维普资讯 http://www.cqvip.com 第38卷第4期 西 安 建 筑 科技 大 学 学 报(自然科学版) Vo1.38 No.4 2006年8月 J.Xi’an Univ.of Arch.&Tech.(Natural Science Edition) Aug. 2006 Fundamental mechanical behavior of innovatively designed hybrid FRP--concrete beams WANG Yah—lei。0U Jin—ping (School of Civil Engineering,Harbin Institute of Technology,Harbin 150090,China) Abstract:In recent years Fiber Reinforced Polymer(FRP)material has been widely applied in the bridge engineering to solve the corrosion of the reinforcing stee1.To make the best use of materials and reduce the initial COSt,hybrid FRP—con— crete structures have been considered as the most effective composite form.The basic design concepts for hybrid FRP— concrete beams and bridge decks consisted of a number of individual hybrid beams are introduced detailedly in this paper. The concepts result in COSt・-effective composite members with pseudo・-ductile characteristics and high stiffness and strength properties.The study presents methods of simplified failure analysis,ductility consideration,and simplified calculations of stiffness and deformation for the proposed hybrid FRP-concrete beams.A preliminary design procedure for the hybrid beams is also presented,based on a complete set of stiffness,strength and ductility design requirements. Key words:FRP;composite beam;fundamental m.echanical behavior;concrete CLC numher:TU398 Document god@:A Arfiele ID:1006—7930(2006)04—0455-08 A major concern for many bridge superstructures is the significant reduction in durability and life expectancy caused by the corrosion of the reinforcing steel and the corresponding deterioration of the concrete.These problems are accelerated by the application of de-icing salts in cold area.The corro— sion will eventually cause enough damage to warrant fl superstructure replacement or retrofitE¨.Ac— cording to the U.S.National Bridge Inventory,nearly 27 percent of 590,000 public bridges were ei— ther structurally deficient or functionally obsolete as of 2003.These facts imply that it is imperative to develop and construct bridge systems that have long—term durability and require low maintenance。 As one of the solutions to this problem,structural applications of fiber reinforced polymer(FRP) composites have recently been attractive in the civil engineering community due to their superior mate— rial properties such as high specific stiffness,high specific strength,and high corrosion resistance. However,all—composite structural systems have specific shortcomings such as high initial costs,low stiffness and existence of catastrophic failure modes.To make the best use of materials and overco— ming the above shortcomings,hybrid FRP-concrete structures have been considered as the most effec— tive composite form[引. This paper presents how composite materials can be combined with fl low--cost construction mate-- rial,concrete,to result in new concepts for the design of lightweight,corrosion immune,and yet in- expensive beams.Simplified analysis methods for fundamental mechanical behavior of the proposed system are studied in detail.This new concept lends itself to low—COSt prefabrication of high-perform— ance structural elements,with fl high potential of use in civil engineering. Received date:2006一O3—3O Foundation item:Western Transportation Science and Technology Funding of China(200431882021) Biography:WANG Yan—lei(1978一),male,Candidate for Ph.D.,Research focused on hybrid FRP—concrete structures,Harbin 150090,P.R.China,Tel:0086—451—86282209,E—mail:hit1127@163.com 维普资讯 http://www.cqvip.com 456 西安建筑科技大学学报(自然科学版) 第38卷 1 Basic Design Concepts To explain the new composite design concept,the traditional reinforced concrete beam concept will be used as fl starting point.Fig.1 shows the basics behind this conceptFor slender concrete .beam under flexural loading it is assumed that the strains vary linearly over the depth with compres— sive strains in the top and tensile strains in the bottom part of the beam.The tensile strength of con— crete is extremely low and is ignored when determining the ultimate load carrying capacityConse— .quently,the main load carrying elements of the beam consist of the concrete compression zone(ap— proximately 2O 一25 of the cross section)and the steel reinforcement ̄ .The two main disadvanta— ges of reinforced concrete beams are the potential corrosion of the reinforcement and the high self- weight.The latter in particular is an important issue given the fact that 75 一80 of the material that contributes to the weight(concrete in the tensile zone)does not directly contribute to the overall load carrying capacity.This is not only inefficient but also wasteful from fl natural resourcas perspec— tive.The low cost of concrete is probably the main reason that this has not been fl major issue in the past. Bcam section S仃ain E bctive section R ̄sultant diagram forc ̄ Fig.1 Reinforce Concrete Beam Concept Fig.2 Proposed Hybrid FRP-concrete Beams .FRP materials can be used to improve this traditional concept in fl number of areasFor example, replacement of the steel reinforcement with FRP reinforcement eliminates the corrosion problam.This replacement is relatively straightforward and is currently being investigated at fl number of research in— stitutions around the world.However,this replacement addresses the corrosion issue onlv,it does not provide any significant reduction in the self weight. To reduced structural self—weight,glass FRP(GFRP)thin—walled box beams were used to re— place the concrete in the tensile zone and the steel reinforcement of common reinforced concrete beams.Thin-walled box sections are the most efficient for beams and are,in fact,verv common1v used in structural applications for FRP beams.Another composite material with fl failure strain less than that of GFRP could be added to the section’s tension zone,so that it will be the first element to fail giving some warning of an important collapse(pseudo—ductility).Since this element will be part of lf flange,it should preferably possess fl high stiffness,too,to increase the section’s rigidityAn ex— .ternally epoxy—bonded thin layer of unidirectional carbon FRP(CFRP)appears to be the best candi— date material for this purpose.A schematic illustration of hybrid FRP—concrete cross—sections for beams in both building and bridge applications is given in fig.2. Furthermore,the bridge deck design is developed based on the traditional plank bridge concept, in which fl number of individual hybrid FRP—concrete beams are placed side by side to create fl bridge deck.The advantages of this concept include: ①No joints between deck and girders(the girders are the deck); 维普资讯 http://www.cqvip.com 第4期 WANG Yan—lei et al:Fundamental mechanical behavior of innovatively designed hybrid FRP-concrete beams 457 ②Significant understanding of the bridge deck behavior can be obtained through testing of individual beams; ③Excellent resistance against flood loading and side impact; ④Significant redundancy in the structure due to large number of beams. In the current design,plate behavior is achieved by adhe— ring the beams together using fl high quality epoxy adhesive. Cross—section of fl bridge deck consisted of six individual beams is shown in fig.3.This bridge deck is provided with fl strong composite laminate in transverse direction along the Fig.3 Cross section of a bridge deck bottom of the beams in order to give the deck adequate trans— verse stiffnass. mplified Anal3 ̄sis for Fundamental Mechanical Behavior 2 SiAn idealization of the proposed cross section and its geometrical variables for hybrid FRP—concrete beam elements are shown in“g.4. The GFRP top flange is used only to support the concrete layer and to provide for the shear transfer to the GFRP webs.This implies that very thin GFRP top flange would suffice,and hence its contri— bution to the bending analysis can be omitted(f2—0).It is assumed C f2 that t ,t f,t1<< .The properties of the different materials con— stituting the cross section are as follows:E 一Young’s modulus of concrete; —compressive strength of concrete;£ 一ultimate corn— ,’ tel CFRP Fig.4 Idealized cross section variables of hybrid FRP—concrete beam pressive strain of concrete;E L=longitudinal Young’s modulus of webs;E T—transverse Young’s modulus of webs;G =shear modu— lus of webs;u L—longitudinal Poisson’s ratio of webs; T:==transverse Poisson’s ratio of webs;rL— web shear fracture strength;E1L—longitudinal Young’s modulus of GFRP bottom flange;G1 shear modulus of GFRP bottom flange;£ =longitudinal tensile failure strain of GFRP bottom flange;OIL— longitudinal Poisson’s ratio of GFRP bottom flange;E f=longitudinal Young’s modulus of CFRP laminate;and£ :longitudinal tensile failure strain of CFRP laminate. 2.1 Basic Assumptions . In the analysis,the classic beam theory approach is implemented,using Timoshenko’s shear flow for the calculation of deformations.The main assumptions are: *Plane sections remain plane in the bending analysis; *Failure prediction is defined by the maximum strain criterion; *A idealized bond between concrete layer and GFRP top flange as well as between GFRP bot— tom flange and CFRP laminate; *Materia1 Droperties for both FRPs are linear elastic,while for concrete the nonlinearity is con— sidered only at failure. 2.2 Simplified Failure Analysis The hybrid section loaded in bending can fail in a number of possible ways.The webs may frac— ture or buckle due to shear stress,resulting in shear failure;one or more of the beam’s elements may fail due to normal stresses,resulting in flexural failure;the beam may buckle laterally(1aterally tor— 维普资讯 http://www.cqvip.com 458 西安建筑科技大学学报(自然科学版) 第38卷 sional buckling);the bond at the GFRP—concrete or GFRP—CFRP interface may fail causing debonding (here we assume that their bonds are reliable):and if the concrete layer is(unreasonably)thick,the concrete may fail in shear(it is very unlikely to occur for reasonable thick concrete layers).The rea— sonable failure modes are discussed next. 2.2.1 Web Shear Fracture As an approximation,the maximum shear stress r x in the webs may be taken to be equal to 1.5 times the average shear stress in the webs,thus r 一1.5V /(2t d) where V 一section’s ultimate shear force. (1) Shear fracture occurs when the maximum shear stress equals the in-plane fracture shear strength of the webs,giving the following: V =(4/3)twd rL 2.2.2 Web Shear Buckling The average web shear buckling stress can be determined theoretically as follows[4j: (2) rL一(4K、4/ L 3T/(£ ),for >1 (3) rbw 一(4K v/D--T ̄H)/(t ),for O<l where DL=E Lt ̄/E12(1一 L T)];DT—E Tt ̄/E12(1一 L wT)]; H=0.5(vwLDT-[-"UwTOL)+2G tk/Et2(1一"UwL T)]; 一x/DLDT/H and the value of K depends on 0 as shown in Tab.1 L . Tab 1 ValRe of 0 vel"su¥K (4) Shear buckling occurs when the average shear stress equals the shear buckling stress,as follows: Vb 一2t d r 2.2.3 Flexural Fallure (5) According to Deskovic’s design concept[ 。 .in a well—designed hybrid FRP—concrete section,the CFRP laminate fails first(thus giving a warning of collapse)at a moment Mcf.The stresses are then redistributed and the section continues to carry an increasing moment until the concrete crushes in compression when its maximum compressive strain equals the ultimate failure strain of concrete£ . Crushing of concrete defines flexural collapse at a moment Mu that is higher than Mcf.The feature of the hybrid beams is referred to as pseudo—ductility,which is shown schematically in fig.5. (1)CFRP laminate fracture(Mlcf) Assuming that the concrete is approximately linear elastic when the CFRP laminate fails,the nor— ealr strain and stress distributions corresponding to Mcf are given in Fig.6.Based on equilibrium of normal forces,the depth of the neutral axis from the concrete bottom fiber y is given as follows: 一(--0.5n bc + 1bd t1+ fbd t f+£ )/( 如+ 1 1+ fbta-4-2t d ) (6) where 一E /E L; 1一E1/E L; f—E f/E L; According to moment equilibrium,Mcf is calculated as Mcf-Ece。bcy +E s twy d.,--y/3一一Y J丢Ew L£ £ ( w-y) (7) (8) Under the condition,the strain at the concrete top fiber£ is less than£ ,that is <( £ 一ccA/(£ +cA) 维普资讯 http://www.cqvip.com 第4期 WANG Yan—lei et al:Fundamental mechanical behavior of innovatively designed hybrid FRP-concrete beams 459 ingMu Concrete laver GFRP webs GFRP bottom lfange CFRP laminate £cf Displacement,A a1 Strains distribution b1 Stresses distribution Fig.5 Pseudo-duictility failure Fig.6 Distributions at CFRP laminate fracture (2)Concrete crushing(Mu) The normal strain and stress distributions corresponding to the ultimate flexural capacity of cross- section Mu are shown in Fig.7.The stresses in the concrete layer are calculated here based on the e- quivalent rectangular stress block approach[川.The depth of the neutral axis from the concrete bottom fiber Y is given as follows: y 一[一(口l届 bc。)/(E L£ +"lbd t1+£ d )]/[(口l届 bc)/(E L£ )+"lbt1+2t d ] (9) where口1一1.0(for normal strength concrete).Note here that because the concrete layer does not extend down to the c neutral axis,the value for岛should be closer to 1 than the 0. yu 7 1—0.8 value used in reinforced concrete analysis. Finally,moment equilibrium gives the flexural ca— pacity of the cross—section,as follows: fl 6。 BOtlOmflange Mo一口l届fobc(d +(2一届)c/2)+E L£ t Y:(d 一 (a)Strain (b)Exact stress (c)Simpliifed stress .), /3)/(c+.), )一E L£ t (d 一.), ) /[3(c+.), )](10) Fig.7 Distributions at flexural failure of coros section Under the constraint that the GFRP bottom flange has not fractured(£l<£l*),that is Y >(d £ 一c£ )/(£ +£ ) (11) 2.2.4 Lateral Instability Although box sections have a high resistance to lateral torsional buckling in general,limits to a section’s height—to—width ratio are often imposed to prevent lateral instability and satisfy other practi一 cal design requirements.Hence,a constraint inequality of the form (c+d )/b<k1 (12) is adopted in this paper,where kl—a constant to be chosen by the designer.Practical experience with composite material sections and thin-walled hybrid sections made of other materials indicates that a number around 3 is a reasonable conservative estimate for k1嘲. 2.3 Ductility Considerations At the cross—section level,ductility can be defined as the ratio of the curvature at failure( )to that at the deviation from the linear elastic response。which is when the CFRP laminate fractures ( ).From Fig.6 and Fig.7,the curvature ductility is calculated as follows: 一 / 一[£cu(d --y)]/[£ (c+.), )] (13) 2.4 Simplified Calculation of Stiffness and Deformation Assuming that all materials constituting the hybrid FRP—concrete section are linear elastic,the flexural rigidity Db of the hybrid section is given by the following: 维普资讯 http://www.cqvip.com 460 西安建筑科技大学学报(自然科学版) 第38卷 Db=E L[” bc。/12+” bc(y+c/2)。+” fbt f( --y)。+nlbtl( --y)。 +f /6+2t d (d /2一 )。] (14) where Y is the depth of the neutral axis from the concrete bottom fiber given by equation(6). In thin—wa11ed box sections under a vertical shear force,the shear load is primarily carried by the webs.Therefore,a simple approximate formula for the shear rigidity Ds gives the following: D 一2G t d (15) Although the effect of shear deformation on the deflection of isotropic beams is usually relatively sma11 compared to the effect of flexural deformations,it can’t be neglected in the case of composite materia1s.So the deformation calculation for composite beam is the sum of flexural deformation and shear deformation. For example,in the case of single-span simple—supported beams for 3-point ben— ding,the maximum deformation in midspan is given as follows: 一(PL。)/(48Db)+(PL)/(4K。D ) (16) where P is the concentrated force applied to the beams,L is the span of beams,K。is the shear coefficient depending on the beam geometry and material properties.Typical values of K。for rectan— gu1ar thin wa11ed composite beams are founded between 0.6一O.71[引.The detailed values of K can be determined using Timoshenko’s shear flow. 3 Design Procedures The issue under consideration here is the selection of the cross section’s geometric variables(d , 6,c,f ,f1 and tc f)in the term of the given material properties,so that a given set of design require— ments is satiffed at the minimum cost. A typical set of design constraints could consist of a bending moment and a shear force to be carried safely by the cross section(strength constraints),as well as a maximum def0rmation limit(stiffness constraint).According to Deskovic’s design concepts for hybrid FRP-concrete beamsE引, some essential concepts towards the development of an appropriate design procedure are discussed next: (a)The webs are designed optimally when they fail simultaneously according to all their associat— ed failure modes(don’t consider the interactions of different failure modes).It suggests that equations (2)一(5)should a11 be satisfied,in dicating that d and t can be obtained in terms of the material prop— erties and the ultimate shear force V : d 一 for >1;d for伊<1 (17) t 一(3V )/(4d r ) (18) (b)CFRP is an expensive material and the primary purpose of its application should be to provide a pseudo-duct订e response in bending.The ductility will increase when the thickness of the CFRP lami— nate decreases.If the thickness of CFRP used is extremely small,the CFRP laminate will fail prema— ture1v at a low load.If the load at failure of the CFRP is a high fraction of the ultimate load at CFRP failure should not be tOO high,warning of an imminent collapse comes tOO late.The previous state— ment suggests that the designers should specify a certain ratio(kz)of the moment at CFRP fracture (Mof)to the ultimate moment(Mu),say M f—k2M (19) where kz<1,k2=O.75 to be chosen in general design. 维普资讯 http://www.cqvip.com 第4期 WANG Yan-lei et al:Fundamental mechanical behavior of innovatively designed hybrid FRP-concrete beams 461 (c)Concrete is a very cheap material and its use in the cross section should be maximized・which suggests that inequality(1 2)should be used in the form of an equation: (f+d )/b—kl (20) Given the above considerations,the proposed design procedure(omitting here for simplicity the application of material and load uncertainty factors)for the hybrid FRP-concrete beams is summarized as follows: (1)Design the cross section for shear by solving equations(17)一(18)for d and t ; (2)Select a very thin CFRP thickness t {; (3)Solve the nonlinear system of equations(10),(19)and(20)for c,6 and t1; (4)Check c whether inequality(8)and(11)are satisfied or not,if c does not satisfy,adjust d (t is also adapted followed d )or tc{and repeat step 3 until the previous two inequalities are satisfied; (5)Calculate the flexural and shear rigidities and hence the deformation at a given load: (6)If the stiffness requirement is not met,increase d (£ is also adapted followed d )and possi- bly td and repeat steps 3-5 until the stiffness is satisfied. 4 Conclusions Combination of different FRP materials with concrete appears to be a feasible way of producing efficient and cost-effective hybrid beams.These beams possess many desirable mechanical behavioral characteristics,such as corrosion resistance,pseudo—ductility and high stiffness,while maintaining a low weight.The proposed Concept for hybrid beams can be thought of as a better way of designing composite profiles using materials by combination and placing exactly where they perform best.Opti— mum combination of materials in structural designing is increasingly becoming a necessity as well as an indispensable part of the structural engineer’s response to the pressure for more durable and light— weight pre-manufactured components. The simplified analysis methods for fundamental mechanical behavior of the proposed hybrid FRP-concrete beams are developed for preliminary design purposes,which can be used in order to ob- tain rough estimates of the design variables.The preliminary design variables can then be refined with the more exact analysis. References [1]THORNBURN J,MUFTI A.Design Recommendations for Externally Restrained Highway Bridge Decks[J].Jour— hal of Bridge Engineering,2001,6(4):243—249. [2]MIRMIRAN A.Innovative Combinations of FRP and Traditional Materials[C]∥Teng J G.Proc.of the Interna— tional Conference on FRP Composites in Civil Engineering,New York:Elsevier Science Ltd.,2001:1289—1298. [3]Van ERP G M,HELDT T,MCCORMICK L,et a1.An Australian Approach to Fibre Composite Bridges[C]∥ Robbins V W.Proc.of the International Composites Conference ACUN4,Sydney:University of Sydney,2002: 145-153. [4]HOLMES M,JUST D J.GRP in Structural Engineering[M].London:Applied Science Publishers Ltd.,1983:259-271. 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(英文审核 杨应华 编辑 白茂瑞) 维普资讯 http://www.cqvip.com 462 西安建筑科技大学学报(自然科学版) 第38卷 创新设计的FRP一混凝土组合梁基本力学性能 王言磊,欧进萍 (哈尔滨工业大学土木工程学院,黑龙江哈尔滨150090) 摘 要:近年来纤维增强聚合物(Fiber Reinforced Polymer,FRP)材料被广泛地用于桥梁结构以解决日益严 重的钢材锈蚀问题.为了充分发挥各种材料的优势和降低造价,FRP一混凝土组合结构被认为是一种最有效的 组合形式.首先阐述了FRP材料与混凝土结合形成一种轻质、耐腐蚀且价格相对便宜的组合梁的创新设计思 路,并提出了由该类型组合梁形成板式桥的新型桥面结构形式;其次,给出了FRP-混凝土组合梁的简化失效 分析方法以及延性、简化刚度和变形的计算公式;最后,基于刚度、强度和延性设计要求,给出了FRP一混凝土 组合梁的初步设计步骤. 关键词:FRP;组合梁;基本力学性能;混凝土 中圈分类号:TU398 文献标识码:A 文章编号:1006—7930(2006)04—0455—08 收稿日期:2006—03—30 基金项目:交通部西部交通科技资助项目(200421882021) 作者简介:王言磊(1978-),男,山东莒南人,博士生,主要从事FRP一混凝土组合结构研究 (上接第449页) E4]CHOI C K,LEE T Y,CHUNG H .Finite Element Method—An Effective Tool for Analysis of Shell Ec]}}Key— note lecture of International Symposium on New Perspectives for Shell and Spatial Structures.Taipei,2003:4-5. [5]于晓野.单层球壳在强震下的破坏机理及抗震设计实用方法研究[D].哈尔滨:哈尔滨工业大学,2005. YU Xiao-ye.Study on Damage Mechanism and Practical Design Methods of Single-layer Latticed Domes under Se— vere Earthquakes[D].Harbin:Harbin Institute of Technology,2005. E6"l沈世钊,支旭东.网壳结构在强震下的失效机理[J].土木工程学报,2005,38(1):11—20. SHEN Shi—zhao,ZHI Xu-dong.Failure Mechanism of Reticular Shells Subjected to Dynamic ActionsEJ].China Civil Engineering Journal,2005,38(1):11-20. (编辑 白茂瑞) Study on the aseismic performance of single-layer reticulated domes yU Xiao—ye。FAN Feng,SHEN Sh i—zhao (School of Civil Engineering,Harbin Institute of Technology,Harbin 150090,China) Abstract:The study on aseimic performance of reticulated shells forms a major part in the anti—seismic investigation of such structures.It greatly deepens the comprehension of possible structural damages in earthquakes,and also provides a base for further mechanism study and practical design.Focusing on single-layer reticulated domes,which are more likely to fail in earthquakes,the authors simulated the dynamic responses of structures under severe earthquakes by numerical meth— ods,and carried out comprehensive and detailed investigation about their aseismic performance.By the world—famous fi— nite element software ABAQUS,lots of parametric studies were taken through changing f/L ratio,roof mass,member sections and earthquake inputs.Based on the statistics and analysis of such indices as critical loads and failure modes, some valuable conclusions and the significant role of f/L ratio in anti—seismic performance for instance are obtained,and the comprehension in this field is pushed forward. Key words:single-layer reticulated domes,aseismff performance,critical loads,{ailure modes Biography:YU Xiao-ye,Candidate for Ph.D.,Harbin 150090,P.R.China,Tel:0086—451—86282080,E’mail:d—boyyxy@yahoo. COm.Cn