SEISMIC DESIGN REQUIREMENTS FOR REINFORCED CONCRETE BUILDINGS

Transcription

1 SEISMIC DESIGN REQUIREMENTS FOR REINFORCED CONCRETE BUILDINGS

2 MODEL BUILDING CODES A model building code is a document containing standardized building requirements applicable throughout the United States. The three model building codes in the United States were: the Uniform Building Code (predominant in the west), the Standard Building Code (predominant in the southeast), and the BOCA National Building Code (predominant in the northeast), were initiated between 1927 and The US Uniform Building Code was the most widely used seismic code in the world, with its last edition published in Up to the year 2000, seismic design in the United States has been based on one these three model building codes. Representatives استشارية هيئة from the three model codes formed the International Code Council (ICC) in 1994, and in April 2000, the ICC published the first edition of the International Building Code, IBC In 2003, 2006, 2009 and 2012, the second, third fourth

3 Initiation Of The Equivalent Static Lateral Force Method The work done after the 1908 Reggio-Messina Earthquake in Sicily by a committee appointed by the Italian government may be the origin of the equivalent static lateral force method, in which a seismic coefficient is applied to the mass of the structure, to produce the lateral force that is approximately equivalent in effect to the dynamic loading of the expected earthquake. The Japanese engineer Toshikata Sano independently developed in 1915 the idea of a lateral design force V proportional to the building s weight W. This relationship can be written as F = C W, where C is a lateral force coefficient, expressed as some percentage of gravity. The first official implementation of Sano s criterion was the specification C = 10 percent of gravity, issued as a part of the 1924 Japanese Urban Building Law Enforcement Regulations in response to the destruction caused by the great 1923 Kanto earthquake. In California, the Santa Barbara earthquake of 1925 motivated several communities to adopt codes with C as high as 20 percent of gravity.

4 Development Of The Equivalent Static Lateral Force Method UBC/IBC Code s UBC UBC 1946 UBC UBC 1958 UBC UBC 1973 UBC UBC 1979 UBC UBC 1985 UBC UBC 1994 UBC 1997 IBC IBC-2012 Lateral Force F = C W F = C W V = ZKCW V = ZIKCSW V = ZIKCSW V = ZICW/Rw V = CvIW/RT V = CsW The first edition of the U.S. Uniform Building Code (UBC) was published in 1927 by the Pacific Coast building Officials (PCBO), contained an optional seismic appendix. The seismic design provisions remained in an appendix to the UBC until the publication of the 1961 UBC. In the 1997 edition of UBC the earthquake load (E) is a function of both the horizontal and vertical components of the ground motion.

5 Safety Concepts Structures designed in accordance with the UBC provisions should generally be able to: 1. Resist minor earthquakes without damage. 2. Resist moderate earthquakes without structural damage, but possibly some nonstructural damage. 3. Resist major earthquakes without collapse, but possibly some structural and nonstructural damage. The UBC intended that structures be designed for life-safety in the event of an earthquake with a 10-percent probability of being exceeded in 50 years. The IBC intends design for collapse prevention in a much larger earthquake, with a 2-percent probability of being exceeded in 50.

6 Seismic Codes Are Based On Earthquake Historical Data The 1925 Santa Barbara earthquake led to the first introduction of simple Newtonian concepts in the 1927 Uniform Building Code. As the level of knowledge and data collected increases, these equations are modified to better represent these forces. In response to the 1985 Mexico City earthquake, a fourth soil profile type,, for very deep soft soils was added to the 1988 UBC, with the factor equal to 2.0. The 1994 Northridge Earthquake resulted in addition of near-fault factor to base shear equation, and prohibition on highly irregular structures in near fault regions. Also, redundancy factor added to design forces. The 1997 UBC incorporated a number of important lessons learned from the 1994 Northridge and the 1995 Kobe earthquake, where four site coefficients use in the earlier 1994 UBC has been extended to six soil profiles, which are determined by shear wave velocity, standard penetration test, and undrained shear strength.

7 Detailing Requirements of ACI Based on R of ACI , for UBC 1991 through 1997, Seismic Zones 0 and 1 are classified as classified as zones of low seismic risk. Thus, provisions of chapters 1 through 19 and chapter 22 are considered sufficient for structures located in these zones. Seismic Zone 2 is classified as a zone of moderate seismic risk, and zones 3 and 4 are classified as zones of high seismic risk. Structures located in these zones are to be detailed as per chapter 21 of ACI Code. For Seismic Design Categories A and B of IBC 2000 through 2012, detailing is done according to provisions of chapters 1 through 19 and chapter 22 of ACI Seismic Design Categories C, D, E and F are detailed as per the provisions of chapter 21. Code/Standard Level of Seismic Risk Low Moderate High IBC SDC A, B SDC C SDC D, E, F UBC zone 0, 1 Zone 2 Zone 3, 4

8 Seismic Design According To 1997 UBC The Static Lateral Force Procedure Major Changes from UBC 1994 (1) Soil Profile Types: The four Site Coefficients S 1 to S 4 of the UBC 1994, which are independent of the level of ground shaking, were expanded to six soil profile types, which are dependent on the seismic zone factors, in the 1997 UBC (S A to S F ) based on previous earthquake records. The new soil profile types were based on soil characteristics for the top 30 m of the soil. The shear wave velocity, standard penetration test and undrained shear strength are used to identify the soil profile types. (2) Structural Framing Systems: In addition to the four basic framing systems (bearing wall, building frame, moment-resisting frame, and dual), two new structural system classifications were introduced: cantilevered column systems and shear wall-frame interaction systems. (3) Load Combinations: The 1997 UBC seismic design provisions are based on strength-level design rather than service-level design. (4) Earthquake Loads: In the 1997 UBC, the earthquake load (E) is a function of both the horizontal and vertical components of the ground motion.

9 Structures having a flexible upper portion supported on a rigid lower portion where both portions of the structure considered separately can be classified as being regular, the average story stiffness of the lower portion is at least ten times the average stiffness of the upper portion and the period of the entire structure is not greater than 1.10 times the period of the upper portion considered as a separate structure fixed at the base. Seismic Design According To 1997 UBC The Static Lateral Force Procedure Applicability The static lateral force procedure may be used for the following structures: All structures, regular or irregular (Table A1), in Seismic Zone no. 1 (Table A-2) and in Occupancy Categories 4 and 5 (Table A-3) in Seismic Zone 2. Regular structures under 73 m in height with lateral force resistance provided by systems given in Table (A-4) except for structures located in soil profile type SF, that have a period greater than 0.70 sec. (see Table A-5 for soil profiles). Irregular structures not more than five stories or 20 m in height.

10 Design Base Shear, V The total design base shear in a given direction is to be determined from the following formula. The total design base shear need not exceed the following: The total design base shear shall not be less than the following:

11 Where V = total design lateral force or shear at the base. W = total seismic dead load In storage and warehouse occupancies, a minimum of 25 % of floor live load is to be considered. Total weight of permanent equipment is to be included. Where a partition load is used in floor design, a load of not less than 50 kg/m 2 is to be included. I = Building importance factor given in Table (A-3). Z = Seismic Zone factor, shown in Table (A-2). R = response modification factor for lateral force resisting system, shown in Table (A- 4). Ca = acceleration-dependent seismic coefficient, shown in Table (A-6). Cv= velocity-dependent seismic coefficient, shown in Table (A-7). T= elastic fundamental period of vibration, in seconds, of the structure in the direction under consideration evaluated from the following equations: For reinforced concrete moment-resisting frames, For other buildings, Design Base Shear, V (Contd.) Alternatively, for shear walls,

12 Design Base Shear, V (Contd.) Where hn= total height of building in meters Ac = combined effective area, in m 2, of the shear walls in the first story of the structure, given by De =the length, in meters, of each shear wall in the first story in the direction parallel to the applied forces. Ai= cross-sectional area of individual shear walls in the direction of loads in m 2

13 Tables And Graphs Table (A-2): Seismic zone factor Z Zone 1 2A 2B 3 4 Z Note: The zone shall be determined from the seismic zone map (Graphs A-1 and A-2). Table (A-3):Occupancy Importance Factors Occupancy Category Seismic Importance Factor, I 1-Essential facilities Hazardous facilities Special occupancy structures Standard occupancy structures Miscellaneous متنوع structures 1.00

14 Tables And Graphs (Contd.) Table (A-4): Structural Systems Lateral- force resisting system description R Height limit Zones 3&4. (meters) Bearing Wall Concrete shear walls Building Frame Concrete shear walls Moment-Resisting Frame SMRF IMRF OMRF Dual Shear wall + SMRF Shear wall + IMRF Cantilevered Column Cantilevered Building column elements Shear-wall Interaction Frame N.L N.L

15 Table (A-5):Soil Profiles

16 Table (A-6): Seismic coefficient Ca Soil Profile Type Tables And Graphs (Contd.) Seismic Zone Factor, Z Z =0.075 Z = 0.15 Z = 0.2 Z = 0.3 SA SB SC SD SE SF See Footnote Footnote: Site-specific geotechnical investigation and dynamic response analysis shall be performed to determine seismic coefficients for soil Profile Type.

17 Table (A-7): Seismic coefficient Cv

18 Tables And Graphs (Contd.) Graph (A-1): Palestine s seismic zone factors (Source: International Handbook of Earthquake Engineering, Mario Paz)

19 Tables And Graphs (Contd.) Graph (A-2): Palestine s seismic zone factors (Source: Annajah National University)

20 Vertical Distribution of Forces Vertical Distribution of Force: The base shear which is evaluated from the following equation, is distributed over the height of the building. Where: The shear force at each story is given The overturning moment is given by

21 Horizontal Distribution of Forces Horizontal Distribution of Force: The design story shear in any direction, is distributed to the various elements of the lateral force-resisting system in proportion to their rigidities. Horizontal Torsional Moment: The torsional design moment at a given story is given by moment resulting from eccentricities between applied design lateral forces applied through each story s center of mass at levels above the story and the center of stiffness of the vertical elements of the story, in addition to the accidental torsion (calculated by displacing the calculated center of mass in each direction a distance equal to 5 % of the building dimension at that level perpendicular to the direction of the force under consideration). Interactions of Shear Walls with Each Other: In the following figure the slabs act as horizontal diaphragms extending between cantilever walls and they are expected to ensure that the positions of the walls, relative to each other, don't change during lateral displacement of the floors. The flexural resistance of rectangular walls with respect to their weak axes may be neglected in lateral load analysis.

22 Horizontal Distribution of Forces (Contd.) The distribution of the total seismic load Fx, or Fy among all cantilever walls may be approximated by the following expressions: Fix = Fix + Fix and Fiy = Fiy + Fiy Where Fix = load induced in wall by inter-story translation only, in x-direction Fiy = load induced in wall by inter-story translation only, in y-direction Fix = load induced in wall by inter-story torsion only, in x-direction Fiy = load induced in wall by inter-story torsion only, in y-direction

23 Horizontal Distribution of Forces (Contd.) The force resisted by wall i due to inter-story translation, in x-direction, is given by The force resisted by wall i due to inter-story translation, in y-direction, is given by The force resisted by wall i due to inter-story torsion, in x-direction, is given by The force resisted by wall i due to inter-story torsion, in y-direction, is given by Where: xi = x-coordinate of a wall w.r.t the C.R of the lateral load resisting system yi = y-coordinate of a wall w.r.t the C.R of the lateral load resisting system ex = eccentricity resulting from non-coincidence of center of gravity C.G and center of rigidity C.R, in x- direction ey= eccentricity resulting from non-coincidence of center of gravity C.G and center of rigidity C.R, in y- direction Fx = total external load to be resisted by all walls, in x-direction Fy = total external load to be resisted by all walls, in y-direction Iix = second moment of area of a wall about x-axis Iiy = second moment of area of a wall about y-axis

24 Classification of Structural Walls According to Chapters 2 and 21 of ACI , structural walls are defined as being walls proportioned to resist combinations of shears, moments, and axial forces induced by earthquake motions. A shear wall is a structural wall. Reinforced concrete structural walls are categorized as follows: Ordinary reinforced concrete structural walls, which are walls complying with the requirements of Chapters 1 through 18. Special reinforced concrete structural walls, which are cast-in-place walls complying with the requirements of 21.2 and 21.7 in addition to the requirements for ordinary reinforced concrete structural walls. Special Provisions For Earthquake Resistance According to Clause of ACI , the seismic risk level of a region is regulated by the legally adopted general building code of which ACI forms a part, or determined by local authority. According to Clauses and of ACI in regions of low seismic risk, provisions of Chapter 21 are to be applied (chapters 1 through 18 are applicable). According to Clause of ACI , in regions of moderate or high seismic risk, provisions of Chapter 21 are to be satisfied. In regions of moderate seismic risk, ordinary or special shear walls are to be used for resisting forces induced by earthquake motions as specified in Clause of the code. According to Clause of ACI , in regions oh high seismic risk, special structural walls complying with 21.2 through are to be used for resisting forces induced by earthquake motions.

25 Classification of Moment Resisting Frames Building Frame System: Based on section 1627 of UBC-1997, it is essentially a complete space frame that provides support for gravity loads. Moment Frames: Based on ACI 2.1, 21.1 and 21.2, are defined as frames in which members and joints resist forces through flexure, shear, and axial force. Moment frames are categorized as follows: Ordinary Moment Frames: Concrete frames complying with the requirements of Chapters 1 through 18 of the ACI Code. They are used in regions of low-seismic risk. Intermediate Moment Frames: Concrete frames complying with the requirements of and in addition to the requirements for ordinary moment frames. They are used in regions of moderate-seismic risk. Special Moment Frames: Concrete frames complying with the requirements of 21.2 through 21.5, in addition to the requirements for ordinary moment frames. They are used in regions of moderate and high-seismic risks.

26 Load Combinations Loads ACI UBC-1997 Dead (D) and Live (L) 1.2 D L 1.32 D L Dead (D), Live (L) and Earthquake (E) 1.2 D L E 1.2 D L E Earthquake Loads Based on UBC , horizontal earthquake loads to be used in the above-stated load combinations are determined as follows: Where: E = earthquake load resulting from the combination of the horizontal component, and the vertical component, E h = the earthquake load due to the base shear, V E v = the load effects resulting from the vertical component of the earthquake ground motion and is equal to the addition of to the dead load effects D Ρ = redundancy factor, to increase the effects of earthquake loads on structures with few lateral force resisting elements (taken as 1.0 where z =0, 1 or 2)

27 Design of Ordinary Shear Walls The shear wall is designed as a cantilever beam fixed at the base, to transfer load to the foundation. Shear force, bending moment, and axial load are maximum at the base of the wall. Types of Reinforcement To control cracking, shear reinforcement is required in the horizontal and vertical directions, to resist in plane shear forces. The vertical reinforcement in the wall serves as flexural reinforcement. If large moment capacity is required, additional reinforcement can be placed at the ends of the wall within the section itself, or within enlargements at the ends. The heavily reinforced or enlarged sections are called boundary elements.

28 Design of Ordinary Shear Walls Shear Design According to ACI , nominal shear strength Vn is given as Where Vc is nominal shear strength provided by concrete and Vs is nominal shear strength provided by the reinforcement. Based on ACI , Vn is limited by the following equation. The shear strength provided by concrete Vc is given by any of the following equations, as applicable. h = thickness of wall d = effective depth in the direction of bending, may be taken as 0.8 lw, as stated in ACI Ag = gross area of wall thickness Nu = factored axial load

29 Design of Ordinary Shear Walls Shear Reinforcement When the factored shear force exceeds ФVc/2, -Horizontal reinforcement ration ρh is not to be less than Spacing of this reinforcement S2 is not to exceed the smallest of lw/5, 3h and 45 cm. - Vertical reinforcement ratio ρn is not to be taken less than Nor According to ACI , when the factored shear force Vu exceeds ФVc, horizontal shear reinforcement must be provided according to the following equation. Spacing of this reinforcement S1 is not to exceed the smallest of lw/3, 3h and 45 cm. Where: Av = Area of horizontal shear reinforcement within a distance S2. Ρh = ratio of horizontal shear reinforcement area to gross concrete area of vertical section. Ρn = ratio of vertical shear reinforcement area to gross concrete area of horizontal section.

30 Design of Ordinary Shear Walls (Contd.) Flexural Design The wall must be designed to resist the bending moment at the base and the axial force produced by the wall weight or the vertical loads it carries. Thus, it is considered as a beam-column. For rectangular shear walls containing uniformly distributed vertical reinforcement and subjected to an axial load smaller than that producing balanced failure, the following equation, developed by Cardenas and Magura in ACI SP-36 in 1973, can be used to determine the approximate moment capacity of the wall. Where: C = distance from the extreme compression fiber to the neutral axis lw = horizontal length of wall Pu = factored axial compressive load fy = yield strength of reinforcement Ф = strength reduction factor

31 Design of Ordinary Shear Walls (Contd.) Reinforcement

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