Chapter8 Seismic Design of Steel Structures of Buildings &.a86sinofs6oltlrs8foostoryfacto 8.1 Behavior of a steel structures 8.1 Failure Modes Lessons from earthquakes 8.1 Failure Modes w6m
1 Chapter 8 Seismic Design of Steel Structures of Buildings 8.1 Behavior of a steel structures 8.2 Design of steel frames for middle and high rise buildings 8.2.1 Structural systems of middle and high rise buildings 8.2.2 Requirement for arrangement of steel structural systems 8.2.3 Calculation of earthquake action 8.2.4 Seismic check for member and section capacity 8.2.5 Detailing requirements for seismic design of members 8.2.6 Seismic check and detailing requirements 8.3 Design of steel structures of one storey factories 8.3.1 Requirements of structural system 8.3.2 Calculation of earthquake action 8.3.3 Seismic check for members and detailing requirement n Lessons from earthquakes n The damage or failure phenomena caused by earthquake can be classified as the following aspects: members, joints, whole systems, and non structural components. 8.1 Behavior of a steel structures 8.1 Failure Modes 8.1 Failure Modes Damage of joints
Damage of frame Q1:What have been learned from these To Svtong Earthquako ings in Past Earthquako tool Solsmi oad Ro Design for Ductile Behavior 8.1 Behavior of a steel member ers to止 H
2 Damage of frame Q1: What have been learned from these damage phenomena of steel building structures during recent earthquakes? (1) Care in the design, detailing, and construction of steel structures is needed to assure satisfactory performance in strong earthquakes. (2) This should lead to the development of building code regulations that specifically address seismic detailing of steel building structures. • Performance of Steel Buildings in Past Earthquakes • Key Philosophy for Seismic Design of Steel Building Structures • Design Earthquake Forces • Steel Seismic Load Resisting Systems • Code Provisions for Seismic Design of Steel Buildings Design of Seismic-Resistant Steel Building Structures: A Brief Overview To Survive Strong Earthquake without Collapse: Design for Ductile Behavior ! Q2: What’s of the most importance to design a steel building economically? H H Ductility = Inelastic Deformation O dy du F y F u F A B F d d The ductility factor of steel beam is defined as du/dy, dy refers to the displacement corresponding to yield load, while du to that corresponding to ultimate. 8.1 Behavior of a steel member
8.1 Behavior of a steel frame 目 03Hblngtieecgerespons8sof "Fuse"Concept should be Adopted! "Fuse"concept for developing ductile behavior 8.1 Behavior of a steel structures Detail 'fus d to kee capacity to prevent from collapse. 8.2 Design of steel frames 8.2.1 Structural systems 8.2.2 Requirement for arrangement of steel structural system 8.23 Calculation of earthquake action 8.2.4 Cheek for the seismic capacity of members and co e te H<110m.30 stories 3
3 Fi F 1 D F n SFi D 8.1 Behavior of a steel frame To Survive Strong Earthquake without Collapse: “Fuse” Concept should be Adopted ! Q3: How to achieve ductile responses of steel building structures? “Fuse” concept for developing ductile behavior • Choose frame elements ("fuses") that will yield in an earthquake. • Detail "fuses" to sustain large inelastic deformations prior to the onset of fracture or instability (i.e. , detail fuses for ductility). • Design all other frame elements to be stronger than the fuses, i.e., design all other frame elements to develop the plastic capacity of the fuses. In general, the common building steel structures are designed to keep elastic when subjected to the influence of frequently occurred earthquake, while when subjected to expected rare earthquake the structure should keep their capacity to prevent from collapse. 8.1 Behavior of a steel structures 8.2 Design of steel frames 8.2.1 Structural systems of middle and high rise buildings 8.2.2 Requirement for arrangement of steel structural systems 8.2.3 Calculation of earthquake action 8.2.4 Check for the seismic capacity of members and connections 8.2.5 Detailing requirements for seismic design of members 8.2.6 Seismic check and detailing requirements of joint and connection Profile Plan 1. Moment resisting frame Moment resisting frame is the structure that is composed of beams and columns. The horizontal forces caused by earthquake are transferred to foundation through beams and columns. H<110m, 30 stories. 8.2.1 Structural systems
Braced frame structure and Frame with s oirt.are theoueh the same t Tube structures at both e ds or one en or a shor s20 .Structural systems of middle and high rise buildin on for building height 五 of the
4 2.Braced frame structure and Frame with shear wall 支撑 支撑 支撑 The main components to resist horizontal earthquake action are the bracing or shear wall. The braced frame structure system can generally divided as concentrically braced frame (CBF) and eccentrically braced frame (EBF). Plan Profile Concentrically braced frame (CBF) ：The central lines of the members: column, beam and brace, which are connected in one joint, are through the same point . Figure 8.8 Concentrically braced frames a) X type b) inverted V type c) V type d) K type e) and f ) diagonal type a) b) c) d) e) f) Eccentrically braced frame (EBF) ：The eccentricity can be arranged in different ways, for example, the brace is shifted from the joint center at both ends or one end, or a short cantilever hung from the beam. a) a a a a a a a b) a a a a a a a a c) a a a a a d) a a a a a the segment marked ‘a’ is called ‘link’ in EBF, which is the main component for energy dissipation. Figure 8.10 Types of eccentrically braced frames 3. Tube structures To the buildings which height exceeds 200 meters, tube structures, mega frame structures are considered more suitable. Plan Plan Profile Figure 8.6 Sections of structures for middle and high rise buildings (a) moment resisting frame (b) braced frame (c) tube structure (d) braced frame tube (e) mage structure 8.2.1 Structural systems of middle and high rise buildings a ) b ) c ) d ) e ) 1. Limitation for building height 2. Limitation for height-width ratio 8.2.2 Requirement for arrangement of steel structural systems (GB50011-2010) tube and mega structure 300 280 180 Concentrically braced frame 220 200 120 frame 110 90 50 IX (0.40g) VII (0.15g) VI, VII (0.1g) Structural systems eccentrically braced frame 240 220 160 VIII 90 180 200 260 (0.20g) (0.30g) 70 150 180 240 Limitation of the ratio 6.5 6.0 5.5 Design intensity of the region VI, VII VIII IX
2agrmgmcaoa 3.Seismic Grading 袋 ≤50 4th 3rd 2nd 5.Laveut ef strustural svstem 504h 3d 2nd 1st ldeal failure mode 6.Resistant lines 2)In CBF st )hekaeteee 5.Enough rigid floor system 8.2.3 Computation of earthquake action e slab cast-in-pl 密 (3)Calculale the earthquake action.Three methods can be used (4)Seismic checkine for the members.comnecters.and bases. (5)Displacement checking (6)Details design
5 3. Seismic Grading 8.2.2 Requirement for arrangement of steel structural systems (GB50011-2010) Height (m) Intensity 6 7 8 9 ≤50 4th 3rd 2nd >50 4th 3rd 2nd 1st 4. Expansion joint To avoid irregularities of the building frame system, the gaps required to the expansion joint in steel structures should be set 1.5 times of that in RC structures. 5. Layout of structural system During the expected rare earthquake, the damage to the structure can be accepted but the structure should keep stable even after the structural damage happens, in order that the gravity loads can be still supported by the structure. 8.2.2 Requirement for arrangement of steel structural systems a) b) The plastic hinge mechanism which occurs in most of beam ends is an ideal failure mode, because it is able to dissipate more energy than the column collapse mode . Figure 8.12 Plastic hinge mechanism Ideal failure mode 2） In CBF structures, the lateral forces are mainly supported by braces because braces provide most of the lateral stiffness. So the bracing is the first resistant line. 3） In EBF structures, the link will be expected to develop plastic hinge firstly, here links conform the first line. 1） Beams should be considered the first line to defend against earthquake in moment resisting frame, therefore, the ‘strong column and weak beam type’ is usually preferred. 6. Resistant lines 5. Enough rigid floor system Composite slab and concrete slab cast-in-place are prior to others because these slabs can afford enough stiffness in the floor plane which assures frames work as an entity. （1）Build a calculation model; （2）Determine the parameters, including earthquake action factor, periods, modes, damping ratio, etc. （3）Calculate the earthquake action. Three methods can be used. （4）Seismic checking for the members, connecters, and bases; （5）Displacement checking; （6）Details design. 8.2.3 Computation of earthquake action Seismic design steps：
8.2.Computation of earthquake action 1.Damping ratio h shear force APoportionofthotatoraloadsharedbybracingand 封 )25 percent of the total base shear.and rted by frame which are 6
6 Seismic design flow chart 8.2.3 Computation of earthquake action 1. Damping ratio n By the experience, the seismic design code recommends that: n when the action of frequently occurred earthquake is to be computed, the damping factor can be taken as 0.04 if the building is less than 50m, n while 0.03 if the building is more than 50m but less than 200m, n and 0.02 when the building exceeds 200m. n when expected rare earthquake is considered, the damping factor is taken as 0.05. 2. P-Delta effect : Second order effect To any story, if the second order moment which is computed by the product of gravity load over the story and the storey drift is greater one tenth of the overturning moment which produced by storey shear force timing the storey drift, PDelta effect should be considered in the computation results. c) Additional moment due to second order effect of vertical load b) Storey moment by horizontal load a) Analsis model Figure 8.14 Second order moment a) 3. Lateral displacement and the limitation of the storey drift The storey drift limitations for middle and high rise building steel structures are 1/250 when considering the frequently occurred earthquake; and n 1/50 when considering the rare occurred earthquake respectively, specified by design code. Mc1 Vb2 Vc1 Mb1 hb a)Moment and shearinpanel zone A Shearinpanel zone Vc=(Mb1+Mb2)/hb-(Vc1+Vc2)/2 Vb=(Mc1+Mc2)/hc-(Vb1+Vb2)/2 A Vb1 Mb2 Vc2 hc Mc2 column beam b)Distortingofpanel zone A Figure 8.15 Distorting of panel zone 4. Proportion of the lateral load shared by bracing and frame Design code asks the lateral load shared by frame is not less than the minimum value of the next two: 1) 25 percent of the total base shear, and 2) 1.8 times of the shear supported by frame which are computed by elastic analysis. 支撑 支撑 支撑
6gminaahotdosigninnerforceot cano d as Equ.(8.I =(the amplirication Tactor) ·厂sethe shear eapacy of link, is the shear 8.2.4 Seismic check for member and section .ofbrace mombor (1-2010) Where. S≤R/YRE S=YGSGE+YESEM+YSE 2设ga9ea49a8awaa。 ers,the co cets0,75 When bending abouts hi1eNIN≤L/AM=AM Tn, whole .-web area and the gross area of the whole section. >
7 5. Provisions about design inner force of beam and column 1） The design end moment of beams can use the moment at the edge of column. moment at the node by analysis beam axis of beam column axis of column Design moment at the beam ends Figure 8.16 Design moment at the beam ends 2） For prevention of the early damage of the beam segment adjacent to link in EBF structures, the design moment of the beam segment should multiply an amplification factor not less than 1.5 in region VIII, or 1.6 in region IX. Thus the design moment is computed as Equ. (8.1). （8.1） c 0 ( ) M M V V lc V ´ ´ or ＝(the amplification factor) In EBF structures, the inner force of column shall be considered to be amplified as Eq. (8.1). 0 M is the computed moment of the beam. c rc are the shear capacity of link V V l 、 ； V is the shear of link； 6. Inner force of brace member (GB50011-2010) The boundary condition of brace in analysis is usually supposed as pin connection. To the brace connecting to link in EBF structure, the design axial force of the brace shall take an amplified factor which is no less than 1.4 for grade 1;1.3 for grade 2 and 1.2 for grade 3 frames. RE S R £ /g 1、Beam and column the load combination in which earthquake action shall be combined S － ； R － the design strength of steel； RE adjusting coefficient for load bearing To beam and column members, the coefficient is 0.75. g － ； 8.2.4 Seismic check for member and section capacity Where, G GE Eh Ehk Ev Evk S = g S + + g g S S （1）The bending capacity of beam and column To I and H shape steel, when bending around its strong section axis ： wh y pc ile N / N £ = 0.13，M M （8.2） whil y pc y p e N / N > 0.13，M = - 1.15(1 N / ) N M （8.3） A A w、 －web area and the gross area of the whole section. n A － net area of the section； p pc ull plasticmoment without axial force and the ultimate moment with axial force existing; M M、 －f y y n ay N、N －design axial force and axial yielding capacity, respectively， ； N = A f Where An－ net area of the section； A A w、 －web area and the gross area of the whole section. p pc ull plastic moment without axial force and the ultimate moment with axial force existing; M M、 －f When bending about its weak axis： y w pc while N / / N £ = A A，M M 2 y w pc w ay y w ay p while N / N > A / A，M = {1-[(N - - A f )/(N A f M )] } （8.4） （8.5） y y n ay N、N －design axial force and axial yielding capacity, respectively， ； N = A f
2entrc6iekaecec ∑W(e-N1A)2n∑W Where plastie section modulus of beam and colu N-design axally compressive force; A-area of column section, ohal stability un Where.o-coefficient,taken as 0.9; K、N-deigherfoceanddcreuruidforoeoftclid (thn ht,depth.thickness o V.=W ( V.min:0.58-NKA1.24Mll-NAa) tue shall be checked by qu 8.2.5 Detailing requirements of members hall no be greater than th 351fm Tabke 8.3 Li yield stress of the brace
8 （2）In order to assure the plastic deformation occurs first at beam end, the following equation shall be checked. except that: 1) the columns in a storey have the shear capacities 25 percentage greater than those in the above storey, or 2) the design axially compressive force does not exceed 0.4 times of its design axial capacity, or 3) the column as an axially compressed member can keep its global stability under two times of design seismic action pc yc c pb yb å å W ( f - ³ N / ) A h W f pc yc c pb yb å å W ( f - ³ N / ) A h W f （8.6） Where: pc pb W W 、 - plastic section modulus of beam and column; N - design axially compressive force; yc yb f f 、 - yield stress of column and beam respectively; c A - area of column section; h ——Coefficient,taken as 1.15 for grade 1 frames, 1.10 for grade 2 frames and 1.05 for grade 3 frames. （3）In CBF structure, if the inversed V type or V type brace is used, the beam connecting with braces should be fabricated in a continuous member to sustain the force transferred by braces．It is checked as a simply supported beam without middle supporting under gravity load and the unbalanced load due to the buckling of one compressed brace. （4）In EBF structure, the shear capacity of link shall be checked by Equ. (8.7). This equation considers the effect of axial force in the link. w RE hile 0.15 / N A l £ £ f，V Vj g （8.7a） w ay p w f w p p min{0.58 ,2 / } ( 2 ) l l l V A f M a A h t t M W f = = - = c RE 0.15 / N A l while > £ f，V Vj g 2 c m w ay p in{0.58 1 [ /( ) ],2.4 [1 /( )]/ } Vl l = A f - - N Af M N Af a （8.7b） Where, j - coefficient, taken as 0.9; V N 、 - design shear force and design axial force of the link; c design shear capacity of the link and its modified capacity considering the influence of axial force; V N l l 、 - p full plastic moment of the link; Ml - w the length, section height, depth, thickness of web and flange of the link; a h 、 、t t 、 － w web area and the gross area of the whole section of the link; A A 、 － p W － plastic section modulus of the link; ay f f 、 － design strength and yield stress of the link; RE g － adjusting coefficient for load bearing of the link, taken as 0.85. 3、 Brace The compressed brace in CBF structure shall be checked by Equ. (8.8a) through (8.8c) br RE （8.8a） n n ay /( ) / 1/(1 0.35 ) ( / ) / N A f f E j y g y l l l p £ = + = （8.8b） Where, N － design axial force of the brace; br A － the area of the brace; j － stability coefficient for axially compressed steel members; y － reduced factor for strength considering the influence of cyclic load; n l － slenderness ratio; （8.8c） ay f － yield stress of the brace; RE g － adjusting coefficient for load bearing of the brace, taken as 0.80. E － Elastic modulus of the brace; 1、Slenderness ratio of column The slenderness ratio of columns shall not be greater than the limitation listed in Table 8.3. The table is made according to Q235 steel. For other steel, the limitation should time the factor of . ay 235/ f 8.2.5 Detailing requirements of members 120 80 60 60 Maximum slenderness ratio to the building over 12 stories 120 120 120 100 Maximum slenderness ratio to the building not exceeding 12 stories Plate element VI VII VIII IX Table 8.3 Limitation for slenderness ratio of columns
hickness ratio for beams and column in the buil ing(GB50011-2010) and colum le 8.4 3.Slenderness ratie and width-thickness ratio of braces ed b Where,人-一 总s水当受 a5÷a6-n学腰心 he io of the hra ble 87 Limi tioo of width-th ess ratio for brace in CB ratio of brace in CB
9 2、Width-thickness ratio of plates in beam and column For the sake of prevent from early local buckling which decrease the deformability of members, the width-thickness ratio shall be limited according to Table 8.4 and Table 8.5. Both two tables are based on Q235 steel and it should be times the factor of to the steel with different yield stress. 235/ ay f 85- 120Nb /(Af ） <=75 80- 110Nb /(Af )< =70 72- 100Nb /(Af )< =65 72-120Nb /(A f)<=60 webs of H and box section plates of box section 30 30 32 36 between two flanges flange of H shaped section 9 9 10 11 Beam and box section plates in box section 33 36 38 40 web of H shaped section 43 45 48 52 flange of H shaped section 10 11 12 13 Column Plate element Grade I Grade II Grade III Grade IV Table 8.5 Limitation of width-thickness ratio for beams and columns in the building(GB50011-2010) 3、Slenderness ratio and width-thickness ratio of braces At the section where plastic hinge is expected the brace should be set at both flanges of the member. The slenderness ratio of the laterally supported members shall meet the requirements decided by the following equations. 1 1 y px px y 235 While 1 0.5 (60 40 ) M M W f W f f - £ £ ，l £ - 1 1 y px px y 235 W 0.5 1.0 (45 10 ) M M W f W f f hile < £ ，l £ - （8.9a） （8.9b） Where， 1 1 y y y is the distance between the lateral support points is the radius of gyration about the out-of-plane axis of the section; l l i i l = － ， 1 1 the moment of the section where the lateral brace is connected, the value taken as positive if the bending moment in same direction among the range of ,or taken as nega M l - tive. ay 235/ f The slenderness ratio of the brace members in CBF structure shall not exceed the limitation listed in Table 8.6. And their widththickness ratio shall not exceed the limitation listed in Table 8.7. Both two tables are based on Q235 steel and it should be times the factor of to the steel with different yield stress. Table 8.6 Limitation of slenderness ratio of brace in CBF Building exceeding 12 stories 120 90 60 tensile member 200 150 150 designed as compressive member 150 120 120 designed as Building not exceeding 12 stories Plate element VI, VII VIII IX The width-thickness ratio of the brace members in CBF structure shall not exceed the limitation listed in Table 8.7. Both two tables are based on Q235 steel and it should be times the factor of to the steel with different yield stress. 235/ ay f Table 8.7 Limitation of width-thickness ratio for brace in CBF of pipe section 42 40 40 38 diameterthickness ratio 31 28 25 23 21 21 19 web of box section web of H section 33 30 27 25 23 23 21 13 11 9 9 8 8 7 outstanding flange VII VIII IX VI VII VIII IX Building exceeding 12 stories Building not exceeding 12 P stories late element
二然 able 8.8 Width-thickness ratio of links and the beams in the same b her he s while p(4/4)<03.a<1.6M/V (5)The ne hale p(A./4)203 a5[15-0.5p(A//V of the li 8.2.6 Seismic check and detailing requirements of joint and connection y(M1+M)/Y≤(4/3)am
10 To the brace in EBF, the slenderness ratio shall not be greater than 120 , and the width-thickness ratio shall follow the provision of the code for design of steel structures. ay 235 / f 4、 Link （1） Due to high dissipating ability is desired, the high strength steel which does not possess good yielding deformation shall not be used as the link. By the experience until now, the link had better use the steel which yield stress is not greater than 345 MPa. ay 235/ f （2） The width-thickness ratio of the link as well as the beam segments in the same bay of the link shall not greater than the limitation of Table 8.8, while when the steel other than Q235 is used the factor of should be multiplied . while N / Af £ 0.14 while N / Af > 0.14 90[1-1.65 / N Af ] 33[2.3 / - N Af ] Table 8.8 Width-thickness ratio of links and the beams in the same bay web outstanding flange 8 Plates limitation （3）Since the brace produces axial force in the link, the length of the link shall be limited by Equ. (8.10) to keep the necessary energy consuming ability if the axial force in the link is greater than 0.16 times of the design axial capacity. w w p hile ( / ) 0.3 1.6 / A A l l r < < ，a M V w w w p hile ( / ) 0.3 [1.15 0.5 ( / )]1.6 / A A l l r r ³ ，a £ - A A M V (8.10a) (8.10b) Where， a －the length of the link, referred to Figure 8.10; r － the proportion of the design axial force and design shear of the link. （4）The web of the link can neither be strengthened by welding a parallel plate nor be holed. The former will be obstacle to develop plastic deformation while the latter will weaken the dissipating ability. （5）The necessary rib should be welded to the link to transfer the shear force and prevent from local buckling. Furthermore, at the both ends of the link the braces should be set on both of the upper and lower flanges to keep the twist of the link. The detailing requirement can refer to the corresponding provisions in the code. thecrosspointof braceandthelink shallbeattheendor insideofthelink ribs beamsegmentsinthe samefloorofthelink brace gusset themiddleribsinthe weboflink thelengthofthelink,a profiledbarwith wideflange thecrosspointof braceandthelink shallbeattheendor insideofthelink beamsegmentsinthe samefloorofthelink thelengthofthelink,a ribs themiddleribsinthe weboflink Figure 8.17 The detailing of link 1、Beam-to-column rigid joint Equ. (8.11) shall be used for the seismic check of the joint to make the yield capacity keeping reasonable. pb1 pb2 p v y(M + £ M )/V f (4 / 3) To H shaped column, Vp b c w = h h t To box column, Vp = 1.8hb h tc w （8.11） （8.12a） （8.12b） Equ. (8.13) shall also be checked w b c t ³ + (h h )/ 90 b1 b2 v RE ( )/ (4/ 3) / M M V f p + £ g （8.13a） （8.13b） 8.2.6 Seismic check and detailing requirements of joint and connection Volume