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ASCE 41 Ad Hoc Committee Supplement Revisions to Chapter 6 of ASCE 41, Seismic Rehabilitation of Existing Buildings

ASCE 41 Ad Hoc Committee Supplement Revisions to Chapter 6 of ASCE 41, Seismic Rehabilitation of Existing Buildings. Kenneth Elwood, UBC (Chair) Craig Comartin, EERI Jon Heintz, ATC Dawn Lehman, Univ of Washington Adolfo Matamoros, Univ of Kansas. Andrew Mitchell, Degenkolb

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ASCE 41 Ad Hoc Committee Supplement Revisions to Chapter 6 of ASCE 41, Seismic Rehabilitation of Existing Buildings

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  1. ASCE 41 Ad Hoc CommitteeSupplement Revisions to Chapter 6 of ASCE 41,Seismic Rehabilitation of Existing Buildings Kenneth Elwood, UBC (Chair) Craig Comartin, EERI Jon Heintz, ATC Dawn Lehman, Univ of Washington Adolfo Matamoros, Univ of Kansas Andrew Mitchell, Degenkolb Jack Moehle, UC Berkeley Mark Moore, R&C Michael Valley, MKA John Wallace, UCLA PEER 2007 Annual Meeting - San Francisco, January 19, 2007

  2. Timeline • EERI/PEER Technical Seminars (Jan-Feb 2006) • ASCE 41 Public Comments on Chapter 6 (Mar 2006) • ASCE 41 Commentary Changes (Apr 2006) • ASCE 41 Ad Hoc Committee • Kick-off meeting, 29 June 2006 • Bi-weekly meetings • Completed revisions, 1 Dec 2006 • ASCE 41 review group provided comments • Final changes completed, 19 Dec 2006 • ASCE 41 voted to consider modifications, 19 Jan 2007 • ASCE 41 Supplement No. 1 released - ?? 2007

  3. Committee Scope • To develop Supplement No. 1 revisions to ASCE 41 to address negative comments withdrawn during the public comment period. • Focus on integrating recent research presented at the EERI/PEER seminars titled, New Information on the Seismic Performance of Existing Concrete Buildings. • Scope limited to Chapter 6 – Concrete. • Although some limited changes are proposed for Chapter 2 to ensure clarity of changes in Chapter 6. • Focused on modifications the committee felt were critical to the outcome of assessments using ASCE 41.

  4. Components addressed • Columns • Substantive changes to modeling parameters, acceptance criteria, and stiffness based on new data. • Beam-Column Joints • Changes to stiffness models. • Slab-Column Connections • Modeling recommendations • Substantive changes to modeling parameters and acceptance criteria based on new data. • Addition of PT slabs • Walls • Substantive changes to modeling parameters and confinement requirements. • Acceptance criteria and alternative criteria • Clarification within Chapter 2

  5. ColumnsSummary of Changes • Effective stiffness modified for low axial loads (6.3.1.2). • Lap splice requirements changed (6.3.5). • Changed format and values in Tables 6-8 and 6-12 to account for flexure-shear failures (6.4.2.2.1). • Added information on probabilities of failure (C6.4.2.2.1).

  6. Columns (Table 6-8)Deformation capacities • Methodology for modifications: • Explicitly account for flexure-shear failure mode. • Account for scatter in data. • Select target probabilities of failure. drift at 20% loss in lateral strength - Flexure failures: Pf < 35% - all others: Pf < 15% Q a drift at loss of axial load capacity - all failure modes: Pf < 15% b D

  7. Columns (Table 6-8)Deformation capacities • Methodology for modifications (cont.) • Columns with low axial loads can sustain gravity loads well beyond lateral-load failure. • Axial-load failure can occur suddenly after lateral load failure for: • columns with high axial loads (P=0.6Agf’c ) • very light transverse reinforcement (r”≤0.0005) • To account for this a and b parameters converge to a single value. • High axial load and very light transverse reinforcement: • Zero plastic rotation capacity!

  8. Proposed Condition i vs.FEMA 356 “controlled by flexure” conforming transverse reinforcement proposed

  9. Proposed Condition i vs.FEMA 356 “controlled by flexure” nonconforming transverse reinforcement proposed

  10. Proposed Condition ii vs.FEMA 356 “controlled by flexure” Nonconforming Conforming

  11. Columns (Table 6-8)Deformation capacities Pf = 30% Pf = 6% • Evaluate “a” for Condition i columns:

  12. Columns (Table 6-8)Deformation capacities Pf = 6% Pf = 0.1% • Evaluate “a” for Condition ii columns:

  13. Columns (Table 6-8)Deformation capacities Pf = 13% Pf = 7% • Evaluate “b” for columns with axial-load failures:

  14. Beam-Column JointsSummary of Changes • Rigid end-zone models (6.4.2.2.1) • Strength section created (6.4.2.3.2) • Definition of “conforming” transverse reinforcement. • Clarifications to Tables 6-9 and 6-10 Substantive changes to Tables 6-9 and 6-10 were discussed by committee, however the committee did not feel the changes were urgent and proposed modifications were better left to a more deliberative process.

  15. Beam-Column JointsRigid end-zone models FEMA 356: Proposed:

  16. Beam-Column JointsRigid end-zone models FEMA 356* Proposed* Column Shear (kips) * Includes beam and column stiffness models, in addition to rigid end zone models. Drift (%) Walker, Lehman, Lowes Drift (%)

  17. Beam-Column JointsRigid end-zone models • Lowes collected a database of 57 beam-column subassemblies from 13 test programs. • kmeas based on first significant load cycle. Lowes and Lehman

  18. Slab-Column Connections (6.4.4)Summary of Changes • Editorial changes. • Expanded commentary on modeling options. • Modification of Tables 6-14 and 6-15 based on new data. • Specific parameters for PT slab-column connections.

  19. Slab-Column Connections (RC) - Comparison with test data Proposed “a” (continuity) Proposed “a” (no continuity) FEMA 356 “a”

  20. Slab-Column Connections (PT) - Comparison with test data Proposed “a” (continuity) Proposed “a” (no continuity) FEMA 356 “a”

  21. Slab-Column Connections “b” values • “b” defined as point of gravity load collapse, thus: • For continuity  b > a • Very limited data available to assess “b”. • For no continuity a = b PT a=b

  22. Walls (6.7)Summary of Changes • Columns under discontinuous shear walls. • Relax confinement requirements. • Increase shear stress limits. • Introduction of tri-linear backbone for walls controlled by shear. • No penalty for walls with one curtain of reinforcement. • Remove limit on reinforcement yield strength.

  23. Walls (6.7.2.2.2)Tri-linear Backbone New Figure 6.1c Q Qy • a/d < 2.5 • walls controlled by shear. • captures shear cracking. • based on model by Sozen and Moehle (1993) e d g 1.0 B C F D E f c A ∆ h

  24. Hidalgo et al. 2002M/Vlw = 0.69 Specimen #8 FEMA 356 proposed Load (kN) 30 35 Displacement (mm)

  25. 3 x Yield Axial collapse Walls (Table 6-19)high axial loads proposed • Recent tests by Wallace suggest failure can occur at low drifts. • assume no residual and reduce “e” to 1% FEMA 356 Lateral Load 1% 1% Drift Ratio

  26. Chapter 2 • Two sections modified: • Deformation and Force-Controlled Actions (2.4.4.3) • ensure clarity of changes in Chapter 6; • maintain consistency between the chapters; • transparency of design intent to the user; • and facilitate development of more liberal acceptance criteria of other materials. • Alternative Modeling Parameters and Acceptance Criteria (2.8) • Address over-estimation of degradation from current procedures.

  27. Alternative Modeling Parameters and Acceptance Criteria FEMA 356, 2.8.3 (1.2): A smooth "backbone" curve shall be drawn through the intersection of the first cycle curve for the (i)th deformation step with the second cycle curve of the (i-1)th deformation step, for all i steps Force Results in exaggeration of strength degradation, which in turn leads to overestimation of displacement demands. Backbone curve Deformation

  28. Alternative Modeling Parameters and Acceptance Criteria Resulting backbone curve applying FEMA 356 2.8.3(1.2) is suspect

  29. Alternative Modeling Parameters and Acceptance Criteria Proposed, 2.8.3 (1.2): A smooth "backbone" curve shall be drawn through each point of peak displacement during the first cycle of each increment of loading (or deformation).

  30. Summary • Columns • Substantive changes to modeling parameters, acceptance criteria, and stiffness based on new data. • Beam-Column Joints • Changes to stiffness models. • Slab-Column Connections • Modeling recommendations • Substantive changes to modeling parameters and acceptance criteria based on new data. • Addition of PT slabs • Walls • Substantive changes to modeling parameters and confinement requirements. • Acceptance criteria and alternative criteria • Clarification within Chapter 2

  31. Columns (6.3.1.2)Effective Stiffness Proposed Figure C6-1: • Proposed model accounts for bar slip from foundation or beam-column joints. • Data suggests effective stiffness is closer to 0.2EIg for low axial loads, but committee did not want to underestimate stiffness for columns in wall buildings. FEMA 356

  32. Columns (6.3.5)Development and splices of reinforcement • FEMA 356 model does not reflect the intent of the ACI development length equation to develop 1.25 times nominal fy. • Committee adopted modified version of model by Cho and Pincheira (2006): Accounts for increasing slip with longer lb expected or lower-boundyield strength Lower bound yield strength

  33. Columns (6.3.5)Development and splices of reinforcement fs / fy nominal

  34. New Table 6-8 ~Flexure failures ~Flexure-shear failures ~Shear failures trans. reinf. ratio High axial load cases

  35. Columns (Table 6-8)Deformation capacities • Condition selected based in ratio of plastic shear demand to shear strength: downgraded downgraded no change Note: The restriction on the effectiveness of transverse reinforcement with 90 degree hooks in regions of moderate and high ductility (Shear and Torsion 6.3.4) has been removed for ASCE41 Supplement 1, but has been maintained for lap spliced transverse reinforcement.

  36. Columns (6.4.2.4)Acceptance Criteria • The following is removed from 6.4.2.4.2: • Not required since shear-critical columns are now considered deformation-controlled and Table 6-8 is used. For columns designated as primary components and for which calculated design shear exceeds design shear strength, the permissible deformation for the Collapse Prevention Performance Level shall not exceed the deformation at which shear strength is calculated to be reached; the permissible deformation for the Life Safety Performance Level shall not exceed three quarters of that value.

  37. Beam-Column JointsDefinition of “Conforming” • “Joint transverse reinforcement is conforming if hoops are spaced at  hc/2 within the joint.” • Based on observation from tests that any reinforcement in the joint will substantially improve the performance.

  38. Beam-Column JointsTable 6-10

  39. New Table 6-14 Based on m values, with b>a Based on m-s values, with b=a Continuity

  40. Slab-Column Connections Continuity reinforcement Considered continuous if: …the area of effectively continuous main bottom bars passes through the column cage in each direction is greater than or equal to 0.5Vg/(ffy). Where the slab is post-tensioned, at least one of the post-tensioning tendons in each direction must pass through the column cage. Recommendations for Design of Beam-Column Connections in Monolithic Reinforced Concrete Structures: ACI 352R-02.

  41. M Elastic column q Column plastic hinge Torsional connection element1 Joint region Slab-beam plastic hinge M Elastic slab beam q Elastic relation for slab beam or column Plastic hinges for slab beams or for torsional element 1Slab-beams and columns only connected by rigid-plastic torsional connection element. Slab-Column Connections Nonlinear Modeling

  42. Walls (6.7.1.2)Columns under discontinuous shear walls Columns under discontinuous shear walls  Use Section 6.4.2 (Columns) • Take advantage of improvements to columns section. • Table 6.8 will be restrictive due to high axial loads. • Consistency.

  43. New Table 6-18 … … … … … … … …

  44. Walls (Table 6-19)Tri-linear Backbone Response dependent on axial load

  45. Hidalgo et al. 2002M/Vlw = 1.0 Specimen #2 FEMA 356 proposed Load (kN) 40mm Displacement (mm)

  46. Walls (6.7.2.3)one curtain of reinforcement The nominal shear strength of a shear wall or wall segment, Vn , shall be determined based on the principles and equations given in Chapter 21 of ACI 318, except that the restriction on the number of curtains of reinforcement shall not apply to existing walls. (MPa)

  47. Walls (6.7.2.3)reinforcement yield strength • The following text is removed from 6.7.2.3: For all shear strength calculations, 1.0 times the specified reinforcement yield strength shall be used. • Factors on yield strength determined by whether action is force or displacement controlled. • Consistency with rest of document.

  48. Walls (6.7.2.3)reinforcement yield strength

  49. Chapter 6Miscellaneous changes • Concrete-encased steel sections (6.1) • Chapter 6 does not apply to these components. • Fig. 6-1 commentary (C6.3.1.2.2) • Impact of rapid strength degradation in Fig 6-1 on displacement demands. • Usable strain limits (6.3.3.1) • Tests for alternative tensile strain limits for reinforcement must include the influence of low-cycle fatigue and spacing of transverse reinforcement.

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