Proposed Revisions to the Acceptance Criteria for Anchor Channels

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ICC-ES Hearing, Los Angeles, 2015 02 10

Proposed Revisions to the Acceptance Criteria for Anchor Channels in Concrete Elements

Subject AC232-0215-R1 (AHG/HS)

Comments by

Werner Fuchs Tobias Schmidt

Source: baulinks.de

AC232-0215-R1 #2

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New load acting in

arbitrary direction

Source: Roik, fib 2011 Source: Roik, fib 2011

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Examples of anchor channels with positive load transfer

• Experience based on products as shown positive load transfer only, no load transfer via adhesion or friction!

• No definition of positive load transfer • No reliability tests to cover minor errors occuring during placement of

the channel on site and fixing of the attachment to the channel exist Conservative approach required in a first step

a) notching channel bolt creating a notch in the channel

b) channel with serrated lips and matching locking channel bolt

Vx Vx

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Hilti’s initiative to complete AC232 with the application of shear loads acting in the longitudinal axis of the channel and seismic loading is supported. Hilti‘s proposal and further improvement during the rebuttal process represents a good start only requiring • limited ‚retrofitting‘ and • amendment to ensure the desired conservatism Hilti‘s rebuttal rejects significant influencing parameters raised during the public discussion without providing new evidence

General

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Installation torque

is vital to ensure a robust connection since it creates a prestressing force which shall exceed the external acting load in service over 50 years

testing without an installation torque or handtight would yield to the development of products which do not need an installation torque

Testing with an installation torque similar to the procedure for post-installed anchors is required

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Prestressing force

is vital to ensure proper functioning of the known products with notching bolts or serrated bolts

Proposed scope also allows other products than notching bolts and serrated channels which might have unknown reliabilty issues other than the installation torque

Continuous inspection is required

Limited experience of installers

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Replacement of the 5kN limit for characteristic lip failure

Hilti agrees to the limitation calculated as 1/3 of the 5%-fractile value

Problem: How to determine the peak load since load displacement curves show different behaviors

1

6

11

16

21

26

0 2 4 6 8 10 12 14 16 18 20

Load

[kN]

Displacement [mm]

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12 14 16 18 20

Load

[kN]

Displacement [mm]

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0

10

20

30

40

50

60

70

0 2 4 6 8 10 12 14 16 18 20

Load

[kN

]

Displacement [mm]

Assessment of the peak load

b1 = width of channel bolt head

b1/2

peak load

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Limitation of anchors on a channel

no information provided on how to properly define the acting load to the resisting anchors CEN/TR approach: n=3 in presence of Vu,x

Limitation to n = 3 anchors if Vu,x acts is required to allow for the use of the design model for headed bolts

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All shear loads subjected to the front anchor channel

No generally applicable investigations with regard to a design model describing the load transfer to the back anchors are available no technical evidence that the stiffness of the connection between anchor and channel is in any case identical to headed studs welded to a steel plate

Front anchor and channel face are decisive for the serviceability

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Conclusions

Open issues in addition to the Hilti proposal shall be considered

Proposals based on technical evidence on how to overcome the open points were provided in the IWB letter dated Jan. 9, 2015

Revise AC232 based on Hilti‘s proposal considering the technical evidence

Proposed Revisions to AC 232


Andreas Beer

AC232-0215-R1 #2


Scope • Transfer of tension loads takes place via interlock between the channel

bolt and the channel lips flange stiffeners, bending of the channel, tension in the anchors, and mechanical interlock with the concrete.

• Shear loads perpendicular to the longitudinal channel axis are partially transferred by compression stresses between the side of the channel and the concrete, and by shear in the anchors which is transferred into the concrete.

• Longitudinal loads shall be transferred by a positive load transfer mechanism (e.g. mechanical interlock between the channel bolt and the channel profile).

Halfen‘s opinion: Only direct load transfer from the bolt to the channel profile is covered (no welding, no grout) Agreement of ICC-ES?


Scope • Serrated anchor channels tolerated as exception • Therefore serrated anchor channels do not feature a

positive load transfer mechanism, rely on friction and are dependent on the level of pretension

• Borderline to positive load transfer mechanism?


Testing • An arbitrary tightening torque (20 Nm) provided by a

proponent is not acceptable. • Testing shall be done with the torque specified by the

manufacturer and stated in the MPII


Thank you for your attention.

ES Committee Meeting – Proposed changes to AC232 I Dr. Philipp Grosser www.hilti.com 2015-02-10 1

ES Committee Meeting

Los Angeles, Sheraton Gateway Hotel

Tuesday, February 10, 2015

Proposal for implementation of longitudinal shear in

AC232 for both static and seismic loading

Subject AC232-0215-R1 (AHG/HS) P. Grosser

AC232-0215-R1 #2


Current version (AC232 October 2014)

Criteria covers static tension and static shear perpendicular to the longitudinal axis

of the channel only

Anchor channels are widely used for

curtain wall applications

Hilti

tension

shear


Hilti

Hilti

Hilti

Hilti Hilti

Hilti


Strong request from customers to cover seismic

Criteria extension for static longitudinal shear and seismic (tension, shear and

longitudinal shear)

long. shear

tension

long. shear

tension

shear

Static loading Seismic loading (SDC C, D, E and F)

shear

Customer statement: An ESR that doesn’t include seismic is a nice sheet of paper however it does not

cover our needs.


Based on input from users, 90% of curtain wall applications require minimal

resistance in the longitudinal direction:

design loads per channel bolt < 670 lb (3kN)

Common solutions used in practice – transfer of longitudinal shear:

Box outs (recessed pockets) / filled channels

Non-serrated channels with notched channel bolts

Welded channels

Installations with additional mechanical attachments

Serrated channels

Independent test results satisfying the requirements of Sec. 104.11 are currently

unavailable. Instead, specifiers and AHJs must rely on

- manufacturer published data

- engineering judgement by specifiers / construction industry


Box outs (recessed pockets)

Common solution, low sensitivity towards installation

(Installation procedure)

Shear in longitudinal direction is

addressed by filling the pockets with

grout or concrete

Hilti


Filled channels (grout or concrete)

Straightforward solution, but eliminates adjustibility of attachment.

Channel is filled end to end. (Typically before curtain wall is installed)

Provides ample shear resistance in longitudinal direction

(much more than typically required)

Installation with fixture (top and front of slab)

Baseplate removed to see grout compaction

Static and cyclic behavior investigated in tests

Hilti Hilti


Non-serrated channels with notched channel bolts

Most common approach, very sensitive to installation conditions

Shear in longitudinal direction is transferred by notches pressed into the

channel lips by means of a torque wrench.

Many open questions on installation and test conditions remain unresolved,

systems are used in practice based on manufacturer information only.


Welded channels

Baseplates welded to the channel profile

comparable to welded embeds – labor intensive

Shear in longitudinal direction is transferred by the welds between

attachment and baseplate welded to the channel profile.

Hilti Hilti


Channels combined with additional mechanical fasteners

Common solution, based on engineering judgement by specifiers

- No guidance given on interaction between anchor channel and fasteners

Shear in longitudinal direction is transferred

by mechanical fasteners.

Example


Serrated channels

No definition for what constitutes a serrated channel. There are several

permutations on the market. Requires application of defined torque.

Shear in longitudinal direction is transferred by matching serrations in the

channel profile and the channel bolt.

Open questions on installation and qualification testing remain unresolved,

systems are used in practice based on manufacturer information only.

Serrations

(Different products on the market)

Graphic taken from

CEN/TR Anchor channels

Graphic taken

from Edilmatic


Efforts to implement longitudinal shear and seismic in AC232 (CAMA task group on AC232)

Proposal submitted to CAMA TG by Hilti for consideration at the June hearings 2013 (letter

dated February 22, 2013) → Proposal rejected due to open questions regarding design and

installation conditions

Improved proposal submitted to CAMA TG by Hilti for consideration at the October hearings 2013 (letter dated June 23, 2013) → Proposal rejected, additional questions raised

Open questions

Level of pretension load in qualification tests (steel – steel relaxation?)

Ratio between pretension and tension load based on different production lots

Determination of peak load in case systems are torqued in qualification tests

Torque tests / Installation tests to determine minimum edge distance

Determination of installation safety

Job-site quality control for systems relying on pretension

The discussion in the CAMA task group over the past three years, while productive, has still not resulted in the implementation of seismic qualification criteria in AC232


Current situation

The use of anchor channels in building structures that are assigned to higher SDCs has increased, and the need for guidance on this issue has become acute. In addition, we are

seeing a trend to employ alternate solutions. Many contractors specifically state anchor channels

will not be considered for future projects. Lack of an ESR w/seismic and costly work-arounds are

driving the industry toward alternative solutions, e.g. welded embeds.

Hilti

Hilti

Hilti

Hilti


Hilti has taken the step of putting forward a conservative but workable proposal to include

recognition for seismic applications in AC232 in response to an urgent need in the field for

clarification on how to address seismic loading, particularly for façade applications.

(excerpt from Hilti letter (rebuttal) to ICC-ES dated January 20, 2015)

→ Open questions related to qualification tests are bypassed by excluding pretension (mandating

positive load transfer mechanisms) and limiting the available strength in the longitudinal direction

to a very low but workable value of 1125lb (5kN).

→ Design proposal patterned on the provisions for headed anchors according ACI 318-11.

→ Conservatively, it is assumed that only three anchors take up the load in longitudinal direction.

Qualification tests without pretension load lead to the

most conservative characteristic resistance

In order to obtain reproducible results, we have

proposed to tighten the nut with a low level of defined

pretension (15ft-lb/20Nm).


Testing of anchor channels oriented perpendicular to the edge and loaded in

shear towards the free edge

For anchor channels with more than two anchors steel failure (connection failure

between channel bolt and channel profile or anchor failure) occured in tests.


Conclusion

We welcome the overtures from Rolf Eligehausen (IEA) and from the University of Stuttgart

(IWB) to develop a more comprehensive solution. However:

The modifications put forward here by IEA and IWB do not improve the current proposal.

Note that they would increase the permissible characteristic resistance (e.g. allowing a

pretension according to Section 5.5), even though the open questions regarding pretension

have not been resolved.

The changes proposed for the scope mis-apprehend the structure of the proposal. Mandating

positive load transfer systems as the baseline and permitting serrated systems (without

definition) under specific circumstances is the best way forward at this time.

The limit of 1125lb (5kN) is a matter of engineering judgment. We can accept alternate

proposals.

The evaluation of load-displacement curves, etc. will be required for the qualification of

systems that rely on a specified torque. With the exception of serrated channels, such

systems are not addressed by the current proposal.

The proposal to extend continuous inspection to systems utilizing positive load transfer

mechanisms is unnecessary. Cast-in-place headed anchors require periodic inspection in

accordance with Chapter 17. Continuous special inspection is required for post-installed

anchors under specific circumstances (adhesive anchors, sustained loads, etc.)

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AC 232Comments to the proposal of Hilti to include shear

loads in direction of the longitudinal channel axis and

seismic loading in seismic design categories C to E

Rolf EligehausenJörg Asmus

Klaus Schmid

AC232-0215-R1 #2

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• We welcome the efforts by company Hilti to

include shear loads in the direction of the

longitudinal channel axis (longitudinal shear) and

seismic loading, because these amendments are

urgently needed.

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• In general we consider the proposal of Hilti as

conservative.

However, it deviates significantly from the

accepted concept for testing, assessing and

special inspection of post-installed anchors.

Furthermore, for anchor channels with many

anchors the design procedure is not clear.

• For these reasons we propose the following main

modifications.

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Proposal Hilti

• Adhesion for the transfer of longitudinal shear loads

Vu,x from the channel bolt via the channel and the

anchors into the concrete is not excluded.

Proposal IEA

• Exclude adhesion as load transfer mechanism for

Vu,x.

Annex A, Section 1.3.2, Loading on anchor channels – load transfer mechanisms

Load transfer mechanism for Vu,x

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Reasons for our proposal:

- Adhesion between concrete and a smooth steel surface isunreliable and may be destroyed by shrinkage, temperature variation, sustained or cyclic loading.

- Adhesion is not accepted in ACI 318 as a load transfermechanism.Example: Smooth round bars need anchorage bymechanical interlock (hook, end anchor, transversewelded bars) and, in general, are not allowed forseismic loading.

Hilti did not comment on this proposal.

Load transfer mechanism for Vu,x

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Annex A, Section 1.3.2 Special Inspection

Proposal Hilti

• Anchor channels with a positive load transfer mechanism (e.g. by

welding) may be installed under periodic special inspection.

• For anchor channels with serrated channel lips and corresponding

locking channel bolts, continuous special inspection shall be

performed during anchor channel installation, including component

attachment and torqueing of channel bolts.

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Proposal Hilti

• In addition, the tests to evaluate Vse,x shall be performed on

anchor channels where the channel bolts are torqued with

Tinst = 20 Nm (15ft-lb).

• Depending on channel bolt diameter, this torque moment may be

<10% of Tinst required in the MPII.

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Proposal IEA

Continuous special inspection shall be performed during installation of all types of anchor channels.

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Reasons for our proposal:a) Load transfer by welding: • Welding of the thin channel lips (t ≈ 1.5 to 3 mm; 0.06 to 0.12 in) is difficult,

especially on recessed anchor channels.• A recces (gap) of 3 mm (0.12 in) is “accepted” in AC232.• Recessed channels are not uncommon on site, even in test members.• Welding needs restrictions on composition of channel steel. Not given in

AC232.

Sketch of a recessed anchor channel


40 mm

≈ 3 mm

concrete surfacefixture to be welded

10.75 mm

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b) Load transfer mechanism other than welding:

• These load transfer mechanisms are not commonly

in use yet.

• Anchor channels with this load transfer mechanism

may or may not be sensitive to installation

inaccuracies.


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c) Load transfer by mechanical interlock (anchor channels with

serrated channel lips)

• Anchor channels with serrated channels lips used in Germany are not

much sensitive to a reduction of the applied installation torque – not

more than most torque controlled expansion anchors which may be

installed under periodic special inspection.

• However, shape and dimensions of serrations are not defined.

Therefore, anchor channels with serrations may be on the market that

are much more sensitive to a reduced torque moment during

installation.

• Therefore, continuous special inspection should also be performed for

these types of anchor channels.


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0,00

0,20

0,40

0,60

0,80

1,00

1,20

0 20 40 60 80 100 120

V u,m

(x%

Tins

t)/V

u,m

(100

%Ti

nst)

[-]

Applied torque [%] (100% =Tinst=180 Nm)

Channel with serrated lips

Channel with notching channel bolts

Influence of ratio applied torque moment to required value on the resistance to longitudinal shear loads (medium channel size, M16). Anchor channels with notching channel bolts are not covered by the proposal of Hilti.


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In the future installation safety tests should be used toevaluate the sensitivity of an anchor channel toinstallation inaccuracies and to decide if continuous orperiodic special inspection is required for a certain anchor channel. This concept is used for post-installed bonded anchors.

These tests will be proposed for the Hearing in June2015.

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Proposal Hilti for testing and anchor channels and

evaluation of measured failure loads:

In service condition tests no. 14 and no. 15 (monotonic and seismic longitudinal shear) the channel bolts shall be torqued with Tinst = 20 Nm (15 ft-lb).

Vsl,x (ESR) = α·Vsl,x (test) with α = 0.33 or 0.5.

Proposal IEA:

In service condition tests no. 14 and no. 15 the channel bolts shall be torqued according to Section 5 (Tinst according to MPII, after 10 minutes reduced to 0.5Tinst to take account of relaxation).

Vsl,x (ESR) = α·Vsl,x (test) with α = 0.33.

Annex A, Section 17.4.3, 17.5.3 and 18.18

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Reasons for our proposal:

a) For post-installed anchors the following concept has been successfully used since many years:

Ø In service condition tests the anchors are installed according to the MPII

-> Characteristic strength Fn

Ø In additional installation safety tests anchors are installed deviating from the MPII, e.g. torque controlled expansion anchors are installed with T=0.5Tinst (MPII)

-> strength reduction factor φ

-> reduction of Fn (if required)


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b) Concept proposed by Hilti deviates significantly from the one valid for post-installed anchors

Ø Depending on channel bolt diameter Tinst = 20 Nm may be ≤ 0.1Tinst(MPII) for anchor channels which must be torqued (e.g. anchor channel with serrated lips).

Ø Vsl,x is evaluated from results of installation safety tests andmultiplied with a reduction factor (influence of installation is taken into account twice).

If torque controlled expansion anchors, approved for cracked concrete, would be tested with T = 0.1Tinst (MPII), many anchors would fail in cracked concrete.

The authors do not see any reason, why the test conditions for anchor channels should be much more stringent than for post-installed torque controlled expansion anchors.


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c) Concept proposed by IEA agrees with the concept valid for post-installed anchors.

The proposed reduction factor α = 0.33 on the characteristic resistances evaluated from tests series no. 14 is considered as a conservative placeholder for the results of installation safety tests.

Such tests will be proposed for the Hearing in June 2015.


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Proposal Hilti:

Ø number of anchors unlimited

Ø Vu,x may be distributed to na ≤ 3 anchors

Ø For anchor channels close to an edge the shear load Vu,x shall be distributed uniformly to the 3 anchors closest to the edge (see Fig. RD1.1.1.5)

Annex A, Section D.3.1.1.5 Distribution of shear load Vu,x

Fig. RD3.1.1.5 Example for the calculation of anchor forces of anchor channels loaded by a shear force acting in the longitudinal direction of the anchor channel

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Proposal IEA:

Ø Limit number of anchors on anchor channels to na ≤ 3, if Vu,x > 0

Ø Distribute Vu,x uniformly to the anchors

Reason for our proposal:The provisions for the distribution of Vu,x are not clear for anchor channels with many anchors.


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2750 mm (9 ft)

1 2 3 4 5 6 7 8 9 121110

250 mm (9.8 in)

Vu,x

Vu,y,1

Nu,1

Vu,y,2 > Vu,y,1

Nu,2 > Nu,1


The designer has several options:a) Distribute Vu,x to anchors 1 to 3.b) Distribute Vu,x to anchors 10 to 12.c) Distribute Vu,x to anchors 1 to 3 and in another verification to anchors 10 to

12.d) Distribute Vu,x to anchor 1.e) Distribute Vu,x to anchors 6 to 8.

While the opitions b) and c) are very conservative, the designer may not believe that Vu,x acting over anchor 1 should be distributed to anchors 10 to 12. Options d) and e) are allowed (when strictly following the test), but are not conservative.

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Ø A more refined model for the distribution of Vu,x to the anchors of an anchor channel with many anchors is needed.

This model is not available yet.

Ø Therefore the number of anchors on an anchor channel should be limited to na ≤ 3 if Vu,x >0.

Ø Anchor channels with na ≤ 3 anchors cover about 90% of all applications.

Ø This approach has been used in Europe for a CEN Technical Report on the design of anchor channels loaded by longitudinal shear forces.


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We recommend to accept the proposal by Hilti with the following main modifications:

a) Annex A, Section 1.3.1: Limit the number of anchors to 3 if Vu,x > 0.

b) Annex A; Section 1.3.2: Exclude adhesion for the transfer of Vu,x from the channel bolt via channel and anchors into the concrete.

c) Annex A; Section 1.3.2: Require continuous special inspection for all types of anchor channels.

d) Annex A, Section 17.4.3 and 17.5.3: Torque the channel bolts according to Section 5.5 (same procedure as for torque-controlled expansion anchors).

e) Annex A, Section 8.18: Vsl,x(ESR) = Vsl,x(test)/3 (accepted by Hilti).

Summary

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ØWhen following our conservative proposal, the approach for testing, assessment and special inspection during installation follows the approach accepted for post-installed anchors since many years.

ØOnly test conditions and acceptance criteria are missing for installation safety tests to evaluate the strength reduction factor φ, a reduction of the value Vsl,x(ESR) (if required) and the type of special inspection (periodic or continuous).

ØThese provisions will replace the current requirements on special inspection and the conservative reduction factor 1/3 in 18.18.

ØCorresponding provisions will be proposed for the Hearing in June 2015.

Summary

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Thank you for your attention.

Comments on Draft for Longitudinal Shear in AC 232

February 2015Florian Julier

AC232-0215-R1 #2

Content:

1. Scope definition

2. Special inspection requirements

3. Torque requirements for Anchor channel qualification

Loads on Anchor channels – longitudinal shear

Dead Load

Existing products in the market:

Toothed/Serrated ChannelsSmooth Channel combined with notching bolts

Hilti proposal for Scope:1.3.2 Loading of Anchor Channels:… the longitudinal loads shall be transferred by a positive load transfermechanism (e.g. mechanical interlock between the channel bolt and thechannel profile).Load transfer in the longitudinal direction:

- shall not rely solely on friction, and- shall not be dependent on the level of pretension in the channel bolt(s)

Exception: Where continuous special inspection is provided duringanchor channel installation, including component attachment and …The anchor channel shall be flush with the concrete surface and thechannel and channel nuts shall be provided with serrations to resistlongitudinal shear.

Exception

Scope – Current Possible Solutions:“ the longitudinal loads shall be transferred by a positive load transfer mechanism (e.g. mechanical interlock between the channel bolt and the channel profile).”

Adhesion and grouting• High temperature behind curtain wall• High temperature changes • Cleanness of Surface

Welding• Welding on jobsite required• No reliable level of quality• Some products are cold formed -no welding

Summary: Adhesion, grouting and welding should be excluded from the scope

Requirements for inspection according IBC:“Exception: Where continuous special inspection is provided during

anchor channel installation…”

2006 IBC section 1704.3.3 requires periodic inspection for bearing type connections

Requirements for inspection - Proposal

“Exception: Where continuous special inspection is provided during anchor channel installation…”

IBC standard requires periodic inspection for bearing type connections

Proposal for a first & conservative step:

Delete the exception and require continuous inspection for all anchor channels with longitudinal loads in AC 232.

Installation Toques for Qualification–Hilti Proposal

7.14 Shear Tests on Anchor Channels Loaded in Longitudinal Channel Axis (Table 4.2, Test No. 15):

7.14.3 General test conditions: The tests shall be performed on anchor channels cast into concrete with two anchors located with the maximum anchor spacing and the distance between the end of the channel and the anchor axis to the minimum value specified by the manufacturer for the tested channel size. Place a PTFE layer (or other friction limiting material of similar friction coefficient) over the entire contact area between fixture and concrete surface. The channel bolt shall be inserted in the channel and pre-tensioned to a maximum of 20Nm (15ft-lb).

Torques for Post Installed Anchor Qualification

In ACI 355.2 and AC 193 reduced installation effort is considered by reducing the MPII specified torque to 0.5 Tinst

Relaxation of anchors is considered by releasing Tinstto 50% after 10 minutes.

Torques for Post Installed Anchors MPII

0

1/8

2/8

2/8

3/8

4/8

5/8

6/8

6/8

7/8

1

0 50 100 150 200

d [in

ch]

Tinst [ft.-Ibf.]

Diameter via Tinst

ESR-2818

ESR-1917

ESR-3037

14,75 ft.-lbf.

Gap 65 ft-lbf

Hilti proposal Tmean15 lb-ft. 80 ft-lbf.

Common sizes for channel bolts are 5/8 & 3/4

Installation Toques for Post Installed Anchors

8.18 Assessment of the Steel Strength Under Shear Load Acting in Longitudinal Channel Axis (Test No. 15 in accordance with Table 4.2):

The measured failure loads shall be normalized according to Section 8.1.4 Eq. (8.3). The 5 percent-fractile of the normalized measured failure loads shall be computed by Eq (8.5). This value shall be multiplied with 0.5 and denoted as Vsl,x and reported in Section 9.5 of this annex.

Hilti proposal reduces nominal value by 0.5 to consider reduced installationeffort, which is conservative.

Additional limitation to a fixed value for very small torque is not in accordance with ACI and ICC codes for post installed anchors.

Summary

1. Scope should be defined – adhesion, grouting and welding excluded

2. Same inspection requirements for all products with longitudinal shear

3. Qualification and torques should follow AC 193 and ACI 355.2

Thank you very much for your attention!

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Documents

Proposed Revisions to the Acceptance Criteria for Anchor Channels