Chapter 13 Steel Conrete Composite Strutural Members 13.1 General Provisions for Steel Conrete Composite Strutural Members This setion states the sope of the speifiation, summarizess referened speifiations, odes and standard douments and provide requirements for materials for steel onrete omposite members. General provisions for omposite setions and shear onnetors are also inluded. 13.1.1 Sope guidelines inludedd in hapter 13 of part 6 of this ode presents the design guidelines for steel onrete omposite members frequently used in medium to high rise buildings. This hapter mainly addresses omposite olumns omposed of rolled or built up strutural steel shapes or HSS, and strutural onrete ating together, and steel beams supporting a reinfored onrete slab so interonneted that the beams and the slab at together to resist bending. Simple and ontinuous omposite beams with shear onnetors and onrete steel enased beams, onstruted with or without temporary shores, are inluded. Seismi provisions for onrete omposite members are also provided. 13.1.2 Referened Speifiations, Codes and Standards douments referened in these provisions shall inlude those listed in Part 6 Chapter 10 Setion 2 with the following additions and modifiations: Amerian Soiety of Civil Engineers ASCE 3 91 Standard for the Strutural Design of Composite Slabs Amerian Welding Soiety AWS D1.1 04 Strutural Welding Code Steel AWS D1.4 98 Strutural Welding Code Reinforing Steel Canadian Standards Assoiation CSA S16 01 Design of Steel Strutures 13.1.3 Material Limitations Conrete and steel reinforing bars in omposite systems shall be subjet to the following limitations. a) For the determination of the available strength, onrete shall have a ompressive strength of not less than 21 MPa nor more than 70 MPa for normal weight onrete and not less than 21 MPa nor more than 42 MPa for lightweightt onrete. b) speified minimum yield stress of strutural steel and reinforing bars used in alulating the strength of a omposite olumn shall not exeed 525 MPa. Higher material strengths are permitted when their use is justified by testing or analysis. 13.1.4 General Provisions In determining load effets in members and onnetions of a struture that inludes omposite members, onsideration shall be given to the effetive setions at the time eah inrement of load is applied. design, detailing and material properties related to the onrete and reinforing steel portions of omposite onstrution shall omply with the reinfored onrete and reinforing bar design speifiations stipulated by the provisions in Part 6 Chapter 5.
Part 6 Strutural Design 13. 1.4.1 Resistane Prior to Composite Ation fatored resistane of the steel member prior to the attainment of omposite ation shall be determined in aordane with Chapter 10 of Part 6. 13. 1.4.2 Nominal Strength of Composite Setions Two methods are provided for determining the nominal strength of omposite setions: the plasti stress distribution method and the strain ompatibility method. tensile strength of the onrete shall be negleted in the determination of the nominal strength of omposite members. 13. 1.4.2.1 Plasti Stress Distribution Method For the plasti stress distribution method, the nominal strength shall be omputed assuming that steel omponents have reahed a stress of F y in either tension or ompression and onrete omponents in ompression have reahed a stress of 0.85 f. For round HSS filled with onrete, a stress of 0.95 f is permitted to be used for onrete omponents in uniform ompression to aount for the effets of onrete onfinement. 13. 1.4.2.2 Strain Compatibility Method For the strain ompatibility method, a linear distribution of strains aross the setion shall be assumed, with the maximum onrete ompressive strain equal to 0.003 mm/mm (in/in). stress strain relationships for steel and onrete shall be obtained from tests or from published results for similar materials. 13. 1.4.2.3 Shear Connetors Shear onnetors shall be headed steel studs not less than four stud diameters in length after installation, or hot rolled steel hannels. Shear stud design values shall be taken as per Setions 13.2.1.7 and 13.3.2.4. Stud onnetors shall onform to the requirements of Setion 13.3.2.4(3) Channel onnetors shall onform to the requirements of Setion 13.3.2.4(4). 13..2 Design of Composite Axial Members This setion states the design guidelines for two types of omposite axial members. se inlude enased omposite olumns and onrete filled hollow strutural setions. 13.2.1 Enased Composite Columns 13.2.1.1 Sope This setion applies to doubly symmetri steel olumns enased in onrete, provided that (1) the steel shape is a ompat or non ompat setion; (2) the ross setional area of the steel ore omprises at least 1 perent of the total omposite ross setion. (3) onrete enasement of the steel ore is reinfored with ontinuous longitudinal bars and lateral ties or spirals. minimum transverse reinforement shall be at least 6 mm 2 per mm of tie spaing. (4) minimumm reinforement ratio for ontinuous longitudinal reinforing, ρ sr A g, shall be 0.004, ρ sr is given by: A ρ sr = A = area of ontinuous reinforing bars, mm 2 = gross area of omposite member, mm sr g m 2 13.2.1
Steel Conrete Composite Strutural Members Chapter 13 13.2.1.2 Compressive Strength design ompressivee strength, φ P n, and allowable ompressive strength, P Ω, for axially loaded enased omposite olumns shall be determined for the limit state of flexural bukling based on olumn slenderness as follows: n φ = 0.75 (LRFD) Ω = 2.00 (ASD) (a) When P 0. 44 e P o P n = P o 0.658 P o Pe 13.2.2 (b) When P < 0. 44 e P o P = 0. 877 n P e 13.2.3 P = A F o s y + A F + 0.85A sr yr f 13.2.4 and e A s = areaa of the steel setion, mm 2 A = areaa of onrete, mm 2 A sr = areaa of ontinuous reinforing bars, mm 2 E E s F y F yr I I s I sr K L w = modulus of elastiity of onrete = modulus of elastiity of steel = 210 MPa f = speified ompressive strength of onrete, MPa = speified minimumm yield stress of steel setion, MPa = speified minimum yield stresss of reinforing bars, MPa = moment of inertia of the onrete setion, mm 4 = moment of inertia of steel shape, mm 4 = moment of inertia of reinforing bars, mm 4 = the effetive length fator determined in aordane with Chapter 10 Part 6 = laterally unbraed length of the member, mmm = weight of onrete per unit volume 1500 w 2500 kg/ EI eff = effetive stiffness of omposite setion, N m EIeff = Es Is + 0.5 Es Isr + C1 E I P e π 2 (E = EI eff ) ( KL) 2 1.5 0.043 w MPa f m 2 m 3 13.2.5 13.2.6 C1 = 0.1+ A s 2 0.3 A + A s 13.2.7
Part 6 Strutural Design 13.2.1.3 Tensilee Strength design tensile strength, φ t P n, and allowable tensile strength, P n /Ω t, for enased omposite olumns shall be determined for the limit state of yielding as P = A F + A F n s y sr yr 13.2.8 0.90 (LRFD) 1.67 (ASD) 13.2.1.4 Shear Strength available shear strength shall be alulated based on either the shear strength of the steel setion alone as speified in Setion 10.7 plus the shear strength providedd by tie reinforement, if present, or the shear strength of the reinfored onrete portion alone. 13.2.1.5 Load Transfer Loads applied to axially loadedd enased omposite olumns shall be transferred between the steel and onrete in aordane with the following requirements: a) When the external fore is applied diretly to the steel setion, shear onnetors shall be provided to transfer the required shear fore, V, as follows: V = required shear fore introdued to olumn, N = area of steel ross setion, mm 2 A s V A = sf V 1 V As F = V y P o b) When load is applied to the onrete of an enased omposite olumn by diret bearing the design bearing strength, φ B P p, and the allowable bearing strength, P p /Ω B, of the onrete shall be: Pp = 1.7 f A B 13.2.11 φ B = 0.65 (LRFD) y P o = nominal axial ompressivee strength without onsideration of length effets, N When the external fore is applied diretly to the onrete enasement, shear onnetors shall be provided to transfer the required shear fore,, as follows: 13.2.10 P o A B = loaded area of onrete, mm Ω B = 2.31 (ASD) m 2 13.2.9 13.2.1.6 Detailing Requirements 13.2.1.6.1 Longitudinal Bars onrete enasement shall be reinfored with longitudinal bars and lateral ties extending ompletely around the strutural steel ore. lear over shall not be less than 40 mm. longitudinal bars shall a) Be ontinuous at framed levels when onsidered to arry load; b) Have an area not less than 0.01 times the total gross ross setional area; ) Be loated at eah orner; and d) Spaed on all sides not further apart than 525t/fy times one half the least dimension of the omposite setion.
Steel Conrete Composite Strutural Members Chapter 13 13.2.1.6.2 Lateral ties lateral ties shall a) Be 15M bars, exept that 10M bars may be used when no side dimension of the omposite setion exeeds 500 mm; and b) Have a vertial spaing not exeeding the least of the following: i) ii) Two thirds of the least side dimension of the ross setion; 16 longitudinal bar diameters; or required, shear onnetors transferring the required shear fore shall be distributed along the length of the member at least a distane of 2.5 times the width of a retangular HSS or 2.5 times the diameter of a round HSS both above and below the load transfer region. maximum onnetor spaing shall be 405 mm. 13.2.1.6.3 Shear Connetors Shear onnetors shall be provided to transfer the required shear fore Setion speified in 13.2.2.5. shear onnetors shall be distributed along the length of the member at least a distane of 2.5 times the depth of the enased omposite olumn above and below the load transfer region. maximum onnetor spaing shall be 405 mm. Connetors to transfer axial load shall be plaed on at least two faes of the steel shape in a onfiguration symmetrial about the steel shape axes. 13.2.1.6.4 Columns with Multiple Built up Shapes If the omposite rosss setion is built up from two or more enased steel shapes, the shapes shall be interonneted with laing, tie plates, batten plates or similar omponents to prevent bukling of individual shapes due to loads applied prior to hardening of the onrete. 13.2.1.7 Strength of Stud Shear Connetors nominal strength of one stud shear onnetor embedded in solid onrete is: = ross setional area of stud shear onnetor, mm 2 = speified minimum tensile strength of a stud shear onnetor, MPa 13.2.2 Conrete Filled Hollow Strutural Setion 13.2.2.1 Setion 13.2.2 applies to omposite members onsisting of steel hollow strutural setions ompletely filled with onrete, providedd that a) ross setional area of the steel HSS shall omprise at least 1 perent of the total omposite ross setion. b) width to thikness ratio of the walls of retangular hollow strutural setions does not exeed 1350 F y ) outside diameter to thikness ration of irularr hollow strutural setions does not exeed 28000/F y d) onrete strength is between 20 and 80 MPa for axially loaded olumns and between 20 and 40 MPa for olumns subjeted to axial ompression and bending. 13.2.2.2 iii) Q = 0. 5 n A s Sope 500 mm. f E Compressive Strength As F u 13.2.12 design ompressivee strength, φ P n and allowable ompressive strength, P n /Ω, for axially loaded filled omposite olumns shall be determined for the limit state of flexural bukling based on Setion 13.2.1.2 with the following modifiations:
Part 6 Strutural Design P A f o = As Fy + AsrFyr + C2 13.2.13 C 2 = 0.85 for retangular setions and 0.95 for irular setions EIeff EsI s + EsI sr + C3 = E I 13.2.14 A s C3 = 0.6 + 2 0.9 A + As 13.2.15 13.2.2.3 Tensile Strength design tensile strength, φ t P n, and allowable tensile strength, P n /Ω t, for filled omposite olumns shall be determined for the limit state of yielding as: P = A F + A F 13.2.16 n s y sr yr φ t = 0.90 (LRFD) Ω t = 1.67 (ASD) 13.2.2.4 Shear Strength available shear strength shall be alulated based on either the shear strength of the steel setion alone as speified in Chapter 10 or the shear strength of the reinfored onrete portion alone. shear strength of reinfored onrete portion may be determined aording to Chapter 6 of Part 6. 13.2.2.5 Load Transfer Loads applied to filled omposite olumns shall be transferred between the steel and onrete. When the external fore is applied either to the steel setion or to the onrete infill, transfer of fore from the steel setion to the onrete ore is required from diret bond interation, shear onnetion or diret bearing. fore transfer mehanism providing the largest nominal strength may be used. se fore transfer mehanisms shall not be superimposed. When load is applied to the onrete of an enased or filled omposite olumn by diret bearing the design bearing strength, φ B P p, and the allowable bearing strength, P p /Ω B, of the onrete shall be: P = 1. 7 f A 13.2.17 p E φ B = 0.65 (LRFD) A B is the loaded area, mm 2 Ω B = 2.31 (ASD) 13.2.2.6 Detailing Requirements required, shear onnetors transferring the required shear fore shall be distributed along the length of the member at least a distane of 2.5 times the width of a retangular HSS or 2.5 times the diameter of a round HSS both above and below the load transfer region. maximum onnetor spaing shall be 405 mm. 13.3 Design of Composite Flexural Members This setion applies to omposite beams onsisting of steel setions interonneted with either a reinfored onrete slab or a steel dek with a onrete over slab. steel beams and the reinfored onrete slab are so interonneted that the beams and the slab at together to resist bending. Simple and ontinuous omposite beams with shear onnetors and onrete enased beams, onstruted with or without temporary shores, are inluded. Design philosophy for omposite olumns subjeted to bending moments is also stated. 6 720 Vol. 2
Steel Conrete Composite Strutural Members Chapter 13 13.3.1 General 13.3.1.1 Defletions Calulation of defletions shall take into aount the effets of reep of onrete, shrinkage of onrete, and inreased flexibility resulting from partial shear onnetion and from interfaial slip. se effets shall be established by test or analysis, pratiable. Consideration shall also be given to the effets of full or partial ontinuity in the steel beams and onrete slabs in reduing alulated defletions. In lieu of tests or analysis, the effets of partial shear onnetion and interfaial slip, reep, and shrinkage may be assessed as follows: a) For inreased flexibility resulting from partial shear onnetion and interfaial slip, the defletions shall be alulated using an effetive moment of inertia given by l 0.25 e = ls + 0.85 p ( It Is ) 13.3.1 I s =moment of inertia of a steel beam, or of a steel joist or truss adjusted to inlude the effet of shear deformations, whih may be taken into aount by dereasing the moment of inertia based on the ross setional areas of the top and bottom hords by 15% or by a more detailed analysis P = fration of full shear onnetion =1.00 for full shear onnetion I t = transformed moment of inertia of omposite beam based on the modular ration n=e/e b) For reep, elasti defletions aused by dead loads and long term live loads, as alulated in Item (a), need to be inreased by 15% and ) For shrinkage of onrete, using a seleted free shrinkage strain, strain ompatibility between the steel and onrete, and an age adjusted effetive modulus of elastiity of onrete as it shrinks and reeps, the defletion of a simply supported omposite beam, joist, or truss shall be alulated as follows: L 2 L 2 ε f A y s = Ψ = 8 8 n s l es 13.3.2 L = span of the beam, joist, or truss ψ = urvature along length of the beam, joist, or truss due to shrinkage of onrete = empirial oeffiient used to math theory with test results (aounting for raking of onrete in tension, the non linear stress strain relationship of onrete, and other fators) = free shrinkage strain of onrete A = effetive area of onrete slab Y = distane from entroid of effetive of effetive area of onrete slab to entroidal axis of the omposite beam, joist, or truss n s = modular ration, E/E E ' = E (1 + χφ) 13.3.3 = age adjusted effetive modulus of elastiity of onrete χ = aging oeffiient of onrete ϕ = reep oeffiient of onrete Ies = Is + 0.85 p 0.25 Its ( ) Is 13.3.4 Bangladesh National Building Code 2012 6 721
Part 6 Strutural Design 13.3.1.2 Design Effetive Width of Conrete effetive width of the onrete slab is the sum of the effetive widths for eah side of the beam enterline, eah of whih shall not exeed: a) one eightb) one half the distane to the enterline of the adjaent beam; or of the beam span, enter to enter of supports; ) the distane to the edge of the slab. 13.3.1.3 Shear Strength available shear strength of omposite beams with shear onnetors shall be determined based upon the properties of the steel setion alone in aordane with Part 6 Chapter 10 Setion 10.7. available shear strength of onrete enas sed and filled omposite members shall be determined based upon the properties of the steel setion alone in aordane with Part 6 Chapter 10 Setion 10.7 or based upon the properties of the onrete and longitudinal steel reinforement. 13.3.1.4 Strength during Constrution When temporary shores are not used during onstrution, the steel setion alone shall have adequate strength to support all loads applied prior to the onrete attaining 75 perent of its speified strength. available flexural strength of the steel setion shall be determined aording to Part 6 Chapter 10 Setion 10.6. 13.3.2 Strength of Composite Beams with Shear Connetors 13.3.2.1 Positive Flexural Strength design positive flexural strength, φ b M n and the allowable positive flexural strength, M n /Ω b, shall be determined for the limit state of yielding as follows: = 0.90 (LRFD) Ω b = 1.67 (ASD) φ b a) For = effetivee moment of inertia of omposite beam, truss, or joist based on the modular ratio n s Its = transformed moment of inertia based on the modular ratio n s h 3. 76 t w E F y M n shall be determined from the plasti stress distribution on the omposite setion for the limit state of yielding (plasti moment). b) For h > 3. 76 t w E F y M n shall be determined from the superposition of elasti stresses, onsidering the effets of shoring, for the limit state of yielding ( yield moment). 13.3.2.2 Negative Flexural Strength design negative flexural strength, φ b M n, and the allowable negative flexural strength, M n /Ω b, shall be determined for the steel setion alone, in aordane with the requirements of Part 6 Chapter 10 Setion 10.6. Alternatively, the available negative flexural strength shall be determined from the plasti stress distribution on the omposite setion, for the limit state of yielding (plasti moment), with φ b = 0.90 (LRFD) Ω b = 1.67 (ASD) provided that: a) steel beam is ompat and is adequately braed aording to Setion 10.6. b) Shear onnetors onnet the slab to the steel beam in the negative moment region. ) slab reinforement parallel to the steel beam, within the effetive width of the slab, is properly developed.
Steel Conrete Composite Strutural Members Chapter 13 13.3.2.3 Strength of Composite Beams with Formed Steel Dek (1) General available flexural strength of omposite onstrution onsisting of onrete slabs on formed steel dek onneted to steel beams shall be determined by the appliable portions of Setion 13.3.2.1 and 13.3.2.2, with the following requirements: a) This setion is appliable to deks with nominal rib height not greater than 75 mm. average width of onrete rib or haunh, w r, shall be not less than 50 mm, but shall not be taken in alulations as more than the minimum lear width near the top of the steel dek. b) onrete slab shall be onneted to the steel beam with welded stud shear onnetors 19 mm or less in diameter (AWS D1.1). Studs shall be welded either through the dek or diretly to the steel ross setion. Stud shear onnetors, after installation, shall extend not less than 38 mm above the top of the steel dek and there shall be at least 13 mm of onrete over above the top of the installed studs. ) slab thikness above the steel dek shall be not less than 50 mm. d) Steel dek shall be anhored to all supporting members at a spaing not to exeed 460 mm. Suh anhorage shall be provided by stud onnetors, a ombination of stud onnetors and ar spot (puddle) welds, or other devies speified by the designer. (2) Dek Ribs Oriented Perpendiular to Steel Beam Conrete below the top of the steel dek shall be negleted in determining omposite setion properties and in alulating A for dek ribs oriented perpendiular to the steel beams. (3) Dek Ribs Oriented Parallel to Steel Beam Conrete below the top of the steel dek may be inluded in determining omposite setion properties and shall be inluded in alulating A. Formed steel dek ribs over supporting beams may be split longitudinally and separated to form a onrete haunh. When the nominal depth of steel dek is 38 mm or greater, the average width, w r, of the supported haunh or rib shall be not less than 50 mm for the first stud in the transverse row plus four stud diameters for eah additional stud. 13.3.2.4 Shear Connetors (1) Load Transfer for Positive Moment entire horizontal shear at the interfae between the steel beam and the onrete slab shall be assumed to be transferred by shear onnetors, exept for onrete enased beams as defined in Setion 10.9.3.3. For omposite ation with onrete subjet to flexural ompression, the total horizontal shear fore, V, between the point of maximum positive moment and the point of zero moment shall be taken as the lowest value aording to the limit states of onrete rushing, tensile yielding of the steel setion, or strength of the shear onnetors: Conrete rushing V = 0. 85 f A 13.3.5a Tensile yielding of the steel setion V = F A 13.3.5b y s Strength of shear onnetors V = Qn 13.3.5 A = area of onrete slab within effetive width, mm 2 Bangladesh National Building Code 2012 6 723
Part 6 Strutural Design A s Q n point of zero moment, N (2) Load Transferr for Negative Moment In ontinuous omposite beams longitudinal reinforing steel in the negative moment regions is onsidered to at ompositely with the steel beam, the total horizontal shear fore between the point of maximum negative moment and the point of zero moment shall be taken as the lower value aording to the limit states of yielding of the steel reinforement in the slab, or strength of the shear onnetors: A r = area of adequately developed longitudinal reinforing steel within the effetive width of the onrete slab, mm 2 F yr = speified minimum yield stress of the reinforing steel, MPa b) Strength of shear onnetors V = Qn (3) Strength of Stud Shear Connetors nominal strength of one stud shear onnetor embedded in solid onrete or in a omposite slab is Q n = 0. 5A s fe Rg R 13.3.7 p AsFu = 1.0; (a) for one stud welded in a steel dek rib with the dek oriented perpendiular to the steel shape; (b) for any number of studs welded in a row diretly to the steel shape; () for any number of studs welded in a row through steel dek with the dek oriented parallel to the steel shape and the ratio of the average rib width to rib depth 1.5 = 0.85; (a) for two studs welded in a steel dek rib with the dek oriented perpendiular to the steel shape; (b) for one stud welded through steel dek with the dek oriented parallel to the steel shape and the ratio of the average rib width to rib depth <1.5 = 0.7 for three or more studs welded in a steel dek rib with the dek oriented perpendiular to the steel shape = 1.0 for studs welded diretly to the steel shape (in other words, not through steel dek or sheet) and having a haunh detail with not more than 50 perent of the top flange overed by dek or sheet steel losuress = 0.75; (a) for studs welded in a omposite slab with the dek oriented perpendiular to the beam and e mid in a omposite slab with the dek oriented parallel to the beam ht 50 mm; (b) for studs welded through steel dek, or steel sheet used as girder filler material and embedded = 0.6 for studs welded in a omposite slab with dek oriented perpendiularr to the beam and e mid ht < 50 mmm e mid d ht = distane from the edge of stud shank to the steel dek web, measured at mid height of the dek rib, and in the load bearing diretion of the stud (in other words, in the diretion of maximum moment for a simply supported beam), mm = weight of onrete per unit volume (1500 w 2500 kg/m 3 ) w = area of steel ross setion, mm 2 = sum of nominal strengths of shear onnetors between the point of maximum positive moment and the a) Tensile yielding of the slab reinforement V = A r F yr = ross setional area of stud shear onnetor, mm 2 = modulus of elastiity of onrete = 0.043 w 1. 5 f, MPa = speified minimum tensile strength of a stud shear onnetor 13.3.6a 13.3.6b
Steel Conrete Composite Strutural Members Chapter 13 (4) Strength of Channel Shear Connetors nominal strength of one hannel shear onnetor embedded in a solid onrete slab is Q n= 0.3( t f + 0.5t w ) L f E 13.3.8 = flange thikness of hannel shear onnetor, mm = web thikness of hannel shear onnetor, mm = length of hannel shear onnetor, mm strength of the hannel shear onnetor shall be developedd by welding the hannel to the beam flange for a fore equal to, onsidering eentriity on the onnetor. (5) Required Number of Shear Connetors number of shear onnetors required between the setion of maximum bending moment, positive or negative, and the adjaent setion of zero moment shall be equal to the horizontal shear fore as determined in Setions 10.9.3.2d(1) and 10.9.3.2d( 2) divided by the nominal strength of one shear onnetor as determined from Setion 10.9.3.2d( (3) or Setion 10.9.3.2d(4). (6) Shear Connetor Plaement and Spaing Shear onnetors required on eah side of the point of maximum bending moment, positive or negative, shall be distributed uniformly between that point and the adjaent points of zero moment, unless otherwise speified. However, the number of shear onnetors plaed between any onentrated load and the nearest point of zero moment shall be suffiient to develop the maximum moment required at the onentrated load point. Shear onnetors shall have at least 25 mm of lateral onrete over, exept for onnetors installed in the ribs of formed steel deks. diameter of studs shall not be greater than 2.5 times the thikness of the flange to whih they are welded, unless loated over the web. minimum enter to enter spaing of stud onnetors shall be six diameters along the longitudinal axis of the supporting omposite beam and four diameters transversee to the longitudinal axis of the supporting omposite beam, exept that within the ribs of formed steel deks oriented perpendiular to the steel beam the minimum enter to enter spaing shall be four diameters in any diretion. maximum enter to enter spaing of shear onnetors shall not exeed eight times the total slab thikness nor 900 mm. 13.3.3 Slab Reinforement 13.3.3.1 General Slabs shall be adequately reinfored to support all loads and to ontrol both raking transversee to the omposite beam span and longitudinal raking over the steel setion. Reinforement shall not be less than that required by the speified fire resistane design of the assembly. 13.3.3.2 Parallel reinforement Reinforement parallel to the span of the beam in regions of negative bending moment of the omposite beam shall be anhored by embedment in onrete that is in ompression. reinforement of slabs that are to be ontinuous over the end support of steel setions or joists fitted with flexible end onnetions shall be given speial attention. Reinforement at the ends of beams supporting ribbed slabs perpendiular to the beam shall be not lesss than two 15M bars or equivalent. 13.3.3.3 Transverse reinforement onrete slab on metal dek Unless it is known from experiene that longitudinal raking aused by omposite ation diretly over the steel setion is unlikely, additional transverse reinforement or other effetive means shall be provided. Suh additional reinforement shall be plaed in the lower part of the slab and anhored so as to develop the yield strength of the reinforement. area of suh reinforement shall be not less than 0.002 times the onrete area being reinfored and shall be uniformly distributed.
Part 6 Strutural Design 13.3.3.4 Transverse reinforement Ribbed slabs a) the ribs are parallel to the beam span, the area of transverse reinforement shall be not less than 0.002 times the onrete over slab area being reinfored and shall be uniformly distributed. b) the ribs are perpendiular to the beam span, the area of transverse reinforement shall be not less than 0.001 times the onrete over slab area being reinfored and shall be uniformly distributed. 13.3.4 Flexural Strength of Conrete Enased and Filled Members nominal flexural strength of onrete enased and filled members shall be determined using one of the following methods: a) superposition of elasti stresses on the omposite setion, onsidering the effets of shoring, for the limit state of yielding (yield moment), φ b = 0.90 (LRFD) Ω b = 1.67 (ASD) b) plasti stress distribution on the steel setion alone, for the limit state of yielding (plasti moment), φ b = 0.90 (LRFD) Ω b = 1.67 (ASD) ) If shear onnetors are provided and the onrete meets the requirements of Setion 10.9.1.2, the nominal flexural strength shall be omputed based upon the plasti stress distribution on the omposite setion or from the strain ompatibility method, φ b = 0.85 (LRFD) Ω b = 1.76 (ASD) 13.3.5 Combined Axial Fore and Flexure interation between axial fores and flexure in omposite members shall aount for stability as required by Chapter C. design ompressive strength, φ P n, and allowable ompressive strength, P n /Ω and the design flexural strength, φ b M n and allowable flexural strength, M n / Ω b, are determined as follows: φ = 0.75 (LRFD) Ω = 2.00 (ASD) φ b = 0.90 (LRFD) Ω b = 1.67 (ASD) (1) nominal strength of the ross setion of a omposite member subjeted to ombined axial ompression and flexure shall be determined using either the plasti stress distribution method or the strain ompatibility method. (2) To aount for the influene of length effets on the axial strength of the member, the nominal axial strength of the member shall be determined by Setion 10.9 with P o taken as the nominal axial strength of the ross setion determined in Setion 10.9.4(13) above. 13.3.6 Speial Cases When omposite onstrution does not onform to the requirements of Setion 13.2 and Setion 13.3, the strength of shear onnetors and details of onstrution shall be established by testing. 13.4 Composite Connetions This Setion is appliable to onnetions in buildings that utilize omposite or dual steel and onrete systems. Composite onnetions shall be demonstrated to have Design Strength, dutility and toughness that is omparable to that exhibited by similar strutural steel or reinfored onrete onnetions that meet the requirements in Part 6 Chapter 10 and Chapter 5, respetively. Methods for alulating the onnetion strength shall meet the requirements in this Setion. 6 726 Vol. 2
Steel Conrete Composite Strutural Members Chapter 13 13.4.1 General Connetions shall have adequate deformation apaity to resist the ritial Required Strengths at the Design Story Drift. Additionally, onnetions that are required for the lateral stability of the building under seismi fores shall meet the requirements in Setion 13.5 based upon the speifi system in whih the onnetion is used. When the Required Strength is based upon nominal material strengths and nominal member dimensions, the determination of the required onnetion strength shall aount for any effets that result from the inrease in the atual Nominal Strength of the onneted member. 13.4.2 Nominal Strength of Connetions Nominal Strength of onnetions in omposite Strutural Systems shall be determined on the basis of rational models that satisfy both equilibrium of internal fores and the strength limitation of omponent materials and elements based upon potential limit states. Unless the onnetion strength is determined by analysis and testing, the models used for analysis of onnetions shall meet the following requirements: (1) When required, fore shall be transferred between strutural steel and reinfored onrete through diret bearing of headed shear studs or suitable alternative devies, by other mehanial means, by shear frition with the neessary lamping fore provided by reinforement normal to the plane of shear transfer, or by a ombination of these means. Any potential bond strength between strutural steel and reinfored onrete shall be ignored for the purpose of the onnetion fore transfer mehanism. (2) nominal bearing and shear frition strengths shall meet the requirements in Part 6 Chapter 6 and 10, exept that the strength redution (resistane) fators shall be as given in Part 6 Chapter 6. Unless a higher strength is substantiated by yli testing, the nominal bearing and shear frition strengths shall be redued by 25 perent for the omposite seismi systems. (3) Design Strengths of strutural steel omponents in omposite onnetions, as determined in Setion 13.2 and Setion 13.3 and the LRFD Speifiation, shall equal or exeed the Required Strengths. Strutural steel elements that are enased in onfined reinfored onrete are permitted to be onsidered to be braed against out of plane bukling. Fae Bearing Plates onsisting of stiffeners between the flanges of steel beams are required when beams are embedded in reinfored onrete olumns or walls. (4) nominal shear strength of reinfored onrete enased steel Panel Zones in beam to olumn onnetions shall be alulated as the sum of the Nominal Strengths of the strutural steel and onfined reinfored onrete shear elements as determined in Part 6 Chapter 10 and Part 6 Chapter 5, respetively. strength redution (resistane) fators for reinfored onrete shall be as given in Part 6 Chapter 6. (5) Reinforement shall be provided to resist all tensile fores in reinfored onrete omponents of the onnetions. Additionally, the onrete shall be on fined with transverse reinforement. All reinforement shall be fully developed in tension or ompression, as appropriate, beyond the point at whih it is no longer required to resist the fores. Development lengths shall be determined in aordane with Part 6 Chapter 6. Connetions shall meet the following additional requirements: (a) When the slab transfers horizontal diaphragm fores, the slab reinforement shall be designed and anhored to arry the in plane tensile fores at all ritial setions in the slab, inluding onnetions to olletor beams, olumns, braes and walls. (b) For onnetions between strutural steel or Composite Beams and reinfored onrete or Reinfored Conrete Enased Composite Columns, transverse hoop reinforement shall be provided in the onnetion region to meet the requirements in Chapter 6 of Part 6 exept for the following modifiations: (i) Strutural steel setions framing into the onnetions are onsidered to provide onfinement over a width equal to that of fae bearing stiffener plates welded to the beams between the flanges. (ii) Lap splies are permitted for perimeter ties when onfinement of the splie is provided by Fae Bearing Plates or other means that prevents spalling of the onrete over. () longitudinal bar sizes and layout in reinfored onrete and Composite Columns shall be detailed to minimize slippage of the bars through the beam to olumn onnetion due to high fore transfer assoiated with the hange in olumn moments over the height of the onnetion. Bangladesh National Building Code 2012 6 727
Part 6 Strutural Design 13.5 Seismi Provisions for Composite Strutural Systems se Provisions are intended for the design and onstrution of omposite strutural steel and reinfored onrete members and onnetions in the Seismi Load Resisting Systems in buildings for whih the design fores resulting from earthquake motions have been determined on the basis of various levels of energy dissipation in the inelasti range of response. 13.5.1 Sope Provisions shall be applied in onjuntion with the AISC Load and Resistane Fator Design (LRFD) Speifiation for Strutural Steel Buildings, hereinafter referred to as the LRFD Speifiation. All members and onnetions in the Seismi Load Resisting System shall have a Design Strength as required in the LRFD Speifiation and shall meet the requirements in these Provisions. appliable requirements in Part 6 Chapter 10 shall be used for the design of strutural steel omponents in omposite systems. Reinfored onrete members subjeted to seismi fores shall meet the requirements in Chapter 5 and 10 of Part 6 exept as modified in these provisions. When the design is based upon elasti analysis, the stiffness properties of the omponent members of omposite systems shall reflet their ondition at the onset of signifiant yielding of the building. 13.5.2 Seismi Design Categories Required Strength and other seismi provisions for Seismi Design Categories, Seismi Use Groups or Seismi Zones and the limitations on height and irregularity shall be as stipulated in the Part 6 Chapter 10. 13.5.3 Loads, Load Combinations, and Nominal Strengths loads and load ombinations shall be as stipulated by the Appliable Building Code. Amplified Seismi Loads are required by these provisions, the horizontal earthquake load E (as defined in Part 6 Chapter 10) shall be multiplied by the over strength fator Ω o presribed by the Part 6 Chapter 10. 13.5.4 Materials 13.5.4.1 Strutural Steel Strutural steel used in omposite Seismi Load Resisting Systems shall meet the requirements in Setion 10.20 of Part 6 in addition Setion 13.1 of Part 6. strutural steels that are expliitly permitted for use in seismi design have been seleted based upon their inelasti properties and weld ability. In general, they meet the following harateristis: (1) a ratio of yield stress to tensile stress not greater than 0.85; (2) a pronouned stress strain plateau at the yield stress; (3) a large inelasti strain apability (for example, tensile elongation of 20 perent or greater in a 2 in. (50 mm) gage length); and (4) good weldability. Other steels should not be used without evidene that the above riteria are met. 13.5.4.2 Conrete and Steel Reinforement Conrete and steel reinforement used in omposite Seismi Load Resisting Systems shall meet the requirements in Part 6 Chapter 5, and the following requirements: (1) speified minimum ompressive strength of onrete in omposite members shall equal or exeed 2.5 ksi (17 MPa). (2) For the purposes of determining the Nominal Strength of omposite members, f shall not be taken as greater than 10 ksi (69 MPa) for normal weight onrete nor 4 ksi (28 MPa) for lightweight onrete. Conrete and steel reinforement used in the omposite Seismi Load Resisting Systems desribed shall also meet the requirements in Part 6 Chapter 6. 13.5.5 Composite Members 13.5.5.1 Composite Floor and Roof Slabs design of omposite floor and roof slabs shall meet the requirements of ASCE 3 91. Composite slab diaphragms shall meet the requirements in this Setion. Details shall be designed to transfer fores between the diaphragm and Boundary Members, Colletor Elements, and elements of the horizontal framing system. 6 728 Vol. 2
Steel Conrete Composite Strutural Members Chapter 13 nominal shear strength of omposite diaphragms and onrete filled steel dek diaphragms shall be taken as the nominal shear strength of the reinfored onrete above the top of the steel dek ribs in aordane with Part 6 Chapter 5. Alternatively, the omposite diaphragm design shear strength shall be determined by in plane shear tests of onrete filled diaphragms. 13.5.5.2 Composite Beams Composite Beams shall meet the requirements in Setion 13.3. Composite Beams that are part of C SMF shall also meet the following requirements: (1) distane from the maximum onrete ompression fiber to the plasti neutral axis shall not exeed: Y on + d b 1700 + Fy 1 Es 13.5.1 Y on = distane from the top of the steel beam to the top of onrete, mm d b = depth of the steel beam, mm F y = speified minimum yield strength of the steel beam, MPa E s = modulus of elastiity of the steel beam, MPa (2) Beam flanges shall meet the requirements in Part 6 Setion 10.20.9.4.2, exept when fully reinforedonrete enased ompression elements have a reinfored onrete over of at least 2 in. (50 mm) and onfinement is provided by hoop reinforement in regions plasti hinges are expeted to our under seismi deformations. Hoop reinforement shall meet the requirements in Part 6 Chapter 6. 13.5.5.3 Reinfored Conrete Enased Composite Columns This Setion is appliable to olumns that: (1) onsist of reinfored onrete enased strutural steel setions with a strutural steel area that omprises at least 4 perent of the total omposite olumn ross setion; and (2) meet the additional limitations in Setion 13.2.2.1. Suh olumns shall meet the requirements in Setion 13.2.2, exept as modified in this Setion. Additional requirements, as speified for intermediate and speial seismi systems in Setions 13.5.5.3.2 and 13.5.5.3.3, shall apply as required. Columns that onsist of reinfored onrete enased strutural steel setions with a strutural steel area that omprises less than 4 perent of the total omposite olumn ross setion shall meet the requirements for reinfored onrete olumns in Part 6 Chapter 5 exept as modified for: (1) steel shape shear onnetors in Setion 13.5.4.3.1 (2). (2) ontribution of the reinfored onrete enased strutural steel setion to the strength of the olumn as provided in Part 6 Chapter 6. (3) seismi requirements for reinfored onrete olumns as speified in the desription of the omposite seismi systems in Setions 13.5.5.3.1 through 13.5.5.3.3. 13.5.5.3.1 Ordinary Seismi System Requirements following requirements for Reinfored Conrete Enased Composite Columns are appliable to all omposite systems: (1) nominal shear strength of the olumn shall be determined as the nominal shear strength of the strutural shape plus the nominal shear strength that is provided by the tie reinforement in the reinforedonrete enasement. nominal shear strength of the strutural steel setion shall be determined in aordane with Setion 10.20 of Chapter 6. nominal shear strength of the tie reinforement shall be determined in aordane with Part 6 Chapter 5. In Part 6 Chapter 5, the dimension b w shall equal the width of the onrete ross setion minus the width of the strutural shape measured perpendiular to the diretion of shear. nominal shear strength shall be multiplied by Ф v equal to 0.75 to determine the design shear strength. (2) Composite Columns that are designed to share the applied loads between the strutural steel setion and reinfored onrete shall have shear onnetors that meet the following requirements: Bangladesh National Building Code 2012 6 729
Part 6 Strutural Design (a) If an external member is framed diretly to the strutural steel setion to transfer a vertial reation V u, shear onnetors shall be provided to transfer the fore V u (1 A s F y /P n ) between the strutural steel setion and the reinfored onrete, As is the area of the strutural steel setion, F y is the speified minimum yield strength of the strutural steel setion, and P n is the nominal ompressive strength of the Composite Column. (b) If an external member is framed diretly to the reinfored onrete to transfer a vertial reation V u, shear onnetors shall be provided to transfer the fore V u A s F y /P n between the strutural steel setion and the reinfored onrete, A s, F y and P n are as defined above. () maximum spaing of shear onnetors shall be 16 in. (406 mm) with attahment along the outside flange faes of the embedded shape. (3) maximum spaing of transverse ties shall be the least of the following: (a) one half the least dimension of the setion (b) 16 longitudinal bar diameters () 48 tie diameters Transverse ties shall be loated vertially within one half the tie spaing above the top of the footing or lowest beam or slab in any story and shall be spaed as provided herein within one half the tie spaing below the lowest beam or slab framing into the olumn. Transverse bars shall have a diameter that is not less than one fiftieth of greatest side dimension of the omposite member, exept that ties shall not be smaller than No. 3 bars and need not be larger than No. 5 bars. Alternatively, welded wire fabri of equivalent area is permitted as transverse reinforement exept when prohibited for intermediate and speial systems. (4) All Load Carrying Reinforement shall meet the detailing and splie requirements in Part 6 Chapter 5. Load Carrying Reinforement shall be provided at every orner of a retangular ross setion. maximum spaing of other load arrying or restraining longitudinal reinforement shall be one half of the least side dimension of the omposite member. (5) Splies and end bearing details for reinfored onrete enased strutural steel setions shall meet the requirements in Chapter 5 of Part 6. If adverse behavioral effets due to the abrupt hange in member stiffness and nominal tensile strength our when reinfored onrete enasement of a strutural steel setion is terminated, either at a transition to a pure reinfored onrete olumn or at the Column Base, they shall be onsidered in the design. 13.5.5.3.2 Intermediate Seismi System Requirements Reinfored Conrete Enased Composite Columns in intermediate seismi systems shall meet the following requirements in addition to those in Setion 13.5.5.3.1: (1) maximum spaing of transverse bars at the top and bottom shall be the least of the following: a) one half the least dimension of the setion b) 8 longitudinal bar diameters ) 24 tie bar diameters d) 12 in. (305 mm) se spaings shall be maintained over a vertial distane equal to the greatest of the following lengths, measured from eah joint fae and on both sides of any setion flexural yielding is expeted to our: a) one sixth the vertial lear height of the olumn b) the maximum ross setional dimension ) 18 in. (457 mm) (2) Tie spaing over the remaining olumn length shall not exeed twie the spaing defined above. (3) Welded wire fabri is not permitted as transverse reinforement in intermediate seismi systems. 13.5.5.3.3 Speial Seismi System Requirements Reinfored onrete enased olumns for speial seismi systems shall meet the following requirements in addition to those in Setions 13.5.4.3.2 and Setions 13.5.4.4.3: (1) required axial strength for Reinfored Conrete Enased Composite Columns and splie details shall meet the requirements in Setion 13.2. 6 730 Vol. 2
Steel Conrete Composite Strutural Members Chapter 13 (2) Longitudinal Load Carrying Reinforement shall meet the requirements in Part 6 Chapter 6. (3) Transverse reinforement shall be hoop reinforement as defined in Part 6 Chapter 6 and shall meet the following requirements: a) minimum area of tie reinforement A sh shall meet the following requirement: Fy As ' = f Ash 0.09hs 1 Pn F 13.5.2 yh h = ross setional dimension of the onfined ore measured enter to enter of the tie reinforement, mm s = spaing of transverse reinforement measured along the longitudinal axis of the strutural member, mm F y = speified minimum yield strength of the strutural steel ore, MPa A s = ross setional area of the strutural ore, mm 2 P n = nominal axial ompressive strength of the Composite Column alulated in aordane with the LRFD Speifiation, N f = speified ompressive strength of onrete, MPa F yh = speified minimum yield strength of the ties, MPa Equation 13.5.2 need not be satisfied if the Nominal Strength of the reinfored onrete enased strutural steel setion alone is greater than 1.0D + 0.5L. b) maximum spaing of transverse reinforement along the length of the olumn shall be the lesser of 6 longitudinal load arrying bar diameters and 152 mm (6 in.). ) When speified in Setions 13.5.5.3.3 (4), (5) or (6), the maximum spaing of transverse reinforement shall be the lesser of one fourth the least member dimension and 102 mm (4 in.). For this reinforement, ross ties, legs of overlapping hoops, and other onfining reinforement shall be spaed not more than 355 mm (14 in.) on enter in the transverse diretion. (4) Reinfored Conrete Enased Composite Columns in Braed Frames with axial ompression fores that are larger than 0.2 times Po shall have transverse reinforement as speified in Setion 13.5.5.3.3(3), over the total element length. This requirement need not be satisfied if the Nominal Strength of the reinfored onreteenased steel setion alone is greater than 1.0D + 0.5L. (5) Composite Columns supporting reations from disontinued stiff members, suh as walls or Braed Frames, shall have transverse reinforement as speified in Setion 13.5.5.3.3(3)() over the full length beneath the level at whih the disontinuity ours if the axial ompression fore exeeds 0.1 times P o. Transverse reinforement shall extend into the disontinued member for at least the length required to develop full yielding in the reinfored onrete enased strutural steel setion and longitudinal reinforement. This requirement need not be satisfied if the Nominal Strength of the reinfored onrete enased strutural steel setion alone is greater than 1.0D + 0.5L. (6) Reinfored Conrete Enased Composite Columns that are used in C SMF shall meet the following requirements: a) Transverse reinforement shall meet the requirements in 13.5.5.3.3(3)() at the top and bottom of the olumn over the region speified in Setion 6.4b. b) strong olumn/weak beam design requirements in shall be satisfied. Column Bases shall be detailed to sustain inelasti flexural hinging. ) minimum required shear strength of the olumn shall meet the requirements in Part 6 Chapter 5. (7) When the olumn terminates on a footing or mat foundation, the transverse reinforement as speified in this setion shall extend into the footing or mat at least 305 mm (12 in.). When the olumn terminates on a wall, the transverse reinforement shall extend into the wall for at least the length required to develop full yielding in the reinfored onrete enased strutural steel setion and longitudinal reinforement. (8) Welded wire fabri is not permitted as transverse reinforement for speial seismi systems. Bangladesh National Building Code 2012 6 731
Part 6 Strutural Design 13.5.5.4 Conrete Filled Composite Columns This Setion is appliable to olumns that: (1) onsist of onrete filled steel retangular or irular hollow strutural setions (HSS) with a strutural steel area that omprises at least 4 perent of the total ompositeolumn ross setion; and (2) meet the additional limitations in Setion 13.2. Suh olumns shall be designed to meet the requirements in Setion 13.2, exept as modified in this Setion. design shear strength of the Composite Column shall be the design shear strength of the strutural steel setion alone. In the speial seismi systems desribed in, members and olumn splies for Conrete Filled Composite Columns shall also meet the requirements in Part 6 Setion 10.20. Conrete Filled Composite Columns used in C SMF shall meet the following additional requirements: (1) minimum required shear strength of the olumn shall meet the requirements in Part 6 Chapter 5. (2) strong olumn/weak beam design requirements shall be met. Column Bases shall be designed to sustain inelasti flexural hinging. (3) minimum wall thikness of onrete filled retangular HSS shall equal b ( 2 ) of eah fae, b is as defined in Part 6 Chapter 10 Table 10.2.1. 13.5.6 Composite Steel Plate Shear Walls (C SPW) Fy / Es for the flat width b 13.5.6.1 Sope This Setion is appliable to strutural walls onsisting of steel plates with reinfored onrete enasement on one or both sides of the plate and strutural steel or omposite Boundary Members. C SPW shall meet the requirements of this setion. 13.5.6.2 Wall Elements 13.5.6.2.1 Nominal Shear Strength nominal shear strength of C SPW with a stiffened plate onforming to Setion 13.5.4.2.2 shall be determined as: Vns = 0. 6A sp F y 13.5.3 V ns = nominal shear strength of the steel plate, N A sp = horizontal area of stiffened steel plate, mm 2 F y = speified minimum yield strength of the plate, MPa nominal shear strength of C SPW with a plate that does not meet the stiffening requirements in Setion 13.5.4.2.2 shall be based upon the strength of the plate, exluding the strength of the reinfored onrete, and meet the requirements in the Part 6 Chapter 10, inluding the effets of bukling of the plate. 13.5.6.2.2 Detailing Requirements steel plate shall be adequately stiffened by enasement or attahment to the reinfored onrete if it an be demonstrated with an elasti plate bukling analysis that the omposite wall an resist a nominal shear fore equal to V ns. onrete thikness shall be a minimum of 102 mm (4 in.) on eah side when onrete is provided on both sides of the steel plate and 200 mm (8 in.) when onrete is provided on one side of the steel plate. Headed shear stud onnetors or other mehanial onnetors shall be provided to prevent loal bukling and separation of the plate and reinfored onrete. Horizontal and vertial reinforement shall be provided in the onrete enasement to meet the detailing requirements in Part 6 Chapter 5. reinforement ratio in both diretions shall not be less than 0.0025; the maximum spaing between bars shall not exeed 455 mm (18 in.). steel plate shall be ontinuously onneted on all edges to strutural steel framing and Boundary Members with welds and/or slip ritial high strength bolts to develop the nominal shear strength of the plate. Design Strength of welded and bolted onnetors shall meet the additional requirements in Part 6 Chapter 10. 6 732 Vol. 2