Aci 318 08-seismic-requirements-l e garcia

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Seismic design ACI 318-08

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Aci 318 08-seismic-requirements-l e garcia

  1. 1. ACI 318-08 - Seismic Requirements -- Luis E. Garcia Seismic Design Chapter 1 General Requirements Requirements in ACI 318-08 Modifications in By: Scope Luis Enrique García President American Concrete Institute – ACI – 2008-2009 Terminology Partner Proyectos y Diseños Ltda. Consulting Engineers Professor Universidad de los Andes Bogotá, Colombia R1.1.9 – Provisions for R1.1.9 – Provisions for earthquake resistance earthquake resistance In this version of ACI 318 (2008), for the first time, earthquake resistance requirements are Commentary was expanded to: defined in function of the Seismic Design Category — SDC required for the structure and not directly associated with the seismic risk zone. Explain changes in terminology used Simplify adoption and interaction of The minimum SDC to use is governed by the ACI 318-08 with model codes and other legally adopted general building code of which documents ACI 318 forms a part. 4 1
  2. 2. ACI 318-08 - Seismic Requirements -- Luis E. Garcia TABLE R1.1.9.1 — CORRELATION BETWEEN SEISMIC-RELATED TERMINOLOGY IN MODEL CODES Chapter 2 Code, standard, or resource document and edition Level of seismic risk or assigned seismic performance or design categories as Notation and Definitions defined in the Code ACI 318-08; IBC 2000, 2003; 2006; NFPA 5000, 2003, 2006; ASCE 7- SCD* SCS SCD There were important changes in 98, 7-02, 7-05; NEHRP 1997, 2000, A, B C D, E, F notation of the whole document and all 2003 individual Chapter notation was moved BOCA National Building Code to Chapter 2. 1993, 1996, 1999; Standard SPC† SPC SPC Building Code 1994, 1997, 1999; A, B C D; E ASCE 7-93, 7-95; NEHRP 1991, 7 93, 7 95; 1994 There are a few new definitions related to Chapter 21. All definitions, old and Uniform Building Code 1991, Seismic Zone Seismic Zone Seismic Zone new, were moved to Chapter 2. 1994, 1997 0, 1 2 3, 4 *SDC = Seismic Design Category as defined in code, standard, or resource document. †SPC = Seismic Performance Category as defined in code, standard, or resource document 5 Chapter 21 Seismic Design Category and Energy Dissipation Capacity Earthquake-resistant structures SDC Denomination Must comply with in Seismic D i S i i Design (Energy di i ti (E dissipation ACI 318-08 Category capacity) Chapter 21 was reorganized in function of A Chapters 1 to 19 and 22 Seismic Design Categories (SDC) A, B, C, and D, E, and F in incremental order from Ordinary ordinary to special: B Chapters 1 to 19, 22, and 21.2 Chapters 1 to 19, 22, A → B → C → D, E, F C Intermediate and 21.3 y 21.4 Chapters 1 to 19, 22, D, E, F Special And 21.5 to 21.13 2
  3. 3. ACI 318-08 - Seismic Requirements -- Luis E. Garcia ACI 318-08 – Chapter 21 Earthquake-resistant structures 21.1 – General requirements Content Scope 21.1 – General requirements 21.2 21 2 – Ordinary moment frames B 21.3 – Intermediate moment frames C Chapter 21 contains provisions considered 21.4 – Intermediate precast structural walls 21.5 – Flexural members of special moment frames to be the minimum requirements for a 21.6 – Special moment frame members subjected to bending and cast-in-place or precast concrete axial load structure capable of sustaining a series of 21.7 – Joints of special moment frames D oscillations into the inelastic range of 21.8 – Special moment frames constructed using precast concrete response without critical deterioration in 21.9 – Special structural walls and coupling beams E g strength. 21.10 – Special structural walls constructed using precast concrete 21.11 – Structural diaphragms and trusses F Therefore, the objective is to provide energy 21.12 – Foundations 21.13 – Members not designated as part of the seismic-force- dissipation capacity in the nonlinear resisting system range of response. TABLE R21.1.1 — SECTIONS OF CHAPTER 21 TO BE SATISFIED IN TYPICAL APPLICATIONS Component resisting Seismic Design Category (SDC) Global Energy Dissipation Capacity earthquake effect, unless A B C D (none) (21.1.1.4) (21.1.1.5) (21.1.1.6) Force elastic otherwise noted Maximum elastic maximum elastic Analysis and design Fe 21.1.2 21.1.2 21.1.2, 21.1.3 force demand displacement demand requirements Materials None None 21.1.4 21.1.7 nonlinear 21.5, 21.6, Frame members 21.2 21.3 21.7, 21.8 Maximum nonlinear Structural walls and Yield strength displacement demand coupling beams None None 21.9 Fy Precast structural walls None 21.4 21.4,† 21.10 None Structural diaphragms and None None 21.11 trusses uy ue um Displacement Foundations None None 21.12 Frame members not In several earthquake resistance Fe u proportioned to resist forces induced by None None 21.13 regulations this is defined through R = = e earthquake motions parameter R Fy uy Anclajes None 21.1.8 21.1.8 3
  4. 4. ACI 318-08 - Seismic Requirements -- Luis E. Garcia Elastic vs. Nonlinear Demand Current seismic design strategy 20 linear elastic nonlinear Given an energy dissipation capacity for the structural 10 material and structural system, defined through an R u 0 value depending of the detailing scheme the design (cm) horizontal seismic force is obtained from: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 -10 -20 time (s) Fe 0.8 Fy = 0.6 linear elastic R 0.4 nonlinear force 0 2 0.2 and the maximum elastic force demand is in turn 0 obtained using Newton’s 2nd Law: (1/W) -0.2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 -0.4 -0.6 Fe = m a s s × S a (T , ξ ) Acceleration response spectrum from the general building code -0.8 time (s) What would happen if energy dissipation capacity is not available? 4
  5. 5. ACI 318-08 - Seismic Requirements -- Luis E. Garcia C.21.1 – General Requirements Nonstructural wall panel in contact Compressive strength of concrete fc′ ≥ 21 MPa with the structure column Specified compressi e strength of light eight compressive lightweight Nonstructural wall concrete ≤ 35 MPa panel separated h from the structure For computing the amount of confinement reinforcement fyt ≤ 700 MPa (= 100,000 psi = 7000 kgf/cm2) ACI 318-08 requires (21.1.2) that interaction between structural and Reinforcing steel must meet ASTM A706. If ASTM nonstructural elements that may a ect t e response du g t e ea t qua e o st uctu a e e e ts t at ay affect the espo se during the earthquake A615 is used, it must meet: Must be taken into account. The actual yield strength based on mill tests does not Rigid members assumed not to be a part of the seismic-force-resisting exceed fy by more than 125 MPa. system are permitted provided their effect on the response of the system is The ratio of the actual tensile strength to the actual yield considered and accommodated in the structural design. strength is not less than1.25 Consequences of failure of structural and nonstructural members that are not a part of the seismic-force-resisting system shall be considered. 5
  6. 6. ACI 318-08 - Seismic Requirements -- Luis E. Garcia Reinforcing steel 21.2 – Ordinary moment frames σ stress MPa actual tensile strength Corresponds to SDC B σu Beams must have at least two continuous longitudinal bars along both top and bottom σy failure faces. These bars shall be developed at the face of support. c1 c2 actual yield strength Columns having clear height less than or equal to E maximum elongation five times the dimension c1 1 yield elongation must be designed for shear in accordance with 21.3.3. (shear requirements for intermediate SDC C) O ε εy strain ε max 21.3 - Intermediate moment frames 21.3 - Intermediate moment frames Requirements for this Section are equivalent to the rest of Chapter 21, but are less strict and have a Reinforcement details in a frame member lesser scope. p shall satisfy beam requirements if the y q factored axial compressive load, Pu , d f d i l i l d does Two alternatives are presented for shear design of not exceed Ag fc′ 10 . beams and columns: Obtain design shear forces as function of When Pu is greater reinforcing details must nominal end moments as done for special meet column requirements. elements, or use twice the shear from analysis This is analysis. equivalent to using the following load When a slab-column system without beams combinations: is part of the seismic-force-resisting U = 1.2D + 1.0L + (1.0E)x2.0 system, reinforcement details in any span resisting moments caused by E must U = 0.9D + (1.0E)x2.0 satisfy 21.3.6. 6
  7. 7. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.3 - Intermediate moment frames 21.3 - Intermediate moment frames For beams: For beams: Moment strength must comply with: g py At both ends of the beam, hoops shall be provided over lengths not less than 2h measured from the face of the supporting member toward midspan. The first hoop shall − be located not more than 50 mm from the face of the Mn − Mn supporting member. Spacing of hoops shall not exceed the 1 smallest of d/4, 8db of the smaller longitudinal bar, 24db Mn ≥ ⋅ ( Mn )max .face of hoop, or 300 mm. Stirrups shall be spaced not more 5 than d/2 throughout the length of the beam. + 1 − Mn ≥ Mn 2h @d/2 2h 3 21.3 - Intermediate moment frames 21.3 - Intermediate moment frames For columns Two-way slabs without beams (slab-column frames) At both ends of the column, hoops shall be provided at spacing so over Reinforcement provided to resist Mslab shall be placed a length 0 measured from the joint within th column strip. ithi the l ti face. Spacing so shall not exceed the smallest of 1/2 of the smallest Not less than 50% of the reinforcement in the column cross-sectional dimension of the strip at supports shall be placed within the effective column, 8db of the smallest slab width defined by lines drawn parallel to the span at 1.5 slab depths from the column face . longitudinal bar enclosed, 24db of the hoop bar, or 300 mm. Outside this length spacing must be the one Continuous bottom reinforcement in the column strip shall be not less than 33% of the top reinforcement at defined in Chapters 7 and 11. 11 the th support in the column strip. t i th l ti Length 0 shall not be less than the largest of the maximum cross- Not less than 25% of the top reinforcement at the sectional dimension of the column;, support in the column strip shall be continuous 1/6 of the clear span of the column, throughout the span. or 450 mm. 7
  8. 8. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.3 - Intermediate moment frames 21.3 - Intermediate moment frames Two-way slabs without beams (slab-column frames) At the critical sections for punching shear, shear caused by factored gravity loads shall not exceed, 0.4φVc where Vc must be calculated as defined in Chapter 11 for prestressed and non prestressed slabs. This requirement may be waived if the slab complies with 21.13.6 21.4 — Intermediate precast structural walls 21.5 — Flexural members of special moment frames Requirements of 21.4 apply to intermediate precast structural walls forming part of the t t t l ll f i t f th A i l force Pu must not exceed 0 10fc′ Ag Axial f t t d 0.10 seismic-force resisting systems. Clear span of element n must be larger In connections between wall panels, or than 4d between wall panels and the foundation, yielding must be restricted to steel elements Ratio bw/h > 0.3 or reinforcement.. Width bw must comply with: Elements of the connection that are not designed to yield must develop at least 1.5Sy. bw > 250 mm larger than the width of the supporting element plus 3h/4 at each side 8
  9. 9. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.5 — Flexural members of special 21.5 — Flexural members of special moment frames moment frames Longitudinal reinforcement Steel ratio for negative and positive reinforcement must not be less than: fc′ 1.4 ρ≥ ≥ 4 ⋅ fy fy but: ρ ≤ 0.025 At least two bars continuous top and bottom. 21.5 — Flexural members of special 21.5 — Flexural members of special moment frames moment frames Longitudinal reinforcement Longitudinal reinforcement Moment strength at each section must be at least: Lap splices are permitted if hoops are provided − Mn − Mn throughout the splice length. Maximum hoop spacing must not exceed d/4 or 100 mm. Mn ≥ 0.25 ⋅ ( Mn )max .face No lap splices are permitted in joints or within 2h of column face or where inelastic action is expected. t d + − Mn ≥ 0.5Mn 9
  10. 10. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.5 — Flexural members of special 21.5 — Flexural members of special moment frames moment frames Hoops must be provided: Shear design: ΔV Δ e 50 mm 50 mm − s ≤d/2 M pr n + M pr (M ) + pr izq . ( − + M pr ) der . (M ) − pr izq . ( + + M pr ) der . ΔVe = ΔVe = n n 2h 2h confinement Mpr computed using fypr = 1.25 fy and φ = 1.0 zones 21.5 — Flexural members of special 21.6 — Special moment frame members moment frames (Vu)vert. left (Vu)ver. right subjected to bending and axial load Pu1 Wu Pu2 General Vu(x) (Vu)ver. left + ΔVe ⎡( V ) − P ⎤ ⎢ u ver.izq. ( u ) ver.der. ∑ u ⎥ n 1 1 + V ⎣ ⎦ Axial force greater than 0.10 ⋅ fc′ ⋅ Ag (Vu)ver. left- ΔVe x The least section dimension that passes (Vu)vert. right - ΔVe through the centroid must be greater than th h th t id tb t th shear envelope 300 mm. (Vu)ver. right+ ΔVe For design, Vc = 0 if ΔVe is more than 50% Ratio b/h > 0.4 of required shear strength, or axial force is less than 0.05f’cAg 10
  11. 11. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.6 — Special moment frame members 21.6 — Special moment frame members subjected to bending and axial load subjected to bending and axial load Transverse reinforcement in confining zones must comply g py Column flexural strength must comply with: with: Spiral columns: f′ ρ s = 0.12 ⋅ c fyt ∑ Mnc ≥ 1.2∑ Mnb Columns with hoops: Mnc Mnc Mnc 0.3 ⋅ s ⋅ bc ⋅ fc′ ⎡⎛ Ag ⎞ ⎤ Mnb Mnb Mnb Mnc Ash = ⋅ ⎢⎜ ⎟ − 1⎥ fyt ⎢⎝ Ach ⎠ ⎥ ⎣ ⎦ Mnb Mnb Mnc Mnc Mnb Mnc Mnb Mnc Mnb Mnc Mnc 0.09 ⋅ s ⋅ bc ⋅ fc′ (a) (b) (c) Ash = fyt 21.6 — Special moment frame members 21.6 — Special moment frame members subjected to bending and axial load subjected to bending and axial load hx ≤ 350 mm Shear design Mpr (M ) ( ) hx hx hx joint transverse + M pr reinforcement as i f t required by 21.7 50 mm pr arriba abajo 0 Ve = hx b ⎧b / 4 hn ⎪ Ve confinement lap splices in s ≤ ⎨6d b long. hn zones central zone ⎪s Mpr corresponds to the maximum moment hc ⎩ 0 ⎧h strength for the axial load range on the ⎪ element (1.25fy and φ=1). Ve cannot be ≤ ⎨ hn 6 ⎧6d b long . ⎧ ⎛ 350 hx ⎞ 350-h 0 ⎪ 450 mm s≤⎨ less than the one obtained from analysis. ⎪100 + ⎜ ⎩ ⎩150 mm ⎪ ⎝ 3 ⎟ ⎠ 0 50 mm s0 = ⎨ For design Vc = 0 if Ve is more than 50% ⎪ ≤ 150 mm joint transverse of the required shear or the axial force ⎪ ≥ 100 mm ⎩ reinforcement as required by 21.7 is less than 0.05f’cAg Mpr 11
  12. 12. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.7 — Joints of special moment frames 21.7 — Joints of special moment frames Computation of the shear demand on the joint: General requirements Ve-col plane to evaluate M pr-c shear Vu column When computing shear strength within the joint in Ts′ = 1.25fy As ′ Cc = Ts = 1.25fy As special frames all longitudinal reinforcement must be presumed to be stressed at 1.25fy. Longitudinal reinforcement terminating at a joint must Cc = Ts′ = 1.25fy As ′ ′ Ts = 1.25fy As be extended to the far face of the column confined core and anchored in tension. beam When th beam l Wh the b longitudinal reinforcement passes it di l i f t Mpr-c Ve-col through the joint , the column dimension parallel to the Beam in both sides: Beam in one side: reinforcement cannot be less than 20db largest ⎧1.25fy ( As )viga − (Ve )col longitudinal bar, for normal weight concrete and 26db Vu = 1.25fy ( As + As )viga − (Ve )col ′ ⎪ ⎪ Vu ≥ ⎨ for lightweight concrete. ⎪ ⎪1.25fy ( As )viga − (Ve )col ⎩ ′ 21.7 — Joints of special moment frames 21.7 — Joints of special moment frames Definition of Aj Shear strength Joints confined in all four faces φ ⋅ Vn = φ ⋅ 1.70 ⋅ fc′ ⋅ A j Joints confined in three faces or in opposite faces h Aj bw φ ⋅ Vn = φ ⋅ 1.25 ⋅ fc′ ⋅ A j bw Other joints h Aj φ ⋅ Vn = φ ⋅ 1.00 ⋅ fc′ ⋅ A j bw x ⎧ bw + 2 x ≤⎨ ⎩ bw + h 12
  13. 13. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.7 — Joints of special moment frames 21.8 — Special moment frames constructed using precast concrete Development for hooks embedded in the The requirements of 21.8 apply for special q pp y p confined core moment frames built using precast critical concrete forming part of the seismic-force- section resistant system. dh The detailing provisions in 21.8.2 and 21.8.3 are intended to produce frames that fy ⋅ d b db respond to design displacements dh = esse t a y e o o t c special o e t essentially like monolithic spec a moment 5.4 fc′ frames. The provisions of 21.8.4 indicate that when not satisfying 21.8.2 or 21.8.3 they must satisfy the requirements of ACI 374.1 21.8 — Special moment frames 21.9 — Special structural walls constructed using precast concrete and coupling beams Special precast moment frames with ductile Terminology connections must comply with all ti t l ith ll requirements for special cast-in-place frames and Vn should not be less than 2Ve. h Special precast moment frames with strong connections are intended to experience flexural yielding outside the connections connections. hw These requirements are applicable independently of any of these two situations. w Vu 13
  14. 14. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.9 — Special structural walls and 21.9 — Special structural walls coupling beams – General requirements and coupling beams Cover Flexure design 20 mm Design for flexure and flexure and axial load for structural walls must be performed using the requirements of Chapter 10. Maximum s h reinforcement spacing s s s Concrete and developed longitudinal reinforcement within effective flange widths boundary elements, widths, elements s ≤ 3h and the wall web shall be considered effective. s ≤ 450 mm s The effects of openings shall be considered. s 21.9 - Special structural walls and 21.9 - Special structural walls and coupling beams coupling beams Flexure design 21.9.2 21 9 2 – Reinforcement Unless a more detailed analysis is performed, The distributed web reinforcement ratios, ρt effective flange widths of flanged sections ( I, L, C and ρ , for structural walls shall not be less or T) may be supposed to extend from the face of than 0.0025, except that if Vu does not exceed the web a distance equal to the smaller of: 0.083A cv λ fc′ (MPa) = 0.27A cv λ fc′ (kgf/cm2), ρt and ρ , may be reduced to the values given (a) 1/2 the distance to an adjacent wall web, in14.3. and Separation of reinforcement must not exceed (b) 25 percent of the total wall height. 450 mm 14
  15. 15. ACI 318-08 - Seismic Requirements -- Luis E. Garcia Minimum steel ratio Difference between wall 14.3.2 – Minimum steel ratio of vertical reinforcement ρ computed over gross section is: and column 0.0012 0 0012 for f deformed bars not larger than Nº 5 ( / ) ó 16M f º (5/8”) (16 mm), with fy not less than 420 MPa. 14.3.6 — Vertical reinforcement 0.0015 for other deformed bars. need not be enclosed by lateral ties 0.0012 for welded wire reinforcement with diameter not if vertical reinforcement area is not larger than16 mm. 14.3.3 - Minimum ratio of horizontal reinforcement area to greater than 0.01 times gross gross concrete area, ρt: concrete area, or where vertical 0.0020 for deformed bars not larger than Nº 5 (5/8”) ó 16M reinforcement is not required as (16 mm), with fy not less than 420 MPa. compression reinforcement. 0.0025 for other deformed bars. 0.0020 for welded wire reinforcement with diameter not larger than16 mm. 21.9 - Special structural walls and 21.9 - Special structural walls and coupling beams coupling beams Nominal shear st e gt must not exceed: o a s ea strength ust ot e ceed At least two curtains of reinforcement l tt t i f i f t Vn = Acv ⎡α c λ fc′ + ρ n fy ⎤ ⎣ ⎦ must be used in a wall if Vu exceeds αc 0.17λ A cv fc′ (MPa) = 0.53λ A cv fc′ where αc is: 0.25 0.17 (kgf/cm2) hw 0 0.5 1.0 1.5 2.0 2.5 w 15
  16. 16. ACI 318-08 - Seismic Requirements -- Luis E. Garcia Wall boundary elements Displacement-based boundary element Boundary elements must be placed at edges and procedure in ACI 318 (21.9.6.2) around openings when inelastic response is This procedure is based on the compressive expected. expected ACI 318-08 gives two alternatives to strain demand at edges of wall when the wall is define if boundary elements are needed: deformed under the maximum expected lateral 1) Section 21.9.6.2 presents a displacement- displacement caused by the design earthquake based procedure. Boundary elements are ground motion. needed or not depending on the compressive Section 21.9.6.2 is based on the assumption that strain at the edge of wall caused by the inelastic response of the wall is dominated by seismic lateral deflection, or flexural action at a critical, yielding section. 2) Section 21.9.6.3 requires boundary elements The wall should be proportioned so that the when the compressive stress at the edge of critical section occurs at the base of the wall and wall caused by the seismic forces exceeds a is applicable only to walls continuous from base threshold value. to top of the structure. Displacement-based boundary element Nonlinear response of a wall procedure in ACI 318 (21.9.6.2) P δ The wall should have a single critical section under flexure and axial load at the base of the wall. The zones of the wall in compression must be provided with specially reinforced boundary elements when the depth of the θ p neutral axis at nominal strength, c, is greater than: c≥ w and δu Plastification ≥ 0.007 p length ⎛ δu ⎞ hw 600 ⋅ ⎜ ⎟ 0 0 Wall Mu My Mcr φu φ y φcr ⎝ hw ⎠ section Moment Curvature 16
  17. 17. ACI 318-08 - Seismic Requirements -- Luis E. Garcia Nonlinear response of wall Nonlinear wall deflection Using Moment-area theorems it is possible to show that the Curvature Deflection Nonlinear Nonlinear lateral deflection caused by curvature up to yield (green at yield at yield curvature deflection (δu−δy) w zone) is: δy b hw and the additional deflection caused by nonlinear rotation (orange zone) is: p θp φy (φu − φy) The total deflection is: Total lateral deflection is then: p φ a (φu− φy) φy We can solve for the ultimate curvature demand and obtain: φu Moment-curvature diagram for wall section What happens at section? M Ultimate curvature demand Mn At level of εcu displacement φu demand Strain At level of εc = 0.003 nominal εs > εy φn strength c εc < 0.003 εs = εy φy At level of cy Mcr yield in tension of extreme h 0 φ reinforcement φcr φy φn φu w 17
  18. 18. ACI 318-08 - Seismic Requirements -- Luis E. Garcia Equation (21-8) deduction Equation (21-8) deduction The rotation at the plastic hinge when the displacement The concrete strain at the extreme fiber in compression demand (δu) takes place is: at ultimate is: We can then obtain the strain at ultimate for the With a plastic hinge length equal to half the wall horizontal displacement demand: length: and Then the curvature at the wall base when the displacement demand occurs is: The value of c for a ultimate strain of εcu = 0.003 is: Equation (21-8) deduction Need for boundary elements in If a 600 value parameter is used instead of 666 in last equation displacement-based procedure ε ε and it is solved for cu a value of cu = 0.0033 is obtained, If equation (21-8) indicates that the value which in turn leads to the following equation: of c is exceeded this is a symptom that exceeded, strains greater than εcu = 0.0033 must be expected and the need to confine the edge of the wall is warranted in order to If the maximum strain at the extreme compression fiber exceeds prevent spalling of the concrete there. εcu = 0.0033 then the value of c obtained from last equation would be exceeded. Thus the form ACI 318 presents it: exceeded In that case ACI 318 prescribes the same If c is greater than the value obtained type and amount of confining transverse boundary elements must be placed reinforcement that for columns. along the length where it is exceeded and a little more. 18
  19. 19. ACI 318-08 - Seismic Requirements -- Luis E. Garcia Boundary elements Displacement-based boundary element displacement-base procedure procedure in ACI 318 (21.9.6.32 Boundary elements must be placed Mn from the critical section up for a εcu distance not less than the larger of w o εs 0.003 Mu/(4Vu). The evaluation is performed for the wall c when subjected to the nonlinear Region where horizontal design displacements boundary corresponding to the design elements must be provided earthquake. The value of δu corresponds to the nonlinear roof horizontal displacement. Stress-based boundary element procedure in ACI 318 (21.9.6.3) Stress-based boundary Boundary elements must be provided at edges element procedure in and around openings of walls when the maximum p g ACI 318 (21 9 6 3) (21.9.6.3) Pu stress at the extreme fiber in compression caused by factored loads that include seismic Mu effects exceeds 0.2 fc′ unless that whole wall is Pu Mu confined as a column. Ptu = Pu − Ag ( Mu − 300 mm ) ≤0 Pcu = + 2 ( w − 300 mm ) w Pu M u ⋅ w fcu = + > 0.2 ⋅ fc′ This procedure had been part of ACI 318 since Ag Iw ⋅ 2 the 1971 version. version In the 1999 version of 318 a modification was The boundary elements can be discontinued introduced in which the need to resist all flexural when the compression stress is less than 0.15 fc′ forces from seismic effects with just the boundary elements was suppressed. 19
  20. 20. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.9 - Special structural walls and Old (pre-1999) procedure coupling beams Boundary elements – Both procedures Boundary elements w resisting all flexural i ti ll fl l When boundary elements are needed (under any of the two procedures) these boundary elements must extend effect that include horizontally from the maximum compression fiber a heb seismic forces distance equal to the greater of : c-0.1 w or c/2. In section with flanges the boundary element must include Pu the effective flange width and must extend at least 300 mm into the web. Transverse reinforcement must be that required for column, Mu but there is no need to comply with equation 21-3. py q P Mu Pu Mu Ptu = u − ≤0 Pcu = + Special transverse reinforcement in the boundary element Ag ( 2 ( − heb ) w − h eb ) w must extend into the foundation element supporting the wall. φ ⋅ P0n = φ ⋅ [0.85 ⋅ fc′ ⋅ (A g − Ast ) + A st ⋅ f y ] Wall horizontal transverse reinforcement must be anchored φ ⋅ Ptn = φ ⋅ A st ⋅ f y into the confined boundary element core. φ ⋅ Pn(max) ≤ 0.80 ⋅ φ ⋅ P0n 21.9 - Special structural walls and coupling beams Coupling beams In ACI 318-08, there are modifications in the requirements for coupling beams in walls. 20
  21. 21. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.10 — Special structural walls constructed using precast concrete Scope— These requirements apply to special structural walls constructed using precast t t l ll t t d i t concrete forming part of the seismic-force- resisting system. Special structural walls constructed using precast concrete shall satisfy all requirements of special cast-in-place structural walls plus those of section 21.10. Special structural walls constructed using precast concrete and unbonded post- tensioning tendons and not satisfying the requirements of 21.10.2 are permitted provided they satisfy the requirements of ACI ITG-5.1. 21.11 — Structural diaphragms and trusses 21.11 — Structural diaphragms and trusses This section contains: prescribed horizontal Requirements for slabs-on-grade , floor and roof slabs forces when they are part of the seismic-force-resisting system must comply with this section. Minimum thickness for diaphragms are given. Gives minimum reinforcement for diaphragms. floor Indicates shear strength for these elements diaphragm Defines when boundary elements must be used in diaphragms. Includes requirements for construction joints within the diaphragm. 21
  22. 22. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.12 — Foundations 21.13 — Members not designated as part of the seismic-force-resisting system This section contains: This section is a response to the extended practice by 21.12.1 — Scope - Foundations resisting earthquake structural designers of designating arbitrarily some of the induced forces or transferring earthquake-induced i d df f i h k i d d structural elements as being parte of the seismic-force- forces between structure and ground. resisting system and part not. Northridge Earthquake 21.12.2 — Footings, foundation mats, and pile caps – affecting the City of Los Angeles in 1994 pointed out great Gives requirements for the anchoring of reinforcement deficiencies in this practice. In ACI 318-95 this section was in vertical elements of the seismic-force-resisting totally revised and it was updated in 1999, 2002, 2005 , and system to these foundation elements. now in 2008. 21.12.3 — Grade beams and slabs-on-ground – Sets minimum dimension s and minimum reinforcement for u d e so a d u e o ce e t o In essence it is a call to the designer to check the these elements, deformation levels that so called “non participating” 21.12.4 — Piles, piers, and caissons – Indicates the type elements are subjected and the minimum reinforcement of effects to take into account in design and the they should comply with. minimum reinforcement allowable for these elements. 21.13 — Members not designated as part of 21.13 — Members not designated as part of the seismic-force-resisting system the seismic-force-resisting system This Section contains two procedures to check non- p participating elements that are not part of the seismic- This section includes new requirements for slab- column frames that are not part of the seismic- force-resisting system: force-resisting system. • When the forces induced by the design displacement Slab-column frames have shown repeatedly their combined with the gravity forces do not exceed the vulnerability under seismic demands. This design strength of the elements, this section indicates vulnerability is specially associated with the the minimum reinforcement to use. punching shear strength of the slab-column joint. • If the strength is exceeded the sections of Chapter 21 The new procedure in ACI 318-08 (Section that are mandatory for these elements are indicated. 21.13.6) indicates when shear reinforcement must be provided in the slab-column joint as a function of the story drift. 22
  23. 23. ACI 318-08 - Seismic Requirements -- Luis E. Garcia 21.13 — Members not designated as part of 21.13 — Members not designated as part of the seismic-force-resisting system the seismic-force-resisting system Story drift cannot exceed the larger of: 0.005 or ⎛ Vug ⎞ ⎜ 0.035 − 0.05 ⎟ ⎝ φVc ⎠ where Vug is the factored gravity punching shear demand and Vc is the punching shear strength. The End 23

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