Wind Design Considerations for Steel Joists and Joist Girders - SEAoT

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Wind Design Considerations for Steel Joists and Joist Girders - SEAoT

Introduction‣ Commercial manufacture of open websteel joists began in 1923‣ The Steel Joist Institute was formed in1928• The use of steel joists has continued togrow year after year for both floors androofs.• Millions of steel joists and Joist Girdersare put in service each year.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 2


General Nature of Wind LoadsTypical Steel Joist and Joist Girder Buildings• Windstorms• Building type – commercial, industrial• Building shape – low rise, rectangular• Roofing systemsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 3


Windstorm Damage to Roof in Texas05 March 2004SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 4


Windstorm Damage to Roof in Texas05 March 2004SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 5


Hurricane Charley Category 4 StormAcross Florida 13-14 August 2004SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 6


Hurricane Charley Category 4 StormAcross Florida 13-14 August 2004SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 7


Population Trends in Hurricane-Prone Regions of the U.S.• Southeast and Gulf of Mexico: Most rapidcoastal growth in recent decades and willcontinue to grow.• Southeast: 8 million (1960) 23 millionprojected (2015)• Gulf of Mexico: 8 million (1960) 22 millionprojected (2015)SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 8


Top 10 Deadliest Hurricanes toStrike the US: 1851-20059,0008,0007,0006,0005,0004,0003,0002,0001,0000LA-Grande Isle (1909)Footnotes:*Could be as high as 12,000.**Could be as high as 3,000.***Midpoint of 1,000 – 2,000 range.****AP total as of Dec. 11, 2005.*****Midpoint of 1,100-1,400 range.Sources: NOAA; InsuranceInformation Institute.372 390 400 408Audrey-SW LA,TX (1957)LA-Last Island (1856)FL Keys (1935)GA/SC (1881)700LA-Cheniere (1893)*****Hurricane Katrinawas the deadliesthurricane to strikethe US since 19281,250 1,323 1,500Katrina (SE LA, MS)****SC/GA Sea Islands (1893)***SE FL/L. Okechobee (1928)**2,500Galvaston (1900)*8,000SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 9


Roof Design to Resist Uplift Loads• Codes and Standards• 2005 SJI Standard Specifications and Code ofStandard Practice• Provisions from 2006 International Building Code• Provisions from ASCE/SEI 7-05• Design of Joist Bearing Seats• Design Example - Placement of Joist Bridging• Summary and ConclusionsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 10


Roof Design to Resist Uplift LoadsThe nominal loads and load combinations shall beas stipulated by the applicable code under whichthe structure is designed, and as shown by theSpecifying Professional in the contract documents.In the absence of a specified building code such asthe International Building Code (IBC 2006), theASCE/SEI 7-05 (ASCE 2005) “Minimum DesignLoads for Buildings and Other Structures” shall beused as the basis for the loads and loadcombinations.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 12


2005 SJI Standard Specification forLongspan Steel Joists, LH-SeriesDeep Longspan Steel Joists, DLH-Series104.12 UPLIFTWhere uplift forces due to wind are a design requirement, theseforces must be indicated on the contract drawings in terms ofNET uplift in pounds per square foot (Pascals). The contractdocuments shall indicate if the net uplift is based upon LRFD orASD. When these forces are specified, they must be consideredin the design of joists and/or bridging. A single line of bottomchord bridging must be provided near the first bottom chordpanel points whenever uplift due to wind forces is a designconsideration.*SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 15


2005 SJI Standard Specification forJoist Girders1004.9 UPLIFTWhere uplift forces due to wind are a design requirement, theseforces must be indicated on the contract drawings in terms ofNET uplift in pounds per square foot (Pascals). The contractdrawings must indicate if the net uplift is based on ASD orLRFD. When these forces are specified, they must beconsidered in the design of Joist Girders and/or bracing. If theends of the bottom chord are not strutted, bracing must beprovided near the first bottom chord panel points wheneveruplift due to wind forces is a design consideration.*SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 16


2005 SJI Code of Standard Practice1.4 DESIGNIn the absence of ordinances or specifications to the contrary,all designs prepared by the specifying professional shall be inaccordance with the Steel Joist Institute Standard SpecificationsLoad Tables & Weight Tables of latest adoption.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 17


2005 SJI Code of Standard Practice6.1 PLANS FURNISHED BY BUYER(a) LoadsThe Steel Joist Institute does not presume to establish theloading requirements for which structures are designed.The Steel Joist Institute Load Tables are based on uniformloading conditions and are valid for use in selecting joist sizesfor gravity loads that can be expressed in terms of "pounds perlinear foot" (kiloNewtons per Meter) of joist. The Steel JoistInstitute Joist Girder Weight Tables are based on uniformlyspaced panel point loading conditions and are valid for use inselecting Joist Girder sizes for gravity conditions that can beexpressed in kips (kiloNewtons) per panel point on the JoistGirder.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 18


2005 SJI Code of Standard Practice6.1 PLANS FURNISHED BY BUYER(a) Loads (cont’d)The specifying professional shall give due consideration to thefollowing loads and load effects:1. Ponded rain water.2. Accumulation of snow in the vicinity of obstructions suchas penthouses, signs, parapets, adjacent buildings, etc.3. Wind.4. Type and magnitude of end moments and/or axial forcesat the joist and Joist Girder end supports shall be shownon the structural drawings. For moment resisting joists orJoist Girders framing near the end of a column, dueconsideration shall be given to extend the column lengthto allow a plate type connection between the top of thejoist or Joist Girder top chord and the column.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 20


Standards and Codes• 2005 SJI Standard Specifications and Codeof Standard Practice• Provisions from 2006 InternationalBuilding Code• Provisions from ASCE/SEI 7-05SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 21


2006 International Building Code• SECTION 1605 LOAD COMBINATIONS• 1605.2 Load combinations using strengthdesign or load and resistance factor design1605.2.1 Basic load combinations1.2D + 1.6(L r or S or R) + (f 1 L or 0.8W)1.2D + 1.6W + f 1 L + 0.5(L r or S or R)0.9D + 1.6W0.9D + 1.0ENOTE: F and/or H loads have been left outof the above equationsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 23


2006 International Building Code• SECTION 1605 LOAD COMBINATIONS• 1605.3 Load combinations using allowablestress design1605.3.1 Basic load combinationsD + (W or 0.7E)D +0.75 (W or 0.7E) + 0.75L + 0.75(L r or S or R)0.6D + W0.6D + 0.7ENOTE: F and/or H loads have been left out ofthe above equationsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 24


Standards and Codes• 2005 SJI Standard Specifications and Codeof Standard Practice• Provisions from 2006 International BuildingCode• Provisions from ASCE/SEI 7-05SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 25


ASCE 7-05 Specified Wind LoadsBasic parameters• Wind speed, importance, exposure• Significance / importance of exposurecategory• Exposure C is “default”, while charts arebased on “B”• The difference is often 30 to 40 percentSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 26


ASCE 7-05 Basic Wind Speed MapSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 27


ASCE 7-05 Specified Wind Loads• It all looks simple when the building structureappears to be a simple rectangle made up oflarge monolithic elements as described inFigure 6-3.• The reality is when the building shape is morecomplex comprised of numerous elements thenit is not as easy to determine the loadings onjoists in corners and Joist Girders that passthrough both edge and corner zones.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 28


ASCE 7-05 Specified Wind LoadsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 29


ASCE 7-05 Specified Wind LoadsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 30


ASCE 7-05 Specified Wind LoadsWhat Constitutes Net Uplift?• For ASD,the “uplift” load combination is 0.6D + W• For LRFD,the “uplift” load combination is 0.9D + 1.6WThe EOR may need to differentiate betweenminimum and maximum dead load.(Note: 0.6D is NOT an allowance for collateral loads)SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 31


ASCE 7-05 Specified Wind LoadsWhat constitutes Net Uplift?DL1.65( ↓) + WL( ↓)( DL + WL)=0.6F A= F A1.65DL + 1.65WL =Amplified DL resistance by 1.65 for uplift is notdesirable!So,DL ↓ + 1.65WL ↑ = F A( ) ( )0.6DL + WL =yyyg0.6F AygF AgyggSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 32


ASCE 7-05 Specified Wind Loads• Maximum Dead Load (for gravity loading)• Minimum Dead Load (for wind uplift)• Collateral Load (also for wind uplift)Collateral loads represent a category of deadloads which are not part of the building structurebut are required for the building’s function.These include: Mechanical equipment, piping,electrical equipment, conduit, sprinkler pipingfire proofing, ceilings, etc.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 33


Wind Loads: Net Uplift Zone Diagram8'24 psf80'15 psf11 psf96 plf8'120'60 plf1460 lbs 1228 lbsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 34


Properly Applying Wind Loads toSteel Joists and Joist Girders• The chart on the following slide is atypical components and cladding roofwind pressures chart provided on thecontract documents.• Roof pressure needs to be converted toNET uplift, or more correctly the result ofthe appropriate load combination for windforces acting upward.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 36


Properly Applying Wind Loads toSteel Joists and Joist GirdersROOF SURFACESEFFECTIVEWIND AREAPOSITIVE PRESSURES(PSF)ZONENEGATIVE PRESSURES(PSF)1 2 3 1 2 310 SF 5.3 5.3 5.3 -13.0 -21.8 -32.820 SF 5.0 5.0 5.0 -12.7 -19.5 -27.250 SF 4.5 4.5 4.5 -12.2 -16.4 -19.7100 SF 4.2 4.2 4.2 -11.9 -14.1 -14.1SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 37


Properly Applying Wind Loads toSteel Joists and Joist Girders• Per ASCE definition of Effective Width, takespan times an effective width that is not lessthan one third the span.Note: This is specifically referenced for the ASCEMethod 2 charts, but it should also apply to ASCEMethod 1 (simplified).SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 38


Properly Applying Wind Loads toSteel Joists and Joist Girders• So for steel joists, a simple rule is that for alljoist spans of 18 foot or greater, use the 100square foot values, i.e. 18 x 6 = 106 > 100 ft. 2• So if a project does not have any spans lessthan 18 feet, there is no need for a detailedchart with values by square foot.• The light weight of joists under 18 foot spansoften allows for a conservative uplift value tobe used rather than a detailed interpolation forthe exact square footage.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 39


Wind Design Considerations forSteel Joists and Joist Girders• Clarifications and Interpretations:• ASCE simplified method described in Section6.4.2.2 provides a formula for net design windpressure. This is NOT the same as SJIsection 5.11 NET uplift.• ASCE net is the sum of internal and externalpressures.• SJI net, is the final resultant pressure, lessappropriate dead load – result of the loadcombinationSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 41


Wind Design Considerations forSteel Joists and Joist GirdersSteel joists are considered components andcladding (C&C).• Joist Girders are considered Main Wind ForceResisting System (MWFRS).Most often, separate MWFRS pressure valuesare not provided for the Joist Girders, and thejoist supplier applies the end reaction (net)uplift forces from the component and claddingjoists to the girders.Is this conservative?SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 42


Wind Design Considerations forSteel Joists and Joist Girders• Other considerations• Overhangs have significant uplift• TCXs “automatically” have same capacity asdownward gravity.• But uplift on overhangs can easily exceed gravity,particularly in coastal areas or hurricane proneregions.• “Kickers” that carry horizontal wind forces needto have both components defined.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 43


Properly Applying Lateral Loads toSteel Joists and Joist Girders• The first consideration relative to the design of thestructure is to determine if rigid frame action isrequired.• For single story structures the optimum framingsystem generally consists of braced frames in bothdirections, and the use of a roof diaphragm systemto transfer wind and seismic loads to the verticalbracing elements.• The specifying professional must specify thenecessary loading and stiffness data to the joistmanufacturer.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 44


Properly Applying Lateral Loads toSteel Joists and Joist Girders• The specifying professional must indicate the typeof joist to column connections so that the joistmanufacturer can provide the joists with thegeometry that meets the design intent.• The joist manufacturer must design the joists inconformance with the SJI Specifications and othercontract requirements specified by the specifyingprofessional.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 45


Properly Applying Lateral Loads toSteel Joists and Joist GirdersSpecification of Required Forces and Moments• Minimum thickness of bottom chord (weldrequirements).• Chord splices must conform to the requirements ofthe 2005 AISC Seismic Provisions, Section 7.3a.• Use IBC Load CombinationsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 46


Properly Applying Lateral Loads toSteel Joists and Joist GirdersAll top chord axial loads and end moments are transmitteddirectly into the columns via the tie plates. No horizontalforces are transferred through the girder seats.MeFFSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 47


Design of Bearing Seats to ResistUplift Loads• Research• 2005 SJI Standard Specifications• Recommended Design ProcedureSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 48


Typical Roof Framing using K-SeriesOpen Web Steel JoistsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 49


End Bearing Seat ConnectionsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 50


Profile of SJI Standard K-SeriesOpen Web Steel JoistsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 51


Components of Uplift Resistance forTest Program• AnchorageWeld• Strength• Ductility• Seat Angle• Strength• DuctilitySEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 52


Joist Seat Test Program Parameters• Vary seat angle size (leg and thickness)• S1 L 1 x 1 x 7/64• S2 L 1-1/2 x 1-1/2 x 1/8• S3 L 2 x 2 x 3/16• S4 L 2 x 2 x 1/4• Vary seat length• 4”, 6”, 8” nominal• Vary anchorage weld length• 1”, 3”, 5” nominalSpecimen Nomenclature SAS-SL-FWS-WLSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 53


Typical Test Specimen ConfigurationTOP CHORDANGLESJOIST SEATWELDPULL PLATEJOIST SEATANGLESBUTT WELDPROVIDEDBETWEEN ANGLES3/4" BASEPLATE9/ 16 " DIA. HOLEFOR ¾” A325N BOLT( 4 PLACES)FILLET WELDS PROVIDEDBETWEEN TOP CHORD TOE ANDSEAT ANGLE AND SEAT ANGLETOE AND TOP CHORDSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 54


Experimental Test SetupINSTRUMENTATIONTEST SPECIMENREACTION PLATESEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 55


End View During and After TestTest Specimen S3-4-1/8-30.30 in. Vertical Displacementat 6.5 kips Applied LoadFailure MechanismSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 56


Typical Load-Deformation BehaviorTest Specimen S3-4-1/8-38.07.0Load (kips)6.05.04.03.02.01.00.0Avg P1 & P2Avg P4 & P70 0.1 0.2 0.3 0.4 0.5Displacement (in.)SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 57


Profile and End View After TestTest Specimen S1-6-1/8-1Yield LinePerimeterSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 58


Yield Line PatternsShort and Long Anchorage WeldsAnchorageWeld (typ.)Yield LineFormation (typ.)AnchorageWeld (typ.)Yield LineFormation (typ.)SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 59


Yield Line Analysis Model forPrediction of Uplift CapacityP u /2Yield LineP u /2aL sL wPlastic HingeaθΔaaSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 60


Yield Line Analysis Using Virtual WorkW e = (P u / 2) ΔW eP uΔW i = M p θ (L yl )W iM pθL yl= External Work= Predicted ultimate uplift load= Distance which the load moves thru= Internal Work= Plastic moment capacity of plate,per unit length = F y Z= Angle through which YL rotates= Length of yield line, the lesser ofL w + πa and L sSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 61


Yield Line Analysis Using Virtual WorkW i + W e = 0(P u / 2) Δ -M p θ (L yl ) = 0But since tan θ = θ for small angles, θ = Δ / aSolving for P u gives:P u = 2 M p L yl / aAssumption of a = 2.3 t provides reasonablygood prediction of ultimate uplift strength ofjoist bearing seatSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 62


Research Programs Recommendations• The flexural resistance of K-Series joistbearing seats can be predicted using ayield line approach.• The yield line model is based onprinciples of basic mechanics, not onempirical curve fitting.• A 5/32” fillet weld is adequate todevelop the flexural strength of the yieldline.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 63


2005 SJI Standard Specification forOpen Web Steel Joists, K-Series5.6 END ANCHORAGE(b) SteelEnds of K-Series Joists resting on steel supports shall beattached thereto with a minimum of two 1/8 inch (3 millmeters)fillet welds 1 inch (25 millmeters) long, or with two 1/2 inch (13millimeters) ASTM – A307 bolts, or the equivalent. When K-Series Joists are used to provide lateral stability to thesupporting member, the final connection shall be made bywelding or as designated by the specifying professional.(c) UpliftWhere uplift forces are a design consideration, roof joistsshall be anchored to resist such forces (Refer to Section 5.11Uplift).SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 64


2005 SJI Standard Specification forLongspan Steel Joists, LH-SeriesDeep Longspan Steel Joists, DLH-Series104.7 END ANCHORAGE(b) SteelEnds of LH- and DLH-Series Joists resting on steel supportsshall be attached thereto with a minimum of two 1/4 inch (6millmeters) fillet welds 2 inches (51 millmeters) long, or with two3/4 inch (19 millimeters) ASTM – A307 bolts, or the equivalent.When LH/DLH-Series Joists are used to provide lateral stabilityto the supporting member, the final connection shall be made bywelding or as designated by the specifying professional.(c) UpliftWhere uplift forces are a design consideration, roof joists shallbe anchored to resist such forces (Refer to Section 104.12Uplift).SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 65


2005 SJI Standard Specification forJoist Girders1004.6 END ANCHORAGE(b) SteelEnds of Joist Girders resting on steel supports shall beattached thereto with a minimum of two 1/4 inch (6 millmeters)fillet welds 2 inches (51 millmeters) long, or with two 3/4 inch(19 millimeters) ASTM – A307 bolts, or the equivalent. In steelframes, bearing seats for Joist Girders shall be fabricated toallow for field bolting.(c) UpliftWhere uplift forces are a design consideration, roof JoistGirders shall be anchored to resist such forces (Refer toSection 1004.9 Uplift).SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 66


P n = 2 M p L yl / aASD Design ProcedureWhere:P n = Nominal uplift capacityM p = Plastic moment capacity of plate perunit length= F y ZZ = t 2 / 4L yl = Length of yield linea = 2.3 tΩ = 1.67 (AISC-ASD safety factor forbending)P n /Ω = Allowable uplift strengthSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 67


LRFD Design ProcedureP n = 2 M p L yl / aWhere:P n = Nominal uplift capacityM p = Plastic moment capacity of plate perunit length= F y ZZ = t 2 / 4L yl = Length of yield linea = 2.3 tφ = 0.90 (AISC-LRFD resistance factor forbending)φP n = Design uplift strengthSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 68


Recommended Bearing Seat Designto Resist Uplift LoadsLength Thickness F yL WM pa L YLP n/ Ω P weldL s(in.) t (in.) (ksi) (in.) (in.-k/in.) (in.) (in.) (kips) (kips)4 0.125 50 1 0.195 0.288 1.903 1.55 3.714 0.125 50 1.5 0.195 0.288 2.403 1.96 5.574 0.125 50 2 0.195 0.288 2.903 2.36 7.424 0.125 50 2.5 0.195 0.288 3.403 2.77 9.284 0.125 50 3 0.195 0.288 3.903 3.18 11.146 0.125 50 4 0.195 0.288 4.903 3.99 14.856 0.125 50 5 0.195 0.288 5.903 4.80 18.564 0.156 50 1 0.304 0.359 2.127 2.16 4.634 0.156 50 1.5 0.304 0.359 2.627 2.67 6.954 0.156 50 2 0.304 0.359 3.127 3.18 9.274 0.156 50 2.5 0.304 0.359 3.627 3.68 11.584 0.156 50 3 0.304 0.359 4.127 4.06 13.906 0.156 50 4 0.304 0.359 5.127 5.21 18.536 0.156 50 5 0.304 0.359 6.127 6.09 23.16SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 69


Recommended Bearing Seat Designto Resist Uplift LoadsLength Thickness F yL WM pa L YLP n/ Ω P weldL s(in.) t (in.) (ksi) (in.) (in.-k/in.) (in.) (in.) (kips) (kips)4 0.188 50 1 0.442 0.432 2.358 2.89 5.584 0.188 50 1.5 0.442 0.432 2.858 3.50 8.374 0.188 50 2 0.442 0.432 3.358 4.11 11.174 0.188 50 2.5 0.442 0.432 3.858 4.72 13.964 0.188 50 3 0.442 0.432 4.358 4.89 16.756 0.188 50 4 0.442 0.432 5.358 6.56 22.336 0.188 50 5 0.442 0.432 6.358 7.34 27.918 0.188 50 6 0.442 0.432 7.358 9.00 33.504 0.250 50 1 0.781 0.575 2.806 4.57 7.424 0.250 50 1.5 0.781 0.575 3.306 5.38 11.144 0.250 50 2 0.781 0.575 3.806 6.19 14.854 0.250 50 2.5 0.781 0.575 4.306 6.51 18.564 0.250 50 3 0.781 0.575 4.806 6.51 22.276 0.250 50 4 0.781 0.575 5.806 9.45 29.706 0.250 50 5 0.781 0.575 6.806 9.76 37.128 0.250 50 6 0.781 0.575 7.806 12.70 44.54SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 70


Recommended Bearing Seat Designto Resist Uplift Loads• The P weld strength given in the preceeding tablesdoes not account for the transverse loading ofthe weld due to uplift and thus could bemultiplied by 1.5.• Where a joist seat has been detailed for a boltedconnection, and for any reason the bolt is notutilized, the empty slot in the bearing seat legseverely diminishes uplift capacity. In such acondition, if a weld and no bolt is to be used on aslotted bearing seat, then the weld should beapplied within the empty slot.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 71


ASD and LRFD Design ExampleSeat Angles – L 1-1/2 x 1-1/2 x 1/8L s =4” L w = 2-1/2” F y = 50 ksiAllowable and Design Uplift StrengthsZ = 0.125 2 / 4 = 0.00391 in. 3 / in.a = 2.3 (0.125) = 0.28750 in.L yl = 2.50 + π (0.2875) = 3.403 in. < L sM p = 50 (0.00391) = 0.1953 in.-kip / in.P n = 2 (0.1953)(3.403) / 0.2875 = 4.62 kipsP n /Ω = 4.62 / 1.67 = 2.77 kipsφP n = 0.9 (4.62) = 4.16 kipsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 72


2005 SJI Standard Specification forOpen Web Steel Joists, K-Series5.11 UPLIFTWhere uplift forces due to wind are a design requirement, theseforces must be indicated on the contract drawings in terms ofNET uplift in pounds per square foot (Pascals). The contractdocuments shall indicate if the net uplift is based upon LRFD orASD. When these forces are specified, they must be consideredin the design of joists and/or bridging. A single line of bottomchord bridging must be provided near the first bottom chordpanel points whenever uplift due to wind forces is a designconsideration.*SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 73


2005 SJI Standard Specification forLongspan Steel Joists, LH-SeriesDeep Longspan Steel Joists, DLH-Series104.12 UPLIFTWhere uplift forces due to wind are a design requirement, theseforces must be indicated on the contract drawings in terms ofNET uplift in pounds per square foot (Pascals). The contractdocuments shall indicate if the net uplift is based upon LRFD orASD. When these forces are specified, they must be consideredin the design of joists and/or bridging. A single line of bottomchord bridging must be provided near the first bottom chordpanel points whenever uplift due to wind forces is a designconsideration.*SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 74


2005 SJI Standard Specification forJoist Girders1004.9 UPLIFTWhere uplift forces due to wind are a design requirement, theseforces must be indicated on the contract drawings in terms ofNET uplift in pounds per square foot (Pascals). The contractdrawings must indicate if the net uplift is based on ASD orLRFD. When these forces are specified, they must beconsidered in the design of Joist Girders and/or bracing. If theends of the bottom chord are not strutted, bracing must beprovided near the first bottom chord panel points wheneveruplift due to wind forces is a design consideration.*SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 75


Design ExampleBuilding Location:Near Orlando, FL in open terrain – minimum slope ¼” / ft.Topography: HomogenousExposure: Category C (Sections 6.5.6.2 and 6.5.6.3)Building Framing and Layout:Flat roof system consisting of steel joists, Joist Girders,and structural roof deck. CMU walls on all four sides withdebris-resistant windows and door infill. Building has aparapet height of less than 3’-0” and is considered aclosed building.Building Classification: Building Category IIImportance Factor = 1.0 (Table 6-1)SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 76


Design ExampleDimensions:Length, l = 121’-4”Width, w = 80’-0”Height, h = 20’-0” above the groundRoof slope is less than or equal to 5 degreesRoof live load deflection is based on L/240Design Roof Loads:Dead Load, D = 15.0 psfRoof Live Load, L r = 20.0 psfTotal Load= 35.0 psfSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 77


ASCE 7-05 Basic Wind Speed MapORLANDOSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 78


Design ExampleBasic wind speed, from Figure 6-1b for Orlando,Florida area – V = 110 mph.Design approach is based on the Simplified Procedure(Method 1) for both Components and Cladding and MainWind Force System since the following conditions exist:• Simple diaphragm building (Section C6.2).• Building shape is basis and has a symmetricalcross section in both directions and a flat roof.• There is no expansion joints in the building.• It’s a low-rise building with a mean roof height, hless than 60 ft. and does not exceed the leasthorizontal dimension (Section 6.2).SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 79


Design Example• Since the building has debris-resistant glazing andhas no dominant opening in any wall it can beclassified as a closed building. (Section 6.5.9.3).• Building has a regular shape.• Rigid building, where height/width,w = 20 ft./80 ft. = 0.25 < 4 (Section C6.2).• The building is not subjected to the topographiceffects of Section 6.5.7• No torsional effects – meets Note 5 of Figure 6-10.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 80


Steel Joist and Joist Girder LayoutSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 81


Wind Zone DefinitionsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 82


Steel Joist Design20K6 Rod Web @ 40’-0”Considering no uplift -2’-0” 18 @ 2’-0” 2’-0”3’-0” 17 @ 2’-0” 3’-0”Bottom Chord = 2 angles 1.5 x 1.5 x 0.137, A = 0.784 in. 2End Web = 5/8 in. dia. round bar , A = 0.307 in. 2SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 83


Steel Joist Design20K6 Rod Web @ 40’-0”With (net) uplift -108 plf84 plf8’-0” 32’-0”Bottom Chord = 2 angles 1.5 x 1.5 x 0.155, A = 0.882 in. 2End Web = 7/8 in. dia. round bar , A = 0.601 in. 2SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 84


Steel Joist DesignDesign DataEnd Web, left end l = 37.49 in.KLrFcrFcrΩ0.9===0.877F( 37.49)0.80.21875e8.02 ksi== 137.10.877( 137.1)PA2π E= 13.35 ksi3.550.601c( 8.02) = 7.21ksi > = = 5.90 ksi ∴OK2Reduce to 90% for eccentricity at bearing seatSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 85


Steel Joist DesignDesign DataBottom Chord, P c = 10.62 kipsllrbyylrzFbcrFcrΩ==96 in.111.324= = 81.40.295= 20.26 ksi=12.16 ksi4 rows – (40)(12)/(4+1) = 96 in.∴controls>10.620.882=12.04 ksi∴ OKSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 86


Placement of Bridging to ResistUplift Loads20K6 Bridging Configuration: Option 15 @ 8’-0”Uplift BridgingErection Stability BridgingA Common Alternative (not for this case)4 Rows Equally Spaced4 Rows Equally Spaced Between Uplift BridgingSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 87


Placement of Bridging to ResistUplift Loads20K6 Bridging Configuration: Option 22 @ 8’-9” 3 @ 7’-6”7’-0”7’-6” 5’-0” 7’-6” 7’-0”Design DataBottom Chord, 2 angles 1.5 x 1.5 x 0.137, A = 0.784 in. 2P c = 10.62 kipsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 88


Placement of Bridging to ResistUplift LoadsAt midspan of the joist:lrFbyycrFor compression, 7’-6” space controls; P c = 10.54 kipslb= 104.85ryyFcrΩ===69.930.84 ksi13.43 ksilrz>=240.295FcrΩ10.540.784===81.418.47 ksi13.44 ksi∴controls10.620.784∴OK13.55 ksiSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 89>=


Placement of Bridging to ResistUplift LoadsWith revised bridging locations at the TC, check spacingllrbbyy= 8' −9"= 105 in.=51'4 + 11050.956== 10.2' >110


Placement of Bridging to ResistUplift Loads20K6 Bridging Configuration: Option 39.8 ft. 3 @ 6.8 ft. 9.8 ft.5 Equal Spaces Between First BC Panel Points5 @ 6.8 ft.End TC space = 9.8 ft.lb= 123 < 145 ∴OKr yy9.8' < 10.2' ∴OKSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 91


Joist Girder Bracing Placement40G8N7K Middle Girder @ 40’-0”Designed based on 2.62 k / PP Uplift Loading(vs. approximately 3.4 k / PP Uplift Loading fromcomponents and cladding joist reactions)No uplift design, i.e. net uplift load = 01 knee brace @ midspan to limit L/r yy < 240With 2.62 k / PP uplift load, same Bottom Chord angles 3 x 3 x 1/4,but now knee brace @ 15 ft. and 25 ft. required.With 3.4 k / PP uplift load, same Bottom Chord angles 3 x 3 x 1/4, butnow 3 knee braces @ 10 ft., 20 ft., and 30 ft. required.SEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 92


Summary and Conclusions• Wind forces can have a significant impact on thedesign and use of steel joists and Joist Girders• The SJI Standard Specifications pertaining to windand uplift have been reviewed; the wind loadrequirements from ASCE 7-05 have been discussed;and the appropriate 2006 IBC load combinationscontaining Wind have been presented.• Particular attention needs to be paid to:• Code specified and calculated wind forces• Seat anchorage welds• Joist bridging and Joist Girder bracing placementSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 93


New Resource Soon AvailableSJI Technical Digest No. 6Structural Design of Steel Joist Roofsto Resist Uplift LoadsSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 94


Any Questions?SJI Website: http://www.steeljoist.orgSEAoT State Conference November 6-8, 2008 Houston TX SEAoT - 95

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