Design of Steel Joists and Roof Decks

1 6/27/20171 Design of Steel Joists and Roof DecksMichael R. Miller, P. PARTNERS. BETTER PRODUCTS. BETTER for Discussion Design Responsibility What is a standard joist ? Assumptions Dimensions Designations Joist Design for Wind Uplift Properly specifying uplift Joist Design by manufacturer for uplift Seat Design Connection designBETTER PARTNERS. BETTER PRODUCTS. BETTER for Discussion Design for lateral loads Girders and Joists in moment frames Connection details Nonuniform loads Design considerations Design options ASD vs LRFD Joist DeflectionBETTER PARTNERS. BETTER PRODUCTS. BETTER for Discussion Steel deck connections for diaphragmBETTER PARTNERS.

2 BETTER PRODUCTS. BETTER Responsibility Is It? Joist supplier? Steel fabricator? Contractor? Other trades? Specifier??BETTER PARTNERS. BETTER PRODUCTS. BETTER Responsibility Steel Joist institute Code of Standard Practice Joist Supplier s Responsibilities Include: Design of standard Joists per Steel Joist institute Specifications Design of special Joists for specified load case(s) Determining bridging requirements Wind uplift Lateral bracing of top chord if required (SSR)6/27/20172 BETTER PARTNERS. BETTER PRODUCTS. BETTER Responsibility Joist Supplier s Responsibilities Do Not Include: Determining framing scheme Calculating wind pressure from wind speed Determining load requirements or load combinations from building codes Determining loads from mechanical drawings Designing lateral bracing system Connection designBETTER PARTNERS.

3 BETTER PRODUCTS. BETTER Review All Verified Items Must Be Answered Engineer of record Loads Structural issues Contractor Building dimensions Attachment locations Steel detailer Bearing details DimensionsBETTER PARTNERS. BETTER PRODUCTS. BETTER Is a Standard Joist? SJI designation Standard dimensions Uniform load only?BETTER PARTNERS. BETTER PRODUCTS. BETTER Assumptions Vulcraft Normally Assumes SJI Standards apply Seat depth Live load deflection is span/360 Camber per chart to offset permanent dead load Joists are laterally braced by roof deck All joist Design requirements are specifically stated on structural drawings Loads are distributed to panel pointsBETTER PARTNERS.

4 BETTER PRODUCTS. BETTER Joist institute SJI Specification is a performance spec References load table Specifies how joist is to be designed No requirement for any physical size Bearing depths Bridging requirementsBETTER PARTNERS. BETTER PRODUCTS. BETTER Joist institute SJI Standard seat depth Flat Bearing K and KCS 2 inches Most LH and DLH5 inches DLH18 or DLH197 inchesIncrease K series whenopposing LH seriesDepths may increase to fitlarger members for high uplift6/27/20173 BETTER PARTNERS. BETTER PRODUCTS. BETTER Joist institute SJI Standard seat depth Most Joist Girders7 inches Left of blue line Large Joist Girders10 inches or more Right of blue lineSee charts in Vulcraft catalogBETTER PARTNERS.

5 BETTER PRODUCTS. BETTER Depths Required Uplift For high uplift, end web size will increase May require different shapes May required increased seat depth Sloped BearingCCBETTER PARTNERS. BETTER PRODUCTS. BETTER Bearing Sloped seats required Assumed 4 bearing Maximum gap is 1/8 Slope greater than 3/8:12 Sloped seats not required Slope is less that 3/8:12 No top chord extension, or top chord only gapBETTER PARTNERS. BETTER PRODUCTS. BETTER Bearing Sloped seats required Long bearing length R type extension Maximum slope for standard bearing will become less than 3/8:12 Similar for LH joistsgapbearingBETTER PARTNERS. BETTER PRODUCTS.

6 BETTER Bearing Sloped seat depth End of base length Not necessarily centerline of bearing Similar for LH joistsseatdepth4 availablebearingc. PARTNERS. BETTER PRODUCTS. BETTER Bearing Increased depth required See charts in Vulcraft catalog Depths shown Minimums (more is OK) Clearance for end web Clearance for extension Contact Vulcraft for extreme slope rates (6:12 or more)6/27/20174 BETTER PARTNERS. BETTER PRODUCTS. BETTER Series? LH? DLH? LH? Joist Girder? Series Determined by: Loading Span Depth?BETTER PARTNERS. BETTER PRODUCTS. BETTER Simple span truss Uniform load From SJI table Indicated by note or diagram Web system can be rod or single angle Pinned joints Axially loaded members Minimum panel shear 25% of end reactionBETTER PARTNERS.

7 BETTER PRODUCTS. BETTER 550 plf maximum load (uniform or equivalent combination) Lyof top chord 36 30 maximum depth L/24 span-depth limit 60 maximum spanBETTER PARTNERS. BETTER PRODUCTS. BETTER , DLH-Series 18 minimum depth 168 maximum depth, but cost increases greatly beyond 144 Shear and moment consistent with SJI tables Same material and production method as K-series L/24 span-depth limitBETTER PARTNERS. BETTER PRODUCTS. BETTER Deeper and longer Same Design approach DLH (Deep Longspan) Load table depths 48 -120 Spans up to 240 SLH (Super Longspan) Formerly a Vulcraft Design Now included in the SJI tablesBETTER PARTNERS.

8 BETTER PRODUCTS. BETTER Girders Primary framing member Design for concentrated loads Unbraced between load locations TC L/ry= 575 Self supporting for construction Otherwise virtually the same as LH6/27/20175 BETTER PARTNERS. BETTER PRODUCTS. BETTER Is the Joist Depth? Sloped top chord? End depth? Centerline depth? Gable Joists ??BETTER PARTNERS. BETTER PRODUCTS. BETTER Depth Designation depth is at centerline Exception is offset double pitch ridgeline depth Not limited to catalog depthsBETTER PARTNERS. BETTER PRODUCTS. BETTER DepthNominal DepthNominal DepthNominal DepthBETTER PARTNERS. BETTER PRODUCTS. BETTER Do I Specify Wind Uplift?

9 Code requirements? Wind speed, Design load, gross/net Roof zones?Units lbs/square foot lbs/linear foot?BETTER PARTNERS. BETTER PRODUCTS. BETTER Uplift Specifications UPLIFT Where uplift forces due to wind are a designrequirement, these forces must be indicated onthe contract drawings in terms of net uplift inpounds per square foot (Pascals). When theseforces are specified, they must be considered inthe Design of Joists and/or bridging. For K-SeriesBETTER PARTNERS. BETTER PRODUCTS. BETTER Uplift Specifications UPLIFT Where uplift forces due to wind are a designrequirement, these forces must be indicated onthe contract drawings in terms of net uplift inpounds per square foot (Pascals).

10 When theseforces are specified, they must be considered inthe Design of Joists and/or single lineofbottom chordbridging must be provided nearthe first bottom chord panel points wheneveruplift due to wind forces is a designconsideration. For K-Series6/27/20176 BETTER PARTNERS. BETTER PRODUCTS. BETTER Net Uplift Responsibility of the Specifying Professional Various approaches are possible Joist supplier needs clear information about what they are expected to do Industry terminology is not always consistent Prefer a net uplift in psfBETTER PARTNERS. BETTER PRODUCTS. BETTER Net Uplift Subtract permanent Dead Load from Gross Uplift IBC 2003, 2006, 2009 ASD + W LRFD + IBC 2012, 2015 ASD + LRFD + PARTNERS.