Wind Drag Coefficients for Highway Signs and Support Structures (2023) / Chapter Skim
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From page 13... ...
13 Findings 2.1 Drag Coefficients for Highway Signs This chapter presents the results of the simulations used to validate the numerical model and to produce the data needed to propose general methodologies for determining wind loads on highway signs, including signs of complex shapes, and their support structures. 2.1.1 Preliminary Simulations In this research, several simulations are performed for a constant-thickness dynamic message sign (Figure 2.1)
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14 Wind Drag Coef cients for Highway Signs and Support Structures Figure 2.1. Main geometrical variables for a thin or thick (dynamic message)
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Findings 15 and the faster-moving ow around the sign. Although the separated shear layers are (anti)
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16 Wind Drag Coef cients for Highway Signs and Support Structures domain. An increase in either of these distances reduces the sign blockage ratio.
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Findings 17 A simulation is also conducted for the canonical case of a 2-D thin plate (a wide rectangular plate spanning the whole width of the computational domain) situated far from any solid boundaries (i.e., no ground surface)
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18 Wind Drag Coefficients for Highway Signs and Support Structures simulations capture the qualitative trend of decreasing Cdʹ with increasing sign depth ratio d/h, as observed in the wind-tunnel experiment. This effect is present at all aspect ratios.
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Findings 19 Chowdhury et al.
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20 Wind Drag Coef cients for Highway Signs and Support Structures As shown in Figure 2.7, the height and width of the main sign are h and b, respectively. e dimensions of the add-on sign are ha and ba.
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Findings 21 of the main variables (h0, hg, mean wind velocity) are the same as those described in Section 2.1.4 and Appendix A.
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22 Wind Drag Coef cients for Highway Signs and Support Structures • For the thick signs (d/h = 0.5) used in Series 2, the predicted Cd0 values are 1.11 (AR = 4)
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Findings 23 • For Series 3, the peak value of Cd/Cd0 for the sign with AR = 1 is close to 1.27 in the experiments and to 1.25 in the simulations. • For Series 4, the peak value of Cd/Cd0 for the sign with AR = 2 is close to 1.25 in the experiment and to 1.23 in the simulations.
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24 Wind Drag Coefficients for Highway Signs and Support Structures shown in Figure 2.1. The thickness of the thin rectangular sign is held constant (d of 0.167 ft)
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Findings 25 • For b/h = 1, the current simulations predict Cd = 1.14 for height h < 8 ft and Cd = 1.22 for h = 20 ft. These values are close to the value (Cd = 1.16)
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26 Wind Drag Coefficients for Highway Signs and Support Structures • Series 3: h = 8 ft; hg = 20 ft; d = 4 ft with 3 degrees inclined upstream face; b/h = 1, 6. • Series 4: h = 8 ft; hg = 16.5 and 23 ft; d = 4 ft; b/h = 1, 6.
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Findings 27 b/h = 6 when 16.5 ft < hg < 23 ft. For both b/h = 1 and b/h = 6, Cd decreases monotonically with increasing hg.
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28 Wind Drag Coefficients for Highway Signs and Support Structures 2.1.7 Rectangular Highway Signs that Include an Add-On Sign Many highway signs include a smaller add-on sign (Figure 2.7)
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Findings 29 The results are qualitatively similar for the case of a wider add-on sign (ba = 7.5 ft, ba/b = 0.25) attached to the main sign (Figure 2.16b and Table 2.8)
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30 Wind Drag Coefficients for Highway Signs and Support Structures • An amplification factor close to 1.05 can then be applied to estimate the drag coefficient for the full sign. These results are consistent with those obtained from the wind-tunnel experiments discussed in Section 2.1.4, which also show that the maximum amplification of drag coefficients for static rectangular signs with an add-on sign is on the order of 5% (compared to drag coefficients measured for the isolated static sign)
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Findings 31 in others a smaller sign is placed next to a sign with a larger area. The main effect investigated is how the drag coefficients for the two signs change with the nondimensional gap spacing 2s/(b1 + b2)
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32 Wind Drag Coefficients for Highway Signs and Support Structures For identical side-by-side signs, the main findings related to the drag coefficient values are as follows: • Drag coefficient first increases with the decrease in distance between the two signs, s or 2s/(b1 + b2)
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From page 33... ...
Findings 33 corresponds to an increase of about 23% compared to the value expected for isolated signs of the same size and thickness (Cd = 1.133)
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34 Wind Drag Coefficients for Highway Signs and Support Structures may be up to 30% larger than the comparable value predicted for the same isolated small sign. Meanwhile, the peak increase of Cd for the larger sign is much smaller.
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Findings 35 Several simulations are performed for rectangular signs attached to an overhead cantilevertype monotube structure. The specific design under consideration is used by the New York State DOT (Figure 2.19)
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36 Wind Drag Coefficients for Highway Signs and Support Structures Figure 2.19. Overhead cantilever-type monotube structure used by New York State DOT (used with permission from the New York State Department of Transportation)
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Findings 37 Figure 2.20. Main geometrical variables for highway sign mounted on overhead cantilever-type truss structure.
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38 Wind Drag Coefficients for Highway Signs and Support Structures • In all simulations, the elevation of the bottom chords falls within the range (18 ft to 26.2 ft) documented by the survey of state DOTs.
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Findings 39 When the thin sign (h = 8.2 ft, b/h = 3.75) is positioned on the back face of the truss relative to the incoming wind direction, the drag coefficient is smaller because of the partial shielding provided by the truss members in front of the sign.
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40 Wind Drag Coefficients for Highway Signs and Support Structures Figure 2.23. Michigan DOT 3-chord truss used in simulations with highway signs supported by an overhead bridge-type 3-chord truss structure (used with permission from the Michigan Department of Transportation)
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Findings 41 (thickness d = 0.17 ft) with height h = 8.2 ft and varying aspect ratio b/h = 1, 3.75, and 8.
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42 Wind Drag Coefficients for Highway Signs and Support Structures Figure 2.25. Iowa DOT truss design used in simulations with highway signs supported by an overhead cantilever-type 4-chord truss structure (used with permission from the Iowa Department of Transportation)
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Findings 43 h (ft)
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44 Wind Drag Coefficients for Highway Signs and Support Structures Figure 2.29. Main geometrical variables for Configuration 2 where a separation rail is present at the edges of traffic lanes.
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Findings 45 • The thickness of the bridge deck is assumed to be hd = 0.67 ft. • The height of the solid barrier rail hbr is assumed to be 3.3 ft for Configuration 1 and 2.83 ft for Configuration 2.
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46 Wind Drag Coefficients for Highway Signs and Support Structures subzone areas are Al, Am, and Au. The wind loads acting on each subzone are computed by using the incoming wind velocity and the corresponding area.
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Findings 47 edge of the sign moves downward in the space between the sign and the region defined by the bridge, the girders, and the rail. This accelerated flow decreases the pressure on the back face of the sign (e.g., based on the Bernoulli equation)
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48 Wind Drag Coefficients for Highway Signs and Support Structures 2.32a: h = 8 ft and b = 30 ft 2.32b: h = 16 ft and b = 30 ft 2.32c: h = 16 ft and b = 8 ft Note: The horizontal lines show the vertical extent of the different subzones depicted in Figure 2.30. Figure 2.32.
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Findings 49 Compared to the lower subzone, the drag coefficients are about 30% larger for the two subzones between the bridge deck and the top of the sign (Table 2.18)
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50 Wind Drag Coef cients for Highway Signs and Support Structures simulation series for Con guration 2 monotonically decreases with increasing ds1 starting with ds1 = 1.0 , which has the largest predicted drag coe cient (Cd = 1.6) (Table 2.19)
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Findings 51 Configuration 2, Series 2: For the second series of Configuration 2 simulations, the drag coefficient for the isolated thin static sign (no bridge behind it)
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52 Wind Drag Coefficients for Highway Signs and Support Structures for instances when the wind is directed toward the front face of the sign. This outcome occurs because the (upper)
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Findings 53 However, for Configuration 1, the drag coefficient for this subzone is smaller than the one for an isolated sign (Cduz ≈ 0.45Cd0) while the opposite is true for Configuration 2 (Cduz ≈ 1.2Cd0)
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From page 54... ...
54 Wind Drag Coefficients for Highway Signs and Support Structures Series 2: The second series of simulations considers the case of a thin rectangular sign placed next to a dynamic message sign on the monotube structure: • The dimensions of the thin rectangular sign are h = 8.2 ft, b1/h = 3.75, d1 = 0.17 ft. • The dimensions of the dynamic message sign are h = 8.2 ft, b2/h = 3.75, d2 = 4 ft.
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Findings 55 The maximum increase of Cd for the thick dynamic message sign in front of the monotube -- Cd = 1.49 when 2s/(b1 + b2) = 0.05 -- is almost 25% compared to the limiting case of an isolated thick sign with no monotube present (Cd = 1.2)
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56 Wind Drag Coefficients for Highway Signs and Support Structures Table 2.23 contains the predicted drag coefficients for the three simulations. Similar to the case of single signs mounted on a truss support structure, the presence of the truss behind the two signs has a very small influence on the drag coefficient for the two signs (less than 3% variation in Cd)
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Findings 57 Two simulations are conducted for identical thin signs with the following dimensions: • d = d1 = d2 = 0.17 ft.
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58 Wind Drag Coefficients for Highway Signs and Support Structures signs (less than 3% variation in Cd)
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Findings 59 good agreement with published experimental data (e.g., Wieselsberger 1921, Fage and Warsap 1929, Achenbach 1968)
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60 Wind Drag Coefficients for Highway Signs and Support Structures For an isolated cylinder, the polar angle at which the flow separates increases from about 90 degrees for Re = 10,000 to around 108 degrees for Re ≈ 500,000. The change of the separation line on the cylinder is the main reason for the reduced value (Cd = 0.59)
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Findings 61 between the two members. Simulations are conducted with the following values of the main variables: • w/d = 1.5, 3, 6, 15, 40.
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62 Wind Drag Coefficients for Highway Signs and Support Structures the values noted for members of circular cross-section are the same as those cited in Section 2.2.1. As expected, only a small reduction of Cd with increasing Reynolds number is observed for L-shaped members (e.g., from Cd = 2.25 for Re = 50,000 to Cd = 2.15 for Re = 1,500,000)
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Findings 63 for Re > 1,500,000, Cd changes rather slowly with increasing Reynolds numbers, the values in Tables 2.27 and 2.28 for Re = 1,500,000 can be used for 1,500,000 < Re < 2,500,000. This approach should allow estimating the drag coefficient for the largest members of sign support structures, assuming design wind velocities up to 130 mph and chord diameters of less than 20 in.
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64 Wind Drag Coefficients for Highway Signs and Support Structures signs with larger h as more of the incoming airflow will be deflected close to the lateral edges of the sign (rather than close to its top and bottom edges)
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Findings 65 Figure 2.41a plots the lateral variation of Cd/Cd0 with y/(bh) 0.5 for a thin static sign (d = 0.17 ft)
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66 Wind Drag Coefficients for Highway Signs and Support Structures The part of the monotube situated very close to the edge of the sign is still shielded, which explains the very small values of Cd/Cd0 for y/(bh)
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Findings 67 be significant is (b1h1) 0.5 for the first sign and (b2h2)
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68 Wind Drag Coefficients for Highway Signs and Support Structures 2.2.4 Wind Loads on Trusses Supporting Highway Signs The series of simulations conducted with one or two highway signs (discussed in Sections 2.1.8 and 2.1.11, respectively) are used to estimate the wind loads acting on the chords and secondary members of truss structures (Figures 2.20, 2.34, and 2.37)
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Findings 69 Drag coefficients for the truss members are estimated for the following simulations performed with static signs and DMS cabinets: • For static signs, the simulations consider signs with height h = 8.2 ft and b/h = 1, 2, 3.75, and 8 and also signs with height h = 20 ft and b/h = 1.5. • For DMS cabinets (d = 4 ft)
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70 Wind Drag Coefficients for Highway Signs and Support Structures in Table 2.29, or as part of a group including the members connecting the front and back faces of the truss (Group E)
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Findings 71 arises because the transverse deflection of the region of accelerating airflow forming near the lateral edge of the sign increases with distance from the sign. The results in Figure 2.47 show that in a good approximation, Cd/Cd0 ≈ 1 for y/(bh)
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72 Wind Drag Coefficients for Highway Signs and Support Structures signs with identical b and h, the nondimensional transverse location where the peak occurs is further away from the sign for dynamic message signs. This effect is larger for signs with a low b/h ratio.
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Findings 73 Secondary members that are part of the back face and are situated in the wakes of the frontface members -- but sufficiently far from the highway sign (e.g., outside of the flow-acceleration region) -- are subjected to smaller forces compared to the corresponding front-face members (Cd/Cd0 ≈ 0.6–0.8)
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74 Wind Drag Coefficients for Highway Signs and Support Structures four chords and the isolated secondary truss members are identical to those in Section 2.2.4.1. A series of simulations addresses the case when the truss supports two identical thin highway signs with the following dimensions: • h1 = h2 = h = 8 ft.
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Findings 75 distance between the two signs (0 < y < s/2) , which means that the interaction between the two signs is negligible in the case when s/b = 1.
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76 Wind Drag Coefficients for Highway Signs and Support Structures (Figures 2.23 and 2.24)
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Findings 77 respectively. The slight reduction in Cd of the lower chord H2 (compared to the higher chord H1)
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78 Wind Drag Coefficients for Highway Signs and Support Structures Secondary truss members that are part of the front face situated away from the traffic sign (e.g., outside of the flow-acceleration regions) are subject to wind loads comparable to those observed when the same member is isolated (Cd/Cd0 = 1, ±0.05)
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From page 79... ...
Findings 79 because in the limit of an angle of inclination between the member and the horizontal plane of zero degrees, only the viscous drag contributes to the total drag force, so one can expect that Cd/Cd0 ≈ 0. For the geometry of the 3-chord truss analyzed here, Cd/Cd0 ≈ 0.6 for the secondary members of the inclined faces (Figure 2.51)
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80 Wind Drag Coefficients for Highway Signs and Support Structures Figure 2.54. Convention used in labeling truss members for the overhead cantilever-type 4-chord truss used by the Iowa DOT.
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Findings 81 Similar to the bridge-type 4-chord truss considered in Section 2.2.4.1, the drag coefficient can be assumed to equal zero over the part of the chord situated behind the highway sign. Although Cd0 is not the same for the two front-face chords (H1 and H2)
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From page 82... ...
82 Wind Drag Coefficients for Highway Signs and Support Structures the highway signs have values of |Cd/Cd0| < 0.15, so one can simply assume that Cd/Cd0≈ 0 for these members. For the simulation conducted with a sign of height h = 8.2 ft and b/h = 1, the width of the flow-acceleration region is only around 3.5 ft for the front face and around 6 ft near the back face (Figure 2.55)
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Findings 83 2.2.4.5 Butterfly-Type 4-Chord Truss Supporting Two Highway Signs Normalized drag coefficients for chords and secondary members are also estimated for an overhead butterfly-type 4-chord truss (Figure 2.36)
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84 Wind Drag Coefficients for Highway Signs and Support Structures group including the members connecting the front and back faces of the truss (Group E)
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Findings 85 • For the case with s/b = 0.25, the peak values of Cd/Cd0 are around 1.6 for H1 and H2 and around 1.0 for H3 and H4. • For all four chords, a constant value of Cd/Cd0 can be assumed over the gap distance between the signs.
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From page 86... ...
86 Wind Drag Coefficients for Highway Signs and Support Structures • Interior-diagonal members that connect the front and back faces but are not close to the vertical column (e.g., E4 and E13) are subject to relatively small drag forces (Cd/Cd0 < 0.4)
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