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Go with the Flow: Design for Windload Patterns at Building Corners
By Stewart Jeske
I recently visited downtown Manhattan on a cool and slightly breezy day. I was walking close to a tall building, and when I rounded the corner, the wind almost knocked me off my feet. The wind pressure difference was stark; absent on one side of the corner and gale force on the other.
This type of difference in wind pressure at corners is well known due to wind tunnel studies dating back to the 1970s. When obstacles (e.g., corners) are introduced into the flow, the side facing the flow has higher incident pressure. The opposite face typically will exhibit flow separation with negative pressures.
Building code requirements for Main Wind Force Resisting Systems (MWFRS) of the building structure address positive pressure loading on one face of a building with an adjacent building face with negative suction loading (side wall pressures). ASCE 7 Minimum Design Loads for Buildings and Other Structures references these requirements. However, the same requirement for combining positive pressure on the windward corner while adding negative pressure on the adjacent corner face isn’t as clear for designing components and cladding. ASCE 7 requires a combination
of positive pressure on windward wall cladding and negative pressure (suction or uplift) for the adjacent roof cladding. This negative pressure zone is identified as a flow separation zone in fluid dynamics (see figure 1).
Positives and Negatives
Many cladding engineers believe the building code does not require the combined application of positive wind pressure to one side of a corner and the negative pressure (flow separation zone) on the adjacent corner face (see figure 2). They’ve indicated that if the combination is not directly spelled out in the building code requirements, then they don’t have to consider it. Good engineering judgment and real-world application of engineering knowledge are requirements of the building code. Just because something is not specifically addressed in the building code does not mean the designer can disregard it. Consider the real design case of a curtainwall with a skylight overhead (see figure 3) The curtainwall and skylight are supported by a structural header designed by the cladding engineer. A load case is required by the building code for the design of the header for positive pressure from the curtainwall combined with the negative roof zone up-lift of the skylight. This is the same case that should be considered at a corner of a curtainwall with a common mullion supporting both faces (see figure 2). The corner mullion should be designed for a combination of positive and negative pressure zones. We typically use .75 positive pressure plus .75 negative pressure as a load combination similar to definitions of loads combined in the MWFRS section of ASCE 7 Wind Load Requirements.
Not Convinced Yet?
Let’s try a couple more logical arguments. We know wind does not choose perpendicular paths to a building (see figure 2, case 2). In this case, the wind approaches the curtainwall corner at a 45-degree angle to a corner. The wind produces a positive pressure load on the incident face. It should be evident from fluid dynamics that the opposite face does receive a negative suction load from a wind separation zone. The combined load to the mullion produces bending about the weak axis of the mullion. This combination typically will control the design of the corner mullion.
Now let’s look at a curtainwall corner at a very sharp angle (see figure 4). This condition is similar to parapet-type windloads prescribed in the code, with positive pressure and negative suction added on the other side. Now slowly increase the sharp angle to a 90-degree corner. At what angle does a designer stop considering the combination of positive load on one face and negative load on the adjacent face? That’s a trick question. The cladding designer should consider the combination of positive pressure on one face of a corner mullion and negative suction on the adjacent face.
It’s critical to address very real turbulent wind design pressure requirements at corners of buildings. Fluid dynamics and wind tunnel studies have pointed to this phenomenon associated with dynamic, positive pressures on a corner with flow separation zones (negative pressure) on opposite sides. Some engineering companies may not be addressing this issue appropriately, and it may lead to system damage and leaks. We highly recommend combining the design pressures to reflect the real-world requirements at corner mullions. We also advocate 3D design modeling of corner framing to address corner loads.
Stewart Jeske is president and owner of JEI Structural Engineering in Kansas City, Mo.
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