Chapter 5 Chapter 5 Earthquake Effect and Seismic Design principles Earthquake Effect and 5.1 Seismic conceptual design Seismic Design principles 6 aionhethoedologyofselsmcacion 5emeCcnranmentesandsneal 5.1 Seismic conceptual design 5.1 Seismic conceptual design The building with good seismic performance can hardly be obtained only by "accurate calculation seismic conceptual design ndnhtrayis 5.1.1 Confieuration Characteristics 5.1.I Configuration Characteristics .of the designer will be tos
1 Chapter 5 Earthquake Effect and Seismic Design principles Chapter 5 Earthquake Effect and Seismic Design principles 5.1 Seismic conceptual design 5.2 Building classification and seismic fortification 5.3 Calculation Methodology of seismic action 5.4 Seismic Check of structural members and structural lateral deformations In the very early stage of building design, the configuration, the basic material, structure, and framing of the building have to be chosen. The architects and the structural engineers should therefore cooperate and thoroughly discuss the matter at this early stage. Seismic design in this stage is generally termed seismic conceptual design 5.1 Seismic conceptual design Bamboo • Since earthquake is a stochastic process, the seismic action that the building might be expected to be suffered in its life time can not be quantified accurately. • A building is not a homogenous block, but a complicated assembly of parts. Considerable simplifications are always needed in the structural analysis. The building with good seismic performance can hardly be obtained only by “accurate calculation’’. 5.1 Seismic conceptual design • The desirable aspects of building configuration are simplicity, regularity, and symmetry in both plan and elevation. • Irregularities, often unavoidable, contribute to the complexity of structural behavior. Any irregularity in the distribution of stiffness or mass is likely to lead to an increased dynamic response. 5.1.1 Configuration Characteristics • The first task of the designer will be to select a structural system most conductive to satisfactory seismic performance within the constraints dictated by architectural requirements. • Configuration is generally defined as building size and shape, and the characteristic of proportion. • The extended definition also includes the location of the structural elements. And location of nonstructural elements. 5.1.1 Configuration Characteristics • Configuration largely determines the ways in which seismic forces are distributed throughout the building, and also influences the relative magnitude of those forces
1.Building Height 2.Aspect Rati a beizht is e 3.Plan Configuration Plan Arrangement F-skapedplan L-shapedplan affect the Sywmetrical foms are preferred. U-shaped plan
2 1. Building Height • As a building grows taller, its period will tend to increase, and a change in period means a change in the building response. • It is easy to visualize the overturning forces associated with height as a seismic problem. • In Chinese Code for Seismic Design of Buildings, height limits are imposed, relating to type of structure and earthquake intensity. With the increasing of earthquake intensity, the allowed maximum height is decreased. Structure Noun Intensity 6、7 Intensity 8 Intensity 9 Frame 5 4 3 / Plate-wall 6 5 4 / Frame-wall, wall 7 6 5 4 Frame-tube 8 7 6 4 Tube in tube 8 8 7 5 Question： What is the purpose for the requirements? 2. Aspect Ratio • The term symmetry denotes a geometrical property of building plan configuration. • Structural symmetry means that the center of mass and center of resistance (center of rigidity) are located at, or close to the same point (unless live loads affect the actual center of mass). • Symmetrical forms are preferred. 3. Plan Configuration T-shaped plan L-shaped plan U-shaped plan Cruciform plan Source from :1980 SEAOC Recommended Lateral Force Requirements and Commentary Plan Arrangement — Irregularity Split levels Other complex shapes Setbacks Multiple tower Unusual high story Unusual low story nonuniform mass distribution, or converse Soft lower levels
Large openings in she Plan Arrangement-Regularity Cable-supported structures Lcng ith Control of t 8,9 fn:Code ae of (Code GRS0011-2010) In Chinese Code for Seismic Design of Buildings Reentrant corner ype of 学增 of 凿中]学 3
3 Large openings in shear walls Interruption of columns Interruption of beams Openings in diaphragms Shear walls in some stories, moment-resisting frames in others Interruption of verticalresisting elements Abrupt changes in size of members Drastic changes in mass/stiffness ratio Cable-supported structures Shells Staggered trusses Buildings on hillsides Plan Arrangement — Regularity Intensity L/B l /Bmax l / b 6，7 6.0 0.35 2.0 8，9 5.0 0.30 1.5 Length-Width ratio —— Control of torsion Source from : Code of Seismic Design of Building （Code GB50011-2010) Plan irregularity Type of irregularity Definition Torsional irregularity The maximum elastic floor displacement or inter-story drift is more than 1.2 times of the corresponding average of two ends of the floor. Irregularity of reentrant corners The projection beyond a reentrant corner is greater than 30 percent of the total plan dimension in the given direction. Diaphragm discontinuity The dimensions and stiffness of diaphragm change abruptly, including those having the effective width of diaphragm less than 50 percent of the typical width, the cutout or open area greater than 30 percent of the gross enclosed floor area, or staggered floor. In Chinese Code for Seismic Design of Buildings Reentrant corner 3 max B 0. B Bmax Bmax 3 max B 0. B Bmax 3 max B 0. B Bmax 3 max B 0. B Bmax 3 max B 0. B Bmax 3 max B 0. B 3 max L 0. L Lmax
Diaphragm Discontinuity 3.Plan Configuration Torsional r actua center of mass 4>0.34A=BL ppp00CJ Plan Configuration In addition.there exists cou ground motion +6>1a + Separated buildings 4.Separation The width of a gap:d Temperature Gap 。Frame System and the podiums and between the diffcrent kn me-wall system,0%d Seismicgap/joint Wall system,da50% Question:
4 Diaphragm Discontinuity b 0.5B B B A0 0.3A A B L • Generally the torsional response will inevitably occur even in the symmetrical structure when attacked by the earthquake. • In general case torsion arises from eccentricity in the building layout. The accidental eccentricity are very likely caused by construction which may change the actual center of resistance, or the distribution of live loads in occupancy which affects the actual center of mass. Torsional irregularity 3. Plan Configuration • The effective force exerted by lateral ground movement acts at the center of mass of each floor creating a torsional moment about the center of structural resistance. • In addition, there exists torsional component in earthquake ground motion. Plan Configuration Seismic force C Separated buildings The L-shaped building • If the ground motion occurs with a north-south emphasis at the L-shaped building, the wing oriented north-south will, tend to be stiffer than the wing oriented east-west. • The north-south wing, if it were a separate building, would tend to be deflect less than the east-west wing, but the two wings are tied together and attempt to move differentially at their notch, pulling and pushing each other. 4. Separation Temperature Gap When the length of a Frame (Cast-in-situ) is larger than 55m, or a wall larger than 45m, it is better to set the Temperature Gap Settlement gap Between the main tower and the podiums and between the different kind of foundations. Seismic gap/joint separating the irregular plan to regular plan，in case to reduce torsion. Source from : Code of Seismic Design of Building （Code GB50011-2010) The width of a gap: d Frame System when H15m，dframe＝70mm in the area of intensity 6, 7,8, 9, every 5m、4m、3m、2m taller, +20mm wider。 Frame-wall system, dframe-wall＝ 70%*dframe Wall system, dwall ＝ 50%*dframe Question: 1. Why the widths of gaps are different ? 2. What is the main reason? 3. How to reduce the unfavorable deformation? Source from : Code of Seismic Design of Building （Code GB50011-2010)
5.Vertical Configuration Vertical irregularity Definition insoft story or weak stery. c2gngaopisnwdauw ontinuity in vertical lateral forre resisting system Lateral stiffnessirregry(sof story) K<07K 口口口 of Abrupt changeof story carrying capacity (weak story) Vertical Configuration ak story insteac <082 月朋月 5
5 • Regularity should prevail in elevation, in both the geometry and the variation of story stiffness and strength, so as not to result in soft story or weak story. • The vertical configuration comprises uniformity and continuity, avoiding drastic changes. • The shape of rectangle, trapezoid, or triangle without abrupt change is preferable. • In Chinese Code for Seismic Design of Buildings, some critical vertical irregularities are defined and quantified. 5. Vertical Configuration Type of irregularity Definition Lateral stiffness irregularity The lateral stiffness of the story is less than 70 percent of that of the story above or less than 80 percent of the average lateral stiffness of the three stories above; The horizontal size of setback is larger than 25 percent of totalsize of the adjacent lower story. Discontinuity in vertical lateral force resisting member Loads applied on the vertical lateral force resisting member (column, shear wall, and brace) are transferred downward by horizontal transfer member (beam, truss etc.). Abrupt change of story carrying capacity The shear strength of the story is less than 80 percent of that of the story above. Vertical irregularity Discontinuity in vertical lateral force resisting system Ki1 Ki i i i u V K 1 0.7 K K i i ui Vi inter-story drift of the ith story shear force of the ith story 1 2 3 0.8( ) 3 i i i i K K K K Ki3 Ki2 Ki1 Ki Lateral stiffness irregularity (soft story) Abrupt changeof story carrying capacity (weak story) Qy,i1 Qy,i , , 1 0.8 Q Q y i y i • Pure cases of soft-story or weak-story failures are rare and generally the same floor is both soft and weak, therefore justifying the use of the term soft/weak story instead. • The soft/weak story problem is commonly magnified by torsional response. • Probably among all urban habitat structural problems, the soft/weak story failures have been responsible for more deaths and destruction than any other. Vertical Configuration