Whose responsibility is it? Design Responsibility. Steel Joist Institute. Standard Assumptions. Common Pitfalls in Steel Joist Specification

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1 Common Pitfalls in Steel Joist Specification Whose responsibility is it? Joist supplier? Steel fabricator? Contractor? Other trades? Specifier? Design Responsibility 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) Standard Assumptions SJI Code of Practice assumes SJI Standards apply Seat depth Live load deflection is span/360 Camber per chart to offset dead load Joists are laterally braced by roof deck All joist design requirements are specifically stated on structural drawings Loads are distributed to panel points Steel 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 requirements 1

2 Design Responsibility LRFD Load Combinations 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 design Special design joists Specifying professional will need to spell out Unfactored loads D dead L live (occupancy & equipment) L r roof live S snow live R rainwater Load combinations ASCE 7 or other cases as deemed appropriate Deflection SJI load tables show a live load which will produce a deflection of L/360 Part of the standard design process More or less severe limits can be specified L/600 (within reason) L/240 for total load Horizontal Movement Scissor joists and arches will move horizontally under load We can control, but not eliminate this deflection Some deflection criteria we can t meet no deflection under live load For KCS joist, Maximum deflection shall be limited to Deflection at end of joist extension shall be limited to Deflection limit of L/600 on long joist Design Responsibility Vulcraft can assist with Optimizing framing scheme Sizing joists for special loads Coordination with sprinkler systems Dimensional information Web layouts Special configurations Seat depths 2

3 Drawing Review All verified items must be answered Engineer of record Loads Structural issues Contractor Building dimensions Attachment locations Steel detailer Bearing details Dimensions Design Calculations For approval? Data used is generally incomplete Loads and dimensions still to be verified Preparation at this point requires extra work for joist supplier For record? Data actually used for joist production Reflects what was supplied No additional steps in joist supplier s work flow What causes dimensional conflicts? Sloped top chords Bearing depths Bearing lengths Depth of Joists with Sloped Chords SJI designation depth is at centerline Exception is offset double pitch ridgeline depth Not limited to catalog depths Joist Depth Bearing Conditions Nominal Depth Nominal Depth Nominal Depth SJI Minimum Bearing Length for Standard Conditions Steel Masonry/ Concrete K and KCS 2 ½ 4 LH02 to 06 incl 2 ½ 6 LH/DLH07 to 17 incl 4 6 LH/DLH18 to 25 incl 6 6 3

4 Dimension Definitions Base length outside of seat to outside of seat Dimension Definitions Base length outside of seat to outside of seat Reaction 4 4 For K series 4 Minimum Bearing on Masonry Prevent joist from loading the face of the wall Bearing Conditions SJI Standard Bearing Length - Masonry or Concrete Bearing Conditions SJI Standard Bearing Length - Masonry or Concrete Assume to be zero 2 K LH, G R 2 K R LH, G Steel Joist Institute SJI Standard Bearing Length - Steel Dimension Definitions Base length outside of seat to outside of seat 2 R K, LH02-06 LH7-17 LH18-25 Always better to have as much bearing as possible up to maximum based on type of joist. TCX 4 4 Overall Length TCX 4

5 Steel Joist Institute Minimum bearing Ensures reaction will not overstress support Not too close to face of wall Not too near toe of beam flange Alternatives EOR verify support design Stagger joist bearing Both can complicate approval process Steel Joist Institute Maximum bearing Ensures web will not foul with support Reaction cannot move back due to geometry Bearing capacity is not normally the issue Alternative Deeper bearing Increases clearance Requires coordination Avoid bearing on top chord extension Wider Supports Base length outside of seat to outside of seat Dimension Definitions Base length outside of seat to outside of seat Reaction What if more bearing is required? Anything past the 4 bearing seat is a tcx 2 TCX 4 Bearing Seat 6 Overall Length Deeper Bearing Base length outside of seat to outside of seat Reaction Deeper Bearing High uplift can require deeper seats Reaction 4 Bearing Seat 6 Clear Bearing Seat depth can be increased, but elevations must be coordinated 4 Bearing Seat Compression in end web too large for rod Must use double angles 5

6 Girder Seats SJI Standard seat depth Most Joist Girders 7 ½ inches Left of blue line Large Joist Girders 10 inches or more Right of blue line Different Bearing Depths in One Location Increase seat depth of smaller member Never try to decrease the seat depth of the larger member Reasons should be clear from previous discussion See pages of Vulcraft manual Extended Ends Frequently same depth as bearing seat SJI type R Bearing seat extends back one panel to develop capacity K series 2 ½ LH series 5 Extended Ends Short extensions may only be top chord SJI type S K series 2 ½ LH series 5 Extended Ends Short extensions may only be top chord SJI type S Bearing seat may extend out for dimensional reasons, but is not considered in design K series 2 ½ LH series 5 Extended Ends Specifying type S can be confusing Is it important that the seat not continue? Or is it only load that is meant? K series 2 ½ LH series 5 6

7 Extended Ends If the loading diagram for any condition is not shown, the joist manufacturer will design the extension to support the uniform load indicated in the K-Series Joist Load Table for the span of the joist. What can happen 10K1 x supports 200 plf total load From the SJI load table, w TL = 550 Top chord extension 3-00 Deflection for 550 plf exceeds L/120 with the largest members available But we could design for 200 plf with no problem Extended Ends Best option for long extension Define loads on contract documents Deflection requirements must include loads What about sloped joist bearing seats? When required? What will the seat depth be? Sloped Bearing Increased depth required See page 42 or 47 of Vulcraft manual Depths shown Minimums (more is OK) Clearance for end web Clearance for extension Contact Vulcraft for extreme slope rates (6:12 or more) Sloped Bearing Sloped 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 gap Sloped seats required Long bearing length R type extension Maximum slope for standard bearing will become less than 3/8:12 Similar for LH joists bearing gap 7

8 Sloped Bearing 4 How to design for concentrated loads? Sloped seat depth End of base length Not necessarily centerline of bearing Similar for LH joists c. l. 5 Seat Depth Double joist? Standard joist? KCS series? Special joist? Add load Load diagram Option 1 Use Double Joists Option 2 Select a Standard Joist Generally conservative Simple Not very efficient Potential problems with framing & bridging L = W TL = 240/120 plf W DRIFT = 120 plf P = 600 lbs ea R L = 6413 lbs R R = 6187 lbs M MAX = ft FT Shear Diagram Compute W EQ and W LL W EQ1 = 2(6413)/44 = 292plf W EQ2 = 8(70584)/44 2 = 292plf = 172 plf W LL Option 2 Select a Standard Joist Using W EQ = 292 plf W LL = 172 plf with a span of from the SJI Load Table Joist TL/LL Weight/ft 22K11 311/ K10 298/ K9 295/ K8 291/ LH05 297/ FT Required shear not covered by selection Option 2 Select a Standard Joist W TL W DRIFT = 240 plf = 120 plf = 600 lbs ea P R MAX = 6413 lbs W EQ = 292 plf 28K9 W TL = 295 plf R = 6490 lbs =0.25(6490) = 1622 lbs V MIN 8

9 Option 3 Select a KCS-Series Steel Joist Option 3 Select a KCS-series Steel Joist JOIST DEPTH DESIGNATION (inches) 10KCS KCS KCS KCS KCS KCS KCS CSS KCS KCS KCS KCS KCS5 16 MOMENT SHEAR APPROX MOM. OF CAPACITY CAPACITY WEIGHT INERTIA (inch-kips) (lbs) (lbs/ft) (in^4) BRD ROW SEC Designed in accordance with the Standard Specifications for K-series joists, plus... Constant moment capacity. Constant shear capacity. Interior webs designed for 100% stress reversal. Top chord end panel designed for axial load based on the force in the end web. No check for secondary bending. Option 3 Select a KCS-series Steel Joist Option 3 Select a KCS-series Steel Joist Moment Shear Determine the required maximum end reaction and moment. Calculate the equivalent uniform load from the maximum moment. Find a KCS-series joist from the Load Table that has an end reaction and moment greater than the required. Check the required number of bridging rows using the standard K-series bridging chart. V M FT 4 12 KCS Series W TL = 240 plf W DRIFT = 120 plf P = 600 lbs ea R MAX = 6413 lbs = 292 plf W EQ 28KCS3 M = 846 in-k R = 8000 lbs = 8000 lbs V MIN Option 3 Select a KCS-series Steel Joist Advantages... Provides alternative to joist supplier provided special designs for joists with concentrated or unusual loading conditions. Allows the Design Professional to know the capacity of the joist being provided for a special loading condition. Can accommodate moving loads or loads that are not possible to locate. 9

10 Option 3 Select a KCS-series Steel Joist Disadvantages... Much heavier than standard designation or special design joists. More expensive per unit than standard K-series joists. (But overall project may be more economical.) Option 3 Select a KCS-series Steel Joist REMEMBER... The equivalent uniform load calculated from the required resisting moment must not exceed 550 plf. If the equivalent uniform load exceeds 550 plf, you have two options : a) Use 2 KCS-series joists to provide a total moment and shear capacity equal to the required. b) Use an LH-series joist selected for the equivalent uniform load. Provide a Load Diagram for this option. Location? RTU Option 4 Specify an Add Load Note that secondary bending is NOT checked For occasions when the TOTAL Concentrated Load to be applied is < 2000 # and the location cannot be determined on the structural drawings Vulcraft will design for: M max = wl 2 /8 + PL/4 and R max = wl/2 + P Option 4 Specify an Add Load Location? RTU The Structural Engineer must provide the following information: 1. The SJI Standard Joist designation, or the uniform Example: total and live loads 20K4SP or 20K200/100SP and 2. The amount of the Add Load SP=Design for additional load of 1000# applied at any panel point. Option 4 Specify an Add Load Option 5 Provide Specific Load Criteria Can an all series be designed for an Add Load? K series YES Girders YES LH series YES KCS series NO DLH series YES K200/100SP /0.5K 1.0/0.5K 20K200/100SP The Structural Engineer must provide the following information: 1. The SJI Standard Joist designation, or the uniform total and live loads 2. The additional point loads to be included 10

11 Specifying Joists with Concetrated Loads Regardless of the option used, secondary bending is not checked for typical concentrated load designs Local Moments Joist chords cannot be designed for any significant local moment Local Moments Connect to both chords so that only axial loads are applied to the joist. What about lateral moments? Load path? Connections? What moments to include? When to make the connection? A structural frame utilizing joist girders with rigid connections at the supports. The rigid connections allow the joist girder to carry moments in addition to the reactions from the joist girder loads. Steel Joist Institute Technical Digest #11 July, 1999 Joist-Girder Frames 11

12 Determine approximate moment of inertia of joist girder: I JG = NPLd N number of joist spaces P panel point load L length in feet d depth in inches Determine approximate moment of inertia of joist girder: Joist Girder Frame Determine area of joist girder cross section Refer to Vulcraft catalog for girder weight A JG = girder weight (plf) / steel density (490 pcf) Rigid Frame with Joist Girder Idealized Rigid Frame Analysis Design information required by joist girder manufacturer. SJI Technical Digest offers two options: 1. Provide end moments and gravity loads 2. Convert the end moments to axial loads based on the joist girder effective depth and provide the chord axial loads and gravity loads. Design information required by joist girder manufacturer. Vulcraft recommends against converting the end moments to axial loads. Therefore.. 12

13 Design information required by joist girder manufacturer. Connection Details Standard joist girder designation Lateral moments due to wind and/or earthquake Live load moments (Axial loads) (Net uplift) Basic Connection Welded Basic Connection TYP. 1/4 5 NO WELD BOTH ANGLES 3/16 6 F M Eccentricity with Basic Connection TYP. 1/4 5 F e Commentary and Suggestions in Vulcraft catalog BOTH ANGLES 3/16 6 Maximum P a 4 k to 10 k 13

14 Welded Basic Connection with Tie Girder Moment Plate TYP. 1/4 5 2 ANGLES BOTH ANGLES 3/16 6 BOTH ANGLES Connection Details Axial Loads & Lateral Moments Can be applied to K-series LH, DLH, SLH-series Joist Girders Connection must transfer the load Moment Connections Consider load path No axial load capacity for 7 ½ inch deep girder seats Sloped seats should not transfer axial load Moment Connections Any load applied after the bottom chord extension is welded creates an end moment Live and dead load moments are induced 14