IBC 2003

Section 1609 of IBC 2003 gives wind requirements specifically focused on a simplified procedure acceptable for enclosed, low-rise, simple diaphragm buildings having a height of less than 60 feet. For other types, it refers to Section 6 of ASCE 7-02 where three methods are given: Method 1- Simplified Procedure, Method 2- Analytical Procedure, and Method 3- Wind Tunnel Procedure. Method 2 is further composed of two different provisions: method for Low-Rise Buildings and for Buildings of Any Height.

Among these methods, only Method 2-Analytical Procedure for Buildings of Any Height is implemented into the program. This method is similar to Method 2 - Analytical Procedure of ASCE 7-98/IBC 2000 wind provision. See Section 6 of ASCE 7-02.

In the Loads – Load Cases command the IBC 2003 Wind option is referred to as "ASCE 7-02 / IBC 2003".

Exposure A is not available in ASCE 7-02 / IBC 2003.

Rigid Buildings of All Height

Calculation of design wind pressures for rigid buildings is given in Section 6.5.12.2.1 and it is calculated from

p = q GC_p- q_i(GC_pi)

(Equation 6-17)

However, it is assumed that internal pressures cancel each other for the main resisting system so that Equation 6-17 is simplified to

p = q GC_p

where

q = 0.00256 K_zK_ztK_dV²I

(Equation 6-15)

and

K_z: Velocity pressure exposure coefficient from Table 6-3. Provided equations in the notes are used

K_zt: Topographic factor. It can be specified by the user or it is obtained from

K_zt= (1+K₁K₂K₃)²

(Equation 6-3)

where K₁, K₂, L_H and γ for K₃ are entered by the user.

Note that the following limits are checked for these variables: 0.15 ≤ K₁ ≤ 0.775, 0.0 ≤ K₁ ≤ 1.0 and 2.5 ≤ γ ≤ 4.0 If only K_zt is entered, this value is applied for all heights. Otherwise, its variation with height is considered as given in Equation 6-3.

K_d: Wind directionality factor from Table 6-4. It is taken as 0.85 if specified by user.

V: Basic wind speed from Figure 6-1

I: Importance factor from Table.6-1

G: Gust factor calculated from Equation 6-4. It can be taken as 0.85 if specified by the user.

C_p: External pressure coefficient from Figure 6.6. A value of 0.8 is used for Windward wall and an interpolated value is used for Leeward wall if needed.

Flexible Buildings

Calculation of design wind pressures for flexible buildings is given in Section 6.5.12.2.3 and it is calculated from

p = qG_fC_p - q_iGC_pi

(Equation 6-19)

However, it is assumed that internal pressures cancel each other for the main resisting system so that Equation 6-19 is simplified to

p = q G_fC_p

where

G_f

the Gust factor for flexible buildings and it is obtained from Equation 6-8

Pressures on parapets for rigid or flexible structures are calculated as follows:

p_p= q_p GC_pn

(Equation 6-20)

where

P_p	=	combined net pressure on parapet
q_p	=	velocity pressure evaluated at top of parapet from Equation 6-15
GC_pn	=	${\begin{matrix} + 1.8 & for windward parapet \\ - 1.1 & for leeward parapet \end{matrix}$

Section 1609.1.2 of IBC 2003 limits the minimum design wind load to 10 psf for both rigid and flexible buildings and this is also enforced in this implementation.

Building natural frequencies can be entered separately for each direction, or they can be calculated by the program.

Load Cases

Load cases for buildings with any heights are created according to design wind load cases given in Figure 6-9 of ASCE 7-02. The figure contains four cases but they are expanded to 12 cases when both X-axis and Y-axis are selected. If only X-axis or only Y-axis is selected, only the corresponding directions of Case 1 and Case 2 are created. It should be noted that eccentricities for X and Y directions are defined respectively with ±0.15B_x and ±0.15B_y for rigid buildings, where B is the building dimension perpendicular to the wind direction. If the building is flexible in that direction, eccentricities are calculated with respect to Equation 6-21. The distance between center of rigidity and center of mass is used in calculation of e_R. The program automatically creates a load case for calculation of center of rigidity if it is a flexible building. The eccentrically applied loads in both directions are shown in Figure Below. These load cases correspond to Case 4 in Fig. 6-9 of ASCE 7-02.

RAM Structural System Help

IBC 2003

Rigid Buildings of All Height

Flexible Buildings

Load Cases

Generated wind load cases for Case 4