The task involved the design of typical signage boards in front of fuel (gas) retail outlets of different total heights. The signages are made in acrylic sheets pasted over Aluminium Composite panel cladding over a structural steel skeleton. This article describes the steps involved in the design of a 7m. high signage.
As per the TOR, the signage boards were to be designed using applicable Euro Codes for a lifespan of 10 years. The structural skeleton was to be designed so that it will be free of rattles and resonant effect of all wind speeds upto 110Kmph.
The predominant design forces are due to wind. Since the vertical loads are small Seismic effects were not considered. Wind loads were computed as per the most severe conditions as in EN1991-1-4:2005 as the location of the signage could be anywhere.
Wind loads on the flag portion of the signage board is asymmetric due to
a) The geometry of the structure itself being asymmetric and
b) Probable instantaneous fluctuations of wind over the surface of the signage.
Following steps illustrate the method used for the structural design of the signage board. Detailed calculations and the formulas used are left out and only the results are indicated.
- Computation of the self weight of the assumed structural members.
- Mean Wind Velocity (45.74m/s) calculated from the Basic Wind Velocity (110Kmph = 30.83 m/s), Directional factor (Cdr =1), Seasonal Factor (Cseason =1), Life Span (10 years), Probability of basic wind speed (10%), Probability Factor (Cprob= 0.902), Terrain Factor (K𝛾= 0.156), Roughness Factor (C𝛾 = 1.158), Orography Factor ( Co = 1.42 assuming location to be on top of hill).
- Calculated Peak velocity pressure ( qp = 2.18 kN/m2) based on Turbulence factor (K1 = 1), its intensity (lv = 0.095), standard deviation (𝜎v =4.342 m/s) and the mean wind velocity.
- The structural dynamic factor (Cd = 1.127) was then calculated from the structural damping (𝛿1 = 0.05), Resonance response (R’ = 0.536), Up-crossing frequency (𝜈 = 3.163 Hz), Average time for mean wind velocity (T’ = 600 secs) and peak factor (Kp =4.04).
- Wind force in the flag portion of the signage ( 29.8 kN) was then calculated as the vectorial sum of the external pressure and the frictional forces, on the basis of the size factor (Cs = 0.969) and the dynamic factor calculated in paragraph d) above.
- The structural meters assumed in step a) were then checked for the stresses resulting from the Bending Moment, Shear Force, Axial Forced the Torsional Moments resulting from the above calculated Wind Force, its point of action (centroid of the flag) and the maximum expected Snow Load (1206N). Connections were also designed once the structural members were proved adequate.
- Deflections were then checked (𝛥h = 39mm) against the permissible limit (Height / 180).
- Critical velocities for Vortex Shedding for wind normal to the surface of the flag (105.5 m/s) and along the flag (19.14 m/s) were then compared with 1.25 times the mean wind velocity (57.17 m/s). Since it was observed the Vortex Shedding occurs at a speed less than the mean velocity for wind long the flag, deflection due to this vortex shedding frequency was then calculated (14mm)and found insignificant.
- As a check for fluttering, the critical wind velocity for divergence (906 m/s) was computed to compare against twice the mean wind velocity and flutter was ruled out.
- Wind velocity for the onset of Galloping was calculated as 449.5 m/s ( fundamental mode, f1x = 6.3 Hz) for wind normal to flag and 88.25 m/s (f1y = 6.7 Hz) for wind along the flag and both values were then compared with 1.25 times the mean wind velocity to exclude any galloping due to wind effects.