1. PEER 2007 Annual Meeting - San Francisco, January 19, 2007
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
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To develop Supplement No. 1 revisions to ASCE 41 to address negative comments withdrawn during the public comment period.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 Beam-Column Joints
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. Columns Summary of Changes
Effective stiffness modified for low axial loads ( ).
Lap splice requirements changed (6.3.5).
Changed format and values in Tables 6-8 and 6-12 to account for flexure-shear failures ( ).
Added information on probabilities of failure (C ).

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 b
drift at loss of axial load capacity
- all failure modes: Pf < 15% 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. nonconforming transverse reinforcement
Proposed Condition i vs. FEMA 356 “controlled by flexure”
nonconforming transverse reinforcement
proposed

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

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

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

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

14. Beam-Column Joints Summary of Changes
Rigid end-zone models ( )
Strength section created ( )
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 Joints Rigid end-zone models
FEMA 356:
Proposed:

16. Beam-Column Joints Rigid 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 Joints Rigid end-zone models
Lowes collected a database of 57 beam-column subassemblies from 13 test programs.
kmeas based on first significant load cycle.
kcalc/kmeas
Proposed
FEMA 356
Mean
1.22
2.59
Min
0.19
0.41
Max
2.52
5.18
cov
0.36
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 BackboneQ Qy
New Figure 6.1c
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. 2002 M/Vlw = 0.69 Specimen #8
FEMA 356
proposed
Load (kN)
Displacement (mm)

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

26. Chapter 2 Two sections modified:
Deformation and Force-Controlled Actions ( )
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, (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 (1.2) is suspect

29. Alternative Modeling Parameters and Acceptance Criteria
Proposed, (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 Beam-Column Joints Slab-Column Connections Walls
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

32. 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

33. 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-bound yield strength
Lower bound yield strength

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

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

36. Columns (Table 6-8) Deformation capacities
Condition selected based in ratio of plastic shear demand to shear strength:
downgraded
no change
downgraded
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.

37. Columns (6.4.2.4) Acceptance Criteria
The following is removed from :
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.

38. Beam-Column Joints Definition 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.

39. Beam-Column Joints Table 6-10

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

41. 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.

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

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

44. New Table 6-18 … … … … … … … …

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

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

47. 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)

48. Walls (6.7.2.3) reinforcement yield strength
The following text is removed from :
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.

49. Walls (6.7.2.3) reinforcement yield strength
Data - Hidalgo et al. 2002
Specimen
hw/lw
Vu Test
Vn ACI 21-7
Vu Test/Vn ACI
(kips) 1
2.0
44.6
43.5
1.02 2
60.8
56.5
1.08 4
72.9
71.1
1.03 6
1.4
69.5
51.1
1.36 8
84.2
75.3
1.12

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

51. Deformation and Force-Controlled Actions
Motivation for changes:
Columns can sustain shear failures without loss of axial load capacity.
This case not permitted by or captured by Fig. 2-3:

52. Deformation and Force-Controlled Actions
New Figure 2-3:

53. Deformation and Force-Controlled Actions
New Figure C2-1:
Although notation used in Fig 2-3 and C2-1 is not ideal, the committee felt further changes in the document would be needed to address this concern.

54. Walls (Table 6-18) confinement and shear stress limit
ACI confinement provisions too restrictive.
High ductility still achieved with:
Ash > 0.75Ash ACI
s < 8db
Moderate ductility still achieved with:
Ash > 0.5Ash ACI
Deformation capacities approximately constant for
Consider as confined
Consider as 80% confined

55. Thomsen & Wallace, 2004 “unconfined boundary”

56. Paulay 1986 high shear stress
Conforming
h = 3.3 m
= ft
Conforming
(3.94”)
(59”)