chamber, acting as a manifold, distributes the outer pressure to the inside of the exterior joints, the joints between glass and frame for example. One such application is the glazing pocket of the window illustrated in Figure 1. A variation on the RS/PE design is that of open joints in the outer plane without dams or baffles. The open joints, often in stone work, rely more heavily upon the inner surface for resistance to water penetration. The horizontal joints in the inner plane are usually staggered relative to those in the outer plane but may have some exposure when crossing joints in the outer plane. The design of both the PE/RS and this variation rely upon wind tunnel test data for assessments of the differential pressures at openings to the cavity as well as both positive and negative