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SEISMIC ANCHORAGEAND APPENDIX D

Efforts and Findings of NCSEA, SEAOC and PUC IT3

Kevin S. Moore, P.E., S.E., SECBCAC Seismic Subcommittee ChairPrincipal

In the beginning…

• Wood framed structures required anchor bolts

• Steel framed structures had anchor bolts

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John Silva

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Early Anchor Bolt Design

• Tabular– Shear and Tension Values from Codified Tables

• Concrete cone pullout and steel yield/fracture limit– Design anchorage considering cone failure of concrete

– Design anchorage considering yield/fracture of bolt

– Minimum design force controls

• Interaction– Consider shear/tension interaction

– Examine all combinations of demands on cone/yield

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Interaction Equation

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ς = 5/3

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1997 UBC Modifications

• Interaction, with consideration of individual failure modes

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2005

• ACI Develops Appendix D

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State of the Practice

• Confusion

• Concern

• Consternation

• However, Appendix D was based on test data

• Actual consideration of failure modes, capacity values that capture both behavior and substrate conditions

• Rigorous, thorough and difficult

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1991 1994 1997 2000 2003 2006 200920??

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SEAOC Instigated Effort

• SEAOC Seismology– Discussed Issue

– Generated Interest

– Leveraged Research Opportunities

– Considered a Singular Case

– Strive to Develop Data; High “Bang for Buck”

– Establish Basis for Code Change in IBC

– SEAONC to SEAOC to NCSEA to IBC

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Laboratory Testing of Anchor Bolts Connecting Wood Sill Plates to Concrete with Minimum Edge Distances

SEAOC 2011 Excellence In Structural Engineering AwardsStudy / Research / Guidelines

practicing engineers practicing researchTest Program Findings:

• When loaded parallel to the concrete edge; the wood governs the connection. The wood sill plate and steel anchor bolt “yield” well before any concrete limit state.

• The connection showed excellent ductility. Beyond the initial “peak” value used for validating capacities, tests often continued on to a second (and higher) peak capacity.

• For 2x4 and 3x4 plates, the average peak capacities were more than 6 times higher than ductile connection design strengths obtained from the equations promulgated in ACI 318-05/08, App. D

• For 2x4 and 3x4 plates, the average peak capacities were more than 4 times higher than the allowable capacities obtained from IBC 2006 (NDS-05).

Project Investigators:

• Andy Fennell, Principal Investigator• Kevin Moore• Phil Line

Project Sponsors:

• Tom VanDorpe• Tom Voss• Gary Mochizuki

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‐2.000

‐1.500

‐1.000

‐0.500

0.000

0.500

1.000

1.500

2.000

Displacement (in.)

Cycles

Displacement Protocol

Approx. equiv. to NDS ASD load

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Research Findings

• When loaded parallel to the concrete edge; the wood governs the connection. The wood sill plate “yields” allowing elongation and ductile response of the anchor bolt well before any concrete limit state.

• The connection showed excellent ductility. Beyond the initial “peak” used for calculating capacities, tests often continued on to a second (and higher) peak.

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Code Change (2012 IBC, 2015 IBC)

• Approved an Exception to:

• IBC 1908.1.9 ACI 318-08, Section D.3.3

• IBC 1905.1.9 ACI 318-11, Section D.3.3.5.3

• In light-frame wood structure bearing or non-bearing walls, for the design of anchors used to attach wood sill plates to foundations or foundation stem walls, it shall be permitted to take the allowable in-plane shear strength of the anchors in accordance with Section 2305.

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Code Change (2012, 2015 IBC) - Paraphrased

• For sill pates of 2x or 3x nominal thickness, the allowable lateral design strength for shear parallel to grain of sill plate anchor bolts is permitted to be determined using the lateral design value for a bolt attaching a wood sill plate to concrete, as specified in AF&PA NDS Table 11E, provided the anchor bolts comply with all of the following:1. The maximum anchor nominal diameter is 5/8”;2. Anchors are embedded into concrete a minimum of 7”;3. Anchors are located a minimum of 1-3/4” from the edge

of the concrete parallel to the length of the wood sill plate; and

4. Anchors are located a minimum of 15 anchor diameters from the edge of the concrete perpendicular to the length of the wood sill plate.

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NIBS BSSC PUC IT3

• National Institute of Building Sciences (NIBS)

• Building Seismic Safety Council (BSSC)

• Provisions Update Committee (PUC)

• Issue Team (IT)

• Investigate Anchorage and Appendix D

– Issue Team 3 - Seismic Anchorage to ConcreteAssessment of ACI-318 Appendix D, and the Exploration of Alternate Methodologies

– Evaluate Appendix D

– Evaluate Alternate Methodologies if Warranted

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Current Efforts (6 Groups)

ACI 318-11, Appendix D Evaluation Tasks1. Recommend appropriate values when warranted

2. Evaluate non-yielding anchorage design for nonstructural components

3. Research fuc/fyc > 1.3 (suggest testing if inadequate data exists)

4. Specification of bolt material

5. Evaluate shear lug provisions

6. Evaluate interaction calculations (for derivation and development)

7. Evaluate stretch length provisions

8. Evaluate yield/ultimate for service loading

9. Evaluate pre-load as part of anchorage design

10. Evaluate creep related to anchorage design/capacity

11. Evaluate steel reinforcement elements as anchorage

12. Evaluate using reinforcing as a ductile element; Chapter 12 as context

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Preliminary Findings (Group 1)

1. Recommend appropriate values when warranted

2. Evaluate non-yielding anchorage design for nonstructural components

• Overstrength for nonductile anchorage is necessary and prudent. Values are provided in tables of ASCE 7-10. However, test data is sparse for support of the values. Without data, it is difficult to recommend alternative values for overstrength.

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Preliminary Findings (Group 2)

3. Research fuc/fyc > 1.3

4. Specification of bolt material

8. Evaluate yield/ultimate for service loading

• Work in progress

• Issues with bolt material specifications (ASTM)

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Preliminary Findings (Group 3)

5. Evaluate shear lug provisions

6. Evaluate interaction calculations

• No referenced shear lug provisions, just design guidelines and guidance per AISC

• Initial effort on interaction calculations indicate that most Appendix D data is conservative related to test results

• Some findings indicate that knowledge of failure mechanism is key in capacity calculations as a singular interaction equation is generally too conservative

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Preliminary Findings (Group 4)

7. Evaluate stretch length provisions

12. Evaluate using reinforcing as a ductile element (Ch. 12)

• Stretch length (area that allows for a bolt to stretch in tension independent of substrate) is a good thing

• Methodology for determination may be presented

• ACI 355 is currently working on improved definitions of ductile and brittle steel elements for anchor applications

• Useable strain is more important than reported strain

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Preliminary Findings (Group 5)

9. Evaluate pre-load as part of anchorage design

10. Evaluate creep related to anchorage design/capacity

• There is no evidence in the research reviewed that suggests pre-tensioning force and the loss of the pre-tensioning force has an effect on the tensile strength of anchors

• Pre-tension and creep have little effect on anchorage when considering seismic loads and anchor capacity

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Preliminary Findings (Group 6)

11. Evaluate steel reinforcement elements as anchorage and evaluate “concrete breakout” phenomenon

• The use of reinforcing to “skirt” anchorage provisions or to create anchors from reinforcing steel is anathema to the intent of the provisions and inappropriate

• Current provisions virtually guarantee the prevention of concrete breakout via unexpectedly low probabilities of failure (5% fractile using expected mean ultimate)

• Further reductions of capacities to ensure certain failure modes may increase conservatism

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What’s Next

• Produce white paper on findings

• Develop example problem to help educate

• Suggest core philosophy challenges that apply to anchorage in general (5% fractile)

• Monitor current research projects– Pankow project on base plate anchorage

– SEAOC project on wood-over-podium hold down anchorage

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