This chapter applies to the determination
of MWFRS wind loads on lowrise buildings using the
envelope procedure.
 Part 1 applies to all lowrise buildings 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 lowrise buildings designated as enclosed simple diaphragm buildings as defined in Section 26.2.
A building whose design wind loads are
determined in accordance with this section 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 28.1.2, 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.
Part 1 Enclosed and Partially Enclosed LowRise Buildings
The steps required for the determination of MWFRS wind loads
on lowrise buildings are shown in Table 28.21.
User Note: Use Part 1 of Chapter 28 to determine the wind pressure on the MWFRS of enclosed, partially enclosed, or open lowrise buildings having a flat, gable, or hip roof. These provisions utilize the envelope procedure by calculating wind pressures from the specific equation applicable to each building surface. For building shapes and heights for which these provisions are applicable, this method generally yields the lowest wind pressure of all analytical methods specified in this standard. 
Table 28.21 Steps to Determine Wind Loads on MWFRS LowRise Buildings  

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 Fig. 26.51A, B, or C Step 3: Determine wind load parameters:
Step 5: Determine velocity pressure, q_{z}, or q_{h}, Eq. 28.31 Step 6: Determine external pressure coefficient, (GC_{p}), using Fig.28.41 for flat and gable roofs
Step 7: Calculate wind pressure, p, from Eq. 28.41 
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_{zt} (Section 26.8),
 Enclosure classification (Section 26.10), and
 Internal pressure coefficient (GC_{pi}) (Section 26.11 ).
Based on the
exposure category determined in Section 26.7.3, a velocity pressure
exposure coefficient K_{z} or K_{h}, as applicable, shall be determined
from Table 28.31.
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_{z} or K_{h},, between those shown in Table 28.31, are permitted, provided that they are determined by a rational analysis method defined in the recognized literature.
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_{z} or K_{h},, between those shown in Table 28.31, are permitted, provided that they are determined by a rational analysis method defined in the recognized literature.
Velocity Pressure Exposure Coefficients, K_{h} and K_{z}  

Table 28.31  
Notes:

Velocity pressure, q_{z}, evaluated at
height z 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 factor for a design application.
q_{z} = 0.00256 K_{z}K_{zt}K_{d}V^{2} (lb/ft^{2})
(28.31)
[In SI: q_{z} = 0.613 K_{z}K_{zt}K_{d}V^{2} (N/m^{2}); V in m/s]
where
K_{d}  =  wind directionality factor defined in Section 26.6 
K_{z}  =  velocity pressure exposure coefficient defined in Section 28.3.1 
K_{zt}  =  topographic factor defined in Section 26.8.2 
V  =  basic wind speed from Section 26.5.1 
q_{h}  =  velocity pressure q_{z} calculated using Eq. 28.31 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 factor for a design application.
Design
wind pressures for the MWFRS of lowrise buildings shall be
determined by the following equation:
p = q_{h}[(GC_{pf})  (GC_{pi})] (lb/ft^{2}) (N/m^{2}) (28.41)
where
p = q_{h}[(GC_{pf})  (GC_{pi})] (lb/ft^{2}) (N/m^{2}) (28.41)
where
q_{h}  =  velocity pressure evaluated at mean roof height h as defined in Section 26.3 
(GC_{pf})  =  external pressure coefficient from Fig. 28.41 
(GC_{pi})  =  internal pressure coefficient from Table 26.111 
The combined
gusteffect factor and external pressure coefficients for
lowrise buildings, (GC_{pf}), are not permitted to be separated.
The design wind pressure for the effect of
parapets on MWFRS of lowrise buildings with flat, gable, or
hip roofs shall be determined by the following equation:
where
p_{p} = q_{p}(GC_{pn}) (lb/ft^{2})
(28.42)
where
p_{p}  =  combined net pressure on the parapet due to the combination 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 
The positive external pressure on the
bottom surface of windward roof overhangs shall be determined
using GC_{p} = 0.7 in combination with the top surface pressures
determined using Fig. 28.41.
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.
Part 2 Enclosed Simple Diaphragm LowRise Buildings
The steps required for the determination of MWFRS wind loads
on enclosed simple diaphragm buildings are shown in Table
28.51.
User Note: Part 2 of Chapter 28 is a simplified method to determine the wind pressure on the MWFRS of enclosed simple diaphragm lowrise buildings having a flat, gable, or hip roof. The wind pressures are obtained directly from a table and applied on horizontal and vertical projected surfaces of the building. This method is a simplification of the envelope procedure contained in Part 1 of Chapter 28. 
The
following wind load parameters are specified in Chapter 26:
 Basic wind speed V (Section 26.5),
 Exposure category (Section 26.7),
 Topographic factor K_{zt} (Section 26.8), and
 Enclosure classification (Section 26.10).
A building whose design wind loads are determined
in accordance with this section shall meet all the conditions
of Section 28.6.2. If a building does not meet all of the conditions of Section 28.6.2, then its MWFRS wind loads shall be determined
by Part 1 of this chapter, by the directional procedure of Chapter
27, or by the wind tunnel procedure of Chapter 31.
For the design of MWFRS the building
shall comply with all of the following conditions:
 The building is a simple diaphragm building as defined in Section 26.2.
 The building is a lowrise building as defined in Section 26.2.
 The building is enclosed as defined in Section 26.2 and conforms to the windborne debris provisions of Section 26.10.31.
 The building is a regularshaped building or structure as defined in Section 26.2.
 The building is not classified as a flexible building 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; and it does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration.
 The building has an approximately symmetrical crosssection in each direction with either a flat roof or a gable or hip roof with θ ≤ 45°.
 The building is exempted from torsional load cases as indicated in Note 5 of Fig. 28.41, or the torsional load cases defined in Note 5 do not control the design of any of the MWFRS of the building.
Simplified design wind pressures,
p_{s}, for the MWFRS of lowrise simple diaphragm buildings
represent the net pressures (sum of internal and external) to be
applied to the horizontal and vertical projections of building
surfaces as shown in Fig. 28.61. For the horizontal pressures
(Zones A, B, C, D), p_{s} is the combination of the windward and
leeward net pressures. p_{s} shall be determined by the following
equation:
where
p_{s} = λ K_{zt} p_{S30}
(28.61)
where
λ  =  adjustment factor for building height and exposure from Fig. 28.61 
K_{zt}  =  topographic factor as defined in Section 26.8 evaluated at mean roof height, h 
p_{S30}  =  simplified design wind pressure for Exposure B, at h = 30 ft (9.1 m) from Fig. 28.61 
Main Wind Force Resisting System  Method 2  h ≤ 60 ft.  

Figure 28.61  Design Wind Pressures  Walls & Roofs  
Enclosed Buildings  
Notes :
Simplified Design Wind Pressure, P_{S30} (psf) (Exposure Bath= 30 ft. with I= 1.0)
Unit Conversions  1.0 ft = 0.3048 m; 1.0 psf = 0.0479 kN/m^{2} 
The load effects of the
design wind pressures from Section 28.6.3 shall not be less than
a minimum load defined by assuming the pressures, p_{s}, for zones
A and C equal to +16 psf, Zones B and D equal to +8 psf, while
assuming p_{s} for Zones E, F, G, and His equal to 0 psf.