FREQUENTY ASKED QUESTIONS TO STRUCTURAL CONNECTIONS

František Wald Czech Technical University in Prague

Czech Republic E-mail: Wald@fsv.cvut.cz

David B. Moore British Constructional Steelwork Association

Great Britain E-mail: David.Moore@Steelconstruction.org

ABSTRACT The European project Continuing Education in Structural Connections (CeStruCo) under Leonardo da Vinci initiative No. CZ/00/B/F/PP-134049 was prepared by partners from seven European countries to disseminate the latest results in research and standardization during the period of transferring the European Pre-Standard into the European Standard. The project has started by a collection of questions from the European practice. The answers to those questions have been prepared in the form of textbooks in the project partners national languages. The educational material is available in textbooks and as an easily accessible Internet/CD (www.fsv.cvut.cz/cestruco) media, and includes video and audio files, slides and worked examples.

1 INTRODUCTION Education has always been seen as an essential part of the introduction and dissemination of new methods for the design of steel connections. One of the first educational packages specialised on connections was produced by Owens and Cheal [1] who prepared educational material for structural connections. This material has been extended and is now incorporated into a European educational package called the European Steel Design Educational Programme, ESDEP [2]. This programme is used today by educational establishments throughout Europe. Other educational packages which build on the work of ESDEP are available some of which include WIVISS [3], a set of lectures on CD, SteelCall [4], a virtual steel designers office, and SSEDTA which consists of a set of basic lectures on PowerPoint for the design of steel and composite elements, see [5]. The project NFATEC - A New and Flexible Approach to Training for Engineers in Construction [6], is transferring the successful lessons of SSEDTA into the Internet based lectures and worked examples. Project SEFIE creates the Eurocode discussion forum on website and strong e-support of the structural steel design.

For more than twenty year the European Convention for Constructional Steelwork’s Technical committee for structural connections (ECCS TC10) has supported the development and implementation of a common set of design rules for steel connections. It is therefore not surprising to find that one of this committee’s priorities is to facilitate the transition of EN1993-1-8: 2004 from a European pre-standard; see [7], [8] and [9], to a full Euro-norm [10]. Part of this activity is the development of the necessary educational material to encourage designers throughout Europe to adopt EN1993-1-8: 2004. Consequently, a programme called “Continuing Education in Structural Connections” (CeStruCo) was formed under the European Commission’s Leonardo initiative to collect commonly asked

questions on the background, implementation and use of prEN1993-1-8 and to publish expert answers to these questions, see Fig. 1. The CeStruCo project was developed from an idea by Mr. Marc Braham (Astron, Luxembourg), Mr. Jan Stark (TU Delft, The Netherlands) and Mr. Jouko Kouhi (VTT, Finland) to provide designers with more detailed information on the background and implementation of the design methods given in prEN1993-1-8: 2004.

Fig. 1: Textbooks in project partner’s languages

The project partners were Aristotle University of Thessaloniki, Greece; Bouwen met Staall, Netherlands; Building Research Establishment Ltd., United Kingdom; Czech Constructional Steelwork Association Ostrava, Czech Republic; Czech Technical University (contractor), Czech Republic; EXCON a.s., Prague, Czech Republic; KREKON Design office, Rotterdam, Netherlands; Luleå University of Technology, Sweden; University of Coimbra, Portugal; Politechnica University of Timisoara, Romania. The project team consist of the following people: Prof. C. C. Baniotopoulos (in charge of Chapters on Welding and on Aluminium); Prof. F. S. K. Bijlaard; Ir. R. Blok (internal review), Mr. J. Brekelmans; Prof. L. S. da Silva (Chapter on Fire design); Prof. D. Dubina (Chapter on Seismic design); Mrs. M. Eliášová; Mr. H. G. A. Evers (Chapter on Good and bad detailing); Mr. D. Grecea (Chapters on Hollow section connections and on Cold-formed connections); Ir. A. M. Gresnigt (Chapter on Moment connections); Dr. V. Janata (internal review); Prof. B. Johansson; Mr. T. Leino;

Mr. T. Lennon; Mr. T. Měřínský (internal review); Dr. D. B. Moore (Chapter on Simple connections and editor); Mrs. A. Santiago; Mr. R. L. Schipholt; Dr. Z. Sokol (Chapter on Structural modelling, Windows help Internet lessons); Ir. C. M. Steenhuis; Dr. M. Veljkovic (Chapter on Bolts); Prof. F. Wald (project promoter, editor and Chapter on Column bases). The material was reviewed externally by Prof. D. Beg, Mr. M. Braham, Prof. J. P. Jaspart, Dr. G. Huber, Mr. J. Kouhi, Prof. F. Mazzolani, Mr. A. J. Rathbone, Prof. J. Studnička, Dr. F. Turcic, Dr. K. Weynand and Mr. N. F. Yeomans.

Q&A 2.1 Deformation criteria for bolt bearing resistance Bearing design is more concerned with avoiding excessive hole deformations than with avoiding actual failure of the connection. Comparison of the design formula for bearing with tests confirms this point. Could you give the background to the deformation criteria that has been adopted in the derivation of the formula?

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The traditional background of most codes indicates the resistance Fexp;1,5 is limited to a deformation of 1,5 mm, see [Owens at al, 1999]. The resistance for the structural members obtained from the tests to failure Fexp;fy/fum is evaluated by reducing the resistance from structural material strength fum to the characteristic yield strength fy, see [Bijlaard et al, 1989]. The procedure is used in form Fexp;fy/fum = 0,9 Fexp;ult fy / fum if a brittle rupture occurs [Snijder et al, 1988a]. The conventional (elastic) limit of resistance Fexp;conv defines the resistance as the intersection of a straight line with the initial stiffness and of a straight line having the slope equal to stiffness divided by ten, which is drawn as a tangent to the non-linear part of the curve, see Figure 2.6, test [Piraprez, 2000]. The conventional resistance depends more on the joint stiffness than on the failure type. Annex D of prEN 1990: 2001 was used for cover plate tests with slotted holes to validate the model of resistance.

0

50

100

150

200

0 5 10 15 20

Deformation, , mm

Force, F, kN

3 mm

exp, convF

exp; 1,5F

δ

Initial stiffnessInitial stiffness /10

Experimental curve of test I 1-3

(for both plates)

exp, fy/fumF

exp, ultF

Figure 2.1 Limits of the resistance of joint; deflection limit Fexp;1,5; ultimate limit Fexp;ult; conventional limit Fexp;conv; reduced limit by steel yield ratio Fexp;fy/fum, [Piraprez, 2000]

Fig. 2: Example from textbook Chapter 2 Bolts

2 COLLECTION OF QUESTIONS The collection was based on publications in national journals and on local seminars. The questions were collected by project internet page and in paper form as well. The questions located during the conversion of ENV 1993-1-1 (including Annexes J [8], K [9], L [7]) into EN 1993-1-8 were taken into account. Together 632 questions were collected related to the topic.

Q&A 7.9 Transfer of Shear Forces by Friction and Anchor Bolts Is it safe to add the friction resistance to the bearing resistances of all the anchor bolts, as clearances in anchor bolt holes are large?

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The shear force model used in prEN 1993-1-8: Clause 6.2.1.2 [prEN 1993-1-8: 2003] is based on the assumption that the bolts loaded by a shear force bend, which results in the development of tension in anchor bolts, see Figure 7.16. The resistance of bolts in bending is small, therefore plastic hinges may form in the bolts. This activates hardening effects of the ductile bolt material, see Figure 7.17. Only the anchor bolts in the compressed part of the base plate may be used to transfer shear force. The shear resistance then consist of the friction resistance at the concrete-base plate interface and the reduced tensile resistance of the bolts Rd.vbRd.tRd.v FnFF += . (7.20) The resistance of the bolts in bearing in the concrete block and in the base plate should be checked separately.

Fv,Rd

δv

Fv

Friction resistance

Nc,Sd

Ff,Rd

δ v,hole δ v

δv,hole

Figure 7.1 The friction resistance and in tension resistance of anchor bolt

Fig. 3: Example from textbook Chapter 7 - Column bases

a)

b)

0,04 rad2,29°

Fig. 4: Example from textbook Chapter 13 - Design cases a) bad solution of column base, b) good design

3 ANSWERING All obtained questions were very good. Answers to 101 selected questions were chosen in the second part of work based on its educative contribution. The agreement between all partners was not reached for all answers. This is the reason that some nice questions are missing in final material. The review of answers was prepared in tree steps: locally between partners, by delegated partners and externally by members of ECCS TC10. An Example from Textbook Chapter 2 Bolts Q&A 2.1 “Deformation criteria for bolt bearing resistance” is shown at Fig. 2. Fig. 3 demonstrates another short from practice the Q&A 7.9 “Transfer of Shear

Forces by Friction and Anchor Bolts”. An Example from Chapter 13 Design cases bad and good solution of column base is presented at Fig. 4.

4 DISSEMINATION Under the third part of the project, dissemination, were prepared the educational materials Textbooks and Internet/CD lectures. To facilitate easy of use questions/answers are split into the following Chapters: Introduction, Bolts, Welding, Structural Modelling, Simple Connections, Moment Resistance, Connections, Column Bases, Seismic Design, Fire Design, Hollow Section Joints, Cold-Formed Member Joints, Aluminium Connections, and Design Cases. Each chapter starts with a brief over-view of the method use in prEN1993-1-8: 2004. This is followed by the commonly asked questions together with their answers. The materials were localised for use in the partner’s national languages, see Fig. 1. 300 copies of textbook in Czech language was printed, 200 copies in Dutch, 1500 in English, 200 in Greek, 1000 in Portuguese, 300 in Rumanian and 200 in Swedish. The translations into Polish and Spanish are under preparation (written in September 2004). The pilot Seminars of the project were organised in project partner’s countries to test the material in local environment and to start to disseminate the material.

Fig. 5: Lesson in Microsoft Windows, example of Chapter Aluminium connections

The pilot Internet Seminar was included in the project. It showed the ability to use in an efficient way the prepared educational material. The pilot Internet seminar was broadcasted to seven project partners from Luleå University. Very robust software MARRATECH was used to establish an interactive communication between partners. The easy to use and simple to managed tool consists of windows showing the view of camera from each partner, a white board and a text messenger. The quality of connection was indicated at each partner window. By clicking on camera window the particular partner’s picture is enlarged to allow high quality projection. The white board allows all partners to draw by mouse and to share with other participants in the session available material such as figures, slides, videos and files (in

MS PowerPoint and Word format). The text message window was very reliable and helped partners to begin communication. The weak point of the Seminar was audio quality which was strongly influenced by the internet traffic. This is an important issue since it is essential part of the lecturing. We all have learned how sensitive is the audio quality, especially in a case of simultaneous broadcasting to various partners. A communication between two partners, using so called node to node communication is more robust and easier to establish. However, our laboratories, well equipped by computers and cameras, have to be upgraded with adequately equipment for video broadcasting. All the partners were pleasantly surprised with the performance of the communication tool under routine Internet connection.

Fig. 6: A tool for nonlinear analysis of steel connections by component method NASCon

5 INTERNET / CD LESSONS

The Internet/CD version, Fig. 5, of project materials is based on Microsoft Windows help format, which is a robust tool for education. The material prepared by RoboHelp tool (14) allows the easy navigation in partner’s languages and in German and French. The Internet/CD lessons are equipped with worked examples, Figs 3 and 4, PowerPoint presentations, slides, videos, worked examples [11] and [12], animations of design cases, educational software for connection modelling, and design tools available round the Europe. Partners were so kind to equipped us with their demo version to show how easy may be nice connection checked by EN1993-1-8:2004. 3000 copies of CD first version were printed at the end of the project in December 2003. The second version, 3000 copies, was disseminated in June 2004 based on evaluation of first ones. Third upgraded version is intended to be published based of interest of structural steel practice in June 2005.

On the CD may be found the NASCon (Non-linear Analysis of Steel Connections) program that offers a computer user-friendly tool for the component method. The tool allows modelling the nonlinear behaviour of different components, see [13]. The video film, see Fig. 7, demonstrates the correct design of T-stub connections and bolted splices to avoid

a fatigue failure of bolts. The related to the fire design is equipped with the PowerPoint lessons including video/audio sequences, see Fig. 8 [14].

Fig. 7: The flow of the stresses in the connection in video film Statically Stressed Bolts in Dynamically Loaded Connections

Fig. 8: Front page of PowerPoint lesson ”Connection Design for Fire Safety”

ACKNOWLEDGMENT The authors wish to dedicate this work to Mr. Martin Steenhuis, our good friend, who worked with us in the field of structural connections for many years, launched this project, and who tragically died in the summer of 2001.

REFERENCES [1] Owens G. W., Cheal B. D., Structural Steelwork Connections, Butterworths, London

1988, ISBN 0-408-01214-5

[2] ESDEP, European Steel Educational Programme, SCI, London, 1994, www.esdep.org

[3] Plank R.J., Wider Vocational Initiative for Structural Steelwork, J. Construct. Steel Research, 46, (1998), pp. 278-279, ISBN 0-08-042997-1

[4] SteelCal, Electronic Education for Steel Construction, steelcal.steel-sci.org

[5] SSEDTA, Structural Steelwork Eurocodes: Development of Trans-National Approach, fp.emberey.plus.com

[6] NFATEC, A New and Flexible Approach to Training for Engineers in Construction, http://www.svf.stuba.sk/kat/KDK/NFATEC_E.htm

[7] ENV 1993-1-1, Design of Steel Structures, Eurocode 3, European Pre-Standard, CEN, Brussels 1992

[8] ENV 1993-1-1-A2, Design of Steel Structures, Annex J, Joint Design, European Pre-Standard, CEN, Brussels 1998

[9] ENV 1993-1-1/A1, Design of Steel Structures, Annex K, Joints in hollow section structures, European Pre-Standard, CEN, Brussels 1994

[10] prEN 1993-1-8, Eurocode 3, Design of Steel Structures, Part 1.8, Design of Joints, European Standard, CEN, Brussels 2004

[11] Jaspart J.P., Renkin S., Guillaume M.L.: European Recommendations for the Design of Simple Joints in Steel Structures, 1st draft of a forthcoming publication of the Technical Committee 10 “Joints and Connections” of the European Convention of Constructional Steelwork, prepared at the University of Liège, September 2003

[12] Wald F. at al, Steel structures 10, Worked examples to Eurocode 3, Czech Technical University in Prague, Prague 2001, p. 146, ISBN 80-01-02308-7

[13] Costa Borges L. A., Probabilistic Evaluation of the Rotation Capacity of Steel Joints Dissertação apresentada à Faculdade de Ciências e Tecnologia para obtenção do grau de Mestre em Engenharia Civil, especialidade de Estruturas, Coimbra, 2003

[14] Wald F., Moore D.B., Veljkovic M., Eliášová M., Continuing education in structural connections, in AISC and ECCS TC 10 Workshop, Amsterdam 2004, in printing

KEYWORDS Connection design, Education, Internet/CD lessons, Frequently asked questions, Bolted connections, Welded connections, Steel joints, Fire design, Seismic design, Aluminium connections, Eurocode 3, Design cases.