同济大学：《建筑结构抗震》课程教学资源（课件讲稿）Chapter 5 Earthquake Effect and Seismic Design principles（1/2）.pdf_大学文库

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

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 H15m，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 • 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 Ki1 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       Ki3 Ki2 Ki1 Ki Lateral stiffness irregularity (soft story) Abrupt changeof story carrying capacity (weak story) Qy,i1 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

7 2.The structure with vertical irregularity • The story shear force of the soft/weak story must be increased by 15%. • The elasto-plastic analysis must be conducted for the tall building exceeding the specified height. • The shear strength of the weak story should not be less than 65 percent of that of the adjacent upper story for the structure with abrupt change of story carrying capacity. 3. The structurewith both plan irregularity and vertical irregularity • The above two terms must be followed simultaneously. • In addition, for the structure with seriously irregularity, time history analysis must be carried out to check the analytic results obtained by modal analysis. • For the building with seriously irregularly configuration, seismic joints can be provided to separate the building into simple and regular individual units. There must be enough clearance at the seismic joints so that the adjoining portions do not pound each other. • The design characteristic of redundancy plays an important role in seismic performance. • Placing resisting members on the perimeter whenever possible is always desirable. • The detailing of connections is a key factor in seismic performance, since the more integrated and interconnected a structure is the more load distribution possibilities there are. • The lower of the density center of a building, the better resistant performance. 5.1.3 Configuration Influences on Seismic Performance Better 5.1.4 Essentials of Structural System 1. Selection of materials and types of construction • Structural materials have their own performance characteristics and should be selected according to the location and condition of the building to be planned in order to accomplish safe, economical, and superior architecture. • High strength-to-weight ratio • High deformability • Low degradation • High uniformity • Reasonable cost • Steel structures • Steel and reinforced-concrete composite structures • Wooden structures • Cast-in-situ reinforced concrete structures • Precast concrete structures • Prestressed concrete structures • Masonry structures • Mixed structures 2. Major Characteristics of Building structures Requirements for seismic structural system 1. It shall have a clear analytical model and reasonable path for seismic action transfer. 2. It should have several lines of defense against earthquakes. It should avoid loss of either earthquake resistance capacity or gravity load capacity of the whole system due to damage to part of the structure or members. 3. It shall possess the necessary strength, adequate deformability, and better energy dissipation ability. Summary

8 4. It should possess a rational distribution of stiffness and strength, avoid weakening of some parts of the structure due to local weakening or abrupt changes; avoid appearance of extremely large concentration of stress and plastic deformation; when weak parts do appear, measures should be taken to enhance their earthquake resistance capacity. 5. It should have similar dynamic characteristics in the direction of individual primary axis. 6. Designing the connections and details of a structure to be earthquake resistant is almost as important as checking the structure’s overall dynamic behavior. Simplicity Regular Symmetry Integral Construction Redundancy Seismic Conceptual Design Three-level 1. Seismic fortification objectives 5.2 Building classification and seismic fortification 1) The first level When buildings designed based on the code are subjected to the influence of frequently occurred earthquakes with an intensity of less than the fortification intensity of the region, they will not be, or will be only slightly damaged and will continue to be serviceable without repair. ------------- No damage under minor earthquake. 2) The second level When buildings are subjected to the influence of earthquakes equal to the fortification intensity of the region, they may be damaged but will still be serviceable after ordinary repair or without repair. ------ Repairable under moderate earthquake 3) The third level When buildings are subjected to the influence of expected rare earthquakes with an intensity higher than the fortification intensity of the region, they will not collapse nor suffer damage that would endangerhuman lives. ------------- No collapse under major earthquake. 1) The first level By the elastic analysis, the carrying capacity of the structure is checked under the fundamental combination of effects of seismic action of minor earthquake and other loads, and the elastic seismic deformation is checked under the action of minor earthquake. Calculation is necessary under the first level. 2. Seismic design method 2) The second level The objective of this level is realized mainly by seismic conceptual design and constructional measures or detailing. 3) The third level The elasto-plastic deformation is checked under the action under rare earthquake. Two-stage In Summary: Level 1 Level 2 Level 3 minor earthquake level moderate earthquake level major earthquake level No damage Repairable No collapse 1) Elastic force 2) Elastic deformation Structural detailing 1) Elastoplastic deformation 2. Seismic design method

9 49 Probability Density Function (PDF) of Earthquake Intensity is look like the 3rd Extreme Value Distribution (地震烈度的概率密度函数符合极限Ⅲ型分布） ( ) ( ) ( ) The maximum value of Earthquake Intencity = ; Earthquake Intencity Shape Parameter; Mean Intensity. I k 1 k k k I f I e 12 I k                          ，； 3. minor / moderate / major earthquake 50 the 3rd Extreme Value Distribution Exceedance Probability (EP) PDF Eq. Intensity 3. minor / major / major earthquake 51 Distribution Function: When ( ) . % EP ( ) . % 1 I F I e 36 8 1 F I 63 2         ：： k I F I e              ( ) EP in Design Reference Period（设计基准期内的超越概率）is 10% Basic Intensity Moderate Earthquake Mean Intensity： Minor Earthquake 3. minor / major / major earthquake EP in Design Reference Period is 2~3% Rare occurred Intensity Major Earthquake 52 Fortification Intensity（设防烈度）： In 50 years, if EP is 10%， the return years is 475。 Frequently occurred Intensity (小震，多遇地震)： In 50 years, if EP is 63.5%， the return years is 50； It is about 1.55 grade lowerthan Fortification Intensity。 Rare Occurred Intensity（大震，罕遇地震）： In 50 years, if EP is 3%~2%，the return years is 1461~2475。 It is about 1 grade higherthan Fortification Intensity。 3. minor / major / major earthquake 53 1. 中国国家标准建筑抗震设防分类标准（GB50223)，根据建筑物重要性、在地震中及地震后破坏对社会和经济造成的影响及在抗震防灾中的作用，将建筑分为四类： • 甲类建筑(特殊设防类) • 乙类建筑(重点设防类） • 丙类建筑(标准设防类) • 丁类建筑(适度设防类) 5.2 建筑的分类与建筑抗震设防 Q: 为什么要分类？如何分类？如何设防？一、建筑抗震设防分类 54 •甲类建筑(特殊设防类)——地震破坏后会对社会有严重影响，对国民经济有巨大损失或有特殊要求的建筑物。专门研究确定。如生命线工程。 •乙类建筑(重点设防类)——地震中使用功能不能中断或需迅速恢复的建筑物及破坏后会对社会有重大影响，对国民经济有重大损失。如医院、中小学校校舍、大型商场或人员集中的公共建筑等。 5.2 建筑的分类与建筑抗震设防