780 CMR: STATE BOARD OF BUILDING REGULATIONS AND

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780 CMR: STATE BOARD OF BUILDING REGULATIONS AND STANDARDSSTRUCTURAL LOADS1610.6.6 Intersecting drifts: When one snowdrift intersects another at an angle as shown inFigure 1610.9, the unit snow load at any pointshall be not less than the greater of the unit loadsfrom the two individual drifts, plus the unit loadof the underlying uniform snow layer.1610.7 Sliding snow from sloped upper roofs:Two cases of drift loading shall be considered forroofs which are located below upper sloped roofs, asshown in Figure 1610.10 and as follows:(a) Case I Drift loading due to snow from theupper roof computed in accordance with780 CMR 1610.6.1, but without load from slidingsnow (Wbu is the full width of the upper roof asshown in Figure 1610.10.)(b) Case II Drift loading due to snow from thelower roof computed in accordance with780 CMR 1610.6.1 and a sliding snow surchargeload as specified below and as shown in Figure1610.10.The maximum intensity of the sliding snow load,Pds , shall be:(Equation 18)where Wa and Ws are defined in Figure 1610.10 andthe coefficient A is defined as follows:(a) For roof surfaces of metal and slate, and forother roof surfaces smoother than mineralsurfaced roofing: If the angle of slope of the upperroof, “a”, as shown in Figure 1610.10 is equal toor greater than 15 degrees (slope 3.2 in 12), A 1.6; if “a” is less than 15 degrees, A 0 (nosliding snow load).(b) For roof surfaces of mineral surfaced roofingor rougher surfaces:If “a” is equal to or greater than 25 degrees(slope 5.6 in 12), A 1.0;if “a” is less than 25 degrees, A 0.The value of Ws , the width of the sliding snowsurcharge, shall be computed as follows:(a) For “a” less than or equal to 45 degrees,(Equation 19)or(Equation 20)whichever is greater.(b) For “a” greater than or equal to 45 (Equation 21)or(Equation 22)whichever is greater1610.7.1 Snow guards: Sliding snow from anadjacent sloping high roof need not be consideredon the low roof if proper snow guards are1611.2.1 Special exposures: Consideration shallbe given to the application of a more severeexposure (e.g., Exposure C instead of ExposuresB or A) when the ground slope near the site of aprovided on the high roof. In this case, thesloping roof with snow guards shall be designedfor the unit snow loads required for a flat roof.1610.8 Snow pockets or wells: Account shall betaken of the load effects of potentially excessivesnow accumulation in pockets or wells of roofs ordecks.1610.9 Snow storage and collection areas:Consideration of potentially excessive snowaccumulation shall be given to portions of structureswhich may be designed or used as snow collectionor storage areas during and after snow removaloperations.780 CMR 1611.0 WIND LOAD1611.1 Wind load zones: The locations of windload zones are shown in the Figures 1611.1A,1611.1B, 1611.1C maps. Zone 1 consists of theCounties of Berkshire, Franklin, Hampshire andHampden; Zone 2 consists of the County ofWorcester; and Zone 3 consists of the Counties ofEssex, Middlesex, Suffolk, Norfolk, Plymouth,Bristol, Barnstable, Dukes and Nantucket.1611.2 Exposures: Exposure is defined as ameasure of terrain roughness and is classfied asfollows:Exposure A: centers of large cities and veryrough, hilly terrain. Exposure A applies fordowntown areas only when the terrain for at leastone half mile upwind of the structure is heavilybuilt up, with at least 50% of the buildings beingin excess of four stories, and when Exposure Bprevails beyond this boundary.Exercise caution in using these reduced windpressures for buildings and structures on highground in the midst of cities or rough terrain.Exposure B: suburban areas, towns, cityoutskirts, wooded areas, and rolling terrain.Exposure B applies only when the terrain for atleast one half mile upwind is a continuous urbandevelopment, forest, wooded area, or rollingterrain.Exposure C: open level terrain with onlyscattered buildings, structures, trees ormiscellaneous obstructions, open water, orshorelines.structure changes abruptly, in order to account forthe resulting higher wind speeds near groundlevel.9/19/97 (Effective 2/28/97) - corrected 780 CMR - Sixth Edition271

780 CMR: STATE BOARD OF BUILDING REGULATIONS AND STANDARDSTHE MASSACHUSETTS STATE BUILDING CODE1611.3 Reference wind velocities: The referencewind velocity for each wind load zone is the“fastest-mile” wind velocity, in miles per hour, at 30feet about the ground ( V30) for Exposure C, asshown in Table 1611.3:ZoneV30 (mph)170280390Table 1611.3Figure 1611.1AWIND LOAD MAP - ZONE 1List of Towns: Wind Load ZonesZone inConwayCummingtonDaltonDeerfieldE. enGranbyGranvilleGreat ldMonroeMonsonMontagueMontereyMontgomeryMount WashingtonNew AshfordNew MarlboroughNew SalemNorth ldSavoySheffieldShelbourneShutesburyS. WendellW. SpringfieldW. liamsburghWiliamstownWindsorWorthington780 CMR - Sixth Edition corrected - 9/19/97 (Effective 2/28/97)

780 CMR: STATE BOARD OF BUILDING REGULATIONS AND STANDARDSSTRUCTURAL LOADSFIGURE 1611.1bWIND LOAD MAP - ZONE ylstonBrookfieldCharltonClintonDouglasDudleyE. ldenList of Towns: Wind Load ZonesZone urySuttonMillvilleTempletonNew BraintreeUptonN. sterOakhamW. BoylstonOxfordW. llipstonWinchedonPrincetonWorcester9/19/97 (Effective 2/28/97) - corrected 780 CMR - Sixth Edition273

780 CMR: STATE BOARD OF BUILDING REGULATIONS AND STANDARDSTHE MASSACHUSETTS STATE BUILDING CODE274780 CMR - Sixth Edition corrected - 9/19/97 (Effective 2/28/97)

780 CMR: STATE BOARD OF BUILDING REGULATIONS AND STANDARDSSTRUCTURAL LOADSFIGURE 1611.1CWIND LOAD MAP - ZONE 3List of Towns: Wind Load ZonesZone nnisDightonDoverDracutDunstableDuxburyE. BridgewaterEasthamEastonEdgartownEssexEverettFall ay Malden9/19/97 (Effective 2/28/97) - corrected 780 CMR - Sixth NahantNantucketNatickNeedhamNew BedfordNewburyNewburyportNewtonNorfolkN. AndoverN. AttleboroN. ReadingNortonNorwellNorwoodOak BluffsOrleans275

780 CMR: STATE BOARD OF BUILDING REGULATIONS AND STANDARDSTHE MASSACHUSETTS STATE BUILDING yngsboroughVineyard dMiltonWellesleyWellfleetWenhamW. BridgewaterWestfordW. NewburyWestonWestportW. chesterWinthropWoburnWrenthamYarmouthwhen it produces more severe effects in thestructural support system. Factors other than 0.7may be used if substantiated by appropriate windtunnel tests.1611.4 Reference wind pressures: Reference windpressures for the various exposures and wind zonesare given in the following Table 1611.4. Thetabulated pressures are combined windward andleeward pressures representing the overall effect ofthe wind on essentially rectangular structures, andaccount for typical gust effects as found in ordinarybuildings. These pressures do not account forbuffeting or channeling caused by positions ofnearby structures, vortex shedding, or wind sensitivedynamic properties of a particular structure.1611.5.2 Wind force distribution: The totalwind force on the vertical surfaces of a structureprescribed in 780 CMR 1611.5 shall bedistributed 6/10 to the windward surfaces (as apositive pressure) and 4/10 to the leeward surfaces(as a suction). Other distributions may be used ifsubstantiated by appropriate wind tunnel tests.1611.5 Wind loads on structures as a whole: Allbuildings and enclosed or partially enclosedstructures shall be designed to withstand a total windload acting on the structure as a whole determinedby applying the appropriate reference wind pressuresgiven in Table 1611.4 or 1611.4a, to the verticalprojected area, normal to the wind direction of thevertical surfaces of the structure, plus theappropriate wind forces on the roof as specified in780 CMR 1611.8. Consideration shall be given towind acting in all directions.1611.6 Vertical parts of structures: Vertical partsof structures that are subjected directly to the wind,and their local supporting elements, shall bedesigned to resist the pressures listed in Table1611.6, normal to the surface, inward or outward.The pressures listed in the table represent thecombined internal and external pressures. A localsupporting element of a vertical part subjecteddirectly to the wind shall be defined as a wallassembly, a stud, a mullion, a girt, or a similar itemwhich distributes the wind load from the verticalpart to the principal structural system of thestructure.1611.5.1Simultaneous wind forces onorthogonal sides: For structures which areessentially rectangular in plan, or whose planshape is made up of rectangular parts, only winddirections normal to the sides of the structure needbe considered, provided that 0.7 times the effectsof the wind acting simultaneously normal toadjacent orthogonal sides shall also be consideredTABLE 1611.4REFERENCE WIND PRESSURE (POUNDS PER SQUARE FOOT)Heightabove grade276Zone 1ExposureZone 2ExposureZone 3ExposureH (feet)ABCABCABC0 - 5011121211171714212150 - 100111218111724142131100 - 150111622142129182637150 - 200131825172433223041200 - 250152027202736253445250 - 300172229223039283748300 - 400192531253342324152400 - 500222834293746364657500 - 600243037334149415161600 - 700273339364452455565780 CMR - Sixth Edition corrected - 9/19/97 (Effective 2/28/97)

780 CMR: STATE BOARD OF BUILDING REGULATIONS AND STANDARDSSTRUCTURAL LOADS700 - 800293541394755485868800 - 900313743414957526272900 - 1000333945445259556574See table 1611.1a for empirical wind pressure formulasTABLE 1611.4aEMPIRICAL WIND PRESSURE FORMULASaZone 1Zone 2Zone 3ExposureExposureExposureABCABCABCp 30p 36p 42p 40p 48p 56p 50p 60p )Note a: Empirical wind pressure formulas may be used in lieu of the reference wind pressures in table1611.1TABLE 1611.6WIND PRESSURES ON PARTS OF STRUCTURESAND LOCAL SUPPORTING ELEMENTSRequired Design PressuresTributary wind load area ofpart or local supportingelementLocation of appliedwind pressureReference pressure of780 CMR 1611.41multiplied byBut not lessthanBut need not begreater thanWithin salient corner2areaAny1.720 psf70 psfBeyond salient cornerareaLess than or equal to 200 sf1.220 psf50 psfBeyond salient cornerareaGreater than 200 sf0.815 psf50 psfNote 1: For partially enclosed structures, where any side is more than 35% open, add a factor of 0.3 to thecoefficients of this column of the tableNote 2: The salient corner shall be defined as the vertical surface located within a distance of 1/10 the leastwidth of the structure, but not more than ten feet, from a prominent (salient) corner.TABLE 1611.8EXTERNAL WIND PRESSURES ON ROOFSExternal Wind Pressure - flat, gable, shed roofs (wind perpendicular to ridge)Roof pitchDegrees0-2020-3030-4040-5050-90Multiples of reference wind pressure of 780 CMR 1611.4Rise/runWindward slopeFlat to 4/124/12 to 7/127/12 to 10/1210/12 to 14/1214/12 to verticalPositive pressureSuctionLeeward 0.50.5External Wind Pressure - arch shaped roofs (wind perpendicular to ridge)Multiples of reference wind pressure of 780 CMR 1611.4Rise to span ratioWindward quarterPositive pressureLess than 2/102/10 to 3/103/10 to 6/100.20.30.6SuctionCenter halfSuctionLeeward quarterSuction0.7-----0.70.81.00.40.40.4External wind pressure - flat, gable, shed or arched shaped roofs (wind parallel to ridge)AllSuction of 0.6 multiplied by the reference wind pressure of 780 CMR 1611.41611.7 Wind loads on roofs: Roofs and theirsupporting structure shall be designed to resist thecombined effects of the external and internal windpressures specified in 780 CMR 1611.8 through9/19/97 (Effective 2/28/97) - corrected 780 CMR - Sixth Edition277

780 CMR: STATE BOARD OF BUILDING REGULATIONS AND STANDARDSTHE MASSACHUSETTS STATE BUILDING CODE1611.11. All pressures specified shall be consideredto act normal to the roof surface. When applying thereference wind pressures of 780 CMR 1611.4 to theprovisions of 780 CMR 1611.8 through 1611.11,the reference wind pressures shall be