美国LRFD钢结构规范介绍(Ⅲ)

ANSI/AISC 360-05美国国家标准钢结构建筑设计规范2005年3月9日发布本规范取代下列规范：1999年12月27日颁布的《钢结构建筑设计规范：荷载和抗力系数设计法》(LRFD)、1989年6月1日颁布的《钢结构建筑设计规范：容许应力设计法和塑性设计法》、其中包括1989年6月1日颁布的附录1《单角钢杆件的容许应力法设计规范》、2000年11月10日颁布的《单角钢杆件的荷载和抗力系数设计法设计规范》、2000年11月10日颁布的《管截面杆件的荷载和抗力系数设计法设计规范》、以及代替上述规范的所有从前使用的相关版本。

本规范由美国钢结构协会委员会（AISC）及其理事会批准发布实施。

本规范由美国钢结构协会规范委员会（AISC）审定，由美国钢结构协会董事会出版发行。

美国钢结构学会One East Wacker Drive，Suite 700芝加哥，伊利诺斯州60601-1802版权©2005美国钢结构学会拥有版权保留所有权利。

没有出版人的书面允许，不得对本书或本书的任何部分以任何形式进行复制。

本规范中所涉及到的相关信息，基本上是根据公认的工程原理和原则进行编制的，并且只提供一般通用性的相关信息内容。

虽然已经提供了这些精确的信息，但是，这些信息，在未经许可的专业工程师、设计人员或建筑工程师对其精确性、适用性和应用范围进行专业审查和验证的情况下，不得任意使用或应用于特定的具体项目中。

本规范中所包含的相关材料，并非对美国钢结构协会的部分内容进行展示或担保，或者，对其中所涉及的相关人员进行展示或担保，并且这些相关信息在适用于任何一般性的或特定的项目时，不得侵害任何相关专利权益。

任何人在侵权使用这些相关信息时，必须承担由此引起的所有相关责任。

必须注意到：在使用其它机构制订的规范和标准时，以及参照相关标准制订的其它规范和标准时，可以随时对本规范的相关内容进行修订或修改并且随后印刷发行。

本协会对未参照这些标准信息材料，以及未按照标准规定在初次出版发行时不承担由此引起的任何责任。

2005版美国钢结构设计规范摘要美国钢结构协会成立于1921年，在1923年发行了第一版美国钢结构建筑设计规范.这本规范基于容许应力设计原则，长达十页，后来又发行了其他版本，一直到1989年的第九版本，但自从第八版本（1978）以后就没什么实质性的变化了。

极限状态设计，在美国又被称为荷载和抗力分项系数设计（LRFD），在第一版本的LRFD规范中被正式介绍，它基于超过15年的大量研究和改进，又被修改过两次，现在使用的是第三版本（1999）。

两本规范的同时存在对美国的设计人员和工业发展都带来了麻烦，AISC因此同意制定一部唯一并且标准统一的钢结构设计规范。

这部规范直到2005年8月13日才被审核通过，介绍了很多重要的概念，包括名义强度准则的使用与适当措施结合以提高可靠性的方法。

在许多其他方面的改进中，框架体系稳定性和支护设计有重大的进步，包括采用塑性准则的新设计方法。

关键词规范可靠性名义强度稳定性标准塑性连接设计组合设计论文纲要1介绍2基本设计理念2.1容许应力设计2.2荷载与阻力因素设计2.2.1强度不足和超载3 2005年AISC说明书3.1 背景3.2 格式规范3.3 基本设计要求4 新规范内容布置4.1内容概述4.2总则4.3设计要求B1 总则B3.6连接点B3.6.1简单连接B3.6.2弯矩连接4.4稳定性设计分析4.4.1稳定性设计要求4.4.2需求强度计算4.5 构件抗拉设计4.6 构件抗压设计4.7 构件抗弯设计4.8 构件抗剪设计4.9 构件组合受力设计和抗扭设计4.10 组合构件设计4.11 连接设计4.12高速钢和箱形构件连接设计5 注释6 摘要参考文献1.介绍1923版美国钢结构设计规范制定的目的是解决那个时候设计人员所面临的一系列问题。

虽然美国材料试验协会（ASTM）制定的钢材和其他材料性能标准是可用的,但仍然没有全国统一的建筑设计规范。

因此，个别州或城市有自己的要求，并且有时候设计特定的建筑甚至有多种规则可以使用，比如，那时候建造的一些桥梁必须遵守由桥梁当局制定的详细的规定，而当局又常常和杰出的设计者或制造商勾结。

SECTION10COLD-FORMED STEEL DESIGNR.L.Brockenbrough,P.E.President,R.L.Brockenbrough&Associates,Inc.,Pittsburgh,PennsylvaniaThis section presents information on the design of structural members that are cold-formedto cross section shape from sheet steels.Cold-formed steel members include such productsas purlins and girts for the construction of metal buildings,studs and joists for light com-mercial and residential construction,supports for curtain wall systems,formed deck for theconstruction offloors and roofs,standing seam roof systems,and a myriad of other products.These products have enjoyed significant growth in recent years and are frequently utilizedin some shape or form in many projects today.Attributes such as strength,light weight,versatility,non-combustibility,and ease of production,make them cost effective in manyapplications.Figure10.1shows cross sections of typical products.10.1DESIGN SPECIFICATIONS AND MATERIALSCold-formed members for most application are designed in accordance with the Specificationfor the Design of Cold-Formed Steel Structural Members,American Iron and Steel Institute,Washington,DC.Generally referred to as the AISI Specification,it applies to members cold-formed to shape from carbon or low-alloy steel sheet,strip,plate,or bar,not more than1-in thick,used for load carrying purposes in buildings.With appropriate allowances,it canbe used for other applications as well.The vast majority of applications are in a thicknessrange from about0.014to0.25in.The design information presented in this section is based on the AISI Specification and its Commentary,including revisions being processed.The design equations are written indimensionless form,except as noted,so that any consistent system of units can be used.Asynopsis of key design provisions is given in this section,but reference should be made tothe complete specification and commentary for a more complete understanding.The AISI Specification lists all of the sheet and strip materials included in Table1.6(Art.1.4)as applicable steels,as well several of the plate steels included in Table1(A36,A242,A588,and A572).A283and A529plate steels are also included,as well as A500structuraltubing(Table1.7).Other steels can be used for structural members if they meet the ductilityrequirements.The basic requirement is a ratio of tensile strength to yield stress not less than1.08and a total elongation of at least10%in2in.If these requirements cannot be met,alternative criteria related to local elongation may be applicable.In addition,certain steelsthat do not meet the criteria,such as Grade80of A653or Grade E of A611,can be used10.110.2SECTION TENFIGURE10.1Typical cold-formed steel members.for multiple-web configurations(roofing,siding,decking,etc.)provided the yield stress istaken as75%of the specified minimum(or60ksi or414MPa,if less)and the tensile stressis taken as75%of the specified minimum(or62ksi or428MPa if less).Some exceptionsapply.Suitability can also be established by structural tests.10.2MANUFACTURING METHODS AND EFFECTSAs the name suggests,the cross section of a cold-formed member is achieved by a bendingoperation at room temperature,rather than the hot rolling process used for the heavier struc-tural steel shapes.The dominant cold forming process is known as roll-forming.In thisprocess,a coil of steel is fed through a series of rolls,each of which bends the sheetprogressively until thefinal shape is reached at the last roll stand.The number of roll standsmay vary from6to20,depending upon the complexity of the shape.Because the steel isfed in coil form,with successive coils weld-spliced as needed,the process can achieve speedsup to about300ft/min and is well suited for quantity production.Small quantities may beproduced on a press-brake,particularly if the shape is simple,such as an angle or channelcross section.In its simplest form,a press brake consists of a male die which presses thesteel sheet into a matching female die.In general,the cold-forming operation is beneficial in that it increases the yield strength of the material in the region of the bend.Theflat material between bends may also showan increase due to squeezing or stretching during roll forming.This increase in strength isattributable to cold working and strain aging effects as discussed in Art.1.10.The strengthincrease,which may be small for sections with few bends,can be conservatively neglected.Alternatively,subject to certain limitations,the AISI Specification includes provisions forusing a section-average design yield stress that includes the strength increase from cold-forming.Either full section tension tests,full section stub column tests,or an analyticalmethod can be employed.Important parameters include the tensile-strength-to-yield-stressCOLD-FORMED STEEL DESIGN10.3 TABLE10.1Safety Factors and Resistance Factors Adopted by the AISI SpecificationCategoryASDsafetyfactor,⍀LRFDresistancefactor,␾Tension members 1.670.95 Flexural members(a)Bending strengthSections with stiffened or partially stiffened compressionflanges 1.670.95 Sections with unstiffened compressionflanges 1.670.90 Laterally unbraced beams 1.670.90 Beams having oneflange through-fastened to deck or sheathing(C-or Z-sections) 1.670.90 Beams having oneflange fastened to a standing seam roof system 1.670.90 (b)Web designShear strength controlled by yielding(Condition a,Art.10.12.4) 1.50 1.00 Shear strength controlled by buckling(Condition b or c,Art.10.12.4) 1.670.90 Web crippling of single unreinforced webs 1.850.75 Web crippling of I-sections 2.000.80 Web crippling of two nested Z-sections 1.800.85 Stiffeners(a)Transverse stiffeners 2.000.85(b)Shear stiffeners 1.50/1.67 1.00/0.90 Concentrically loaded compression members 1.800.85 Combined axial load and bending(a)Tension component 1.670.95(b)Compression component 1.800.85(c)Bending component 1.670.90/0.95 Cylindrical tubular members(a)Bending 1.670.95(b)Axial compression 1.800.85 Wall studs(a)Compression 1.800.85(b)Bending 1.670.90/0.95 Diaphragm construction 2.00/3.000.50/0.65 Welded connections(a)Groove weldsTension or compression2500.90 Shear,welds 2.500.80 Shear,base metal 2.500.90 (b)Arc spot weldsShear,welds 2.500.60 Shear,connected part 2.500.50/0.60 Shear,minimum edge distance 2.500.60/0.70 Tension 2.500.60 (c)Arc seam weldsShear,welds 2.500.60 Shear,connected part 2.500.60 (d)Fillet weldsWelds 2.500.60 Connected part,longitudinal loadingWeld length/sheet thicknessϽ25 2.500.60 Weld length/sheet thicknessՆ25 2.500.55 Connected part,transverse loading 2.500.6010.4SECTION TENTABLE10.1Safety Factors and Resistance Factors Adopted by the AISI Specification(Continued)CategoryASDsafetyfactor,⍀LRFDresistancefactor,␾(e)Flare groove weldsWelds 2.500.60 Connected part,longitudinal loading 2.500.55 Connected part,transverse loading 2.500.55 (f)Resistance welds 2.500.65 Bolted connections(a)Minimum spacing and edge distance*When Fu /FsyՆ1.08 2.000.70When Fu /FsyϽ1.08 2.220.60(b)Tension strength on net sectionWith washers,double shear connection 2.000.65 With washers,single shear connection 2.220.55 Without washers,double or single shear 2.220.65(c)Bearing strength 2.220.55/0.70(d)Shear strength of bolts 2.400.65(e)Tensile strength of bolts 2.00/2.250.75 Screw connections 3.000.50*Fu is tensile strength and Fsyis yield stress.ratio of the virgin steel and the radius-to-thickness ratio of the bends.The forming operation may also induce residual stresses in the member but these effects are accounted for in the equations for member design.10.3NOMINAL LOADSThe nominal loads for design should be according to the applicable code or specificationunder which the structure is designed or as dictated by the conditions involved.In the absenceof a code or specification,the nominal loads should be those stipulated in the AmericanSociety of Civil Engineers Standard,Minimum Design Loads for Buildings and Other Struc-tures,ASCE7.The following loads are used for the primary load combinations in the AISISpecification:DϭDead load,which consists of the weight of the member itself,the weight of allmaterials of construction incorporated into the building which are supported by the mem-ber,including built-in partitions;and the weight of permanent equipmentEϭEarthquake loadLϭLive loads due to intended use and occupancy,including loads due to movable objectsand movable partitions and loads temporarily supported by the structure during mainte-nance.(L includes any permissible load reductions.If resistance to impact loads is takeninto account in the design,such effects should be included with the live load.)COLD-FORMED STEEL DESIGN 10.5L r ϭRoof live load S ϭSnow loadR r ϭRain load,except for ponding W ϭWind loadThe effects of other loads such as those due to ponding should be considered when signif-icant.Also,unless a roof surface is provided with sufficient slope toward points of free drainage or adequate individual drains to prevent the accumulation of rainwater,the roof system should be investigated to assure stability under ponding conditions.10.4DESIGN METHODSThe AISI Specification is structured such that nominal strength equations are given for various types of structural members such as beams and columns.For allowable stress design (ASD),the nominal strength is divided by a safety factor and compared to the required strength based on nominal loads.For Load and Resistance Factor Design (LRFD),the nominal strength is multiplied by a resistance factor and compared to the required strength based on factored loads.These procedures and pertinent load combinations to consider are set forth in the specification as follows.10.4.1ASD RequirementsASD Strength Requirements.A design satisfies the requirements of the AISI Specification when the allowable design strength of each structural component equals or exceeds the required strength,determined on the basis of the nominal loads,for all applicable load combinations.This is expressed asR ՅR /⍀(10.1)n where R ϭrequired strengthR n ϭnominal strength (specified in Chapters B through E of the Specification )⍀ϭsafety factor (see Table 10.1)R n /⍀ϭallowable design strength ASD Load Combinations.In the absence of an applicable code or specification or if the applicable code or specification does not include ASD load combinations,the structure and its components should be designed so that allowable design strengths equal or exceed the effects of the nominal loads for each of the following load combinations:1.D2.D ϩL ϩ(L r or S or R r )3.D ϩ(W or E )4.D ϩL ϩ(L r or S or R r )ϩ(W orE )Wind or Earthquake Loads for ASD.When the seismic load model specified by the applicable code or specification is limit state based,the resulting earthquake load (E )is permitted to be multiplied by 0.67.Additionally,when the specified load combinations in-clude wind or earthquake loads,the resulting forces are permitted to be multiplied by 0.75.However,no decrease in forces is permitted when designing diaphragms.10.6SECTION TENComposite Construction under ASD.For the composite construction offloors and roofs using cold-formed deck,the combined effects of the weight of the deck,the weight of thewet concrete,and construction loads(such as equipment,workmen,formwork)must beconsidered.10.4.2LRFD RequirementsLRFD Strength Requirements.A design satisfies the requirements of the AISI Specificationwhen the design strength of each structural component equals or exceeds the requiredstrength determined on the basis of the nominal loads,multiplied by the appropriate loadfactors,for all applicable load combinations.This is expressed asRϽ␾R(10.2)u nwhere Ruϭrequired strengthRnϭnominal strength(specified in chapters B through E of the Specification)␾ϭresistance factor(see Table10.1)␾R nϭdesign strengthLRFD Load Factors and Load Combinations.In the absence of an applicable code or specification,or if the applicable code or specification does not include LRFD load combi-nations and load factors,the structure and its components should be designed so that design strengths equal or exceed the effects of the factored nominal loads for each of the following combinations:1.1.4DϩL2.1.2Dϩ1.6Lϩ0.5(Lr or S or Rr)3.1.2Dϩ1.6(Lr or S or Rr)ϩ(0.5L or0.8W)4.1.2Dϩ1.3Wϩ0.5Lϩ0.5(Lr or S or Rr)5.1.2Dϩ1.5Eϩ0.5Lϩ0.2S6.0.9DϪ(1.3W or1.5E)Several exceptions apply:1.The load factor for E in combinations(5)and(6)should equal1.0when the seismic loadmodel specified by the applicable code or specification is limit state based.2.The load factor for L in combinations(3),(4),and(5)should equal1.0for garages,areasoccupied as places of public assembly,and all areas where the live load is greater than 100psf.3.For wind load on individual purlins,girts,wall panels and roof decks,multiply the loadfactor for W by0.9.4.The load factor for Lr in combination(3)should equal1.4in lieu of1.6when the rooflive load is due to the presence of workmen and materials during repair operations.Composite Construction under LRFD.For the composite construction offloors and roofs using cold-formed deck,the following additional load combination applies:1.2Dϩ1.6Cϩ1.4C(10.3)S Wwhere DSϭweight of steel deckCWϭweight of wet concreteCϭconstruction load(including equipment,workmen,and form work but excluding wet concreteCOLD-FORMED STEEL DESIGN10.7 10.5SECTION PROPERTY CALCULATIONSBecause of theflexibility of the manufacturing method and the variety of shapes that can bemanufactured,properties of cold-formed sections often must be calculated for a particularconfiguration of interest rather than relying on tables of standard values.However,propertiesof representative or typical sections are listed in the Cold-Formed Steel Design Manual,American Iron and Steel Institute,1996,Washington,DC(AISI Manual).Because the cross section of a cold-formed section is generally of a single thickness of steel,computation of section properties may be simplified by using the linear method.Withthis method,the material is considered concentrated along the centerline of the steel sheetand area elements are replaced by straight or curved line elements.Section properties arecalculated for the assembly of line elements and then multiplied by the thickness,t.Thus,the cross section area is given by AϭLϫt,where L is the total length of all line elements;the moment of inertia of the section is given by IϭIЈϫt,where IЈis the moment ofinertia determined for the line elements;and the section modulus is calculated by dividingI by the distance from the neutral axis to the extremefiber,not to the centerline of theextreme element.As subsequently discussed,it is sometimes necessary to use a reduced oreffective width rather than the full width of an element.Most sections can be divided into straight lines and circular arcs.The moments of inertia and centroid location of such elements are defined by equations from fundamental theory aspresented in Table10.2.10.6EFFECTIVE WIDTH CONCEPTThe design of cold-formed steel differs from heavier construction in that elements of mem-bers typically have large width-to-thickness(w/t)ratios and are thus subject to local buck-ling.Figure10.2illustrates local buckling in beams and columns.Flat elements in com-pression that have both edges parallel to the direction of stress stiffened by a web,flange,lip or stiffener are referred to as stiffened elements.Examples in Fig.10.2include the topflange of the channel and theflanges of the I-cross section column.To account for the effect of local buckling in design,the concept of effective width is employed for elements in compression.The background for this concept can be explainedas follows.Unlike a column,a plate does not usually attain its maximum load carrying capacity at the buckling load,but usually shows significant post buckling strength.This behavior isillustrated in Fig.10.3,where longitudinal and transverse bars represent a plate that is simplysupported along all edges.As the uniformly distributed end load is gradually increased,thelongitudinal bars are equally stressed and reach their buckling load simultaneously.However,as the longitudinal bars buckle,the transverse bars develop tension in restraining the lateraldeflection of the longitudinal bars.Thus,the longitudinal bars do not collapse when theyreach their buckling load but are able to carry additional load because of the transverserestraint.The longitudinal bars nearest the center can deflect more than the bars near theedge,and therefore,the edge bars carry higher loads after buckling than do the center bars.The post buckling behavior of a simply supported plate is similar to that of the grid model.However,the ability of a plate to resist shear strains that develop during bucklingalso contributes to its post buckling strength.Although the grid shown in Fig.10.3a buckledinto only one longitudinal half-wave,a longer plate may buckle into several waves as illus-trated in Figs.10.2and10.3b.For long plates,the half-wave length approaches the widthb.After a simply supported plate buckles,the compressive stress will vary from a maximum near the supported edges to a minimum at the mid-width of the plate as shown by line1of10.8SECTION TENTABLE10.2Moment of Inertia for Line ElementsSource:Adapted from Cold-Formed Steel Design Manual,American Iron and Steel Institute,1996,Washington,DC.COLD-FORMED STEEL DESIGN10.9FIGURE10.2Local buckling of compression elements.(a)In beams;(b)incolumns.(Source:Commentary on the Specification for the Design of Cold-Formed Steel Structural Members,American Iron and Steel Institute,Washington,DC,1996,with permission.)Fig.10.3c.As the load is increased the edge stresses will increase,but the stress in the mid-width of the plate may decrease slightly.The maximum load is reached and collapse is initiated when the edge stress reaches the yield stress—a condition indicated by line2of Fig.10.3c.The post buckling strength of a plate element can be considered by assuming that after buckling,the total load is carried by strips adjacent to the supported edges which are at a uniform stress equal to the actual maximum edge stress.These strips are indicated by the dashed lines in Fig.10.3c.The total width of the strips,which represents the effective width of the element b,is defined so that the product of b and the maximum edge stress equals the actual stresses integrated over the entire width.The effective width decreases as the applied stress increases.At maximum load,the stress on the effective width is the yield stress.Thus,an element with a small enough w/t will be able to reach the yield point and will be fully effective.Elements with larger ratios will have an effective width that is less than the full width,and that reduced width will be used in section property calculations.The behavior of elements with other edge-support conditions is generally similar to that discussed above.However,an element supported along only one edge will develop only one effective strip.Equations for calculating effective widths of elements are given in subsequent articles based on the AISI Specification.These equations are based on theoretical elastic buckling theory but modified to reflect the results of extensive physical testing.10.10SECTION TENFIGURE10.3Effective width concept.(a)Buckling of grid model;(b)buckling ofplate;(c)stress distributions.10.7MAXIMUM WIDTH-TO-THICKNESS RATIOSThe AISI Specification gives certain maximum width-to-thickness ratios that must be adhered to.For flange elements,such as in flexural members or columns,the maximum flat width-to-thickness ratio,w/t ,disregarding any intermediate stiffeners,is as follows:Stiffened compression element having one longitudinal edge connected to a web or flange element,the other stiffened by (a)a simple lip,60(b)other stiffener with I S ϽI a ,90(c)other stiffener with I S ՆI a ,90Stiffened compression element with both longitudinal edges connected to other stiffened elements,500Unstiffened compression element,60In the above,I S is the moment of inertia of the stiffener about its centroidal axis,parallel to the element to be stiffened,and I a is the moment of inertia of a stiffener adequate for the element to behave as a stiffened element.Note that,although greater ratios are permitted,stiffened compression elements with w /t Ͼ250,and unstiffened compression elements with w/t Ͼ30are likely to develop noticeable deformations at full design strength,but ability to develop required strength will be unaffected.For web elements of flexural members,the maximum web depth-to-thickness ratio,h/t ,disregarding any intermediate stiffeners,is as follows:Unreinforced webs,200Webs with qualified transverse stiffeners that include (a)bearing stiffeners only,260(b)bearing and intermediate stiffeners,30010.8EFFECTIVE WIDTHS OF STIFFENED ELEMENTS10.8.1Uniformly Compressed Stiffened ElementsThe effective width for load capacity determination depends on a slenderness factor ␭defined as1.052wƒ␭ϭ(10.4)ͩͪΊt E͙kwhere k ϭplate buckling coefficient (4.0for stiffened elements supported by a web alongeach longitudinal edge;values for other conditions are given subsequently)ƒϭmaximum compressive stress (with no safety factor applied)E ϭModulus of elasticity (29,500ksi or 203000MPa)FIGURE 10.4Illustration of uniformly compressed stiffened element.(a )Actual element;(b )stress on effective element.(Source:Specification for the Design of Cold-Formed Steel Structural Members,Amer-ican Iron and Steel Institute,Washington,DC,1996,with permission.)For flexural members,when initial yielding is in compression,ƒϭF y ,where F y is the yield stress;when the initial yielding is in tension,ƒϭthe compressive stress determined on the basis of effective section.For compression members,ƒϭcolumn buckling stress.The effective width is as follows:when ␭Յ0.673,b ϭw (10.5)when ␭Ͼ0.673,b ϭ␳w(10.6)where the reduction factor ␳is defined as␳ϭ(1Ϫ0.22/␭)/␭(10.7)Figure 10.4shows the location of the effective width on the cross section,with one-half located adjacent to each edge.Effective widths determined in this manner,based on maximum stresses (no safety factor)define the cross section used to calculate section properties for strength determination.How-ever,at service load levels,the effective widths will be greater because the stresses are smaller,and another set of section properties should be calculated.Therefore,to calculate effective width for deflection determination,use the above equations but in Eq.10.4,sub-stitute the compressive stress at design loads,ƒd .10.8.2Stiffened Elements with Stress GradientElements with stress gradients include webs subjected to compression from bending alone or from a combination of bending and uniform compression.For load capacity determination,the effective widths b 1and b 2illustrated in Fig.10.5must be determined.First,calculate the ratio of stresses␺ϭƒ/ƒ(10.8)21where ƒ1and ƒ2are the stresses as shown,calculated on the basis of effective section,with no safety factor applied.In this case ƒ1is compression and treated as ϩ,while ƒ2can be either tension (Ϫ)or compression (ϩ).Next,calculate the effective width,b e ,as if the element was in uniform compression (Art.10.8.1)using ƒ1for ƒand with k determined as follows:3k ϭ4ϩ2(1Ϫ␺)ϩ2(1Ϫ␺)(10.9)Effective widths b 1and b 2are determined from the following equations:FIGURE10.5Illustration of stiffened element with stress gradient.(a)Actual element;(b)stress on ef-fective element varying from compression to tension;(c)stress on effective element with non-uniform com-pression.(Source:Specification for the Design of Cold-Formed Steel Structural Members,American Iron and Steel Institute,Washington,DC,1996,with permission.)bϭb/(3Ϫ␺)(10.10)1ebϭb/2(10.11)2eThe sum of b1and b2must not exceed the width of the compression portion of the webcalculated on the basis of effective section.Effective width for deflection determination is calculated in the same manner except that stresses are calculated at service load levels based on the effective section at that load.FIGURE 10.6Illustration of uniformly compressed unstiffened element.(a )Actual element;(b )stress on effective element.(Source:Specification for the Design of Cold-Formed Steel Structural Members,American Iron and Steel Institute,Washington,DC,1996,with permission.)10.9EFFECTIVE WIDTHS OF UNSTIFFENED ELEMENTS10.9.1Uniformly Compressed Unstiffened ElementsThe effective widths for uniformly compressed unstiffened elements are calculated in the same manner as for stiffened elements (Art.10.8.1),except that k in Eq.10.4is taken as 0.43.Figure 10.6illustrates the location of the effective width on the cross section.10.9.2Unstiffened Elements and Edge Stiffeners with Stress GradientThe effective width for unstiffened elements (including edge stiffeners)with a stress gradient is calculated in the same manner as for uniformly loaded stiffened elements (Art.10.9.1)except that (1)k in Eq.10.4is taken as 0.43,and (2)the stress ƒ3is taken as the maximum compressive stress in the element.Figure 10.7shows the location of ƒ3and the effective width for an edge stiffener consisting of an inclined lip.(Such lips are more structurally efficient when bent at 90Њ,but inclined lips allow nesting of certain sections.)10.10EFFECTIVE WIDTHS OF UNIFORMLY COMPRESSED ELEMENTS WITH EDGE STIFFENERA commonly encountered condition is a flange with one edge stiffened by a web,the other by an edge stiffener (Fig.10.7).To determine its effective width for load capacity determi-nation,one of three cases must be considered.The case selection depends on the relation between the flange flat width-to-thickness ratio,w/t ,and the parameter S defined asS ϭ1.28͙E /ƒ(10.12)For each case an equation will be given for determining I a ,the moment of inertia required for a stiffener adequate so that the flange element behaves as a stiffened element,I S is the moment of inertia of the full section of the stiffener about its centroidal axis,parallel to the element to be stiffened.A ЈS is the effective area of a stiffener of any shape,calculated by methods previously discussed.The reduced area of the stiffener to be used in section property calculations is termed A S and its relation to A ЈS is given for each case.Note that for edge stiffeners,the rounded corner between the stiffener and the flange is not considered as part of the stiffener in calculations.The following additional definitions for a simple lip stiffener illustrated in Fig.10.7apply.The effective width d S Јis that of the stiffener calculated ac-cording to Arts.10.9.1and 10.9.2.The reduced effective width to be used in section propertyFIGURE 10.7Illustration of element with edge stiffener.(a )Actual element;(b )stress on effective element and stiffener.(Source:Specification for the Design of Cold-Formed Steel Structural Members,American Iron and Steel Institute,Washington,DC 1996,with permission.)calculations is termed d S and its relation to d S Јis given for each case.For the inclined stiffener of flat depth d at an angle ␪as shown in Fig.10.7,32I ϭ(d t sin ␪)/12(10.13)S A Јϭd Јt(10.14)S S Limit d /t to 14.Case I:w /t ՅS /3For this condition,the flange element is fully effective without an edge stiffener so b ϭw ,I a ϭ0,d S ϭd S Ј,A S ϭA ЈS .Case II:S /3Ͻw /t ϽS43I ϭ399t {[(w /t )/S ]Ϫ͙k /4}(10.15)a u where k u ϭ0.43.The effective width b is calculated according to Art.10.8.1using the following k :。

简介美国冷弯薄壁型钢规范全面升级在美国钢框架联盟(Steel Framing Alliance)网站下单2个多月后，今天公司终于收到了刚刚升级完成的全套北美冷弯薄壁型钢结构设计规范（(North American Standard for Cold-formed Steel Framing 2007Edition).之前据说这次升级幅度很大，反映了最近几年北美冷弯薄壁型钢结构界的最新研究成果，下班回家后大致翻了一下，果然动作很大。

先简要摘录如下：一是有更新又有新增；这是再原有基础上更新的标准:•AISI S200-07: North American Standard for Cold-Formed Steel Framing – General Provisions •AISI S211-07: North American Standard for Cold-Formed Steel Framing – Wall Stud Design •AISI S212-07: North American Standard for Cold-Formed Steel Framing – Header Design •AISI S213-07: North American Standard for Cold-Formed Steel Framing – Lateral Design •AISI S214-07: North American Standard for Cold-Formed Steel Framing – Truss Design •AISI S230-07: Standard for Cold-Formed Steel Framing –Prescriptive Method for One and Two Family Dwellings以下是这次新增的标准:•AISI S201-07: North American Standard for Cold-Formed Steel Framing – Product Data •AISI S210-07: North American Standard for Cold-Formed Steel Framing – Floor and Roof System Design二是这次将加拿大和墨西哥规范一起统一了起来，真正形成了北美标准；三是内容进行了很大的更新，特别是Lateral Design分册，有了很大的扩充；四是发现North American Specification and Commentary fo the Design of Cold-Formed Steel Structural Members 2007 Edition先于North American Standard for Cold-Formed Steel Framing 标准发布了。

中美两国钢结构抗震设计对比分析钢结构建筑应用日益广泛，在设计时仍需考虑地震作用，就中国和美国在钢结构抗震设计方面的不同进行了对比分析。

标签：钢结构；抗震设计；中国和美国doi：10.19311/ki.16723198.2016.12.0941美国钢结构抗震设计的发展1923年，美国钢结构协会制定了第一个钢结构设计规范，该规范是以容许应力为基本原则的设计法，经过多次修改，在1961年，其格式与内容基本上形成了固定模式。

1986年，AISC规范委员会提出了以概率理论为基础编写的荷载和抗力分项系数钢结构设计规范，简称LRFD。

以概率理论为基础编写的ASCE/SEI 7—05，作为美国各种设计理论依据，后该理论被不断修改与改进。

美国工程结构抗震设计大体上分为三种，国家标准、协会标准以及地方标准。

发展过程大致经过初创、发展、统一几个阶段。

1925年，出现了第一个建筑结构抗震设计规范UBC，紧接着又出现了NBC，SBC。

美国的抗震目标是把地震伤害降到最小化，即那些专门为人民提供生命安全、财产安全保障的设施，要按照他们的作用进行改造，加强它们的防震性能，使它们在震后也能正常的运行工作。

该抗震目标把抗震强度分为两个等级，即“设计地震”和“最大考虑地震”。

“最大考虑地震”是指五十年的超越概率为百分之二的地震；“设计地震”的加速度是“最大强度地震”的三分之二。

2中国抗震设计规范中国抗震规范提出的抗震目标为三水准，即“小震不坏，中震可修，大震不倒”。

第一水准是指，当某地区所受到的地震强度伤害低于该地区所预防的强度时，遇到这种地震，震后可以不用修复，继续正常使用；第二水准是指，当某地区所受到的地震强度伤害等于该地区所预防的地震强度时，建筑物可能会受到小的或局部损伤，只需进行简单的修理甚至不用修理，就可以继续使用；第三水准是指，当某地区遇到的地震强度远大于他所预防的强度时，不会导致房屋坍塌或危及到人的生命财产安全。

其中，小震五十年的超越概率为63.2%；中震是指五十年的超越概率为10%，相当于美国的“设计地震”等级；大震是指五十年的超越概率为2%到3%，相当于美国的“最大考虑地震”等级。