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PCI 6 th Edition

PCI 6 th Edition. Building Systems (Seismic). Presentation Outline. Building System Loads Seismic Structural Integrity LFRS – Walls LFRS – Frames Diaphragms. Seismic Changes. Based on new changes to ASCE 7 and ACI 318 Based current seismic research and observations. Seismic Changes.

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PCI 6 th Edition

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  1. PCI 6th Edition Building Systems (Seismic)

  2. Presentation Outline • Building System Loads • Seismic • Structural Integrity • LFRS – Walls • LFRS – Frames • Diaphragms

  3. Seismic Changes • Based on new changes to ASCE 7 and ACI 318 • Based current seismic research and observations

  4. Seismic Changes • Some of these changes are: • Recognition of jointed panel construction • Recognition of strong and ductile connections in precast frames • Recognition and requirements for connections in precast walls

  5. Seismic Changes • Additional changes are: • Modification of drift computation and limiting drift • Deformation compatibility of elements • Additional soil type classifications • Special considerations locations near seismic faults • Consideration of redundancy and reliability in strength design requirements

  6. Seismic Changes • Design Forces are Based on Risk • Previous codes based on 10% chance of exceedance in 50 years • IBC 2000, 2003 codes based on 2% chance of exceedance in 50 years

  7. Seismic Risk • Soil factors • Other regions of high seismic risk - not just west coast anymore

  8. Practically every precast, prestressed concrete structure designed under IBC 2000 will require some consideration of seismic effects.

  9. Seismic Performance Objectives • Current design - minor damage for moderate earthquakes • Accepts major damage for severe earthquakes • Collapse is prevented of severe events

  10. Seismic Performance Objectives In order to achieve the design objectives, the current code approach requires details capable of undergoing large inelastic deformations for energy dissipation.

  11. Seismic Design Approach • Emulation • No special requirements for low seismic risk • Chapter 21 requirements for moderate and high seismic risk • Non-emulative design • PRESSS • Acceptance criteria for frames

  12. Earthquake Loads – Equivalent Lateral Force Method • Base Shear, V V= Cs·W Where: Cs - Seismic Response Coefficient W - Total Weight

  13. Equivalent Lateral Force Method Limitations • This method may not apply to buildings with irregularities in Seismic Design Categories D, E, or F

  14. Earthquake Loads – Total Weight, W • Dead Load of structure plus: • 25% of reduced floor live load in storage areas • live load in parking structures not included • Partition load if included in gravity dead • Total weight of permanent equipment • 20% of flat roof snow load, pf where pf > 30 psf

  15. Seismic Response Coefficient, Cs • Function of • Spectral response acceleration • Site soil factors • Building Period • Response modification factors • Importance factor

  16. Seismic Response Coefficient, Cs • Step 1 - Determine SS and S1 • Step 2 - Determine site Soil Classification • Step 3 - Calculate Response Accelerations • Step 4 - Calculate the 5% Damped Design Spectral Response Accelerations • Step 5 - Determine the Seismic Design Category • Step 6 - Determine the Fundamental Period • Step 7 - Calculate Seismic Response Coefficient

  17. Step 1 – Determine SS and S1 • From IBC Map • From local building codes • IBC 2003 CD-ROM • Based on • Longitude / Latitude • Zip Code

  18. Step 2 – Determine Site Soil Classification • If site soils are not known use Site Class D • Figure 3.10.7 (a) (page 3-111) • From soil reports

  19. Step 3 – Calculate Response Accelerations • SMS = Fa·SS • SM1 = Fv·S1 Where: • Fa and Fv are site coefficients from Figure 3.10.7 (b) and (c) (page 3-111) • SS spectral accelerations for short periods • S1 spectral accelerations for 1-second period • All values based on IBC 2003

  20. Step 4 – Calculate the 5%-Damped Design Spectral Response Accelerations • SDS = (2/3)SMS • SD1 = (2/3)SM1

  21. Step 5 – Determine the Seismic Design Category • Table 3.2.4.1. • Sometimes this restricts the type of Seismic Force Resisting System (SFRS) used (see Figure 3.10.8) (page 3-112)

  22. Step 6 – (Approximate Period) Determine the Buildings Fundamental Period Where: Ct = 0.016 for moment resisting frame systems of reinforced concrete 0.020 for other concrete structural systems x = 0.9 for concrete moment resisting frames 0.75 for other concrete structural systems hn = distance from base to highest level (in feet)

  23. Step 6 – (Exact Period) Determine the Buildings Fundamental Period Rayleigh’s formula Where: wi = dead load weight at Floor i δi = elastic displacement at Floor i Fi = lateral force at Floor i g = acceleration of gravity n = total number of floors

  24. Step 7 – Determine Seismic Response Coefficient, Cs Lesser of Where: R = Response Modification Factor Figure 3.10.8 (page 3-112) Ι = Seismic Importance Factor

  25. Step 7 – Determine Cs Minimum Value of Cs Special Cases In Seismic Design Categories E and F Cs = 0.044·SDS·Ι

  26. Vertical Distribution of Lateral Force Where: Fx = Force per floor Cvx = Vertical distribution factor V = Base shear k = 1 - buildings with a period ≤ 0.5 sec = 2 - buildings with a period > 2.5 sec hi and hx = height from base to Level i or x wi and wx = Level i or x portion of total gravity load

  27. Location of Force in Plane • Accidental Torsion • calculated by assuming that the center of mass is located a distance of 5% of the plan dimension perpendicular to the applied load on either side of the actual center of mass • Total torsion = sum of the actual torsion plus the accidental torsion

  28. Elastic Displacement Amplification Factor, dx Stability Coefficient Limits, q P-D Effects Seismic Drift Requirements

  29. Drift Limits • Figure 3.10.9 (page 3-113)

  30. Drift Amplification Factor, dx Where: δx = Amplified deflection of Level x δxe = Deflection of Level x determined from elastic analysis, includes consideration of cracking Cd = Deflection amplification factor (Figure 3.10.8) Ι = Seismic Importance Factor

  31. Stability Coefficient, θ Where: Px = Total vertical unfactored load including and above Level x ∆ = Difference of deflections between levels x and x-1 Vx = Seismic shear force acting between levels x and x-1 hsx = Story height below Level x Cd = Deflection amplification factor

  32. Stability Coefficient, θ The stability coefficient is limited to: Where: β = ratio of shear demand to shear capacity between Levels x and x-1

  33. P-D Effects • To account for P-∆ effects, the design story drift is increased by (1− θ)-1 • If θ < 0.10, P-∆ effects may be neglected

  34. Reliability Factor, ri • Required in High Seismic Design Categories D, E, and F • The Earthquake Force is increase by a Reliability Factor, ri • 1.5 Maximum Required Value ri =1.0 for structures in Seismic Design Categories A, B and C

  35. Reliability Factor, ri For Moment Frames Where, for each level: Ai = floor area rmaxi = For moment frames, the maximum of the sum of the shears in any two adjacent columns divided by the story shear. For columns common to two bays with moment-resisting connections on opposite sides, 70% of the shear in that column may be used in the column shear summary.

  36. Reliability Factor, ri For Shear Walls Where, for each level: Ai = floor area rmaxi = For shear walls, the maximum value of the product of the shear in the wall and 10/lw divided by the story shear.

  37. U = 1.4(D+F) U = 1.2(D+F+T) + 1.6(L+H) U = 1.2D +1.6(Lr or S or R) + (1.0L or 0.8W) U = 1.2D + 1.6W + 1.0L + 0.5(Lr or S or R) U = 1.2D + 1.0E + f1L + 0.2S U = 0.9D + 1.6W + 1.6H U = 0.9D + 1.0E + 1.6H f1 = 1.0 Parking garages = 1.0 Live load ≥ 100 psf on public assembly floors = 0.5 All others Load Combinations

  38. Modification for Vertical Acceleration Notice Building weight increase as Ground move Up • E = ρ·QE ± 0.2·SDS·D Seismic Load Combinations Become • U = (1.2 + 0.2·SDS)D + ρ·QE + f1L + 0.2S • U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H • Where • QE = Horizontal Seismic Force

  39. Modification for Vertical Acceleration Notice Building weight decreases as Ground move Down • E = ρ·QE ± 0.2·SDS·D Seismic Load Combinations Become • U = (1.2 + 0.2·SDS)D + ρ·QE + f1L + 0.2S • U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H

  40. Overstrength Factor, Wo • Components within the Diaphragm • Chord ties • Shear Steel • Connectors • Ωo = 2.0 - Seismic Design Categories C, D, E and F • Ωo = 1.0 - Seismic Design Categories A and B

  41. Special Load Combinations • U = 1.2D + fi·L + Em • U = 0.9D + E Where: Em = Wo·QE + 0.2·SDS·D and Wo = Overstrength Factor

  42. Overstrength Factor, Wo • Connections from Diaphragms to Seismic Force Resisting System (SFRS) • Ωo = Seismic Design Categories C and higher Figure 3.10.8 (page 3-112)

  43. Structural Integrity Requirements • All members must be connected to the Lateral Force Resisting System (LFRS) • Tension ties must be provided in all directions • The LFRS is continuous to the foundation • A diaphragm must be provided with • Connections between diaphragm elements • Tension ties around its perimeter • Perimeter ties provided • Nominal strength of at least 16 kips • Within 4 ft of the edge • Column splices and column base connections must have a nominal tensile strength not less than 200Ag in pounds

  44. Structural Integrity Requirements • Precast vertical panels connected by a minimum of two connections • Each connection is to have a nominal strength of 10 kips • Precast diaphragm connections to members being laterally supported must have a nominal tensile strength not less than 300 lbs per linear ft • Connection details allow volume change strains • Connection details that rely solely on friction caused by gravity loads are not to be used

  45. Lateral Force Resisting Systems (LFRS) • Rigid frames and shear walls exhibit different responses to lateral loads

  46. Influential Factors • The supporting soil and footings • The stiffness of the diaphragm • The stiffness LFRS elements and connections • Lateral load eccentricity with respect to center of rigidity of the shear walls or frames

  47. Shear Wall Systems • Most common lateral force resisting systems • Design typically follows principles used for cast-in-place structures

  48. International Building Code(IBC)Requirements • Two categories of shear walls • Ordinary • Special

  49. ACI 318-02 Requirements • Created an additional intermediate category, but has assigned no distinct R, Ωo and Cd

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