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SEISMIC DESIGN REQUIREMENTS FOR REINFORCED CONCRETE BUILDINGS
SEISMIC DESIGN REQUIREMENTS FOR REINFORCED CONCRETE BUILDINGS
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 1950.
• The US Uniform Building Code was the most widely used seismic code in the world, with its last edition published in 1997.
• Up to the year 2000, seismic design in the United States has been based on one these three model building codes.
• Representatives from the three model codes formed the استشارية هيئةInternational Code Council (ICC) in 1994, and in April 2000, the ICC published the first edition of the International Building Code, IBC-2000. In 2003, 2006, 2009 and 2012, the second, third fourth and fifth editions of the IBC followed suit.
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.
Development Of The Equivalent Static Lateral Force Method
• 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.
UBC/IBC Code s Lateral Force
UBC 1927- UBC 1946 F = C’W
UBC 1949- UBC 1958 F = C’W
UBC 1961- UBC 1973 V = ZKCW
UBC 1976- UBC 1979 V = ZIKCSW
UBC 1982- UBC 1985 V = ZIKCSW
UBC 1988- UBC 1994 V = ZICW/Rw
UBC 1997 V = CvIW/RT
IBC- 2000- IBC-2012 V = CsW
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.
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.
Based on R1.1.1.9.1 of ACI 318-08, 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 318-08 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 318-08. Seismic Design Categories C, D, E and F are detailed as per the provisions of chapter 21.
Detailing Requirements of ACI 318-08
Code/Standard Level of Seismic Risk
Low Moderate High
IBC 2000-2012 SDC A, B SDC C SDC D, E, F
UBC 1991-1997 zone 0, 1 Zone 2 Zone 3, 4
Major Changes from UBC 1994
(1) Soil Profile Types:
The four Site Coefficients S1 to S4 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 (SA to SF) 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.
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.
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
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:
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/m2 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,
Alternatively, for shear walls,
Design Base Shear, V (Contd.)
Where
hn= total height of building in meters
Ac = combined effective area, in m2, 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
m2
Design Base Shear, V (Contd.)
Table (A-2): Seismic zone factor Z
Note: The zone shall be determined from the seismic zone map (Graphs A-1 and A-2).
Table (A-3):Occupancy Importance Factors
Tables And Graphs
Zone 1 2A 2B 3 4
Z 0.075 0.15 0.20 0.30 0.40
Occupancy Category Seismic Importance Factor, I
1-Essential facilities 1.25
2-Hazardous facilities 1.25
3-Special occupancy structures 1.00
4-Standard occupancy structures
1.00
5-Miscellaneous متنوع structures 1.00
Table (A-4): Structural Systems
Tables And Graphs (Contd.)
Lateral- force resisting system description
R Height limit Zones 3&4.
(meters)
Bearing Wall Concrete shear walls
4.5 48
Building Frame Concrete shear walls
5.5 73
Moment-Resisting Frame
SMRF
IMRF
OMRF
8.5
5.5
3.5
N.L
---- ----
Dual Shear wall + SMRF
Shear wall + IMRF
8.5
6.5
N.L
48
Cantilevered Column Building
Cantilevered column elements
2.2 10
Shear-wall Frame Interaction
5.5 48
Table (A-5):Soil Profiles
Table (A-6): Seismic coefficient Ca
Footnote: Site-specific geotechnical investigation and dynamic response analysis shall be performed to determine seismic coefficients for soil Profile Type .
Tables And Graphs (Contd.)
Soil Profile Type
Seismic Zone Factor, Z
Z =0.075 Z = 0.15 Z = 0.2 Z = 0.3
SA 0.06 0.12 0.16 0.24
SB 0.08 0.15 0.20 0.30
SC 0.09 0.18 0.24 0.33
SD 0.12 0.22 0.28 0.36
SE 0.19 0.30 0.34 0.36
SF See Footnote
Table (A-7): Seismic coefficient Cv
Graph (A-1): Palestine’s seismic zone factors (Source: International Handbook of Earthquake Engineering , Mario Paz)
Tables And Graphs (Contd.)
Graph (A-2): Palestine’s seismic zone factors (Source: Annajah National University)
Tables And Graphs (Contd.)
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
Vertical 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.
Horizontal Distribution of Forces
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
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
Horizontal Distribution of Forces (Contd.)
According to Chapters 2 and 21 of ACI 318-02, 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 1.1.8.3 of ACI 318-02, the seismic risk level of a region is regulated by the legally adopted general building code of which ACI 318-02 forms a part, or determined by local authority.
According to Clauses 1.1.8.1 and 21.2.1.2 of ACI 318-02 in regions of low seismic risk, provisions of Chapter 21 are to be applied (chapters 1 through 18 are applicable).
According to Clause 1.1.8.2 of ACI 318-02, 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 21.2.1.3 of the code.
According to Clause 21.2.1.4 of ACI 318-02, in regions oh high seismic risk, special structural walls complying with 21.2 through 21.10 are to be used for resisting forces induced by earthquake motions.
Classification of Structural Walls
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 21.2.2.3 and 21.12 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.
Classification of Moment Resisting Frames
Earthquake Loads
Based on UBC 1630.1.1, 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,
Eh = the earthquake load due to the base shear, V
Ev = 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)
Load Combinations
Loads ACI 818-02 UBC-1997
Dead (D) and Live (L) 1.2 D + 1.6 L 1.32 D + 1.1 L
Dead (D), Live (L)
and Earthquake (E)
1.2 D + 1.0 L + 1.0 E 1.2 D + 1.0 L + 1.1 E
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.
Design of Ordinary Shear Walls
Shear Design
According to ACI 11.1.1, 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 11.10.3, 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 11.10.4
Ag = gross area of wall thickness
Nu = factored axial load
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 0.0025. 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 0.0025.
According to ACI 11.10.9.1, 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.
Design of Ordinary Shear Walls
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
Design of Ordinary Shear Walls (Contd.)
Reinforcement
Design of Ordinary Shear Walls (Contd.)
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