For plant structures designed under Indian codes, the program calculates the wind load per IS 875 Part 3 Wind Load on Buildings and Structures, Fourth Revision (2002). A static wind pressure is applied to the structure by the program using the following general procedure.

1. Calculate Basic Wind Speed, V_b, based on mapped values (Figure 1 in Section 5) or the table supplied in Appendix A of IS 875.
2. Calculate Probability Factor (risk coefficient), k₁, from Table 1 in Section 5 of IS 875.

3. Calculate the k₂ Factor (Table 2 in IS 875) based on structure height and terrain category. Separate k₂ values are determined at different height intervals for vertical vessels as well as for each horizontal vessel in a stacked configuration.

4. Calculate the Topography Factor, k₃ per Appendix C in IS 875.

5. Calculate the Design Wind speed for each height considered (IS 875, Clause 5.3) as:

   $V_z=V_b{k}_1{k}_2{k}_3$
   Note: The cyclonic factor, k₄, is typically not significant for foundations and is assumed as 1.0 by the program. However, the C_f value can be multiplied by the desired k₄ value to effectively account for structures in cyclonic regions.

6. Calculate the wind pressure for each height considered (IS 875, Clause 5.4) as:

   $p_z=0.6V_z^2$

7. Calculate the design wind pressure for each height as:

   $p_d=K_a{K}_d{K}_c{p}_z$

   Where:

   K_d = wind directionality factor, as determined in Cl. 5.4.1.

   K_a = area averaging factor, as determined in Cl. 5.4.2.

   K_c = combination factor, taken from Table 19 in Section 6.

8. For each height, the exposure area, A, is consider. This area is calculated differently for vertical and horizontal vessels.

   

   Exposure areas for vertical and horizontal vessels

9. Calculate Force coefficient, C_f, which is considered for structures without openings.

   Note: For vertical vessel, the equipment is symmetric and thus the wind load need only be evaluated from only one direction. Therefore, only one set of C_f is calculated. For a horizontal vessel, the wind is considered from both the longitudinal (x) and transverse (z) directions. So, two sets of C_fn are calculated.

10. Calculate the Wind force, F_n, from Clause 6.3.3.2 of IS 875 for each wind height considered as:

    $F_n=C_{fn}\times p_d\times K\times A$

    Where K = 1 for horizontal and vertical vessels.