IBC 2000
IBC Section 1609.1.1 Determination of wind loads adopts by reference the wind requirements of Section 6 of ASCE 7-98. It also gives some exceptions, which permits the use of alternate methodologies. None of these alternate methods have been be implemented.
ASCE 7-98 gives three methodologies: Section 6.4 Method 1 - Simplified Procedure, Section 6.5 Method 2 - Analytical Procedure, and Section 6.6 Method 3 - Wind Tunnel Procedure. Only Method 2 has been implemented. Method 2 includes provisions for "Buildings and other structures" (Section 6.5.6.2.1) and provisions for "Low-rise buildings" (Section 6.5.6.2.2). The provisions for Low-rise buildings have not been implemented. Refer to ASCE 7-98 for the Sections, Tables, Figures, and Equations cited below.
In the Loads – Load Cases command in RAM Frame, the IBC wind option is referred to as ASCE 7-98 / IBC 2000.
Buildings with a frequency greater than or equal to 1 Hz are classified as "Rigid" structures; those with a frequency less than 1 Hz are classified as "Flexible".
Rigid Structures
| p = q GCp - qi(GCpi) | (Equation 6-15) |
However, since as implemented in the program the internal pressures are assumed to cancel for the main force resisting system, it can be simplified to:
| p = qGCp n | (Equation 6-15) |
where
| q = 0.00256KzKztKdV2I | (Equation 6-13) |
- Kz is the Velocity Pressure Exposure Coefficient and is given in Table 6-5; the equations given in the Notes to that table are used.
- Kzt is the Topographic Factor input by the user.
- Kd is the Wind Directionality Factor. It is given in Table 6-6 as 0.85. It is only applied if specified by the user. See the footnote to that table for applicability. There is a corresponding option in the Load Combination generation command; the selection there should correspond to the selection here.
- V is the Basic Wind Speed. It is taken from Fig. 6-1 and is input by the user.
- I is the Importance Factor. It is taken from Table 6-1 and is input by the user.
- G is given by Equation 6-2. See Section 6.5.8.1. G = 0.85 can also be specified.
- Cp is given by Fig. 6-3. Values for Windward Wall and Leeward Wall are given. The values are determined by the program.
Section 6.1.4.1, Main Wind Force Resisting System, requires that a minimum design wind load of 10 psf be applied over the entire projected area. This force is used if it controls over that calculated using Eq. 6-15.
Flexible Structures
| p = qGfCp - qi(GCpi) (Equation 6-17) |
However, since the internal pressures are assumed to cancel for the main force resisting system in the types of structures in the program, it can be simplified to:
| p = qGfCp | (Equation 6-17) |
where Gf is given by Equation 6-6. See Section 6.5.8.2.
The other variables are the same as for Rigid structures.
Section 6.1.4.1, Main Wind Force Resisting System, requires that a minimum design wind load of 10 psf be applied over the entire projected area. This force is used if it controls over that calculated using Eq. 6-17.
Load Cases
Section 6.5.12.3, Full and Partial Loading, requires that for buildings with mean roof height greater than 60ft, full and partial load cases must be created. These are given in Fig. 6-9. Four Cases are shown, but these expand to 10 cases when both the X-axis and Y-axis are specified by the user. If only the X-axis or only the Y-axis is selected, only the corresponding directions of Cases 1 and 2 are created; Cases 3 and 4 are not.