土木工程专业英语6.5.1.doc

6. 5 Ductile Design of Concrete Frame Structures6. 5.1 Ductility Iksign PhilosophyThe seismic design philosophy relies on providing sufficient ductility to the structure by which the structure can dissipate seismic energy. A reinforced concrete structure with sufficient ductility has the following advantages：(a) A ductile reinforced concrete structure may take care of overloading, load reversals, impact and secondary stresses due to differential settlement of foundation.(b) A ductile reinforced concrete structure gives the occupant sufficient time to vacate the structure by showing large deformation before its final collapse. Accordingly，the loss of life is minimized with the provision of sufficient ductility.(c) Properly designed ductile joints are capable of resisting forces and deformations at the yielding of steel reinforcement. Therefore, these sections can reach their respective moment capacities, which is one of the assumptions in the design of reinforced concrete structures by limit state method.The ductile design of reinforced concrete members should ensure both strength and ductility. Strength of members can be assured by proper design of the sections following limit stat method. To achieve an appropriate degree of ductility, the following failure mode are expected： (a) Brittle failure modes should be suppressed； and (b) beams should fail before columns.Flexural yielding of bending members is ductile. Shear failure is brittle and should be avoided under seismic loading. To prevent shear failure occurring before bending failure it is good practice to design so that the flexural steel in a member yields while the shear reinforcement is working at a stress less than yield (say 90%).The structural ductility is achieved in the form of inelastic rotations in reinforced concrete members. The inelastic rotations spread over definite regions called as plastic hinges. During inelastic deformations, the actual material properties are beyond elastic range and hence damages in these regions are obvious. The plastic hinges are “expected” locations where the structural damage can be allowed to occur due to inelastic actions involving large defomuitions. Hence, in seismic design the damages in the form of plastic hinges are accepted to be formed in bcams rather than in columns as shown in Figure 6. 18. Mechanism with beam yielding is characteristic of strong column-weak-beam behavior in which the imposed inelastic rotational demands can be achieved reasonably well through proper detailing practice in beamsFigure 6. 18 rame mechanismTherefore, in this mode of behavior, it is possible for the structure to attain the desired inelastic response and ductility. On the other hand, if plastic hinges are allowed to form in columns, the inelastic rotational demands imposed are very high that it is very difficult to be catered with any possible detailing. The mechanism with such a feature is called column yielding or storey mechanism 、乂岣2 FiRure 6. 19 Columand-beam assemblyOne of the basic requirements of design that the columns above andjoint should have sufficient flexural strength when the adjoining beams develop flexural over-strength at their plastic hinges. This column to beam flexural strength ratio is an important parameter to ensure that possible hinging occurs in bcams rather than in columns.The moment capacities of beams and columns are such that the column momentsoppose the beam moments as shown in Figure 6. 19. To obtain a strong-column- weak-beam design，GB 50011-2001 requires that the design flexural strength of thecolumns framing into a joint excecd the design flexural strength of the beams framing into the joint. ThereforeSM*- = YlMixtm(6.6.1 Wherey! Aumn = sum of moments at joint faces corresponding to design flexural strength of columns framing into jointyj Mbfam = sum of moments at joint faces corresponding to design flexural strength of beams framing into jointjjt=column to beam flexural strength ratio, taken as 1. 4 for Grade 1，taken as1. 2 for Grade 2，and taken as 1. 1 for Grade 3. The sum of moment capacity of the lower end of column at the first storey of Grade 1，2 and 3 shall be multiplied by an amplifying factor of 1. 5，1. 25 and 1. 156. 5. 2 Ductility DetailingFor ensuring ductility, specific recommendations are to be followed regarding the materials, dimensions, minimum and maximum percentages of reinforcement.1. Frame BeamsBeams in frames must have a clear span-to-effective depth ratio of at least 4，a width-to-depth ratio of at least 0. 25，and a web width of not less than 200mm. The minimum clear span-to- effective depth ratio helps ensure that flexural rather than shear strength dominates member behavior under inelastic load reversals. Minimum web dimensions help provide adequate copfinement for the concrete, whereas thc width of beam relative to the column is limited to provide adequate moment transfer between bearns and columns.In accordance with seismic design code GB 50011-2001, both top and bottom minimum flexural steel is required. Thc minimqm tension reinforcement ratio should not be less than that given by Table 6. 6. I，with a minimum of two reinforcing bars, top and bottom, through out the member. In addition，the positive to negative moment resistance ratio shall not be less than 0. 5 for frames assigned to Grade 1 and 0. 3 for frames assigned Grade 2 or Grade 3，nor be less than the calculated necessary ratio. Neither the negative nor the positive moment resistance at any section along the member length shall be less than one-quarter of the maximum moment resistance provided at the face of either end joint. These criteria are designed to provide for ductile behavior throughout the member, although the minimum of two reinforcing bars on the top and bottom is based principally on construction requirements. A maximum reinforcement ratio of 0. 025 is set to limit problems with steel congestion and to ensure adequate member size for carrying shear that is governed by the flexural capacity of the member.Table 6. 6. 1 Minimum tension reinforcement ratio ( %) in frame beamsAseismic GradePosition in BeamSupportIn spanGradc IGrcatcr of 0. 4 and 80 f%/fyGreater of 0. 3 and 65 fi/frGrade HGreater of 0. 3 and 65 /,Greater of 0. 25 and 55 f%/fyGrade ffl/WGreater of 0. 25 and 55 fx/fyGrcatcr of 0. 2 and 45 /,/fyTo obtain ductile performance, the location of splices is limited They maybe not be used within joints* within the closely spaced transverse reinforcement area of beams. Lap splice must be enclosed by hoops or spirals，with a maximum of IOOmm or 5 times the diameter of the smaller spliced bar. Welded and mechanical connections may be used, provided that they are not used within the closely spaced transverse reinforcement area.Transverse reinforcement is required throughout beams in frames resisting earthquake-induced force. Closely spaced transverse reinforcement in the form of hoops must be used over a length from the joint face toward mid-span, at the both ends of the beam. These hoops are intended to prevent buckling of the longitudinal bars in the compression zone in plastic hinge regions where both the top and bottom reinforcement can be subjected to yielding in tension and compression due to reversed cyclic flexure. Bars that buckle in compression and are subsequently stressed to yield in tension usually rupture. The length of closely spaced transverse reinforcement area, the maximum spacing of the hoops and the minimum diameter of the hoop bars should be taken according to Table 6. 6. 2; when the tension reinforcement ratio at the end of thc beam is greater than 2% the minimum diameter of hoop bars listed in the table should be increased by 2mm.Table 6. 6. 2 Construction requirements for closely spaced transverse reinforccment area at beam ends in framesAseismic GradeIwngth of Closcly Spaccd Tran5verse Reinforccmcnt Arca / mmMax. Spacing of Iioops /mmMinimum Diameter of Hoop Bars / mmGradc IGrcatcr of 2 hi and 500Smallest o( hy,/A%6d and 10010Grade nGreatcr of b and 500Smallest of Ab/4. 8d and 1008Grade QlGreater of hh and 500Smalle5t of Ab/4.8d and 1008Grade 汉Grcatcr of 1. 5 ki and 500Smallest of Ab/48d and 1008NOTES： hi is thc beam depth； J is the diameter of the smallest longitudinal bar.2. Frame ColumnTo help ensure constructability and adequate confinement of the concreteseismic design code GB 50011-2001 requires that columns in frames have (a) a minimum cross-section dimension of at least 300mm，（b) a ratio of the largest cross-section dimension to the perpendicular dimension of no more than 3 . and(c) the shear-to-span ratio of no greater than 2.The axial compressive force in the column due to design load effects shall not exceed, nAc fc, where n is taken as 0. 7 for Grade 1，taken as 0. 8 for Grade 2， and taken as 0. 9 for Grade 3.The column reinforcement ratio hased on the gross section shall not exceed0. 05. Welded splices and mechanical connections in columns must satisfy the same requirement specified for frame beams* whereas lapped splices must be designed for tension and are permitted only within the center half of column.GB 50011-2001 specifies the use of minimum transverse reinforcement over the length of the closely spaced transverse reinforcement area from each joint face. The maximum spacing and the minimum diameter cf hoops should satisfy the stipulation in Table 6. 6. 3. The length of the closely spaced transverse reinforcement area may not be less than (a) the larger cross-section dimension of the column, (b) one-sixth the column height, or (c) 500mm. As for the corner columns of aseismic Grade I and II，columns with a shear-to-span ratio of no more than 2，and short columns with a clear span-to- effective depth ratio of no more than 4，the hoops should be closely spaced along the full column height.Table 6.6.3 Construction requirements for closely spaced Iransvcrscreinforcement area at column cnds in framesAscismic GradeMax. Spacing of Hoops /mmMinimum Diameter of Hoop Bars /mmGrade ISmaller of 6J and 10010Gradc flSmaller of 81 and 1008Gradc fflSmaller of 8d and 150 100 for column root)8Gradc WSmaller of 8d and 150 (100 for column root)5 8 for column root)NOTES： d is thc diameter of the smallest longitudinal bar. 3.Joints Detailing （1） Confinement and transverse joint reinforcement The successful performance of a beam-column joint depends strongly on the lateral confinement of the joint. Confinement has three benefits： (a) the core concrete is strengthened and its strain capacity improved, (b) the vcrtical columnbars are prevented from buckling outward, and (c) its structural integrity is also ensured under cyclic loading.Difficulties arise due to rcinforcement congestion whcn trying to achieve high ductility in framed structures, and the detailing of beam-column joints to withstand strong cyclic loading remains a difficult problem. Often, confinement around a joint is provided by hoops. The maximum spacing and minimum diameter of hoops in the core area of the frame joint should be taken according to Table 6. 6. 3.(1) Anchorage and development of beam reinforcementFor interior joints, normally the flexural reinforcement in a beam entering one face of the joint is continuous through the joint to become the tlexural steel for the beam entering the opposite face. Therefore, for loading associated with interior joints, pullout is unlikely. However, for exterior or corner joints, where one or more of the beams dont continue beyond thc joint, a problem of baranchorage exists. The criticalsection for development of theyield strength of the beamsteel is at the face of thecolumn. Column dimensionseldom permit development ofthe steel entering the joint bystraight embedment alone. Ifa bar entering the joint needto develop its strength A1 /yat the face of the joint, itshould have 90 degree hookextending toward and beyondthe mid-depth of the joint.Figure 6. 20 Anchorage and splice of longitudinal reinforcing bars in frame jointIn an interior joint of themiddle floors* the developmentlength of the bottom bars of the beam in the interior joint should not be less than Itg. for aseismic Grade I or fl , and the length extendsbeyond the mid-depth of the column should not be less than 5d (Figure 6. 20).In an external joint of top floor of the frame，the longitudinal steel bars ofthe column should extend into the top of the column. The anchorage lengih starting from