Wind Drag Coefficients for Highway Signs and Support Structures (2023) / Chapter Skim
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Pages 87-100
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From page 87... ...
87 C H A P T E R 3 3.1 Overall Procedure to Determine Wind Loads The wind load acting on a highway sign or members of its support structure along the wind direction is determined from the equation = ρF C V Ad0.5 (3.1) 2 where V is the mean wind velocity, ρ is the air density, A is the projected area in a plane perpendicular to the wind direction, and Cd is the drag coefficient.
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From page 88... ...
88 Wind Drag Coefficients for Highway Signs and Support Structures 3.2 Wind Drag Coefficients for Estimation of Wind Loads Applied Normal to the Highway Signs The method to determine the wind drag coefficient for highway signs starts by determining the base drag coefficient for an isolated rectangular sign, Cd0. One or more modification factors are then applied to this base value to account for specific situations in which the sign is used and to obtain the actual drag coefficient Cd used in determining the wind load.
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From page 89... ...
Proposed Methods for Estimating Wind Drag Coefficients and the Associated Wind Loads 89 3.2.2 Wind Drag Coefficient Cd for a Highway Sign The base drag coefficient Cd0 is modified with factors that consider different situations. In the most general case, the wind drag coefficient Cd is determined as =C K K K K Cd t a p S d (3.5)
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From page 90... ...
90 Wind Drag Coefficients for Highway Signs and Support Structures 3.2.2.2 Factor Ka to Account for the Effect of an Add-On Sign Based on the simulation results in Section 2.1.7 (Figures 2.16 and 2.17 and Tables 2.8 and 2.9) , it is proposed to use the factor Ka = 1.05, which accounts for the effect of an add-on sign.
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From page 91... ...
Proposed Methods for Estimating Wind Drag Coefficients and the Associated Wind Loads 91 3.2.3 Wind Drag Coefficient Cd for a Sign Mounted on a Grade Separation Structure Based on the simulation results in Section 2.1.11 (Tables 2.18, 2.19, 2.20, 2.21) , the wind loads over the different subzones and the total wind load acting on a highway sign mounted on a grade separation structure that includes a barrier rail (Configuration 1, Figure 3.5)
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From page 92... ...
92 Wind Drag Coefficients for Highway Signs and Support Structures Once the drag coefficient for the isolated sign, Cd0, is determined from Figure 3.1, the drag coefficients for the different subzones and the full sign are calculated as described below. If h − hlz ≤ hd + hbr: • If the wind is directed toward the front face of the sign: Cdlz = 1.3Cd0, Cdmz = 1.45Cd0 for Configuration 1.
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From page 93... ...
Proposed Methods for Estimating Wind Drag Coefficients and the Associated Wind Loads 93 • The diameter of the monotube is replaced by the diameter of the vertical column. • The height h of the sign is replaced by its width when estimating the modification factor Ks.
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From page 94... ...
94 Wind Drag Coefficients for Highway Signs and Support Structures Because both Re and AR are nondimensional variables, the units of V, dc, Lc, and vair need to be consistent (e.g., ft/s, ft, ft, and ft2/s)
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From page 95... ...
Proposed Methods for Estimating Wind Drag Coefficients and the Associated Wind Loads 95 For two side-by-side signs, use the following rule for each half-gap (0.5s) portion of the monotube adjacent to a sign (Figure 3.11)
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From page 96... ...
96 Wind Drag Coefficients for Highway Signs and Support Structures Based on results reported in Section 2.2.4, the modification factor Ku for individual truss chords is determined as follows: • For the front chords (facing the wind) of 2-chord, 3-chord, and 4-chord trusses: Use Ku = 0.9 for the bottom chord, and use Ku = 1.0 for the top chord.
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From page 97... ...
Proposed Methods for Estimating Wind Drag Coefficients and the Associated Wind Loads 97 If more than one sign is present, use the following rule for each half-gap (0.5s) portion of the chord adjacent to a sign (as shown in Figure 3.11)
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From page 98... ...
98 Wind Drag Coefficients for Highway Signs and Support Structures • Km = 1.20 for the (shielded) back-face members of 4-chord trusses.
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From page 99... ...
Proposed Methods for Estimating Wind Drag Coefficients and the Associated Wind Loads 99 Based on results reported in Section 2.2.4.4, assume the following values for the factor Kmg: • For gusset plates situated behind a sign, use Kmg = 0. • For gusset plates on the front face, use Kmg = 1.1.
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From page 100... ...
100 Wind Drag Coefficients for Highway Signs and Support Structures one should assume that Cd = Cd0 for both columns. Even in the case when the wind load is applied parallel to the support structure, the shielding effect on the downstream column can be neglected due to the large ratio of the distance between the two columns and the diameter of each column.
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