47.5 MB - The Whole Building Design Guide

Building shape: The highest uplift pressures occur at roof cornersbecause of building aerodynamics (i.e., the interactionbetween the wind and the building). The roof perimeter has asomewhat lower load compared to the corners, and the field ofthe roof has still lower loads. Exterior walls typically have lowerloads than the roof. The ends (edges) of walls have higher suctionloads than the portion of wall between the ends. However,when the wall is loaded with positive pressure, the entire wall isuniformly loaded. Figure 3-8 illustrates these aerodynamic influences.The negative values shown in Figure 3-8 indicate suctionpressure acting upward from the roof surface and outward fromthe wall surface. Positive values indicate positive pressure actinginward on the wall surface.Aerodynamic influences are accounted for by using external pressurecoefficients in load calculations. The value of the coefficientis a function of the location on the building (e.g., roof corner orfield of roof) and building shape as discussed below. Positive coefficientsrepresent a positive (inward-acting) pressure, and negativecoefficients represent negative (outward-acting [suction]) pressure.External pressure coefficients for MWFRS and C&C arelisted in ASCE 7.Building shape affects the value of pressure coefficients and,therefore, the loads applied to the various building surfaces. Forexample, the uplift loads on a low-slope roof are larger than theloads on a gable or hip roof. The steeper the slope, the lower theuplift load. Pressure coefficients for monoslope (shed) roofs, sawtoothroofs, and domes are all different from those for low-slopeand gable/hip roofs.Building irregularities, such as re-entrant corners, bay window projections,a stair tower projecting out from the main wall, dormers,and chimneys can cause localized turbulence. Turbulence causeswind speed-up, which increases the wind loads in the vicinity ofthe building irregularity, as shown in Figures 3-9 and 3-10. Figure3-9 shows the aggregate ballast on a hospital’s single-ply membraneroof blown away at the re-entrant corner and in the vicinityof the corners of the wall projections at the window bays. Theirregular wall surface created turbulence, which led to wind speedupand loss of aggregate in the turbulent flow areas.3-14 MAKING CRITICAL FACILITIES SAFE FROM High Wind