SJI First Editio CJ-Series Stadard Specificatios for Composite Steel Joists, Weight Tables, Bridgig Tables ad Code of Stadard Practice A Brief Overview Authors: Perry S. Gree, Techical Director, 3127 Mr. Joe White Ave. Myrtle Beach, SC 29577, Phoe: 843-626-1995, Fax: 843-626-5565, psgree@steeljoist.org Michael Wiarta, Egieerig Maager, New Milleium Buildig Systems, 6115 Couty Road 42, Butler, IN 46721 michael.wiarta@ewmill.com INTRODUCTION The Steel Joist Istitute (SJI) has developed a stadard specificatio for the desig of composite steel joists, the CJ-Series, i respose to a growig eed to have a cosistet desig methodology for all Steel Joist Istitute (SJI) member compaies. I 2006, the Stadard Specificatios for Composite Steel Joists was ANSI approved ad it is ow cotaied i the 2007 First Editio Composite Steel Joist Catalog show i Figure 1 [SJI, 2007]. This publicatio cotais the followig mai sectios: The Stadard Specificatios for Composite Steel Joists, CJ-Series, Code of Stadard Practice for Composite Steel Joists, Resposibilities of the Desig Professioal, Desig Guide LRFD Weight Tables ad Bridgig Tables for Normal Weight (145 pcf, 2350 kg/m 3 ) ad Light Weight (110 pcf, 1800 kg/m 3 ) Cocrete, Composite Joist Desig Examples, Accessories ad Details, Fire-Resistace Ratigs for Composite Steel Joists, ad OSHA Safety Stadards for Steel Erectio. The Stadard Specificatios for Composite Steel Joists (subsequetly referred to as the Stadard Specificatios) cover the desig, maufacture, ad use of simply-supported, uiformly loaded ope-web composite steel joists. The desig methodology that was adopted by the SJI is based o Load ad Resistace Factor Desig (LRFD). Basic Descriptio of Product The term Composite Steel Joists refers to ope web, parallel chord, load-carryig members suitable for direct support of oe-way floor or roof systems. Members may cosist of hot-rolled or cold-formed steel, icludig cold-formed steel whose yield stregth has bee attaied by cold workig. CJ-Series joists are lightweight, shop-maufactured steel trusses. Shear coectio betwee the joist top chord ad overlyig cocrete slab allows the steel joist ad slab to act together as a itegral uit after the cocrete has adequately cured. Shear coectio typically cosists of 3/8, 1/2, 5/8, or 3/4 ich (9.5, 12.7, 15.9, or 19 mm) diameter shear s welded through the steel deck to the top chord members of the uderlyig steel joist. The desig of CJ-Series joist chord sectios is based o a yield stregth of 50 ksi (345 MPa). Web sectios are based o a yield stregth of at least 36 ksi (250 MPa), but ot greater tha 50 ksi (345 MPa). Applicable cocrete stregths are either 4 or 5 ksi (27.6 or 34.5 MPa). A composite steel joist desigatio is determied by its omial depth, the letters CJ, followed by the total factored uiform composite load, factored uiform composite live load, ad
fially the factored uiform composite dead load. For example, a 24CJ1400/800/240 has a omial depth measured from the upper surface of the steel top chord to the uderside of the bottom chord of 24 iches (610 mm), total factored composite load-carryig capacity of 1400 plf (20.4 kn/m), factored composite live load capacity of 800 plf (11.7 kn/m), ad factored composite dead load capacity of 240 plf (3.5 kn/m). The depth of the bearig seat at the eds of uderslug CJ-Series joists ca vary from 2.5 to 7.5 iches (64 to 191 mm) depedig o the joist spa, depth, or load-carryig capacity. A suggested bearig depth for a give composite steel joist desigatio is icluded i the Desig Guide LRFD Weight Table for Composite Steel Joists, CJ-Series for NORMAL WEIGHT CONCRETE as well as for LIGHT WEIGHT CONCRETE. Composite Steel Joist products ca be furished as uderslug (top chord bearig) or with square eds (bottom chord bearig). FIGURE 1 Newly released First Editio Composite Steel Joist Catalog cotaiig the Stadard Specificatios for Composite Steel Joists, CJ-Series, Weight Tables ad Bridgig Tables, ad the Code of Stadard Practice COMPOSITE JOIST DESIGN REQUIREMENTS Chord Desig The momet capacity of a composite steel joist ca be calculated usig the simple model show i Figure 2. The distace betwee the cetroid of the tesio bottom chord ad the cetroid of the cocrete compressive stress block, d e, is computed usig a cocrete stress of 0.85f c ad a effective cocrete width, b e, take as the sum of the effective widths for each side of the joist ceterlie, each of which shall be the lowest value of the followig: 1. oe-eighth of the joist spa, ceter-to-ceter of supports; 2. oe-half the distace to the ceter-lie of the adjacet joist; 3. the distace to the edge of the slab.
a = M /(0.85 f c b e d e ) t c, i. (mm) (1) d e = d j y bc + h deck + t c a/2, i. (mm) (2) FIGURE 2 COMPOSITE STEEL JOIST FLEXURAL MODEL a = depth of cocrete compressive stress block, i. (mm) b e = effective width of cocrete slab over the joist, i. (mm) d j = steel joist depth, i. (mm) f c = specified miimum 28 day cocrete compressive stregth, ksi (MPa) h deck = height of steel deck, i. (mm) M = omial momet capacity of the composite joist, kip-i. (N-mm) t c = thickess of cocrete slab above the steel deck, i. (mm) y bc = vertical distace to cetroidal axis of bottom chord measured from the bottom of the bottom chord, i. (mm) Whe the steel deck ribs are perpedicular to the steel joists, the cocrete below the top of the steel deck is eglected whe determiig sectio properties ad i calculatig the cocrete compressive stress block. The cotributio of the steel joist top chord to the momet capacity of the composite system is igored. With the cetroid of the top chord beig close to the ceter of the compressive stress block i the overlyig cocrete slab, the top chord develops a small momet couple. If oe icludes the top chord i the total momet capacity this would result i a large icrease i shear coectio requiremets that geerally is ot ecoomical for the small gai i momet capacity. The first top chord ed pael member is desiged for the full factored load requiremets as a o-composite member. M φ (3) u M φm = miimum desig flexural stregth of composite sectio as determied from Equatios 4, 5, 6, ad 7. kip-i. (N-mm) = required flexural stregth determied from applied factored loads, kip-i. (N-mm) M u
The desig flexural stregth of the composite sectio, φm, shall be computed as the lowest value of the followig limit states: Bottom Chord Tesile Yieldig, Bottom Chord Tesile Rupture, Cocrete Crushig, ad Shear Coector Stregth. a) Bottom Chord Tesile Yieldig: φ t = 0. 90 φ M = φtabfyd e (4) b) Bottom Chord Tesile Rupture: φ tr = 0. 75 φ M = φtra Fu de (5) c) Cocrete Crushig: φ cc = 0. 85 φ M = φcc 0.85f ' c bet cde (6) d) Shear Coector Stregth: φ = 0. 90 φ M = φ NQde 0.50φtAbFyd e (7) A bc = cross-sectioal area of steel joist bottom chord, i. 2 (mm 2 ) A = et cross-sectioal area of the steel joist bottom chord, i. 2 (mm 2 ) b e = effective width of cocrete slab over the joist, i. (mm) d e = vertical distace from the cetroid of steel joist bottom chord to the cetroid of resistace of the cocrete i compressio, i. (mm) F u = tesile stregth of the steel joist bottom chord, ksi (MPa) F y = specified miimum yield stress of steel joist bottom chord, ksi (MPa) N = umber of shear s betwee the poit of maximum momet ad zero momet Q = omial shear capacity of oe shear, kips (N) t c = miimum thickess of the cocrete slab above the top of the metal deck, i. (mm) I additio to the chord requiremets specified above, the miimum horizotal flat leg width ad miimum thickess of top chord shall be as specified i Table 1. This will allow the proper istallatio of headed steel shear s i accordace with AWS D1.1 Sectios 7 ad C7, Stud Weldig [AWS, 2004]. TABLE 1 MINIMUM TOP CHORD SIZES FOR INSTALLING WELDED SHEAR STUDS Shear Stud Diameter, i. (mm) Miimum Horizotal Flat Leg Width, i. (mm) Miimum Leg Thickess, i. (mm) 0.375 (10) 1.50 (38) 0.125 (3.2) 0.500 (13) 1.75 (44) 0.167 (4.2) 0.625 (16) 2.00 (51) 0.209 (5.3) 0.750 (19) 2.50 (64) 0.250 (6.3) Web Desig Testig has verified that the web members of a composite steel joist behave i essetially the same maer as web members foud withi a traditioal o-composite steel joist. Webs must be desiged so that they have sufficiet stregth to trasfer the vertical shear from the applied loads to the eds of the composite joist. Webs of CJ-Series joists are desiged for a miimum vertical shear equal to 25% of the factored ed reactio. I additio, tesio webs that are
cotrolled by the above miimum shear requiremet are also checked for a stress reversal (compressive force) resultig from a half-spa live load applied to the joist. Equatio (8) was itroduced ito the Stadard Specificatios to satisfy this requiremet. ( 1.6 w ) L L Vc mi = (8) 8 w L = ufactored live load due to occupacy ad moveable equipmet, plf (kn/m) L = desig legth for the composite steel joist (Spa 0.33 ft., Spa 102 mm) V c mi = miimum factored compressive desig shear i tesio web members, lb (kn) Iterior vertical web members used i modified Warre type joist cofiguratios are desiged to resist the gravity loads supported by the member plus 2.0 percet of the composite bottom chord axial force. Shear Stud Desig Shear trasfer betwee the cocrete slab ad a CJ-Series joist is typically accomplished by the istallatio of headed shear s welded through the steel deck to the uderlyig steel joist top chord. The typical steel joist top chord cosists of double agles with a horizotal gap of 1 ich (25 mm) betwee chord agles as show i Figure 3. Shear s are ideally istalled o alteratig top chord agles versus istallig all shear s o the same top chord agle. This will result i a more uiform shear trasfer ito both joist top chord agles. The Stadard Specificatios provide a shear capacity, Q, coservatively assumig that all shear s are placed o the Weak side of the deck ceter stiffeig rib, i.e. o the side of the deck stiffeig rib closest to the poit of maximum bedig momet for the joist spa. Therefore, the Specifyig Professioal does ot eed to be cocered as to which side of the deck ceter stiffeig rib the shear s are beig welded. The defiitio of Weak ad Strog shear positio ca be foud i the 2005 AISC Commetary o the Specificatio for Structural Steel Buildigs, Fig. C-I3.4 [AISC, 2005] ad illustrated i Figure 4. (A) (B) FIGURE 3 SHEAR STUD POSITIONS ON TOP CHORD FIGURE 4 SHEAR STUD LAYOUT A) WEAK AND B) STRONG POSITION The Stadard Specificatios limit the ratio of the shear diameter/flage thickess to a maximum of 3.0, but requires a reductio i shear capacity whe the /top chord thickess ratio falls i betwee 2.7 3.0. Prior testig of shear s o thi flages [Goble, 1968] idicated that whe the ratio of the diameter of the /flage thickess exceeds 2.7, shear s do ot develop their full shear capacity.
For s i 1.5, 2, or 3 i. (38, 51, or 76 mm) deep decks with d t top chord 2. 7 : [ 0.5A f E, R R A F ] Q = Mi, kips (N) (9) c c p g u For s i 1.5, 2, or 3 i. (38, 51, or 76 mm) deep decks with.7< d t 3. 0: 2 top chord d Q = Mi 0.5A f c Ec, R pr ga Fu 1.5 2.7, kips or t top chord d = Mi 0.5A f c Ec, R pr ga Fu 6670 2.7 (N) (10) t top chord A = cross sectioal area of shear, i. 2 (mm 2 ) d = diameter of shear, i. (mm) E c = modulus of elasticity of the cocrete, ksi (MPa) f c = specified miimum 28 day cocrete compressive stregth, ksi (MPa) F u = miimum tesile stregth of, 65 ksi (450 MPa) Q = shear capacity of a sigle shear, kips (N) R p = shear coefficiet from Table 2 R g = 1.00 for oe per rib or staggered positio s = 0.85 for two s per rib side-by-side; 0.70 for three s per rib side-by-side t top chord = thickess of top chord horizotal leg or flage, i. (mm) Shear s, after istallatio, shall exted ot less tha 1 1/2 i. (38 mm) above the top of the steel deck ad there shall be at least 1/2 i. (13 mm) of cocrete cover above the top of the istalled s. Sice pait may potetially hider the istallatio of welded shear s to the joist top chord, the Stadard Specificatios for Composite Steel Joists states that the stadard shop practice is to supply composite steel joists upaited. TABLE 2 VALUES FOR R P Deck Height Wr @ mid-height 3/8 i. (10 mm) Dia. Stud 1/2 i. (13 mm) Dia. Stud 5/8 i. (16 mm) Dia. Stud 3/4 i. (19 mm) Dia. Stud 1 i. (25 mm) 1.9 i. (48 mm) 0.55 0.55 0.50 0.45 1.5 i. (38 mm) 2.1 i. (53 mm) 0.55 0.50 0.45 0.40 1.5 i. (38 mm) Iverted 3.9 i. (99 mm) 0.85 0.60 0.60 0.60 2 i. (51 mm) 6 i. (152 mm) --- 0.55 0.50 0.45 3 i. (76 mm) 6 i. (152 mm) --- 0.50 0.50 0.50 Notes: 1) Wr @ mid-height = Average deck rib width of deck rib cotaiig the shear. 2) The deck is assumed to be orieted perpedicular to the joists. CAMBER All CJ-Series composite steel joists are cambered for 100% of the ufactored o-composite dead load durig maufacturig. This amout of camber will be completely removed durig costructio with the applicatio of the dead loads from the joists, bridgig, steel deck, ad
cocrete slab. Typical composite joist bearig seats provide egligible rotatioal restrait at the eds of the joist, hece, CJ-Series joists are modeled as pi-eded members whe maufactured with typical uderslug bearig seats. This is i cotrast to composite wide flage beams where coectios at the eds of the beams may oly permit approximately 85% of the camber to be removed uder the full o-composite dead load. BRIDGING Top ad bottom chord bridgig is required for the support of all composite steel joists. This bridgig may be horizotal, diagoal, or a combiatio of both depedig o the spa, depth, ad load-carryig capacity of the CJ-Series joists. For spas up through 60 feet (18.3 meters), welded horizotal bridgig may be used except where the row of bridgig earest the ceter is required to be bolted diagoal bridgig as idicated o the SJI joist maufacturer s joist placemet plas. Whe the spa of the composite steel joist is over 60 feet (18.3 meters), but ot greater tha 100 feet (30.5 meters), hoistig cables shall ot be released util the two rows of bridgig earest the third poits are completely istalled. Whe the spa exceeds 100 feet (30.5 meters) hoistig cables shall ot be released util all rows of bridgig are completely istalled. For spas over 60 feet (18.3 meters) all rows of bridgig shall be diagoal bridgig with bolted coectios at the chords ad itersectios. The umber of rows of bottom chord bridgig shall ot be less tha the umber of top chord rows. Rows of bottom chord bridgig are permitted to be spaced idepedetly of rows of top chord bridgig. Bridgig must be properly spaced ad achored to support the deckig ad the employees prior to the attachmet of the deck to the top chord. The maximum spacig of lies of bridgig, l br shall be the lesser of, d j d j l br = 100 + 0.67 d j + 40 r y,i. or = 100 + 0.026 d j + 0.48 r y (mm) (11) L L l br = 170r y (12) d j = steel joist depth, i. (mm) L = desig legth for the composite joist, ft. (m) r y = out-of-plae radius of gyratio of the top chord, i. (mm) Coectio of bridgig to the chords of a composite steel joist shall be made by positive mechaical meas or by weldig. Eds of all bridgig lies termiatig at walls, beams, or double joists boxed by diagoal bridgig shall be achored. Coectio of the horizotal ad diagoal bridgig to the joist chord or bridgig termius poit shall be capable of resistig the omial top chord horizotal force, P br give i (13). P br = 0.0025 A t F costructio, lbs (N) (13) 2 π E = 0.9 br l r y Fcostructi o 2 12.2 ksi or 84.1MPa (14)
= 8 for horizotal bridgig; 2 for diagoal bridgig A t = cross sectioal area of joist top chord, i. 2 (mm 2 ) E = Modulus of Elasticity of steel = 29,000 ksi (200,000 MPa) l br = is determied from (11) or (12) ry F costructio = assumed omial stress i top chord due to costructio loads, ksi (MPa) CODE OF STANDARD PRACTICE FOR COMPOSITE STEEL JOISTS I additio to the ew Stadard Specificatios for Composite Steel Joist, CJ-Series, the Steel Joist Istitute has also published a ANSI approved Code of Stadard Practice for Composite Steel Joists (COSP). The COSP has bee developed with good egieerig practice ad idustry stadards i mid as the goverig stadard iterpretatio of cotracts that iclude the purchase of Composite Steel Joists. I geeral, the Plas used for biddig purposes shall have sufficiet iformatio to allow for a accurate estimate, ad shall show the followig: Desigatio, locatios, ad elevatios of all the materials. Joist depth ad sizes, icludig ay special desig ad cofiguratio requiremets. Type ad depth of floor deck. Cocrete uit weight, omial compressive stregth, ad total depth of cocrete slab. Loads ad their locatios. Locatios of all partitios ad opeigs. No pait o the joist. The Estimate for the Composite Steel Joists shall iclude: Composite Steel Joists. Joist exteded eds, ceilig extesios, ad exteded bottom chords used as struts. Bridgig ad bridgig achors. Although ot required as part of a stadard bid estimate, a approved SJI member compay may also quote ad idetify additioal items such as headers, shear coectors, ceterig materials ad attachmets, erectio bolts, momet plates, etc. The joist maufacturer awarded with a particular cotract shall furish Composite Steel Joist placemet plas to show the materials as specified i the cotract documets, ad to be utilized for field istallatio of the materials i accordace with the specific project requiremets. The Composite Steel Joist placemet plas shall iclude the followig: Listig of all applicable loads used i the desig of the Composite Steel Joists. Coectio requiremets for joist supports, field splices, ad bridgig attachmets. Deflectio criteria ad desig camber for each of the composite joists. Shear istallatio plas with sizes, quatity, ad locatios of all shear coectors o the Composite Steel Joists. Size, locatios, ad coectios of all bridgig. Joist headers.
Composite Joist Floor Desig Parameters Checklist As a aid the Specifyig Professioal, the COSP icludes the SJI Composite Joist Floor Desig Parameters Checklist. This form, located at the back of the Code of Stadard Practice, ca be used to covey all the required stadard desig iformatio for the Composite Steel Joists to the joist maufacturer. The form idetifies all the composite joist geometry requiremets, cocrete ad steel deck requiremets, omial loadigs, ad the correspodig factored loadigs, as well as ay camber ad deflectio requiremets. The Checklist covers most of the basic loadig requiremets for the structure ad the load combiatio cases. If there is loadig criteria that is too complex to be placed o the form, the Specifyig Professioal eeds to effectively commuicate these loadig requiremets by meas of load schedule, load diagram, ad load combiatio to be used. The Specifyig Professioal shall show o the structural drawigs ad give due cosideratio to the followig special loads ad load effects: Sow drift loads. Type ad magitude of axial loads. Due cosideratio shall be give to supply a trasfer plate/agle to avoid resolvig this force through joist seats. Type ad magitude of ed momets. Due cosideratio shall be give to exted the colum legth ad allow a momet plate coectio betwee top of the joist top chord ad the colum, sice joists have a limited capacity for resolvig this force through the joist bearig seat coectio. Structural bracig loads. Poded rai water. Wid uplift. Cocetrated loads. CONCLUSIONS Composite Steel Joists may be used to provide a ecoomical shallow floor system with reduced floor-to-floor heights, icreased flexibility i layig out floor plas uiterrupted by closely spaced colums, ad allow the routig of HVAC ducts, plumbig, electrical coduits, ad telecommuicatios through the ope-web system of the joists. The Steel Joist Istitute has recetly published the First Editio Catalog for the CJ-Series that cotais the Stadard Specificatios for Composite Joists, Weight Tables, Bridgig Tables, ad Code of Stadard Practice to assist Specifyig Professioals with the desig ad selectio of Composite Steel Joists. A copy of this catalog ca be obtaied from the Steel Joist Istitute s website at http://www.steeljoist.org/publicatios or by cotactig the SJI office. REFERENCES AISC, Steel Costructio Maual, Thirteeth Ed. America Istitute of Steel Costructio, Chicago, IL, 2005 AWS, Structural Weldig Code Steel, AWS D1.1/D1.1M, 20 TH Ed., America Weldig Society, Miami, FL, 2006 Goble, G. G. Shear Stregth of Thi Flage Composite Specimes, AISC Egieerig Joural, Vol. 5, No. 2, April 1968 SJI, Composite Steel Joist Catalog, First Editio, Steel Joist Istitute, Myrtle Beach, SC, 2007