Assessment of Fasteners to Concrete: A Tribute to Rolf Eligehausen

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ASSESSMENT OF FASTENERS TO CONCRETE A TRIBUTE TO ROLF ELIGEHAUSEN

Lennart Elfgren

¹

*, Rasoul Nilforoush

¹

, Martin Nilsson

¹

, Ulf Ohlsson

¹

1

Luleå University of Technology, Luleå, Sweden

*Corresponding Author Email: lennart.elfgren@ltu.se

ABSTRACT

Some examples are given of assessment of fastenings to concrete structures and the work started by Rolf Eligehausen in fib Task Group 2.9 “Fastenings to structural concrete and masonry”. Studies have been made on e.g. the influence of creep on adhesive anchors and of surface reinforcement and size effects on headed anchors.

1 Introduction

There is often a need to assess the capacity of existing structures. However, the basis for a good assessment is the knowledge of how the structures behave and can be modelled. A large step in di- rection of establishing this for fasteners was taken when Rolf Eligehausen in 1987 initiated a Task Group on Fastenings to Reinforced Concrete and Masonry Structures.

2 Task Group on Fastenings to Reinforced Concrete and Masonry Structures

The first meeting of the Task Group took place in Stuttgart in November 1987, see Figure 1. It was organized as Task Group VI/5 within Comité Européen du Beton (CEB). The goals of the Group were:

(a) to compile and compare the available research results on the behaviour of fastenings systems (b) to propose a consistent approach based on current empirical and theoretical models for the design of fastenings

(c) to develop design methods that account for the effects of fastenings and the loads they carry on the behaviour of the structures to which they are attached.

The group met about twice a year and a first state of the art report was published in 1991, CEB (1991)

^1,2

. Then a first guideline was published in 1995, CEB (1995)

³

. Revised versions of the gui- deline were published in 1997 and in 2011 by the new organization fib (Fédération internationale du béton – International Federation for Structural Concrete) in fib Bulletin 58 (2011)

⁴

. The new organization was a merge between CEB and FIP (Fédération Internationale de la Précontrainte - International Federation for Prestressing). The Task Group has in-between been renamed to Task Group 2.9 “Fastenings to structural concrete and masonry”. A text book was also published, first in

3rd International Symposium on

Connections between Steel and Concrete

Stuttgart, Germany, September 27th -29th , 2017

216

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German and later in English, Eligehausen et al. (2006)

⁵

. Work is now in progress to make a revision of the guide to include new aspects as assessment of existing anchors.

Figure 1. Photo from first meeting of CEB Task Group on fastenings in Stuttgart, November 4

^th

, 1987. From left: Lennart Elfgren, Johann Tshositsch, Rüdiger Tewes, Klaus Latenser, Werner Fuchs, Kent Gylltoft, Vicky Covert, Rolf Eligehausen, Hans-Dieter Seghezzi,

Elisabet Vintzéleau, B Blache, Paul Hollenbach and Harry Wievel.

3 Anchor bolts for foundations

Works for anchor bolts in machine foundations was started in Sweden in 1978, Elfgren et al. (1980, 1982)

^6,7

. The idea to use adhesives for the bonding was brought up and a study visit was made to Rolf Eligehausen in Stuttgart and to producers of anchors. We then started tests on fatigue and longtime properties, Elfgren et al. (1988)

⁸

4 Fracture Mechanics

A way to understand the size effect in anchor bolts was to use the fracture mechanics theory.

RILEM had two consecutive Task Groups on this and they arranged round robin tests and analyses of anchors, Elfgren et al. (1989, 1998, 2001)

^{9, 10, 11}

, see Figure 2. In the theory of fracture mechanics, the ratio of the elastic energy to the fracture energy was studied. Based on such studies Eligehausen & Sawade (1989)12 proposed a formula for the capacity F

_max

[N] of an anchor to be

F_max = 2,1∙ (E_c

G

_f

)

^1/2

∙ h

_v^3/2

(1) where E

_c

is the modulus of elasticity of the concrete [N/m

²

], G

_f

is the fracture energy of the concrete [Nm/m

²

] and h

_v

is the embedment depth of the anchor [m]. Here the exponent of the depth is reduced to 1,5 from the earlier used value of 2, to consider a size effect on the tensile ca- pacity of anchors. The size effect predicts that at ultimate load, the tensile stresses in the concrete averaged over the fracture surface decreases as the thickness of concrete component increases (Eligehausen et al. 2006)

⁵

. This idea was much spread later, see e.g. Ohlsson (1995)

¹³

, Eligehausen et al. (1998)

¹⁴

.

Stuttgart, Germany, September 27th -29th , 2017 First Author, Second Author and Third Author

later in English, Eligehausen et al (2006)⁵. Work is now in progress to make a revision of the guide to include new aspects as assessment.

Figure 1. Photo from first meeting of CEB Task Group on fastenings in Stuttgart, November 4th, 1987. From left: Lennart Elfgren, Johann Tshositsch, Hans Rüdiger Tewes, Klaus Latenser, Werner

Fuchs, Kent Gylltoft, Vicky Covert, Rolf Eligehausen, NN, Elisabet Vintzéleau, B Blache , Paul Hollenbach and Harry Wievel.

3 Anchor bolts for foundations

Works for anchor bolts in machine foundations was started in Sweden in 1978, Elfgren et al (1980, 1982)^6,7. The idea to use adhesives for the bonding was brought up and a study visit was made to Rolf Eligehausen in Stuttgart and to producers of anchors. We then started a tests on fatigue and longtime properties, Elfgren et al (1988)⁸

4 Fracture Mechanics

A way to understand the size effect in anchor bolts was to use fracture mechanics. RILEM had two consecutive Task Groups on this and they arranged round robin tests and analyses of anchors, Elfgren et al (1989, 1998, 2001)^{9, 10, 11}, see Figure 2. In the theory of fracture mechanics, the ratio of the elastic energy to the fracture energy was studied. Based on such studies Eligehausen & Sawade (1989)¹² proposed a formula for the capacity Fmax [N] of an anchor to be

Fmax = 2,1∙ (Ec Gf)^1/2 ∙ hv3/2 (1) where Ec is the modulus of elasticity of the concrete [N/m²], Gf is the fracture energy of the concrete [Nm/m²] and hv is the embedment depth of the anchor [m]. Here the exponent of the depth is reduced to 1,5 from the earlier used value of 2 reflecting a smaller size effect than earlier when the load was supposed to be proportional to the square of the depth hv (proportional to the area of the break-out cone). This idea was much spread, see e.g. Ohlsson (1995)¹³, Eligehausen et al (1998)¹⁴

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Figure 2: Round Robin Analysis and Tests, Elfgren et al. (1998,2001)

^9,11

.

The second RILEM group had its first meeting in Stuttgart 1993. They primarily studied tension in reinforced concrete prisms, Elfgren & Noghabai (2001)

¹⁵

.

5 Assessment of Structures

The assessment of structures is often divided into three phases: Initial, Intermediate and Enhan- ced and you stop when you get satisfying results. An example of a general procedure is shown in Figure 3, see e.g. Schneider (1994)

¹⁶

, Schneider & Vrouwenvelder (2017)

¹⁷

, SB-LRA (2007)

¹⁸

. ISO 13822 (2010)

¹⁹

and Paulsson et al. (2016)

²⁰

. When assessing the capacity of anchors in a special structure, e.g. in a power plant applications, there can also be a need to subdivide the phases in three steps as: (1) a global seismic analysis, (2) a local pull-out analysis of an anchor, and (3) an updated global analysis including piping and anchor stiffness.

In methods based on the reliability, the probability p

_f

is studied for the case that the Load Effect (E) is larger than the Resistance (R), see e.g. Figure 4. When the curves overlaps and E > R there is a certain risk for failure, see e.g. Schneider (1997)

¹⁶

, EC Reliability (2005)

²¹

.

The variabilities of the load effect and the resistance have a great effect on the load that can be applied to a structure. If by testing, it can be shown that the variability of the resistance can be narrowed; the load-carrying capacity can be increased considerably. This is an argument for producers and construction companies to keep track of the variability in the capacity of installed anchors by e.g. proof loading procedures.

218

First Author, Second Author and Third Author

Fig Figure 2. Round Robin Analysis and Tests, Elfgren et al. (1998,2001)^9,11.

The second RILEM group had its first meeting in Stuttgart 1993. They primarily studied tension in reinforced concrete prisms, Elfgren & Noghabai (2001)¹⁵.

5 Assessment of Structures

The assessment of structures is often divided into three phases: Initial, Intermediate and Enhanced and you stop when you get satisfying results. An example of a general procedure is shown in Figure 3, see e.g. Schneider (1994)¹⁶, Schneider & Vrouwenvelder (2017)¹⁷,SB-LRA (2007)¹⁸. ISO 13822 (2010)¹⁹ and Paulsson et al (2016)²⁰. When assessing the capacity of anchors in a special structure, there can also be a need to subdivide the phases in three steps as e.g. in a power plant: (1) a global seismic analysis, (2) a local pull-out analysis of an anchor, and (3) an updated global analysis including piping and anchor stiffness,

In methods based on the reliability, the probability pf is studied for the case that the Load Effect E is larger than the Resistance, R, see Figure 4. When the curves overlaps and E > R there is a certain risk for failure, see e.g. Schneider (1997)¹⁶, EC Reliability (2005)²¹

The variabilities of the load effect and the resistance have a great effect on the load that can be applied to a structure. If by testing it can be shown that the variability of the resistance can be narrowed the load-carrying capacity can be increased considerably.

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Figure 3: Flow chart for assessment of existing bridges and other structures. Three phases are identified: Initial, Intermediate and Enhanced depending on the complexity of the questions

involved, Schneider (1994)

¹⁶

, Paulsson et al (2016)

²⁰

.

Stuttgart, Germany, September 27th -29th , 2017 First Author, Second Author and Third Author

Figure 3. Flow chart for assessment of existing bridges and other structures. Three phases are identified: Initial, Intermediate and Enhanced depending on the complexity of the questions

involved, Schneider (1994)¹⁶, Paulsson et al (2016)²⁰. First Author, Second Author and Third Author

Figure 3. Flow chart for assessment of existing bridges and other structures. Three phases are identified: Initial, Intermediate and Enhanced depending on the complexity of the questions

involved, Schneider (1994)¹⁶, Paulsson et al (2016)²⁰.

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6 Recent work

Long time sustained loading tests on adhesive anchors were started in Sweden in 1981. Two types adhesive anchors (type A and B) were exposed to various in-service conditions and subjected to sustained tension loads of 15, 30 and 45 kN (i.e. approximately 23, 47 and 70% of their mean ulti- mate short-time capacities, respectively) over more than 28 years. The experiments were termina- ted in 2013 and the final results and evaluations reported in Nilforoush et al. (2016)

²²

. The curves of creep displacement versus time for the tested adhesive anchors are shown in Figure 5. The test results showed that the creep deformation increases by increasing the sustained load level. Results indicate that the tested bonded anchors did not fail indoors when subjected to sustained loads up to 47% of their mean ultimate short-time capacity. However, the long-term performance was substantially impaired outdoors, presumably due to temperature and humidity variations, leading to failure for sustained loads higher than 23% of the anchors’ mean ultimate short-time capacity.

Based on the results of long-term experiments, the reliability and suitability of the current testing and approval provisions for qualifying adhesive anchors subjected to sustained tension loads was evaluated and several recommendations were provided (see Nilforoush et al. 2016

²²

).

Figure 5: Creep displacement versus time for M16 adhesive anchors of type A and B exposed to various in-service conditions and different sustained tension load levels (Figures reprinted

from Nilforoush et al. 2016

²²

).

Work on modelling of the influence of surface reinforcement, member thickness, anchors head size and cracked concrete has recently been carried out in collaboration with Stuttgart. The full descrip- tions and evaluations of the numerical studies on single cast-in-place headed anchors are given in Nilforoush et al. (2017a, b)

^23,24

. Based on these studies, it was found that the tensile breakout capacity of headed anchors increases with increasing member thickness; anchor head size and/or if orthogonal surface reinforcement is present (see Figure 6). Based on the numerical results, the CC method was refined by incorporating three modification factors to account for the influence of anchor head size, member thickness and surface reinforcement.

220

Lennart Elfgren, Rasoul Nilforoush, Martin Nilsson and Ulf Ohlsson

6 Recent work

Long time sustained loading tests on adhesive anchors were started in Sweden in 1981. Two types adhesive anchors (type A and B) were exposed to various in-service conditions and subjected to sustained tension loads of 15, 30 and 45 kN (i.e. approximately 23, 47 and 70% of their mean ultimate short-time capacities, respectively) over more than 28 years. The experiments were terminated in 2013 and the final results and evaluations reported in Nilforoush et al. (2016)²². The curves of creep displacement versus time for the tested adhesive anchors are shown in Figure 5. The test results showed that the creep deformation increases by increasing the sustained load level.

Results indicate that the tested bonded anchors did not fail indoors when subjected to sustained loads up to 47% of their mean ultimate short-time capacity. However, the long-term performance was substantially impaired outdoors, presumably due to temperature and humidity variations, leading to failure for sustained loads higher than 23% of the anchors’ mean ultimate short-time capacity. Based on the results of long-term experiments, the reliability and suitability of the current testing and approval provisions for qualifying adhesive anchors subjected to sustained tension loads was evaluated and several recommendations were provided (see Nilforoush et al. 2016²²).

(a) M16 anchors of type A (b) M16 anchors of type B

Figure 5: Creep displacement versus time for M16 adhesive anchors of type A and B exposed to various in-service conditions and different sustained tension load levels (Figures reprinted

from Nilforoush et al. 2016²²).

Work on modelling of the influence of surface reinforcement, member thickness, anchors head size and cracked concrete has recently been carried out in collaboration with Stuttgart. The full descriptions and evaluations of the numerical studies on single cast-in-place headed anchors are given in Nilforoush et al. (2017a, b)^23,24. Based on these studies, it was found that the tensile breakout capacity of headed anchors increases with increasing member thickness; anchor head size and/or if orthogonal surface reinforcement is present (see Figure 6). Based on the numerical results, the CC method was refined by incorporating three modification factors to account for the influence of anchor head size, member thickness and surface reinforcement.

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Nilforoush et al. (2017c)

²⁵

carried out also supplementary experimental studies to verify the nu- merical results and evaluate the validity of the proposed refined model. The experimental results showed very good agreements with the numerical results.

The proposed refined model may be used for the design of new cast-in-place headed anchors as well as for the assessment of existing anchors.

Recently, Nilforoush et al. (2017d)

²⁶

studied the tensile behavior of single cast-in-place headed anchors in plain and steel fibre-reinforced normal- and high strength concrete base materials. The experiments showed an increase of approximately 25-50% on the tensile breakout capacity when steel fiber is present in concrete.

Figure 6. (a) Load-deflection curves of anchor bolts (h

_ef

= 200mm) in uncracked and pre-cracked plain and reinforced concrete slabs, (b) Load-deflection curves of anchor bolts (h

_ef

= 200mm) with various head sizes in plain concrete members (Figures reprinted from Nilforoush et al. 2017 a, b

^23,24

).

7 Summary

Some examples have been given for the assessment of fastenings to concrete structures and on the influence of sustained tension loads on the long-term behaviour of adhesive anchors and on influ- ence of surface reinforcement, anchor head size and embedment depth of headed anchors.

8 Acknowledgements

Friendly support from Rolf Eligehausen over many years of cooperation is greatly appreciated. The financial support from the Swedish Council for Building Research, Energiforsk and Elsa and Sven Thysells Stiftelse is thankfully acknowledged as well as cooperation with many other colleagues in the Task Group of Fasteners to Structural Concrete and Masonry.

Stuttgart, Germany, September 27th -29th , 2017 Lennart Elfgren, Rasoul Nilforoush, Martin Nilsson and Ulf Ohlsson

Nilforoush et al. (2017c)²⁵carried out also supplementary experimental studies to verify the numerical results and evaluate the validity of the proposed refined model. The experimental results showed very good agreements with the numerical results.

The proposed refined model may be used for the design of new cast-in-place headed anchors as well as for the assessment of existing anchors.

Recently, Nilforoush et al. (2017d)²⁶ studied the tensile behavior of single cast-in-place headed anchors in plain and steel fibre-reinforced normal- and high strength concrete base materials. The experiments showed an increase of approximately 25-50% on the tensile breakout capacity when steel fiber is present in concrete.

0 100 200 300 400

0.0 2.0 4.0 6.0 8.0 10.0

Load [kN]

Displacement [mm]

Uncracked (unreinforced) Uncracked (Lightly-reinf.) Uncracked (Over-reinf.) Cracked (Lightly-reinf.) Cracked (Over-reinf.) CC method (Uncracked) CC method (Cracked)

0 100 200 300 400

0.0 1.0 2.0 3.0 4.0 5.0

Load [kN]

Displacement [mm]

Small head Medium head Large head CC Method

(a) (b)

Figure 6. (a) Load-deflection curves of anchor bolts (hef = 200mm) in uncracked and pre-cracked plain and reinforced concrete slabs, (b) Load-deflection curves of anchor bolts (hef = 200mm) with various head sizes in plain concrete members (Figures reprinted from Nilforoush et al. 2017 a, b^23,24).

7 Summary

Some examples have been given for the assessment of fastenings to concrete structures and on the influence of sustained tension loads on the long-term behaviour of adhesive anchors and on influence of surface reinforcement, anchor head size and embedment depth of headed anchors.

8 Acknowledgements

Friendly support from Rolf Eligehausen over many years of cooperation is greatly appreciated. The financial support from the Swedish Council for Building Research, Energiforsk and Elsa and Sven Thysells Stiftelse is thankfully acknowledged as well as cooperation with many other colleagues in the Task Group of Fasteners to Structural Concrete and Masonry.

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References:

1. CEB (1991): Fastenings to Reinforced Concrete and Masonry Structures. State-of-art-report - Part I and Part II, CEB Bull. No 206 & 207, 2 volumes 562 pp. Comité Euro-International du Béton 2. CEB (1994): Fastenings to concrete and masonry structures, state of the art report. Comité Euro- International du Béton (CEB). Bull. No. 216. Printed rev. ed. of CEB Bulletins 206 and 207.

3. CEB (1995) Design of fastenings in concrete – Design Guide - Parts 1 to 3. Comité Euro-Inter- national du Béton (CEB), CEB Bulletin d‘Information 226. Revised as Bull. 233.

4. fib Bulletin 58 (2011): Design of anchorages in concrete – Guide to good practice, fib special activity group 4, International federation for concrete (fib), Bulletin 58, 2011, 280 pp.

5. Eligehausen, R., Mallée, R and Silva, J.F.: Anchorage in Concrete Construction, Ernst & Sohn, 2006

6. Elfgren, L., Broms, C.E., Johansson, H.E. and Rehnström, A. (1980): Anchor Bolts in Reinforced Concrete Foundations. Short Time Tests. Res. Report TULEA 1980:36, University of Luleå, Sweden, 7. Elfgren, L., Cederwall, K., Gylltoft, K., and Broms C.E. (1982): Fatigue of anchor bolts in reinforced concrete foundations. IABSE Report 37,1982, pp. 463-470.

8. Elfgren, L.; Anneling, R.; Eriksson, A. and Granlund, S.-O. (1988) Adhesive anchors. Tests with cyclic and long-time loads. Swed. Nat. Testing Institute, SP-RAPP 1987:39, Borås

9. Elfgren, L (Editor) (1989): Fracture mechanics of concrete structures. From theory to applica- tions. Chapman & Hall, London, London 1989, 407 pp (ISBN 0 412 30680 8).

10. Elfgren, L., Editor (1998): Round Robin Analyses and Tests of Anchor Bolts in Concrete Structures. RILEM TC 90-FMA. Research Report 1998:14, Luleå University of Technology, 446 pp.

11. Elfgren, L., Eligehausen, R. and Rots, J. (2001): Anchor bolts in concrete structures:

Summary of round robin tests and analysis arranged by RILEM TC 90-FMA ‘Fracture Mechanics of Concrete-Applications’. Materials and Structures, Vol. 34, No 8, pp 451-457.

12. Eligehausen, R and Sawade, G (1989): A fracture mechanics based description of the pull-out behaviour of headed anchors embedded in concrete. Section 13.2 in “Fracture mechanics of con- crete structures. From theory to applications”, Editor L Elfgren, Chapman & Hall, London 1989, pp 281-289. (ISBN 0 412 30680 8).

13. Ohlsson, U (1995): Fracture Mechanics Analysis of Concrete Structures. Doctoral Thesis 1995:179D. Luleå University of Technology, 113 pp. Available at http://ltu.diva-portal.org/

14. Eligehausen, R., Bouska, P., Cervenka, V. and Pukl, R. (1998) Size effect of the concrete cone failure load of anchor bolts. In Ref. 10, pp. 7-1—7-20 Available at http://ltu.diva-portal.org/

15. Elfgren, L. and Noghabai, K, Editors (2001): Tension of Reinforced Concrete Prisms.

Round Robin Analysis and Tests on Bond. RILEM TC 147-FMB. Research Report 2001:13, LTU.

16. Schneider, J. (1994): Sicherheit und Zuverlässigkeit im Bauwesen. Grundwissen für Ingenieure. Mit H.-P. Schlatter. vdf Hochschulverlag ETH Zürich, 188p., ISBN 978-3-7281-2167-3.

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17. Schneider, J. & Vrouwenvelder, T. (2017): Introduction to Safety and Reliability of Structures. 3rd Ed. Structural Engineering Documents No 5, (IABSE), c/o ETH Zürich, Switzerland, 164 pp.

18. SB-LRA (2007): Load and Resistance Assessment of Railway Bridges. Guideline developed in the EC-FP6 Project Sustainable Bridges 428 pp. Available at www.sustainablebridges.net

19. ISO 13822 (2010): Basis for design of structures – Assessment of existing structures, Genéve ISO, 46 pp.

20. Paulsson, B., Bell, B., Schewe, B., Jensen, J. S., Carolin, A., and Elfgren, L. (2016). Results and Experiences from European Research Projects on Railway Bridges. 19th IABSE Congress Stockholm 21-23 September 2016, Zürich, 2016, pp. 2570 – 2578. ISBN 978-3-85748-144-4.

Available at http://ltu.diva-portal.org/

21. EC Reliability (2005): Implementations of Eurocodes. Handbook 2. Reliability backgrounds, Leonardo da Vinci Pilot Project, 254 pp. Available at

http://www.eurocodes.fi/1990/paasivu1990/sahkoinen1990/handbook2%5B1%5D.pdf

22. Nilforoush, R., Nilsson, M., Söderlind, G. and Elfgren, L. (2016): Long-Term Performance of Adhesive bonded Anchors. ACI Structural Journal, ISSN 0889-3241, Vol. 113, No 2, pp. 251-261.

23. Nilforoush, R., Nilsson, M., Elfgren, L., Ozbolt, J., Hofmann, J., and Eligehausen, R. (2017a):

Tensile Capacity of Anchor Bolts in Concrete: Influence of Member Thickness and Anchor’s Head Size. ACI Structural Journal, MS S-2016-220, paper in press.

24. Nilforoush, R., Nilsson, M., Elfgren, L., Ozbolt, J., Hofmann, J., and Eligehausen, R. (2017b):

Influence of Surface Reinforcement, Member Thickness and Cracked Concrete on Tensile Capacity of Anchor Bolts. ACI Structural Journal, MS S-2016-236, paper in press.

25. Nilforoush, R., Nilsson, M., Elfgren, L. (2017c): Experimental Evaluation of Influence of Member Thickness, Anchor-Head Size, and Orthogonal Surface Reinforcement on the Tensile Capacity of Headed Anchors in Uncracked Concrete. Journal of Structural Engineering, ASCE, MS-STENG 6117, paper in press.

26. Nilforoush, R., Nilsson, M., Elfgren, L. (2017d): Experimental Evaluation of Tensile Behavi- our of Single Cast-in-Place Anchor Bolts in Plain and Steel Fibre-Reinforced Normal- and High Strength concrete. Engineering Structures, Vol. 147 (2007), pp. 195-206.