ASCE 41‐13 Seminar Analysis Procedures ASCE 41-13 Seismic Evaluation and Retrofit of Existing Buildings Analysis Procedures Robert Pekelnicky, PE, SE Degenkolb Engineers, San Francisco Some material courtesy of Mark Moore, SE ASCE 41-13 Analysis Provision Linear Static Procedure (LSP) Linear Dynamic Procedure (LDP) – Response Spectrum – Response History Method Nonlinear Static Procedure (NSP) Nonlinear Dynamic Procedure Reproduction not permitted without permission 1 ASCE 41‐13 Seminar Analysis Procedures ASCE 41 Building Analysis Displacement-based philosophy Capacity design principles Tie the building together Primary and secondary components Component Designation Primary Components • • • Main lateral force-resisting elements Must be included in the analysis Can be existing or new elements Secondary Components (< 25% total stiffness) • • • • • Typically existing elements Do not contribute to the lateral strength and/or stiffness Do support gravity load Must accommodate structures deformations Can yield provided gravity load support not lost Reproduction not permitted without permission 2 ASCE 41‐13 Seminar Analysis Procedures Component Designation Nonstructural Components • • Cladding and partitions Shall be considered structural elements if their stiffness exceeds 10% of the total stiffness of the primary system at that story Primary or Secondary 7-Story Concrete Building Perimeter concrete moment frame Flat-slab interior gravity framing Slab-column gravity systems resist 54% of longitudinal and 72% of transverse base shear! Reproduction not permitted without permission 3 ASCE 41‐13 Seminar Analysis Procedures Capacity-Based Design Designate specific elements to yield, which are called Deformation-Controlled elements Every other element of the structure should not yield, rupture, or otherwise fail, those are called Force-Controlled elements Structure dissipates seismic energy through controlled yielding No brittle failures occur which could lead to instability Displacement Based Design V = C1C2CmSaW Elastic Response C1 C2 Pseudo Lateral Force CmSaW Force Yield Capacity, or QCE Inelastic Structural Response Displacement Reproduction not permitted without permission Sd Expected Maximum (Target) Displacement (t = C0C1C2Sd) 4 ASCE 41‐13 Seminar Analysis Procedures Displacement Based Design CP,s= 0.75CP,s Force / Moment Qud CP,p= 0.75CP,p IO,p= 0.67CP,p mIO= 0.75IO,p/y mLS,p= 0.75LS,p/y Qce mCP,p= 0.75CP,p/y mLS,s= 0.75LS,s/y mCP,s= 0.75CP,s/y y IO,p/s u LS,s CP,s LS,p CP,p Displacement / Rotation Deformation Compatibility • Secondary elements must be checked at maximum displacement of primary elements in linear procedures for earthquake induced deformations and gravity loads • Secondary elements MUST be modeled in nonlinear analysis per Section 7.2.3.3 Mathematical models for use with nonlinear procedures shall include the stiffness and resistance of primary and secondary components. The strength and stiffness degradation of primary and secondary components shall be modeled explicitly. Reproduction not permitted without permission 5 ASCE 41‐13 Seminar Analysis Procedures Knowledge Factor • Factor to account for uncertainty in collection of asbuilt information • κ = 0.75 or 1.0 depending on data collection requirements • Additional value of κ = 0.90 for minimum data collection with material strengths listed in design documents – Life Safety or lower performance level – Linear analysis procedures Damping Requirements • 5% of critical unless otherwise specified (10% for wood with cross-walls) • Damping for response spectra and damping of building system • All provisions combined and moved to one section to avoid confusion (see Section 7.2.3.6) • Damping of building system dependent on selected analysis procedure • More guidance and additional references provided for user Reproduction not permitted without permission 6 ASCE 41‐13 Seminar Analysis Procedures Linear Analysis Procedures Similar to new building design No R-factor, use unreduced loads Evaluated each deformation-controlled element specifically with an m-factor Qud ≤ m**Qce or DCR = Qud/(*Qce) ≤ m M-factors based on component ductility and performance level = Knowledge factor Force-controlled elements designed for capacity of deformation-controlled elements Limitations on Use of Linear Procedures Not permitted for two defined types of irregularity • Weak Story – Total DCR above less than 125% Total DCR below • Torsion – Total DCR on one side of center of rigidity 150% Total DCR on the other side Unless all DCR < 3.0 and associated component mfactor Increase from 2.0 in ASCE 41-06 Reproduction not permitted without permission 7 ASCE 41‐13 Seminar Analysis Procedures Limitations on Use of Linear Procedures Linear Static Procedure not permitted when: • • • • • T greater than 3.5Ts Abrupt changes in lateral system dimensions Soft story condition Story torsional stiffness irregularity Nonorthogonal lateral systems Out-of-Plane Wall Strength & Anchorage Diaphragm Anchorage Out‐of‐Plane Strength Force-controlled actions Revised design force equations Equivalency to ASCE 7-10 ka factor used to account for diaphragm flexibility 1.0 ≤ ka ≤ 2.0 kh factor used to account for force distribution over height of buildings with rigid diaphragms Reproduction not permitted without permission 8 ASCE 41‐13 Seminar Analysis Procedures Nonlinear Analysis Procedures Nonlinear Static Procedure • Displace the structure to the maximum estimated roof displacement • Permit yielding and force redistribution • Evaluate nonlinear deformation of each component versus specified limits Nonlinear Dynamic Procedure • Use actual or simulated EQ ground motions • Simulates structures response to the earthquake • Evaluate nonlinear deformation of each component versus specified limits Nonlinear Analysis Procedures • NSP vs. NDP – NSP permitted to be used when μstrength < μmax and higher mode effects are insignificant • Deformation-Controlled Actions – Material chapters with modeling parameters and deformation limits • Force-Controlled Actions – Capacity based design (limit state analysis) Reproduction not permitted without permission 9 ASCE 41‐13 Seminar Analysis Procedures Nonlinear Analysis Procedures Reclassification of Force-Controlled Actions • Force-controlled components may be treated as Type 3 deformation curve in nonlinear procedures • Must maintain adequate gravity load path to ensure local stability • All remaining components still must meet acceptance criteria for given performance objective Nonlinear Analysis Procedures Consideration of Torsional Effects • Nonlinear 3D building model • Minimize number of analysis load cases • Accidental torsion need only to be included for higher Seismic Hazard Level (2 or more levels) • Displacement amplification factor η can be calculated for individual component actions instead of entire diaphragm level Reproduction not permitted without permission 10 ASCE 41‐13 Seminar Analysis Procedures Nonlinear Analysis Procedures Gravity Loads • Only one load combination D+L+S Nonlinear Static Procedure • Main procedure essentially unchanged • Simplified NSP removed • NIST Study cautions on use of NSP Reproduction not permitted without permission 11 ASCE 41‐13 Seminar Analysis Procedures Nonlinear Dynamic Procedure • Significant advances in nonlinear response history analysis methods since FEMA 273 • Earthquake shaking represented by ground motion acceleration histories • Clarification and more enforceable provisions to aid the AHJ or peer reviewer • NDP requires considerable engineering judgment and experience Nonlinear Dynamic Procedure • New requirements for inclusion of concurrent seismic effects – Near-field vs. far-field sites for 3D models • Required number of earthquake record pairs and rotation • Can reduce large number of analysis load cases if appropriate • New rules for determination of forces and deformations Reproduction not permitted without permission 12 ASCE 41‐13 Seminar Analysis Procedures Nonlinear Dynamic Procedure Nonlinear Dynamic Procedure Acceleration history scaling • Use SRSS of pairs • Average of all SRSS pairs greater than target spectrum • Spectral matching permitted • Alternate scaling procedures permitted with peer review Reproduction not permitted without permission 13 ASCE 41‐13 Seminar Analysis Procedures Nonlinear Dynamic Procedure ASCE 41 Provides this: But this is needed! Nonlinear Dynamic Procedure ASCE 41 Provides this: Reproduction not permitted without permission But this is needed! 14 ASCE 41‐13 Seminar Analysis Procedures Nonlinear Dynamic Procedure Additional Resources ASCE 41-13 Seismic Evaluation and Retrofit of Existing Buildings Your Thoughts & Questions? Reproduction not permitted without permission 15
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