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

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### 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

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

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- To develop Supplement No. 1 revisions to ASCE 41 to address negative comments withdrawn during the public comment period.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.

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

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).

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

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!

Proposed Condition i vs.FEMA 356 “controlled by flexure”

nonconforming transverse reinforcement

proposed

Columns (Table 6-8)Deformation capacities

Pf = 13%

Pf = 7%

- Evaluate “b” for columns with axial-load failures:

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.

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 (%)

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

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.

Slab-Column Connections (RC) - Comparison with test data

Proposed “a” (continuity)

Proposed “a” (no continuity)

FEMA 356 “a”

Slab-Column Connections (PT) - Comparison with test data

Proposed “a” (continuity)

Proposed “a” (no continuity)

FEMA 356 “a”

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

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.

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

Axial collapse

Walls (Table 6-19)high axial loadsproposed

- 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

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.

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

Alternative Modeling Parameters and Acceptance Criteria

Resulting backbone curve applying FEMA 356 2.8.3(1.2) is suspect

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).

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

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

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

Columns (6.3.5)Development and splices of reinforcement

fs / fy nominal

New Table 6-8

~Flexure failures

~Flexure-shear failures

~Shear failures

trans. reinf. ratio

High axial load cases

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.

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.

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.

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.

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

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.

Walls (Table 6-19)Tri-linear Backbone

Response dependent on axial load

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)

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.

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.

Deformation and Force-Controlled Actions

- Motivation for changes:
- Columns can sustain shear failures without loss of axial load capacity.
- This case not permitted by 2.4.4.3 or captured by Fig. 2-3:

Deformation and Force-Controlled Actions

New Figure 2-3:

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.

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
- s < 8db
- Deformation capacities approximately constant for

Consider as confined

Consider as 80% confined

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