This chapter applies to the determination of MWFRS wind loads on enclosed, partially enclosed, and
open buildings of all heights using the directional procedure.
 Part 1 applies to buildings of all heights where it is necessary to separate applied wind loads onto the windward, leeward, and side walls of the building to properly assess the internal forces in the MWFRS members.
 Part 2 applies to a special class of buildings designated as enclosed simple diaphragm buildings, as defined in Section 26.2, with h ≤ 160 ft (48.8 m).
A building whose design wind loads are
determined in accordance with this chapter shall comply with
the following conditions:
 The building is a regularshaped building or structure as defined in Section 26.2.
 The building does not have response characteristics making it subject to acrosswind loading, vortex shedding, or instability due to galloping or flutter, or it does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration.
The provisions of this chapter take into consideration the load magnification effect caused by gusts in resonance with alongwind vibrations of flexible buildings. Buildings
not meeting the requirements of Section 27.1.2, or having unusual
shapes or response characteristics, shall be designed using recognized literature documenting such wind load effects or shall use
the wind tunnel procedure specified in Chapter 31.
There shall be no reductions in velocity pressure due to apparent shielding afforded by buildings and other
structures or terrain features.
The wind load to be
used in the design of the MWFRS for an enclosed or partially
enclosed building shall not be less than 16 lb/ft^{2} (0.77 kN/m^{2})
multiplied by the wall area of the building and 8 lb/ft^{2}
(0.38 kN/m^{2}) multiplied by the roof area of the building projected onto a vertical plane normal to the assumed wind direction. Wall and roof loads shall be applied simultaneously. The
design wind force for open buildings shall be not less than 16
lb/ft^{2} (0.77 kN/m^{2}) multiplied by the area A_{f}.
Part 1 Enclosed, Partially Enclosed, and Open Buildings of all Heights
The steps to determine the wind loads on the MWFRS for
enclosed, partially enclosed, and open buildings of all heights
are provided in Table 27.21.
Table 27.21 Steps to Determine MWFRS Wind Loads for Enclosed, Partially Enclosed, and Open Buildings of All Heights 


Step 1:  Determine risk category of building or other structure, see Table 1.51 
Step 2:  Determine the basic wind speed, V, for the applicable risk category, see Figure 26.51A, B, or C 
Step 3:  Determine wind load parameters: ➢ Wind directionality factor, K_{d}, see Section 26.6 and Table 26.61 ➢ Exposure category, see Section 26.7 ➢ Topographic factor, K_{ɀt}, see Section 26.8 and Figure 26.81 ➢ Gusteffect factor, G, see Section 26.9 ➢ Enclosure classification, see Section 26.10 ➢ Internal pressure coefficient, (GC_{pi}), see Section 26.11 and Table 26.111 
Step 4:  Determine velocity pressure exposure coefficient, K_{ɀ} or K_{h}, see Table 27.31 
Step 5:  Determine velocity pressure q_{z} or q_{h}, see Eq. 27.31 
Step 6:  Determine external pressure coefficient, C_{p} or C_{N}: ➢ Fig. 27.41 for walls and flat, gable, hip, monoslope, or mansard roofs ➢ Fig. 27.42 for domed roofs ➢ Fig. 27.43 for arched roofs ➢ Fig. 27.44 for monoslope roof, open building ➢ Fig. 27.45 for pitched roof, open building ➢ Fig. 27.46 for troughed roof, open building ➢ Fig. 27.47 for alongridge/valley wind load case for monoslope, pitched or troughed roof, open building 
Step 7:  Calculate wind pressure, p, on each building surface: ➢ Eq. 27.41 for rigid buildings ➢ Eq. 27.42 for flexible buildings ➢ Eq. 27.43 for open buildings 
User Note: Use Part 1 of Chapter 27 to determine wind pressures on the MWFRS of enclosed, partially enclosed or an open building with any general plan shape, building height, or roof geometry that matches the figures provided. These provisions utilize the traditional "all heights" method (direc tional procedure) by calculating wind pressures using specific wind pressure equations applicable to each building surface. 
The following wind load parameters shall be determined in accordance with Chapter 26:
Basic wind speed, V (Section 26.5),
Wind directionality factor, K_{d} (Section 26.6),
Exposure category (Section 26.7),
Topographic factor, K_{ɀt} (Section 26.8),
Gusteffect factor (Section 26.9),
Enclosure classification (Section 26.10), and
Internal pressure coefficient, (GC_{pi}) (Section 2611).
Based on the
exposure category determined in Section 26.7.3, a velocity pressure exposure coefficient K_{ɀ} or K_{h}, as applicable, shall be determined from Table 27.31. For a site located in a transition zone
between exposure categories that is near to a change in ground
surface roughness, intermediate values of K_{ɀ} or K_{h}, between
those shown in Table 27.31 are permitted, provided that they
are determined by a rational analysis method defined in the
recognized literature.
Velocity pressure, q_{ɀ}, evaluated at
height ɀ shall be calculated by the following equation:
where
The numerical coefficient 0.00256 (0.613 in SI) shall be used except where sufficient climatic data are available to justify the selection of a different value of this coefficient for a design application.
q_{z} = 0.00256K_{ɀ}K_{ɀt}K_{d}V² (lb/ft²)
(27.31)
[In SI: q_{ɀ} = 0.613K_{ɀ}K_{ɀt}K_{d}V²(N/m²); V in m/s]
where
K_{d}  =  wind directionality factor, see Section 26.6 
K_{ɀ}  =  velocity pressure exposure coefficient, see Section 27.3.1 
K_{ɀt}  =  topographic factor, see Section 26.8.2 
V  =  basic wind speed, see Section 26.5 
q_{z}  =  velocity pressure calculated using Eq. 27.31 at height ɀ 
q_{h}  =  velocity pressure calculated using Eq. 27.31 at mean roof height h. 
The numerical coefficient 0.00256 (0.613 in SI) shall be used except where sufficient climatic data are available to justify the selection of a different value of this coefficient for a design application.
Design wind pressures for the MWFRS of buildings of all
heights shall be determined by the following equation:
where
q and q_{i} shall be evaluated using exposure defined in Section 26.7.3. Pressure shall be applied simultaneously on windward and leeward walls and on roof surfaces as defined in Figs. 27.41, 27.42, and 27.43.
p = qGC_{p} — q_{i}(GC_{pi}) (lb/ft²) (N/m²)
(27.41)
where
q  =  q_{ɀ} for windward walls evaluated at height ɀ above the ground 
q  =  q_{h} for leeward walls, side walls, and roofs, evaluated at height h 
q_{i}  =  q_{h} for windward walls, side walls, leeward walls, and roofs of enclosed buildings and for negative internal pressure evaluation in partially enclosed buildings 
q_{i}  =  q_{ɀ} for positive internal pressure evaluation in partially enclosed buildings where height ɀ is defined as the level of the highest opening in the building that could affect the positive internal pressure. For buildings sited in windborne debris regions, glazing that is not impact resistant or protected with an impactresistant covering shall be treated as an opening in accordance with Section 26.10.3. For positive internal pressure evaluation, q_{i}, may conservatively be evaluated at height h(q_{i} = q_{h}) 
G  =  gusteffect factor, see Section 26.9 
Cp  =  external pressure coefficient from Figs. 27.41, 27.42, and 27.43 
(GCpi)  =  internal pressure coefficient from Table 26.111 
q and q_{i} shall be evaluated using exposure defined in Section 26.7.3. Pressure shall be applied simultaneously on windward and leeward walls and on roof surfaces as defined in Figs. 27.41, 27.42, and 27.43.
Design wind pressures for the MWFRS of flexible buildings shall be determined from the following equation:
where q, q_{i}, C_{p}, and (GC_{pi}) are as defined in Section 27.4.1 and G_{f} (gusteffect factor) is determined in accordance with Section 26.9.5.
p = qG_{f}C_{p} – q_{i}(GC_{pi})(lb/ft²) (N/m²)
(27.42)
where q, q_{i}, C_{p}, and (GC_{pi}) are as defined in Section 27.4.1 and G_{f} (gusteffect factor) is determined in accordance with Section 26.9.5.
The net design pressure for the MWFRS of open
buildings with monoslope, pitched, or troughed roofs shall be
determined by the following equation:
where
Net pressure coefficients, C_{N}, include contributions from top and bottom surfaces. All load cases shown for each roof angle shall be investigated. Plus and minus signs signify pressure acting toward and away from the top surface of the roof, respectively.
For free roofs with an angle of plane of roof from horizontal θ less than or equal to 5° and containing fascia panels, the fascia panel shall be considered an inverted parapet. The contribution of loads on the fascia to the MWFRS loads shall be determined using Section 27.4.5 with q_{p} equal to q_{h}.
p = q_{h}GC_{N}
(27.43)
where
q_{h}  =  velocity pressure evaluated at mean roof height h using the exposure as defined in Section 26.7.3 that results in the highest wind loads for any wind direction at the site 
G  =  gusteffect factor from Section 26.9 
C_{N}  =  net pressure coefficient determined from Figs. 27.44 through 27.47 
Net pressure coefficients, C_{N}, include contributions from top and bottom surfaces. All load cases shown for each roof angle shall be investigated. Plus and minus signs signify pressure acting toward and away from the top surface of the roof, respectively.
For free roofs with an angle of plane of roof from horizontal θ less than or equal to 5° and containing fascia panels, the fascia panel shall be considered an inverted parapet. The contribution of loads on the fascia to the MWFRS loads shall be determined using Section 27.4.5 with q_{p} equal to q_{h}.
The positive external pressure on the
bottom surface of windward roof overhangs shall be determined
using C_{p} = 0.8 and combined with the top surface pressures
determined using Fig. 27.41.
The design wind pressure for the effect of parapets on MWFRS of rigid or flexible buildings with flat, gable, or
hip roofs shall be determined by the following equation:
where
p_{p} = q_{p}(GC_{pn}) (lb/ft^{2})
(27.44)
where
p_{p}  =  combined net pressure on the parapet due to the com bination of the net pressures from the front and back parapet surfaces. Plus (and minus) signs signify net pressure acting toward (and away from) the front (exterior) side of the parapet 
q_{p}  =  velocity pressure evaluated at the top of the parapet 
(GC_{pn})  =  combined net pressure coefficient 
=  +1.5 for windward parapet  
=  –1.0 for leeward parapet 
Main Wind Force Resisting System – Part 1  All Heights  
Velocity Pressure Exposure Coefficients, K_{h} and K_{z}^{}  
Table 27.31  
Notes:

Main Wind Force Resisting System – Part 1  All Heights  
Figure 27.41  External Pressure Coefficients, C_{p}  Walls & Roofs 
Enclosed, Partially Enclosed Buildings  

Main Wind Force Resisting System – Part 1  All Heights  
Figure 27.41 (cont.)  External Pressure Coefficients, C_{p}  Walls & Roofs  
Enclosed, Partially Enclosed Buildings  
Notes:
#For roof slopes greater than 80°, use C_{p} = 0.8

Main Wind Force Resisting System – Part 1  All Heights  
Figure 27.42  External Pressure Coefficients, C_{p}  Domed Roofs 
Enclosed, Partially Enclosed Buildings and Structures  
Notes:

Main Wind Force Resisting System and Components and Cladding  Part 1 
All Heights  
Figure 27.43  External Pressure Coefficients, C_{p}  Arched Roofs  
Enclosed, Partially Enclosed Buildings and Structures  
*When the risetospan ratio is 0.2 ≤ r ≤ 0.3, alternate coefficients given by 6r  2.1 shall also be used for the windward quarter. Notes:

Main Wind Force Resisting System  Part 1  0.25 ≤ h/L ≤ 1.0  
Figure 27.44  Net Pressure Coefficient, C_{N}  Monoslope Free Roofs θ ≤ 45°, γ = 0°, 180° 

Open Buildings  
Notes:

Main Wind Force Resisting System  Part 1  0.25 ≤ h/L ≤ 1.0  
Figure 27.45  Net Pressure Coefficient, C_{N}  Pitched Free Roofs θ ≤ 45°, γ = 0°, 180° 

Open Buildings  
Notes:

Main Wind Force Resisting System  0.25 ≤ h/L ≤ 1.0  
Figure 27.46  Net Pressure Coefficient, C_{N}  Troughed Free Roofs θ ≤ 45°, γ = 0°, 180° 

Open Buildings  
Notes:

Main Wind Force Resisting System Part 1  0.25 ≤ h/L ≤ 1.0  
Figure 27.47  Net Pressure Coefficient, C_{N}  Free Roofs θ ≤ 45°, γ = 90°, 270° 

Open Buildings  
Notes:

Main Wind Force Resisting System  Part 1  All Heights  
Figure 27.48  Design Wind Load Cases  
Notes:

The MWFRS of buildings
of all heights, whose wind loads have been determined under the
provisions of this chapter, shall be designed for the wind load
cases as defined in Fig. 27.48.
EXCEPTION: Buildings meeting the requirements of Section D.2 of Appendix D need only be designed for Case 1 and Case 3 of Fig. 27.48.
The eccentricity e for rigid structures shall be measured from the geometric center of the building face and shall be considered for each principal axis (e_{X}, e_{Y}). The eccentricity e for flexible structures shall be determined from the following equation and shall be considered for each principal axis (e_{X}, e_{Y}):
where
I_{z̄}, g_{Q}, Q, g_{R}, and R shall be as defined in Section 26.9
The sign of the eccentricity e shall be plus or minus, whichever causes the more severe load effect.
EXCEPTION: Buildings meeting the requirements of Section D.2 of Appendix D need only be designed for Case 1 and Case 3 of Fig. 27.48.
The eccentricity e for rigid structures shall be measured from the geometric center of the building face and shall be considered for each principal axis (e_{X}, e_{Y}). The eccentricity e for flexible structures shall be determined from the following equation and shall be considered for each principal axis (e_{X}, e_{Y}):
(27.45)
where
e_{Q}  =  eccentricity e as determined for rigid structures in Fig. 27.48 
e_{R}  =  distance between the elastic shear center and center of mass of each floor 
I_{z̄}, g_{Q}, Q, g_{R}, and R shall be as defined in Section 26.9
The sign of the eccentricity e shall be plus or minus, whichever causes the more severe load effect.
Part 2 Enclosed Simple Diaphragm Buildings with h ≤ 160 ft (48.8m)
The procedure specified herein
applies to the determination of MWFRS wind loads of enclosed
simple diaphragm buildings, as defined in Section 26.2, with a
mean roof height h ≤ 160 ft (48.8 m). The steps required for the
determination of MWFRS wind loads on enclosed simple diaphragm buildings are shown in Table 27.51.
User Note: Part 2 of Chapter 27 is a simplified method for determining the wind pressures for the MWFRS of enclosed, simple diaphragm buildings whose height h is ≤ 160 ft (48.8 m). The wind pressures are obtained directly from a table. The building may be of any general plan shape and roof geometry that matches the specified figures. This method is a simplifica tion of the traditional "all heights" method (directional procedure) contained in Part 1 of Chapter 27. 
Main Wind Force Resisting System – Part 2  h ≤ 160 ft.  
Figure 27.51  Building Class  
Enclosed Simple Diaphragm Buildings  
Note: Roof form may be flat, gable, mansard or hip

In addition to the requirements in Section
27.1.2, a building whose design wind loads are determined in
accordance with this section shall meet all of the following conditions for either a Class 1 or Class 2 building (see Fig. 27.51):
Class 1 Buildings:
Class 2 Buildings:
Class 1 Buildings:
 The building shall be an enclosed simple diaphragm building as defined in Section 26.2.
 The building shall have a mean roof height h ≤ 60 ft (18.3 m).
 The ratio of L/B shall not be less than 0.2 nor more than 5.0 (0.2 ≤ L/B ≤ 5.0).
Table 27.51 Steps to Determine MWFRS Wind Loads Enclosed Simple Diaphragm Buildings [h ≤ 160 ft. (48.8 m)] 


Step 1:  Determine risk category of building or other structure, see Table 1.51 
Step 2:  Determine the basic wind speed, V, for applicable risk category, see Figure 26.51A, B, or C 
Step 3:  Determine wind load parameters: ➢ Exposure category B, C, or D, see Section 26.7 ➢ Topographic factor, K_{ɀt}, see Section 26.8 and Figure 26.81 ➢ Enclosure classification, see Section 26.10 
Step 4:  Enter table to determine net pressures on walls at top and base of building respectively, p_{h}, p_{O}, Table 27.61 
Step 5:  Enter table to determine net roof pressures, P_{ɀ}, Table 27.62 
Step 6:  Determine topographic factor, K_{ɀt}, and apply factor to wall and roof pressures (if applicable), see Section 26.8 
Step 7:  Apply loads to walls and roofs simultaneously. 
 The topographic effect factor K_{ɀt} = 1.0 or the wind pressures determined from this section shall be multiplied by K_{ɀt} at each height ɀ as determined from Section 26.8. It shall be permitted to use one value of K_{ɀt} for the building calculated at 0.33h. Alternatively it shall be permitted to enter the pressure table with a wind velocity equal to V where K_{ɀt} is determined at a height of 0.33h.
Class 2 Buildings:
 The building shall be an enclosed simple diaphragm building as defined in Section 26.2.
 The building shall have a mean roof height 60 ft < h ≤ 160 ft (18.3 m < h ≤ 48.8 m).
 The ratio of L/B shall not be less than 0.5 nor more than 2.0 (0.5 ≤ L/B ≤ 2.0).
 The fundamental natural frequency (Hertz) of the building shall not be less 75/h where h is in feet.
 The topographic effect factor K_{ɀt} = 1.0 or the wind pressures determined from this section shall be multiplied by K_{ɀt} at each height ɀ as determined from Section 26.8. It shall be permitted to use one value of K_{ɀt}, for the building calculated at 0.33h. Alternatively it shall be permitted to enter the pressure table with a wind velocity equal to V where K_{ɀt} is determined at a height of 0.33h .
Refer
to Chapter 26 for determination of basic wind speed V (Section
26.5) and exposure category (Section 26.7) and topographic
factor K_{ɀt} (Section 26.8).
The design procedure specified
herein applies to buildings having either rigid or flexible diaphragms. The structural analysis shall consider the relative stiffness of diaphragms and the vertical elements of the MWFRS.
Diaphragms constructed of wood panels can be idealized as flexible. Diaphragms constructed of untopped metal decks, concretefilled metal decks, and concrete slabs, each having a spantodepth ratio of 2 or less, are permitted to be idealized as rigid for consideration of wind loading.
Diaphragms constructed of wood panels can be idealized as flexible. Diaphragms constructed of untopped metal decks, concretefilled metal decks, and concrete slabs, each having a spantodepth ratio of 2 or less, are permitted to be idealized as rigid for consideration of wind loading.
Net wind pressures for the walls and roof surfaces shall be
determined from Tables 27.61 and 27.62, respectively, for the
applicable exposure category as determined by Section 26.7.
For Class 1 building with L/B values less than 0.5, use wind pressures tabulated for L/B = 0.5. For Class 1 building with L/B values greater than 2.0, use wind pressures tabulated for L/B = 2.0.
Net wall pressures shall be applied to the projected area of the building walls in the direction of the wind, and exterior side wall pressures shall be applied to the projected area of the building walls normal to the direction of the wind acting outward according to Note 3 of Table 27.61, simultaneously with the roof pressures from Table 27.62 as shown in Fig. 27.61.
Where two load cases are shown in the table of roof pressures, the effects of each load case shall be investigated separately. The MWFRS in each direction shall be designed for the wind load cases as defined in Fig. 27.48.
EXCEPTION: The torsional load cases in Fig. 27.48 (Case 2 and Case 4) need not be considered for buildings that meet the requirements of Appendix D.
For Class 1 building with L/B values less than 0.5, use wind pressures tabulated for L/B = 0.5. For Class 1 building with L/B values greater than 2.0, use wind pressures tabulated for L/B = 2.0.
Net wall pressures shall be applied to the projected area of the building walls in the direction of the wind, and exterior side wall pressures shall be applied to the projected area of the building walls normal to the direction of the wind acting outward according to Note 3 of Table 27.61, simultaneously with the roof pressures from Table 27.62 as shown in Fig. 27.61.
Where two load cases are shown in the table of roof pressures, the effects of each load case shall be investigated separately. The MWFRS in each direction shall be designed for the wind load cases as defined in Fig. 27.48.
EXCEPTION: The torsional load cases in Fig. 27.48 (Case 2 and Case 4) need not be considered for buildings that meet the requirements of Appendix D.
Main Wind Force Resisting System – Part 2  h ≤ 160 ft. 

Figure 27.61  Wind Pressures – Walls and Roof  Application of Wind Pressures See Tables 27.61 and 27.62 
Enclosed Simple Diaphragm Buildings  

Main Force Resisting System – Part 2  h ≤ 160 ft. Application of Wall Pressures 

Table 27.61  Wind Pressures – Walls  
Enclosed Simple Diaphragm Buildings  
Notes to Wall Pressure Table 27.61:

Table 27.61 MWFRS – Part 2: Wind Loads – Walls Exposure B 
Table 27.61 MWFRS – Part 2: Wind Loads – Walls Exposure C 
Table 27.61 MWFRS – Part 2: Wind Loads – Walls Exposure D 
The effect of horizontal wind loads applied to
all vertical surfaces of roof parapets for the design of the MWFRS
shall be based on the application of an additional net horizontal
wind pressure applied to the projected area of the parapet surface
equal to 2.25 times the wall pressures tabulated in Table 27.61
for L/B = 1.0. The net pressure specified accounts for both the
windward and leeward parapet loading on both the windward
and leeward building surface. The parapet pressure shall be
applied simultaneously with the specified wall and roof pressures
shown in the table as shown in Fig. 27.62. The height h used
to enter into Table 27.61 to determine the parapet pressure shall
be the height to the top of the parapet as shown in Fig. 27.62
(use h = h_{p}).
Main Wind Force Resisting System  Part 2  h ≤ 160 ft. Application of Parapet Wind Loads  See Table 27 .61 

Table 27.62  Parapet Wind Loads  
Enclosed Simple Diaphragm Buildings  

Main Wind Force Resisting System  Part 2  h ≤ 160 ft. Application of Roof Pressures 

Table 27.62  Wind Pressures  Roof  
Enclosed Simple Diaphragm Buildings  
Notes to Roof Pressure Table 27.62:

Main Wind Force Resisting System  Part 2  h ≤ 160 ft. 

Table 27.62  Wind Pressures  Roof  Application of Roof Pressures 
Enclosed Simple Diaphragm Buildings  
Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

Table 27.62
MWFRS – Part 2: Wind Loads – Walls Exposure C 

The effect of vertical wind loads on
any roof overhangs shall be based on the application of a positive
wind pressure on the underside of the windward overhang equal
to 75% of the roof edge pressure from Table 27.62 for Zone 1
or Zone 3 as applicable. This pressure shall be applied to the
windward roof overhang only and shall be applied simultaneously with other tabulated wall and roof pressures as shown in
Fig. 27.63.
Main Wind Force Resisting System  Part 2  h ≤ 160 ft. Application of Roof Overhang Wind Loads – See Table 27.62 

Table 27.63  Roof Overhang Wind Loads  
Enclosed Simple Diaphragm Buildings  
