structures conference required in design.ppt
- 1. 2007 Structures Congress Selected New Provisions of ASCE/SEI 7-2005: Jim Harris J. R. Harris & Company Denver, Colorado May 18, 2007 John Hooper Magnusson Klemencic Associates Seattle, Washington
- 2. 2007 Structures Congress Seismic Design Under ASCE 7-2005 2 Overview • Reorganization • New maps; Long period map • New systems, revised R factors and limitations • Diaphragm assumptions • Redundancy factor • Dynamic analysis triggers • Near fault spectral shape applicability • Modal response spectrum analysis • Simplified design method
- 3. 2007 Structures Congress Seismic Design Under ASCE 7-2005 3 ASCE 7-05 Reorganization Goals of Seismic Section Reorganization 1. To improve clarity and use 2. Reduce depth of section numbering from 6 max typical to 4 max typical (i.e. Sec. 9.5.2.5.2.2 is now Sec. 12.5.3) 3. Simplify table and figure numbering (i.e. Table 9.5.2.5.1 is now Table 12.6-1) 4. Create logical sequence of provisions aim at the structural engineering community 5. Improve headings and clarify ambiguous provisions
- 4. 2007 Structures Congress Seismic Design Under ASCE 7-2005 4 ASCE 7-05 Reorganization 1. Changed major subjects to Chapters rather than Sections (similar to the IBC) 2. Replaced Section 9 with Chapters 11-23 3. Incorporated the material appendices 4. Put the Chapters into a logical sequence 5. Rewrote ambiguous headings 6. Examined and rewrote sections to eliminate ambiguity 7. Provided Cross Reference Table C-11-1…02 to 05
- 5. 2007 Structures Congress Seismic Design Under ASCE 7-2005 5 Comparison of Contents ASCE 7-2002 Sections 1. General 2. Combinations of Loads 3. Dead Loads 4. Live Loads 5. Soil and Hydrostatic …and Flood Loads ASCE 7-2005 Chapters 1. General 2. Combinations of Loads 3. Dead Loads, Soil … and Hydrostatic 4. Live Loads 5. Flood Loads
- 6. 2007 Structures Congress Seismic Design Under ASCE 7-2005 6 Comparison, continued ASCE 7-2002 Sections 6. Wind Loads 7. Snow Loads 8. Rain Loads 9. Earthquake Loads 10. Ice Loads A. Supplemental (QA) B. Serviceability ASCE 7-2005 Chapters 6. Wind Loads 7. Snow Loads 8. Rain Loads 9. (not used) 10. Ice Loads 11. - 23. Seismic A & B. QA & Existing
- 7. 2007 Structures Congress Seismic Design Under ASCE 7-2005 7 Seismic Contents ASCE 7-2002 9.1 General Provisions 9.2 Definitions/ Symbols 9.3 (not used) 9.4 Ground Motion 9.5 Structural Design Criteria, Analysis, and Procedures ASCE 7-2005 11. Seismic Design Criteria 12. Seismic Design Requirements for Building Structures 13. Seismic Design Requirements for Nonstructural Comp.
- 8. 2007 Structures Congress Seismic Design Under ASCE 7-2005 8 Seismic Contents, continued ASCE 7-2002 9.6 Arch, Mech, Elect Comp and Sys 9.7 Foundations 9.8 Steel 9.9 Concrete 9.10 Composite Struct. 9.11 Masonry ASCE 7-2005 14. Material Specific 15. Nonbuilding Structures 16. Response Hist Anal 17. Seismic Isolation 18. Damping Systems 19. Soil-Struct. Interact.
- 9. 2007 Structures Congress Seismic Design Under ASCE 7-2005 9 Seismic Contents, continued ASCE 7-2002 9.12 Wood 9.13 Seismic Isolation 9.14 Nonbuilding Structures A9. Quality Assurance ASCE 7-2005 20. Site Classification 21. Site-Specific Ground Motions 22. Maps 23. Reference Docs 11A. Quality Assurance 11B. Existing Buildings
- 10. 2007 Structures Congress Seismic Design Under ASCE 7-2005 10 11 Seismic Design Criteria 11.1 General 11.2 Definitions 11.3 Notation 11.4 Seismic Ground Motion Values 11.5 Importance Factor 11.6 Seismic Design Category 11.7 Design Req’ts for Category A 11.8 Geologic Hazards & Geotechnical Invest.
- 11. 2007 Structures Congress Seismic Design Under ASCE 7-2005 11 11.1 Purpose “…specified earthquake loads are based upon post-elastic energy dissipation in the structure, and because of this fact, the requirements for design, detailing, and construction shall be satisfied even for structures and members for which load combos w/o EQ exceed those with EQ…”
- 12. 2007 Structures Congress Seismic Design Under ASCE 7-2005 12 11.4 Seismic Ground Motions 1 Determine basic values from maps for bedrock conditions 2, 3 Classify soil conditions at site and determine site coefficients 4 Determine site-adjusted values 5 Take two-thirds for use in design 6 Construct design response spectrum 7 Site-specific studies permitted/required
- 13. 2007 Structures Congress Seismic Design Under ASCE 7-2005 13 Mapped Acceleration Parameters • Two updated sets of basic maps for the response spectrum accelerations – SS for spectral response acceleration at 0.2 secs – S1 for spectral response acceleration at 1.0 secs • New map for long period transition: TL in seconds
- 14. 2007 Structures Congress Seismic Design Under ASCE 7-2005 14 SS and S1 are the mapped 2% in 50 year spectral accelerations for firm rock SDS and SD1 are the design level spectral accelerations (modified for site and “expected good performance”) Ground Motion Parameters & Seismic Hazard Mapped Contours of SS
- 15. 2007 Structures Congress Seismic Design Under ASCE 7-2005 15 General Comparison of Maps With ASCE 7-02 • Changes everywhere, but mostly minor • Deterministic area around New Madrid With UBC 97 • Lots of change • Lower in most areas • Higher in high hazard areas, except near fault in California • Three maps, not one
- 16. 2007 Structures Congress Seismic Design Under ASCE 7-2005 16 Long Period Transition Maps (Fig 22.15)
- 17. 2007 Structures Congress Seismic Design Under ASCE 7-2005 17 Site Specific Studies • Clarification of two types: – Basic ground motion hazard at a point in rock – Site amplification in overburden soil • First type never required, but permitted; limits placed upon results • Second type encouraged; required in some instances
- 18. 2007 Structures Congress Seismic Design Under ASCE 7-2005 18 Cumulative Nature of Provisions by Seismic Design Category A B C D E F
- 19. 2007 Structures Congress Seismic Design Under ASCE 7-2005 19 Seismic Design Category A • 11.7 is a self-contained section; defines E • Horizontal force = 1% of dead load • Load path for horizontal forces – connections = 5% of weight of smaller part • Beam, truss connections = 5% D + L • Anchor concrete and masonry walls – 280 pounds per foot
- 20. 2007 Structures Congress Seismic Design Under ASCE 7-2005 20 Geologic Hazards and Geotechnical Investigations • SD Category E and F: – Do not locate on active fault • SD Category C: – Evaluate slope instability, liquefaction, differential settlement, surface displacement • SD Category D, E, F: – More detail than C plus lateral pressures on basement walls and retaining walls
- 21. 2007 Structures Congress Seismic Design Under ASCE 7-2005 21 Overview • Reorganization • New maps; Long period map • New systems, revised R factors and limitations • Diaphragm assumptions • Redundancy factor • Dynamic analysis triggers • Near fault spectral shape applicability • Modal response spectrum analysis • Simplified design method
- 22. 2007 Structures Congress Seismic Design Under ASCE 7-2005 22 12 Seismic Design of Building Structures 1 Design Basis 2 Structural Systems 3 Diaphragm Flexibility; Configuration; & Redundancy 4 Load Effects & Combinations of Loads 5 Direction of Loading 6 Selection of Analysis 7 Modeling Criteria 8 ELF Method 9 Modal RS Method 10 Diaphragms, Chords Collectors 11 Structural Walls (out-of-plane) 12 Drift and Deformation 13 Foundation Design 14 Simplified Alternate
- 23. 2007 Structures Congress Seismic Design Under ASCE 7-2005 23 12.1.1 Basic Requirements • Strength, Stiffness, Energy Dissipation • Design Motion in Any Horizontal Direction • Construct Mathematical Model • Evaluate Model for Effects – Limitations on methods of evaluation – Modifications to internal forces (R, 0) – Modifications to deformations (Cd) • Alternate Procedures Must Be Consistent
- 24. 2007 Structures Congress Seismic Design Under ASCE 7-2005 24 Minimum Connection Forces • Very Similar to 11.7 for SD Category A • Continuous Load Path: “…from point of application to final point of resistance…” – 0.133 SDS WP 0.05 WP – Does not apply to overall design of SFRS (Seismic Force Resisting System) • Beams, Trusses to Support 5% of D + L
- 25. 2007 Structures Congress Seismic Design Under ASCE 7-2005 25 12.2 Structural System Requirements 1. System identification and limitations 2. Combos of systems: different direction 3. Combos of systems: same direction 4. Combos of systems: detailing 5. Specific system requirements
- 26. 2007 Structures Congress Seismic Design Under ASCE 7-2005 26 System Identification • “Basic lateral and vertical SFRS shall conform to one (or a permitted combo) of the systems from Table 12.2-1…” • “Selected SFRS shall be designed and detailed per referenced requirements…” • SFRS not from table permitted only if analytical and test data establish basis
- 27. 2007 Structures Congress Seismic Design Under ASCE 7-2005 27 Basic system parameters Obtain from table and use in seismic design: • R Response Modification Factor - measure of system inelastic capabilities • Cd Deflection Amplification Factor - increase elastic to total o System Overstrength Factor - accounts for actual strength greater than design strength; used to protect vulnerable items.
- 28. 2007 Structures Congress Seismic Design Under ASCE 7-2005 28 R factor comparisons System 97 02 05 Special RC Shear Wall (bearing) 4.5 5 5 Ordinary RC Shear Wall (bearing) 4.5 4 4 Special RC Shear Wall (bldg frm) 5.5 6 6 Ordinary RC Shear Wall (bldg frm) 5.5 5 5 Intermed Precast Shear Wall (b f) -- -- 5 Ord Precast Shear Wall (b f) -- -- 4
- 29. 2007 Structures Congress Seismic Design Under ASCE 7-2005 29 R factor comparisons System 97 02 05 Special RM Shear Wall (bearing) 4.5 5 5 Intermed RM Shear Wall (bearing) 4.5 3.5 3.5 Ord RM Shear Wall (bearing) 4.5 2 2 Special RM Shear Wall (bldg frm) 5.5 5.5 5.5 Intermed RM Shear Wall (bldg frm) 5.5 4 4 Ord RM Shear Wall (bldg frm) 5.5 2.5 2.5
- 30. 2007 Structures Congress Seismic Design Under ASCE 7-2005 30 R factor comparisons System 97 02 05 Special Steel Concentric BF 6.4 6 6 Ordinary Steel Concentric BR 5.6 6 3.25 Special RC Shear Wall (bldg frm) 5.5 6 6 Ordinary RC Shear Wall (bldg frm) 5.5 5 5 Eccentrically Braced Frame (with) 7 8 8 Eccentrically Bracked Frame (w/out) 7 7 7
- 31. 2007 Structures Congress Seismic Design Under ASCE 7-2005 31 R factor comparisons System 97 02 05 Light Frame with SWP (bearing) 5.5 6 6.5 Light Frame with other (bearing) 4.5 2 2 Light Frame with SWP (bldg frm) 6.5 6.5 7 Light Frame with other (bldg frm) 5 2.5 2.5 Light Frame with straps (bearing) 2.8 4 4
- 32. 2007 Structures Congress Seismic Design Under ASCE 7-2005 32 Height Limits • Most shear wall and braced frame systems limited to 160 feet high in SD Categories D and E, and to 100 feet in SD Category F • These limits can be increase to 240 feet and 160 feet, respectively for some structures – No line resists more than 60% of base shear – Torsional force < 20% of total force in the line • Many exceptions, especially for nonbuilding
- 33. 2007 Structures Congress Seismic Design Under ASCE 7-2005 33 Concrete Shear Wall - Frame • Limited to SD Category B • Ordinary detailing for wall and frame • Analyze for interaction and provide as a minimum – Walls strong enough for 0.75 Vx at each story – Frames strong enough for 0.25 Vx at each story
- 34. 2007 Structures Congress Seismic Design Under ASCE 7-2005 34 12.3 Diaphragms, Configuration, and Redundancy 1. Diaphragm Flexibility in Analysis 2. Identification of Irregularities in System Configuration 3. Limitations on and Penalties for Irregularities 4. Redundancy – Significant changes from prior edition
- 35. 2007 Structures Congress Seismic Design Under ASCE 7-2005 35 Diaphragm Flexibility Assume Flexible if • Wood or steel deck with concrete or masonry walls • 1 or 2 family residential if light frame • Compute ΔDia > 2 * δvert Assume Rigid if • Concrete slab (or filled deck) with span to depth < 3 and no horizontal irregularity Otherwise: • Must analyze system including actual stiffness of diaphragm!
- 36. 2007 Structures Congress Seismic Design Under ASCE 7-2005 36 Configuration: Basic Parameters Size Proportion Shape
- 37. 2007 Structures Congress Seismic Design Under ASCE 7-2005 37 Plan Irregularities 1a Torsional irregularity corner > 1.2 center 1b Extreme torsional irregularity corner > 1.4 center Note: torsional irregularity not checked for flexible diaphragms 2 Re-entrant corners Both projections > 15% of respective sides 3 Diaphragm discontinuity 50% change in a level or from level to level 4 Out-of-plane offsets absolute 5 Nonparallel systems absolute Type Measure
- 38. 2007 Structures Congress Seismic Design Under ASCE 7-2005 38 Vertical Irregularities 1a Stiffness-Soft Story Story stiffness < 70% above 1b Stiffness- Extreme Soft Story Story stiffness < 60% above 2 Weight (Mass) More than 150% adjacent story Note: 1 and 2 dropped if no story drift exceeds 130% of story above 3 Vertical Geometric Length of SFRS >130% of that in adjacent story 4 In-Plane Discontinuity Offset > length of element or a reduction in stiffness below 5a Capacity-Weak Story Lat strength < 80% of above 5b Extreme Weak Story Lat strength < 65% of above Type Measure
- 39. 2007 Structures Congress Seismic Design Under ASCE 7-2005 39 Configuration Limitations • Horiz 1b not permitted in SD Cat E+ • Vert 1b, 5a not permitted in SD Cat E+ • Vert 5b not permitted in SD Cat D+ • Vert 5b limited to 2 stories or 30 feet in SD Cat B or C, unless weak story strength capable of 0 times design force
- 40. 2007 Structures Congress Seismic Design Under ASCE 7-2005 40 Configuration Penalties • Horiz 4 and Vert 4 (column, slab, beam, or truss elements supporting discontinuous elements) to resist 0 force (all SD Categories) • Horiz 1, 2, 3, 4 and Vert 4 have 25% increase in force for connection of diaphragm to vert element and collectors in SD Cat D+; also req’d for collectors except those already designed for 0 force
- 41. 2007 Structures Congress Seismic Design Under ASCE 7-2005 41 Overview • Reorganization • New maps; Long period map • New systems, revised R factors and limitations • Diaphragm assumptions • Redundancy factor • Dynamic analysis triggers • Near fault spectral shape applicability • Modal response spectrum analysis • Simplified design method
- 42. 2007 Structures Congress Seismic Design Under ASCE 7-2005 42 Redundancy factor Reliability Factor 1.3 or 1.0 ρ 1.0 ρ Seismic Design Category B or C D, E or F is always 1.0 for drift and P-delta calcs and for design of: • Nonstructural components • Nonbuilding structures not similar to buildings • Members designed for 0 forces • Diaphragms • Structures with damping systems
- 43. 2007 Structures Congress Seismic Design Under ASCE 7-2005 43 Redundancy Factor = 1.3 Unless following loss does not an extreme torsional irregularity and does not reduce story strength by more than 33%: • Braced frame: removal of a single brace • Moment frame: loss of moment resistance at both ends of a single beam (or at base of a single cantilever column) • Shear walls: removal of any single pier with h/l > 1.0 (or collector to such a pier)
- 44. 2007 Structures Congress Seismic Design Under ASCE 7-2005 44 12.6 Analysis Method Selection Methods Defined • Equivalent (Static) Lateral Force: ELF • Modal Response Spectrum: MRS • Seismic Response History (Linear and Nonlinear): SRH (Defined in section 16) Alternate classifications: Static / Dynamic Max / Dynamic History Linear / Nonlinear
- 45. 2007 Structures Congress Seismic Design Under ASCE 7-2005 45 What type of Analysis? • The answer depends on: – what performance level you are hoping to achieve – the configuration of the structure – how accurate you need to be • A wide range of choices are available-
- 46. 2007 Structures Congress Seismic Design Under ASCE 7-2005 46 Superior Performance Levels • Behavior will be essentially elastic – For regular structures with short periods, linear static procedures are fine – For regular structures with long periods and all irregular structures - linear dynamic procedures are better, response spectra accurate enough Joe’s Beer! Beer! Food! Food! Beer! Beer! Food! Food! Joe’s
- 47. 2007 Structures Congress Seismic Design Under ASCE 7-2005 47 Poorer Performance Levels • Inelastic behavior is significant (elastic analyses are the wrong approach!) – For structures dominated by first mode response, pushover analysis may be adequate – For structures with significant hire mode response, nonlinear time history necessary Beer! Beer! Food! Food! Joe’s
- 48. 2007 Structures Congress Seismic Design Under ASCE 7-2005 48 Methods Permitted • SD Cat B and C: any defined method • SD Cat D+: ELF permitted for – Occ Cat I/II < 3 stories – Occ Cat I/II of light frame < 4 stories – Reg structures with T < 3.5 TS – Reg structures of light frame any T – Irreg structure with T < 3.5 TS limited to horiz types 2, 3, 4, or 5 and vert types 4, 5a, or 5b • Other SD Cat D+ must use MRS or SRH
- 49. 2007 Structures Congress Seismic Design Under ASCE 7-2005 49 Design Response Spectrum 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 1 2 3 4 5 6 7 TS T0 Period, seconds Spectral Acceleration, g 0.4SDS Sa = SD1 / T Sa = SDS(0.4 + 0.6 T/T0) Sa = SD1 TL / T2 Drawn for SS = 1.0, Fa = 1.0 S1 = 0.4, Fv = 1.5 TL = 4
- 50. 2007 Structures Congress Seismic Design Under ASCE 7-2005 50 Design Response Spectrum 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 1 2 3 4 5 6 7 TS T0 Period, seconds Spectral Acceleration, g 0.4SDS Sa = SD1 / T Sa = SDS(0.4 + 0.6 T/T0) Sa = SD1 TL / T2 Drawn for SS = 1.0, Fa = 1.0 S1 = 0.4, Fv = 1.5 TL = 4 Sa = 0.5 S1
- 51. 2007 Structures Congress Seismic Design Under ASCE 7-2005 51 12.8 ELF Method of Analysis 1. Seismic Base Shear: V = CSW 2. Period Determination 3. Vertical Distribution of Seismic Forces 4. Horizontal Distribution of Forces 5. Overturning 6. Story Drift Determination 7. P-Delta Effects
- 52. 2007 Structures Congress Seismic Design Under ASCE 7-2005 52 Seismic Coefficient Basic rules are the design spectrum adjusted for R and I: Also, where S1 > 0.6: minimum) new ( 01 . 0 2 1 1 s L D D DS s C I R T T S I R T S I R S C I R S Cs 1 5 . 0
- 53. 2007 Structures Congress Seismic Design Under ASCE 7-2005 53 Adjustments to Base Shear • Soil-Structure Interaction per Section 19 is permitted • Low rise buildings in high ground motion areas: If stories < 6 and T < 0.5 seconds Can use SS = 1.5 max
- 54. 2007 Structures Congress Seismic Design Under ASCE 7-2005 54 12.8.2 Period of Vibration • Follow modeling criteria in 12.7 to compute T • Upper bound for forces: a uT C T SD1 Cu > 0.4 1.4 0.3 1.4 0.2 1.5 0.15 1.6 <0.1 1.7
- 55. 2007 Structures Congress Seismic Design Under ASCE 7-2005 55 Approximate Period x n t a h C T Structure Ct x 100% Moment Frames: Steel Concrete 0.028 0.016 0.8 0.9 Eccentrically Braced 0.03 0.75 All others 0.02 0.75
- 56. 2007 Structures Congress Seismic Design Under ASCE 7-2005 56 What is hn? – Concept of Base hn hn Base RC frame RC wall Masonry wall RC wall Base
- 57. 2007 Structures Congress Seismic Design Under ASCE 7-2005 57 Alternate Estimates for Ta • 100% moment frames up to 12 stories with story heights at least 10 feet: , Ta = 0.1N • Shear walls of concrete or masonry: n w a h C T 0019 . 0 x i i i i i n B w D h A h h A C 1 2 2 83 . 0 1 100
- 58. 2007 Structures Congress Seismic Design Under ASCE 7-2005 58 Vertical Distribution of Force Equivalent static force at level x: V C F vx x n i k i i k x x vx h w h w C 1 where wi , wx: Portion of W assigned to level i or x hi , hx: Height of level i or x above base k sets the shape of distribution and depends on T Story Shear: n x i i x F V
- 59. 2007 Structures Congress Seismic Design Under ASCE 7-2005 59 Shape of Vertical Distribution 1 ≤ k ≤ 2: Varies with T For T ≤ 0.5, k = 1 (linear distribution) For T ≥ 2.5, k = 2 (parabolic distribution; impact of higher modes) For 0.5 < T < 2.5, k = 2 or k = 0.75 + T/2 (interpolation)
- 60. 2007 Structures Congress Seismic Design Under ASCE 7-2005 60 ELF - Story Shears F V n x i i x F V Sum the story forces from the top down Distribution of story forces intended to give proper envelope of maximum story shears for a regular building. It does not give envelope of maximum story forces.
- 61. 2007 Structures Congress Seismic Design Under ASCE 7-2005 61 ELF - Horizontal Distribution • Distribute story shear to vertical elements per relative stiffness of vertical elements and diaphragm • Account for computed (inherent) torsion - eccentricity between mass and resistance • Add accidental torsion, except for flexible diaphragms • Amplify torsion if torsionally irregular
- 62. 2007 Structures Congress Seismic Design Under ASCE 7-2005 62 Torsional Irregularity For S.D. Category C, D, E or F accidental eccentricity must be multiplied by Ax 0 . 3 2 . 1 2 max avg x A where avg max min extreme irregular avg 4 . 1 2 . 1 max
- 63. 2007 Structures Congress Seismic Design Under ASCE 7-2005 63 ELF - Overturning Moments . Story force times height to level under consideration. • Overestimate where higher modes are significant • Prior “codes” allowed up to a 20% reduction in tall buildings • Now require modal analysis for such structures, thus this provision is now deleted • Moment can be reduced 25% at foundation, permitting some rocking F V M x i n x i i x h h F M
- 64. 2007 Structures Congress Seismic Design Under ASCE 7-2005 64 Definition of Drift I C xe d x Structural displacement, where, xe Elastic deflection calculated from design forces d C Deflection amplification factor Importance factor I No reduction for ASD, but, can ignore limit on T
- 65. 2007 Structures Congress Seismic Design Under ASCE 7-2005 65 Stability: P-Δ Effects P Δ Deflection introduces P-Δ moment which increases deflection, which increases moment ….. Structure must be designed to prevent collapse due to P-Δ effects
- 66. 2007 Structures Congress Seismic Design Under ASCE 7-2005 66 Stability: P-Δ Effects • Determine stability coefficient, θ, for each story d sx x x C h V P • If θ > 0.10 at any level, then all design forces and moments must be increased by factor 1+ad 1 d a where • Check 25 . 0 5 . 0 max d C
- 67. 2007 Structures Congress Seismic Design Under ASCE 7-2005 67 Derivation of Stability Factor θ δf h V P k V P V δ Undeformed A Deformed B Response C Equilibrium at B: 0 h k P Vh f f Define: 0 , o f f o h V P Vh then V k Rearranging terms: 1 1 o o o f Vh P
- 68. 2007 Structures Congress Seismic Design Under ASCE 7-2005 68 P-Delta • What if your analysis program “includes” P-Delta and you don’t want to make a second set of output? max must still be checked • Compute * from displacements that include P-Delta, then max * 1 *
- 69. 2007 Structures Congress Seismic Design Under ASCE 7-2005 69 12.9 Modal Response Spectrum Analysis Method 1. Minimum Number of Modes 2. Adjustment of Response Parameters by R/I (forces) and Cd/I (displacements) 3. Combining Modes for Total Response 4. Scaling of Design Values 5. Horizontal Shear Distribution, Torsion 6. P-Delta 7. Soil Structure Interaction
- 70. 2007 Structures Congress Seismic Design Under ASCE 7-2005 70 M.R.S. Analysis • Include enough modes to obtain a combined modal mass participation of at least 90% of the actual mass in each of the orthogonal directions of response • Short period branch of spectrum is usable • Divide spectrum by (R/I) to obtain force responses • Multiply each displacement by (Cd/I)
- 71. 2007 Structures Congress Seismic Design Under ASCE 7-2005 71 Combining Modal Results • Basic rule is Square Root of Sum of Squares (SRSS) • Complete Quadratic Combination (CQC) always permitted (see ASCE 4) • CQC required where modal periods are closely spaced or where translational and torsional modes are cross correlated
- 72. 2007 Structures Congress Seismic Design Under ASCE 7-2005 72 Design Response Spectrum 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 1 2 3 4 5 6 7 TS T0 Period, seconds Spectral Acceleration, g 0.4SDS Sa = SD1 / T Sa = SDS(0.4 + 0.6 T/T0) Sa = SD1 TL / T2 Drawn for SS = 1.0, Fa = 1.0 S1 = 0.4, Fv = 1.5 TL = 4
- 73. 2007 Structures Congress Seismic Design Under ASCE 7-2005 73 Scaling Modal Results • Compute limiting base shear, V, by ELF; if T exceeds CuTa, then use T = CuTa • Compare 85% of this force with combined modal base shear, Vt • If Vt < 0.85V then multiply all combined response quantities from modal analysis by 0.85V / Vt
- 74. 2007 Structures Congress Seismic Design Under ASCE 7-2005 74 M.R.S. Analysis • Critical direction of load applies (orthogonal combinations) • Inherent torsion automatically included • Accidental torsion: two choices: – Offset mass to achieve accidental eccentricity - Include static torsion as a load case • P-Delta applies as for ELF • Soil Structure Interaction analysis permitted
- 75. 2007 Structures Congress Seismic Design Under ASCE 7-2005 75 12.12 Drift and Deformation 1. Story Drift – Satisfy limits per table; occupancy is factor – If irregular, include torsion effect in SD Cat C+ – Divide allowable by for MF in SD Cat D+ 2. Diaphragm Deflection 3. Building Separation 4. Compatibility for SD Category D+
- 76. 2007 Structures Congress Seismic Design Under ASCE 7-2005 76 Drift Ratio Limits Structure Occupancy Category UBC I or II III IV 4 stories, no masonry 0.025 0.020 0.015 0.025* Masonry cantilever 0.010 0.010 0.010 Other masonry 0.007 0.007 0.007 All other 0.020 0.015 0.010 0.020*
- 77. 2007 Structures Congress Seismic Design Under ASCE 7-2005 77 Deformation Compatibility Applies to • SD Category D+ • All structural components not in SFRS • Check capacity for gravity load combined with effects induced from design drift; rational analysis of restraint required • ACI 318 Chap 21 acceptable alternate
- 78. 2007 Structures Congress Seismic Design Under ASCE 7-2005 78 Overview • Reorganization • New maps; Long period map • New systems, revised R factors and limitations • Diaphragm assumptions • Redundancy factor • Dynamic analysis triggers • Near fault spectral shape applicability • Modal response spectrum analysis • Simplified design method
- 79. 2007 Structures Congress Seismic Design Under ASCE 7-2005 79 12.14 Simplified Alternate 1. General (Limitations/Eligibility) 2. Design Basis (& Load Combinations) 3. SFRS Identification (& Combinations) 4. Diaphragm Flexibility 5. Direction of Loading 6. Design & Detailing: Load path connections, collectors, wall anchorage 7. ELF Analysis
- 80. 2007 Structures Congress Seismic Design Under ASCE 7-2005 80 Limitations • Occupancy Category I or II • Site Class A, B, C, or D • 1, 2, or 3 stories • Bearing Wall or Building Frame System – Braced frames or shear walls – No unbraced (moment) frames • “Regular”
- 81. 2007 Structures Congress Seismic Design Under ASCE 7-2005 81 Torsional Regularity • Flexible diaphragms: – Overhang (cantilever) < depth / 5 – Controls displacement at edge – Controls torsion in non-flexible • Non-flexible diaphragm – Eccentricity < 15% width of diaphragm – Minimum torsional stiffness
- 82. 2007 Structures Congress Seismic Design Under ASCE 7-2005 82 Torsion m i i n j j j m i i i k b b e d k d k 1 1 2 1 1 1 1 2 2 2 1 2 1 1 ) 05 . 0 ( 5 . 2 •k1i - lateral stiffness, wall “i” parallel to 1 •k2j - lateral stiffness, wall “j” parallel to 2 •d1i, - the distance from the wall “i” to the center of rigidity, perpendicular to axis 1 •d2j is the distance from the wall “j” to the center of rigidity, perpendicular to axis 2 •e1 is the distance perpendicular to axis 1 between the center of rigidity and the center of mass •b1 is the width of the diaphragm perpendicular to axis 1 •m is the number of walls in direction 1 •n is the number of walls in direction 2
- 83. 2007 Structures Congress Seismic Design Under ASCE 7-2005 83 Regularity • Limit skewed alignments to 15 degrees • Use simplified method for design in both horizontal directions • No in-plane or out-of-plane offsets – Exception: shear walls in 2 story light frame – Must use Ω0 ( = 2.5 for all structures) • No weak stories (80% rule)
- 84. 2007 Structures Congress Seismic Design Under ASCE 7-2005 84 Seismic Design Category • Limited to Occupancy groups I and II • Only use SDS, therefore SDS < 0.33 gives Category B SDS < 0.50 gives Category C SDS > 0.50 gives Category D • Can have Category E if S1 is high
- 85. 2007 Structures Congress Seismic Design Under ASCE 7-2005 85 Combined Effects • Vertical Seismic Load = • Combine positive vertical seismic load where gravity and effect of horizontal seismic add; combine negative vertical seismic load where gravity offsets effect of horizontal seismic • Orthogonal combinations not required 0 2 . S W DS
- 86. 2007 Structures Congress Seismic Design Under ASCE 7-2005 86 R Factor Table • Includes wood shear walls, all concrete and masonry walls and all steel bracing systems – special, ordinary, intermediate, plain, detailed, etc – and composite steel/concrete walls • Includes specific citations to reference standards for detailing the systems • No Cd or Ω0 factors here
- 87. 2007 Structures Congress Seismic Design Under ASCE 7-2005 87 Detailed Rules • Connections: 0.2SDSwi or 0.05wi • Openings and corners in shear panels • Collectors: Ω0 = 2.5 (except light frame) • Diaphragms: – use the story force – provide continuous ties • Anchor concrete/masonry walls (flexible)
- 88. 2007 Structures Congress Seismic Design Under ASCE 7-2005 88 Detailed Rules • No orthogonal combinations • Redundancy factor = 1.0 • Bearing walls: out of plane = 0.4SDSwc • Nonstructural components – same as any other building
- 89. 2007 Structures Congress Seismic Design Under ASCE 7-2005 89 Simplified ESF Analysis • Base Shear • Story Force • Story factor F = 1.0 for one story 1.1 for two stories 1.2 for three stories • Same acceleration at all levels of building (very simple!) • No I factor, No period T V F S R W DS F F S R w i DS i
- 90. 2007 Structures Congress Seismic Design Under ASCE 7-2005 90 Simplified Response Acceleration • Use only SDS; don’t use S1S • Equation • Site response amplifier Fa = 1.0 for rock 1.4 for soil S F S DS a S 2 3
- 91. 2007 Structures Congress Seismic Design Under ASCE 7-2005 91 Drift and Separation • Do not have to check drift • Use 1% drift for purposes of building separation, nonstructural component compatibility, etc, unless actually computed
- 92. 2007 Structures Congress Seismic Design Under ASCE 7-2005 92 Overview • Introduction to Earthquake Engineering • Basic Criteria in ASCE 7 – 2005 – Ground Motions – Response Spectrum – Occupancy and Seismic Design Categories • Seismic Design for Buildings – Basic Requirements – System Requirements – Analysis – Diaphragms, Walls, Foundations – Simplified Method
- 93. 2007 Structures Congress Seismic Design Under ASCE 7-2005 93 Errata • Go to the SEI website www.seinstitute.org • Go to the “Publications” in the bar at the top • Click on the “Errata” tab • Download pdf files for ASCE 7-05 (and any other structural standards you may need)
Editor's Notes
- #18: Cumulative nature of Provisions. Since SDC based on SUG and Sa, Provisions address detailing for performance and design load levels. Large jump in requirements from A to B and from C to D. SDC A - seismic design not required, but structural integrity highly recommended. SDC D - higher seismic areas and where structure's use fills vital public need. (May turn back slide to stress these points.)
- #27: Framing system in buildings in SPC B and above must be classed and appropriate R, 0, and Cd factors (Table 3.3) selected. R, Response Modification Factor, is a measure of ability of structure to dissipate earthquake energy inelastically; varies widely based on material and system. 0 is a system overstrength factor used to predict the maximum force that might develop in a structure subject to strong ground motion and designed according to the Provisions. Cd factor used to magnify displacements from elastic analysis for reduced forces to reflect expected total deflection, elastic and inelastic. The estimate for R is intended to be somewhat of a lower estimate. The estimate for the other two is intended to be somewhat of an upper estimate.
- #35: Note: this will be discussed in more detail in another lesson devoted to diaphragms. Diaphragm flexibility is defined for several common cases. However, this definition does not cover all structures. Some cases with intermediate flexiblity should be checked both ways. Also refer to example in Guide of one story frame with steel deck. These definitions (essentially) appeared in the 1988 UBC, but many professionals specializing in wood framed structures were not cognizant of that fact. Following the publishing of design examples by SEAOC in about 1998, a controversy arose in the profession. It is likely that wood diaphragms supported by wood shear walls will be allowed to be analyzed as if the diaphragm were flexible.
- #36: All issues affect seismic forces. Specific classification of irregularities based on shape used for requirements in Provisions. Regularity or irregularity of building determines minimum analysis procedure. Recall ELF based upon ideally uniform building. Irregularity does not affect analysis procedures until SDC D; but, irregularities must be identified even in SPCs B and C, since some irregularities create requirements on subjects other than analysis.
- #37: Building irregular if has at least one of irregularities listed. Design reference sections for plan irregularities are as noted on slide. Each type of irregularity illustrated in a subsequent lesson.
- #38: Building irregular if has at least one of irregularities listed. Vertical irregularities can cause serious problems; can create force distributions which vary significantly from distribution described in Chapter Four (based on regular structure). Each type of irregularity illustrated in a subsequent lesson.
- #42: The quantitative requirement for redundancy is termed a reliability requirement. Note that it works two ways: a force penalty is imposed on systems with less than desirable redundancy, and a minimum amount of redundancy is required for certain systems and SDC’s.
- #43: Northridge earthquake experience was impetus for introducing . There were many moment resisting frames with one bay per frame and two frames per direction, thus giving a system with poor redundancy.
- #56: Since many calculations depend on h, height above base, definition of base of structure greatly affects design force determination. NEHRP Provisions provide definition in Chapter Two: "the level at which the horizontal seismic ground motions are considered to be imparted to the building." Dynamic response is key factor; gross changes in stiffness, such as foundation wall, can be considered the base. Examples on slide show that grade level is not necessarily base of structure. Configuration of basements affect determination of base: (a) shows flexible superstructure on rigid foundation; (b) shows structure with negligible change in stiffness at grade. See also example in Chapter 3 of Guide.
- #59: When k = 1, vertical distribution of lateral forces is linear; when k = 2, distribution is parabolic. For periods between 0.5 and 2.5 seconds, k = 0.75+.5T. Note: k used to account for higher mode effects.
- #60: Present Sections 5.3.5, distribution of shears and moments. Note: this distribution of story forces gives envelope of story shears from modal analyses of uniform and regular buildings. Story forces are not themselves an envelope.
- #61: Wording established before age of 3D frame analysis. Amplification factor for torsion is similar to a ratio of spectral acceleration to ground acceleration. It applies only for rigid diaphragm buildings. For buildings with the vertical elements of the lateral force resisting system concentrated in a core area, this amplification can be very significant.
- #62: 3 dimensional modal analysis automatically accounts for amplification of computed torsion, but not accidental. Maximum Ax = 3.0.
- #63: Note: ELF story shears are upper bounds; may produce overly conservative estimates of overturning moment. Thus, reductions in overturning moment have been traditionally allowed. The period limit of 3.5 Ts for use of the ELF method effectively renders such a device unnecessary.
- #64: Note: drift defined only on a story basis, not an overall basis.
- #65: Introduce P-delta effect.
- #66: Note: Cd in denominator means that inelastic displacements are ignored; stability analysis based on initial elastic stiffness. Upper limit on theta (thetamax) in provisions introduced to address this problem. thetamax = 0.5/(Cd) < 0.25 but not shown on slide. It is based upon stability at full residual displacement.
- #67: Procedure only takes into account structural P-delta effect; local, "P-", effect missed since derivation ignores curvature in vertical member (thus underestimates true response). For frame with rigid girders and flexible columns, could be as much as 22% greater.
- #73: Note: in general, the base shear for modal analysis less than that for ELF. Dynamic analysis is preferred over static analysis. This particular comparison has changed in the 1997 edition.
- #74: Most standard analysis programs can do these items. Note: many analysts include accidental torsion by applying horizontal moment rather than actually offsetting mass, but this depends on the capability of the particular software. In addition to masses, 3-D analysis with the rigid diaphragm assumption requires calculation of mass moments of inertia. This is illustrated in Chapter 3 of Guide.