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Calculation of Wind Loads on Structures according to ASCE 7-10Permitted ProceduresThe design wind loads for buildings and other structures, including the MainWind-Force Resisting System (MWFRS) and component and claddingelements thereof, shall be determined using one of the procedures asspecified in the following section. An outline of the overall process for thedetermination of the wind loads, including section references, is provided inFigure (1).Main Wind-Force Resisting System (MWFRS)Wind loads for MWFRS shall be determined using one of the followingprocedures:(1) Directional Procedure for buildings of all heights as specified in Chapter27 for buildings meeting the requirements specified therein;(2) Envelope Procedure for low-rise buildings as specified in Chapter 28 forbuildings meeting the requirements specified therein;(3) Directional Procedure for Building Appurtenances (rooftop structuresand rooftop equipment) and Other Structures (such as solid freestandingwalls and solid freestanding signs, chimneys, tanks, open signs, latticeframeworks, and trussed towers) as specified in Chapter 29;(4) Wind Tunnel Procedure for all buildings and all other structures asspecified in Chapter 31.218

Figure (1): Dtermination of Wind Loads219

Directional ProcedureStep 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 26.5-1A, B or C (United States). Basic wind speed is a threesecond gust speed at 10 m above the ground in Exposure C.Step 3: Determine wind load parameters: Wind directionality factor, 𝐾𝑑 , see Table 26.6.1220

Table 26.6.1: Wind directionality factor, 𝑲𝒅 Exposure category, for each wind direction considered, the upwindexposure shall be based on ground surface roughness that is determinedfrom natural topography, vegetation, and constructed facilities.Surface Roughness B: Urban and suburban areas, wooded areas, or otherterrain with numerous closely spaced obstructions having the size of singlefamily dwellings or larger.Surface Roughness C: Open terrain with scattered obstructions havingheights generally less than 9.1 m. This category includes flat open countryand grasslands.Surface Roughness D: Flat, unobstructed areas and water surfaces. Thiscategory includes smooth mud flats, salt flats, and unbroken ice. Topographic factor, 𝐾𝑧𝑑 , see Figure 26.8-1.𝐾𝑧𝑑 (1 𝐾1 𝐾2 𝐾3 )2, where 𝐾1 , 𝐾2 and 𝐾3 are given in Fig. 26.8-1. Forflat terrains, 𝑲𝒛𝒕 . 𝟎 .221

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Gust factor, G:The gust effect factor for a rigid building is permitted to be taken as 0.85. Enclosure classification:Open Building: A building having each wall at least 80 percent open. Thiscondition is expressed for each wall by the equation Ao 0.8 Ag whereAo total area of openings in a wall that receives positive external pressureAg the gross area of that wall in which Ao is identifiedPartially Enclosed Building: A building that complies with both of thefollowing conditions:1. The total area of openings in a wall that receives positive externalpressure exceeds the sum of the areas of openings in the balance of thebuilding envelope (walls and roof) by more than 10 percent.2. The total area of openings in a wall that receives positive externalpressure exceeds (0.37 m2) or 1 percent of the area of that wall, whichever issmaller, and the percentage of openings in the balance of the buildingenvelope does not exceed 20 percent.Enclosed Building: It is a building that is not classified as open or partiallyenclosed. Internal pressure coefficient, 𝐺𝐢𝑝𝑖 , see Table 26.11-1.Table 26.11-1; Internal Pressure Coefficient223

Step 4: Determine velocity pressure exposure coefficient, 𝐾𝑧 π‘œπ‘Ÿ πΎβ„Ž , seeTable 27.3-1. Note that πΎβ„Ž is constant and calculated for mean height of thebuilding, while 𝐾𝑧 varies with heights measured from the base of thebuilding.Step 5: Determine velocity pressure, π‘žπ‘§ π‘œπ‘Ÿ π‘žβ„Ž , see equation below.π‘žπ‘§ 0.613 𝐾𝑧 𝐾𝑧𝑑 𝐾𝑑 𝑉 2224

where:π‘žπ‘§ velocity pressure calculated at height z, (N/m2)π‘žβ„Ž velocity pressure calculated at mean roof height h, (N/m2)𝐾𝑑 wind directionality factor𝐾𝑧 velocity pressure exposure coefficient𝐾𝑧𝑑 topographic factor𝑉 basic wind speed, in m/sStep 6: Determine external pressure coefficients, 𝐢𝑝 (Figure 27.4-1)225

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Step 7: Determine wind pressure, p, on each building surface (enclosed andpartially enclosed).𝑝 π‘žπΊ 𝐢𝑝 π‘žπ‘– (𝐺 𝐢𝑝𝑖 )Design wind load cases are shown in Figure 27.4-8.227

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Example:It is required to calculate the lateral wind loads acting on the 8-storybuilding, considering the wind is acting first in the North-South direction.The building which is used as headquarter for police operation, is 30 m x 15m in plan as shown in the figure (enclosed), and located right on the GazaBeach (flat terrain).Note: Use a basic wind speed of 100 Km/hr and ASCE 7-10 DirectionalProcedure.PlanElevation229

Step 1: Building risk category: Based on Table 1.5-1, building risk category is IV.Step 2: Basic wind speed: It is given as 100 km/hr.Step 3: Building wind load parameters:K d 0.85 (wind directionality factored evaluated from Table 26.6.1) Exposure category is DK zt 1.0 (Topographic factor for flat terrain) Gust factor, G , is 0.85 for rigid buildings Building is enclosed Internal pressure coefficient for enclosed buildings, 𝐺𝐢𝑝𝑖 , is 0.18Step 4: Velocity pressure coefficients, K h and K z :K h 1.384 (Interpolating from Table 27.3-1) and K z varies with heightStep 5: Determine velocity pressure, qh and qz : qh 0.613 Kh K zt Kd V 22 100,000 556.43 N / m2 0.613 1.384 1.0 0.85 60 60 2qz 0.613 K z K zt K d V2 100,000 402.05 K z N / m2 0.613 K z 1.0 0.85 60 60 Step 6: External pressure coefficients, C p :ForL/ B 15 0. 530and using Figure 27.4.1, the external pressurecoefficients are shown in the figure.230

Step 7: Wind pressure, p :For the windward walls,p qz G C p qi G C pi qz 0.85 0.8 556.43 0.85 0.18 0.68 qz 85.13 N / m2 (max)For the leeward walls,p qh G C p qi G C pi 556.43 0.85 0.5 556.43 0.85 0.18 321.62 N / m2 (max)For the side walls,p qh G C p qi G C pi 556.43 0.85 0.7 556.43 0.85 0.18 416.21 N / m2 (max)Height, metersKzqzp0 to 4.6 m4.6 to 6.1m6.1 to 7.6 m7.6 to 9.1 m9.1 to 12.2 m12.2 to 15.2 m15.2 to 18 m18 to 21.3 m21.3 to 24.4 m24.4 to 25 62.34467.82231

Requirements for Structural IntegrityA structure is said to have structural integrity if localized damage does notspread progressively to other parts of the structure. Experience has shownthat the overall integrity of a structure can be substantially enhanced byminor changes in detailing of reinforcement.The 1989 ACI Code introduced section 7.13. which provides details toimprove the integrity of joist construction, beams without stirrups andperimeter beams. These requirements were updated, and shown below. In detailing of reinforcement and connections, members of a structureshall be effectively tied together to improve integrity of the overallstructure. In joist construction, at least one bottom bar shall be continuous andshall be anchored to develop fy at the face of supports. Beams along the perimeter of the structure shall have continuousreinforcement consisting of:(a) at least 1/6 of the tension reinforcement required for negativemoment at the support, but not less than 2 bars;(b) at least ΒΌ of the tension reinforcement required for positivemoment at mid span , but not less than 2 bars.- The above reinforcement shall be enclosed by close stirrups or hoopsalong the clear span of the beam.- Where splices are needed to provide the required continuity, topreinforcement shall be spliced at or near mid span and bottomreinforcement shall be spliced at or near the support. Splices shall beClass B tension lap splices or mechanical or welded splices. In other than perimeter beams, structural integrity reinforcementshall be in accordance with (a) or (b):(a) At least 1/4 of the positive moment reinforcement required atmid span, but not less than 2 bars.(b) Longitudinal reinforcement shall be enclosed by closed stirrupsor hoops along the clear span of the beam.232

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Diaphragm Key ComponentsDiaphragm Slab:It is the component of the diaphragm which acts primarily to resist shearforced developed in the plane of the diaphragm.Diaphragm Chords:They are components along the diaphragm edges with increased longitudinaland transverse reinforcement, acting primarily to resist tension andcompression forces generated by bending in the diaphragm.Diaphragm Collectors:They are components that serve to transmit the internal forces within thediaphragm to elements of the lateral force resisting system. They shall bemonolithic with the slab, occurring either within the slab thickness or beingthickened.Diaphragm Struts:They are components of a structural diaphragm used to provide continuityaround an opening in the diaphragm. They shall be monolithic with the slab,occurring either within the slab thickness or being thickened.Distribution of Forces:For rigid diaphragms the distribution of forces to vertical elements will beessentially in proportion to their relative stiffness with respect to each other.234

Diaphragm Chord / Beam dCompressive StressSupportSupportTensile Stress17235

Horizontal Diaphragm BoundariesBoundariesBoundariesBoundaryInterior shearwallBoundariesBoundariesBoundariesDiaphragm boundaries may not justoccur at the perimeter of thediaphragm. Interior shear walls anddrag members create diaphragmboundaries.12236

Requirements for Structural DiaphragmsFloor and roof slabs acting as structural diaphragms to transmit designactions induced by earthquake ground motions shall be designed inaccordance with section 18.12 of ACI Code.1- Scope:Diaphragms are used in building construction are structural elements such asfloors and roofs that provide some or all of the following actions: Support for building elements such as walls, partitions, andcladding resisting horizontal forces but not acting as part of thebuilding vertical lateral force resisting system. Transfer of lateral forces from the point of application to thebuilding vertical lateral force resisting system. Connection of various components of the building lateral forceresisting system with appropriate stiffness so the building respondsas intended in the design.2- Minimum Thickness of Slab: Concrete slabs serving as structural diaphragms used to transmitearthquake forces shall not be less than 5 cm thick.3- Reinforcement: The minimum reinforcement ratio for structural diaphragms shallnot be less than the shrinkage and temperature reinforcement ratio.Reinforcement spacing each way shall not exceed 45 cm Diaphragm chord members and collector elements withcompressive stresses exceeding 0.2 f c at any section shall havetransverse reinforcement over the length of the element as perspecial moment resisting frame transverse reinforcement. Thespecial transverse reinforcement is allowed to be discontinued at asection where the calculated compressive stress is less than0.15 f c . Stresses are calculated for the factored forces using alinearly elastic model and gross-section properties of the elementsconsidered.237

All continuous reinforcement in diaphragms, chords and collectorelements shall be anchored and spliced in accordance with theprovisions for tension reinforcement discussed in joints of specialmoment frame provisions.4- Design Forces:The seismic design forces for structural diaphragms shall be obtained fromthe lateral load analysis in accordance with the design load combinations.5- Shear Strength:Nominal shear strength Vn of structural diaphragms shall not exceed Vn Acv 0.53 f c n f y 2.12 Acvf c 6- Boundary Elements: Boundary elements of structural diaphragms shall beproportioned to resist the sum of the factored axial forces actingin the plane of the diaphragm and the force obtained fromdividing the factored moment at the section by the distancebetween the boundary elements of the diaphragm at thatsection. Reinforcement for chord and collectors at splices and anchoragezones shall have either:(a) A minimum center-to-center spacing of three longitudinalbar diameters, but not less than 4 cm, and a minimumconcrete cover of 2.5 d b , but not less than 5 cm;(b) Transverse reinforcement as required per minimum shearreinforcement in beams, except where compressive stressesexceed 0.2 f c .238