Tall Building Initiative Response Evaluation Helmut Krawinkler Professor Emeritus Stanford University On behalf of the Guidelines writers: Y. Bozorgnia, C.B. Crouse, R.O. Hamburger, R. Klemencic, H. Krawinkler, J.O Malley, J.P. Moehle, F. Naeim, J.P. Stewart Quake Summit 2010, October 8, 2010

Performance Objectives Demonstrate that structure will be capable of essentially elastic response and limited damage under Service-level Earthquake shaking (mean RP = 43 years = 50/30) Demonstrate, with high confidence, that structure will respond to Maximum Considered Earthquake (MCE) shaking without loss of gravity-load-carrying capacity without inelastic straining of important lateral-force resisting elements to a level that will severely degrade their strength; and without experiencing excessive permanent lateral drift or development of global structural instability.

Quake Summit 2010, October 8, 2010 MCE Level Evaluation Objective: provide, implicitly, adequate life safety protection Protection against collapse Protection against life threatening falling hazards Protection against aftershocks & condemnation Use 3-D nonlinear response history analysis – for at least 7 ground motion pairs Use a realistic model of the structural system Follow capacity design principles (enforced in acceptance criteria) Minimum base shear not required

Quake Summit 2010, October 8, 2010 Acceptance Criteria at Component Level Force-controlled actions with severe consequences: F u ≤ F n,e F u = smaller of 1.5 times mean Mean + 1.3 but ≥ 1.2 times mean F n,e = nominal strength based on expected material properties  = resistance factor

Quake Summit 2010, October 8, 2010 Acceptance Criteria at Component Level Deformation controlled actions: No specific limitations, but use realistic model of component behavior, including deterioration, or limit maximum deformation to a conservative (low) value  u. If  >  u in any one analysis: Strength in this action should drop to zero Effect on related strength properties should be evaluated

Quake Summit 2010, October 8, 2010 Mean of max. transient drift in every story ≤ 3.0% Max. transient drift in every story ≤ 4.5% Mean of max. residual drift in every story ≤ 1.0% Max. residual drift in every story ≤ 1.5% Loss in story strength at max. drift should not be more than 20% Acceptance Criteria at System Level

Quake Summit 2010, October 8, 2010 System Modeling Issues Incorporate all components and all behavior modes (e.g., shear in RC) that “significantly” affect prediction of seismic response Might require post-analysis review and re-analysis Flexibility of floor diaphragms should be modeled if deemed important Analysis should provide information needed to quantify diaphragm forces Podium and backstay effects must be represented realistically P-Delta effects must be included Include real torsion, but no requirement for accidental torsion

Quake Summit 2010, October 8, 2010 Wall Hinging at the Base Loading Story Shear  y =V y /W Story OTM

Quake Summit 2010, October 8, 2010 NRHA force demands may be very different from elastic “expectations” Maximum moment in shear wall

Quake Summit 2010, October 8, 2010 NRHA force demands may be very different from elastic “expectations” Maximum shear force in shear wall

Quake Summit 2010, October 8, 2010 Component Modeling Deterioration in strength and stiffness must be considered if it significantly affects the response of the structure to the MCE ground motions Or – conservative estimates must be made of strength and deformation capacities

Quake Summit 2010, October 8, 2010 Modes of Deterioration

Quake Summit 2010, October 8, 2010 Basic Observation The cyclic envelope curve is different from the monotonic backbone curve

Quake Summit 2010, October 8, 2010 Resource Document ATC-72-1 Interim Guidelines on Modeling and Acceptance Criteria for Seismic Design and Analysis of Tall Buildings

Quake Summit 2010, October 8, 2010 GENERAL MODELING ISSUES Types of Models Deterioration P-Delta effects Damping Uncertainties PROPERTIES OF NONLINEAR STRUCTURAL COMPONENTS Steel beams and columns Steel panel zones Axially loaded steel braces RC beams, columns, and joints PLANAR AND CORE WALL SYSTEMS AND COMPONENTS Planar walls, flanged walls, core walls Coupling beams Slab-columns and connections FLOOR DIAPHRAGMS, COLLECTORS, AND PODIUM AND BACKSTAY EFFECTS Rigid, semi-rigid, and flexible diaphragms Podium and backstay effects Resource Document ATC-72-1

Quake Summit 2010, October 8, 2010 Source: G. Deierlein

Quake Summit 2010, October 8, 2010 Use of Strain-based Models (Fiber & Curvature Models) Argument for their use: whenever lumped plasticity models are not available Columns subjected to biaxial bending and large axial force Shear walls with (and without?) openings Spandrel beams?

Quake Summit 2010, October 8, 2010 Use of Strain-based Models (Fiber & Curvature Models) Arguments against their use: RC: Rebar buckling? Rebar fracture? Bond slippage and pullout? Shear? Steel: Local instabilities? Fracture? Joint panel zones? Need to account for cyclic deterioration

Quake Summit 2010, October 8, 2010 Use of Concentrated Plasticity (Spring) Models Rotational spring models if inelastic behavior mode is bending Translational spring models if inelastic behavior mode is shear Arguments for their use Can capture deterioration characteristics if good calibrations are available from experimental data Are relatively simple Arguments against their use Are approximate Not available for many important failure modes

Quake Summit 2010, October 8, 2010 ASCE 41 models may be used if deemed appropriate They were intended to be used in conjunction with pushover analysis They were not intended to be used for hysteresis modeling The sharp drop from C to D is not representative of reality except for brittle failure modes They may not be applicable to many “new” components

Quake Summit 2010, October 8, 2010 Component Models with Deterioration (see ATC-72) Q-HYST Degrading Stiffness Flag-Shaped Bi-Linear Hysteresis 1. Monotonic (initial) backbone curve: 2.Cyclic deterioration parameter 3.Description of hysteresis loops F  F c F r =  F y  y  c  r  u K e  p  pc F y K s K

Quake Summit 2010, October 8, 2010 Modeling Option #1 – ATC-72 Use of monotonic backbone curve and explicit incorporation of cyclic deterioration Option 1

Quake Summit 2010, October 8, 2010 Modeling Option #2 – ATC-72 Use of cyclic envelope curve as modified backbone curve, and no incorporation of cyclic deterioration – limit  u to max. observed in test Option 2 Mod. B.C. from exp. env. curve

Quake Summit 2010, October 8, 2010 Modeling Option #3 – ATC-72 Use of factors to generate modified backbone curve from monotonic backbone curve, and no incorporation of cyclic deterioration - capping strength F c * = 0.9 F c - plastic deformation capacity  p * = 0.7 p - post-capping deformation capacity  pc * = 0.5 p - residual strength F r * = 0.7F r - ultimate deformation capacity  u * = 1.2 c

Quake Summit 2010, October 8, 2010 Modeling Option #3 Option 3 Mod. B.C. from Monotonic B.C

Quake Summit 2010, October 8, 2010 Modeling Option #4 – ATC-72 No deterioration at all in analytical model ultimate deformation capacity  u * corresponding to 80% of capping strength on descending branch of Options 2 or 3 Option 4

Quake Summit 2010, October 8, 2010 Comparison of ATC-72 Modeling Options Option 1 Option 4 Option 2 Mod. B.C. from exp. env. curve Option 3 Mod. B.C. from Monotonic B.C

Quake Summit 2010, October 8, 2010  y  c  p 0.5  pc   ’ c  ’ p =0.7  p 1.5  c M c 0.8M c Initial backbone curve Modified backbone curve, Option 3 Ultimate rotation, Option 4  pu Ultimate rotation, Option 3 Penalties for Options 3 and 4

Quake Summit 2010, October 8, 2010 What is new? No radical changes Explicit formulation of performance objective and acceptance criteria at two levels of ground motions (SLE & MCE) Consideration of deterioration in component properties – if it is important Or acceptance of “penalty” in component modeling Consistent design and performance evaluation process

Quake Summit 2010, October 8, 2010 I think we are making progress