06 Design Workshop 2017 - Adhesive Anchors JAB...

Adhesive Anchorages

Joe Black | Structural Wall Engineer

May 17, 2017

Bridge Office | mndot.gov/bridge

History

Bridge Office | mndot.gov/bridge5/17/2017

Important Events

• Boston Big Dig Failure ‐ 2006

• Creep, short anchors, improper mixing, etc.

• Initial updates to ACI (2011) and ICC (2006)

• Manufacture change from ASD to LRFD

• MnDOT response 2006 – Present

• No anchors in sustained tension

• Investigated metal railing

• Updated design practices (ACI 318 and AC308)

• Changed Special Provisions5/17/2017 3Bridge Office | mndot.gov/bridge

No Anchors in Sustained Tension

• MnDOT issued the following guidance in September of 2007:

“Effective immediately, MnDOT is prohibiting the use of adhesive anchors in all sustained tensile‐load applications…”

• Simply put “DON’T DO IT!”

• Use undercut anchors if needed

5/17/2017 4Bridge Office | mndot.gov/bridge

Undercut Anchors


T‐1 Rail Investigation


A Semi Hits a Bridge Metal Rail…


And MnDOT Responded…

• 48 bridges tested based on design speed

• Of the 48 bridges tested:

• 2 bridges found with no defects

• 11 bridges found with short anchors but no bond issues

• 11 bridges found with bond issues and no short anchors

• 24 bridges had issues with both length and bond

• Installation deficiencies were the primary cause


Design


MnDOT Utilized Design Codes

• AASHTO LRFD Bridge Design Specifications

• ACI 318

• AC308 (ICC‐ES)


AASHTO LRFD Bridge Design Specification

• Currently silent on the subject

• Will add language in next edition which:

• Defers to ACI 318

• Waves the exclusion of impact loads per ACI 318 17.1.4


AC308

• Published by ICC

• Most immediate response to Big Dig Disaster

• Currently defers to ACI 318 with some minor modifications

• Frequent updates


ACI 318

• First added adhesive anchors to Appendix D in 2011 code

• Incorporated anchors into Chapter 17 of the current design code (ACI 318‐14)

• ACI 318‐11 Appendix D resistance factors used by MnDOT in design

• Splitting failure not dealt with directly


7 Modes of Anchor Failure

• Steel rupture in tension

• Steel shear

• Concrete breakout in tension

• Concrete breakout in shear

• Splitting

• Bond failure

• Pryout failure


Steel Failure Modes

• For steel tension check, use AASHTO 6.13.2.11

• For steel shear check, use AASHTO 6.13.2.7

• Account for interaction effects as per AASHTO• If the shear loads exceeds 33% of shear resistance, modify the tension in accordance with AASHTO Eq 6.13.2.11‐2


Concrete Breakout in Tension

• Calculate resistance in accordance with ACI 318 17.4.2

• Assume concrete is cracked for this failure mode

• Take care when assuming breakout is restrained by reinforcing


Concrete Breakout in Shear

• Calculate resistance in an accordance with ACI 318 17.5.2

• Assume concrete is cracked for this failure mode

• Take care when assuming breakout is restrained by reinforcing


Splitting Failure

• Modification factors to account for splitting effects (cp,N and cp,Na) are found in ACI 318 17.4.2.7 and 17.4.5.5 respectively

• A description of the reason for these modification factors is found in ACI 318 R17.4.2.7

• ACI equations “are based on the assumption that the basic concrete breakout strength can be achieved if the minimum edge distance equals [the critical edge distance]”. However, certain anchor types “may” not achieve the basic tension capacity if a minimum edge distance equal to the critical edge distance is not provided.


Splitting Failure

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X‐Section Plan View


AC308 vs ACI 318

• Equations presented in ACI may be overly conservative

• The depth of the member and the presence of reinforcing reduce the likelihood that concrete splitting may occur

• To account for the effect that the member depth has on the capacity, use AC308 4.2.2 to calculate the critical edge distance, cac


Bond Failure

• Calculate bond strength using ACI 318 17.4.5

• Consider carefully condition of bond to concrete (cracked or uncracked)

• When in doubt, consider as a cracked condition


Cracking Assumption with Adhesives

• Uncracked bond strength (uncr) vs cracked bond strength (cr)


Bond Failure

• MnDOT uses values different then those found in ACI 318 Table 17.4.5.2

• Characteristic Bond Strength for Threaded Rod Applications:

• uncr = 1000 psi and cr = 500 psi, or

• uncr = 1500 psi and cr = 750 psi

• Characteristic Bond Strength for Reinforcing Bar Applications:

• uncr = 1000 psi and cr = 500 psi


Pryout Failure

• Calculate resistance using ACI 318 17.5.3

• Pryout is a shear failure mode which is estimated using tensile failure values

• Always consider pryout when shear is a valid load case


Resistance Factors


Anchor Category

• Anchor Category – Defined as the systems sensitivity to installation and overall reliability

• Category 1 = Low sensitivity to installation and high reliability

• Category 2 = Moderate sensitivity to installation and moderate reliability

• Category 3 = High sensitivity to installation and low reliability


Resistance Factors

• Use ACI 318‐11 Appendix D.4.3 and D.4.4

• Apply Category 1 resistance factors for concrete breakout in tension

• Apply Category 2 resistance factors for bond failure mode


Concrete Condition

• Anchor Condition – Defined by the presence or absence of supplementary reinforcing.

• Condition A = Concrete with supplementary reinforcement

• Condition B = Concrete without supplementary reinforcement

• Any regularly reinforced section with appropriate temperature and shrinkage steel and/or distribution steel may be considered Condition A.

• Always use Condition B for pryout strength


Resistance Factor for Pryout

• a∙Nag is calculated using ACI 318 17.4.5

• c∙Ncbg is calculated using ACI 318 17.4.2

• Vcp is calculated using ACI 318 17.5.3

• When a∙Nag > c∙Ncg then use Ncbg to calculate Vcp and use cas the resistance factor for pryout

• Otherwise use Nag to calculate Vcp and use a as the resistance factor for pryout


D.4.3


D.4.3


Apply to Ncg

Apply to Nag

a∙Nag > c∙Ncg

Apply to Vcp

a∙Nag < c∙Ncg

Apply to Vcp

Extreme Event

• Set all resistance factors to 1.0 for Extreme Event loading


Interaction Effects

• Apply ACI 318 17.6 when considering combined tension and shear.

• Do not consider steel failure modes using ACI 318 interaction because it is already considered in AASHTO equations


Interaction Effects


Plans and Special Provisions


Standard Plan Note

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See MnDOT Standard Figure 5‐397.162 for actual note


Standard Plan Note

• Similar note is to be included in all plans containing adhesive anchors

• Some important questions to ask:

• What diameter of bar or rebar mark is the anchor?

• What is the required uncracked concrete bond strength to meet design parameters?

• What is the required effective embedment?

• If encountered, should workers drill through reinforcing?

• WHAT IS MY REQUIRED PROOF LOAD?


Proof Load

• When proof loading an anchor, edge effects may be reduced and group effects are not considered

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• Set to the minimum of:

• The factored nominal concrete breakout of a single anchor (c∙Ncb)

• The factored nominal bond strength of a single anchor (a∙Na)

• 80% of the yield of the anchor dowel or rod


Special Provisions

• “Adhesive anchorage installers must hold current ACI‐CRSI Adhesive Anchor Installer Certification credentials. Installers are required to check depth, diameter and condition of the drilled hole, clean the hole, and install the anchorage per the Manufacturer’s Printed Installation Instructions (MPII).”

• I.E., the installer should know what he/she is doing

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See ‘SB2016‐2402.6WP’ entitled ‘Metal Railing ("Duplex Coated" using Hot‐dipped Galvanizing and Paint Coating)’ for all changes


Installation Proof Testing METHOD 1

• Prior to installation, 5 anchors are tested to the proof load given in the plans to verify installation procedure

• 10% (no less than 10) of the anchors are tested after installation to verify installation

• Additional anchorages tested per each failure

• No more than 5% concrete breakout failures permitted

• Concrete breakout failure is defined in the special provision

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Installation Proof Testing METHOD 2

• No testing of system prior to installation

• 10% plus 5 (no less than 15) of the anchors are tested after installation to verify installation

• Additional anchorages tested per each failure

• No more than 5% concrete breakout failure permitted

• Concrete breakout failure is defined in the special provision

• Ultrasonic Testing required on all tested anchors and an additional 5% (no less than 3) to verify embedment


Special Provisions

• The number of anchorages tested for each method is provided by the designer.

• New Approved Products List generated as part of release

• Similar changes will be made to all adhesive anchorage applications, including rebar dowel applications

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Thank you!

Joe [email protected]

651‐366‐4485

5/17/2017 Bridge Office | mndot.gov/bridge 43

06 Design Workshop 2017 - Adhesive Anchors JAB...

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