| doi:10.3850/978-981-08-6218-3_SS-We009 |
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EXTENSION OF EN 1993-1-8 JOINT DESIGN RULES TO HIGH STRENGTH STEEL GRADES
A. M. Girão Coelho1,2,3,a and F. S. K. Bijlaard1,b
1Structural and Building Engineering, Delft University of Technology, Delft, The Netherlands.
aa.m.girao@clix.pt
bf.s.k.bijlaard@tudelft.nl
2Polytechnic Institute of Coimbra, Department of Civil Engineering, Coimbra, Portugal.
3Institute of Computers and Systems Engineering of Coimbra (INESC-Coimbra), Coimbra, Portugal.
EXTENDED ABSTRACT
In current European Standard EN 1993, the moment-rotation behaviour of beam-to-column joints made from steel grades with a yield stress > 460 N/mm2 are obtained from elastic design procedures. The strength of the joint basic components is thus limited to the yield resistance rather than the plastic resistance. With the recent developments of higher strength steel grades, the need for these restrictions should be revisited. This paper collects the principal results from research investigations into the characterization of the rotational response of moment joints with components made from high strength steel in the framework of the component method. Using these findings, suggestions for revising the Standard are made.
Steel industry has recently made an effort to re-engineer structural steels to have improved performance in tensile stress, toughness, weldability and corrosion for use in bridge and building applications. This has led to the development of a new class of steels, generally classified as high strength steels (minimum yield stress greater than 460 N/mm2). The material properties of high strength steels are different from mild carbon steel in several key areas of the uniaxial stress-strain relationship. These steels have neither a well-defined yield plateau nor a substantial strain-hardening modulus as compared with mild steel grades. In general, high strength steels are also less ductile than mild steels. These characteristic differences in the mechanical properties of high strength steels are at issue in light of the assumptions that current European code of practice for the design of steel structures (EN 1993 – Eurocode 3) makes in its prediction of the ultimate response of structural steel members and joints. Part 1-12 of EN 1993 that specifically deals with the design rules for members and joints made from these new classes of steel does not permit inelastic design procedures for such elements because there is not sufficient background research work that shows that these accommodate plastic deformations. The strength of members and joints is thus limited to the elastic resistance (yield in the outer fibre) rather than the plastic resistance. With the recent developments of higher strength steel grades, the need for these restrictions should be revisited.
The research summarized in this paper represents key outcomes from an extensive series of investigations into the nonlinear behaviour of moment joints fabricated from high strength steel grades, from a strength and ductility standpoints. The aim of the programme is to investigate whether high strength steel joints with yield stresses in the range from 460 MPa to 960 MPa can be designed according to existing rules of the EN 1993-1-8 (Design of joints) or whether these rules need to be modified to include high strength steel. These objectives are addressed below through detailed experimental testing and nonlinear finite element (FE) analyses of joint components.
High strength steels are most efficient when they develop their full yield stress, i.e. when structural elements are designed on the basis of strength. Research studies at Delft University of Technology have indicated that the benefits of the use of these steels can be utilized in plastically designed braced frames, for which stiffness, in the form of deflections or drift limits for complying with serviceability limit states, does not govern design. EN 1993 adopts the semicontinuous/ partially-restrained philosophy for the design of this type of framing in recognition of the economic benefits. Designing steel and composite frames on a semi-continuous/partiallyrestrained basis requires the modelling of beam-to-column joints. The characteristics of a beam-to- column joint are best described by the moment-rotation relationship. Current design practice adopts the component method for predicting this relationship. The design basis consists of first identifying all active components for a given structural joint, including the fasteners, end plate in bending (modelled as a T-stub), column web panel in shear, column web in transverse compression caused by bearing, etc. The individual component force-deformation response is then characterized and finally those elements are assembled in form of a mechanical model made up of extensional springs and rigid links. This spring assembly is treated as a structure whose forcedeformation behaviour is used to generate the moment-rotation curve of the full joint and predict the three key properties of moment resistance, rotation capacity and initial rotational stiffness.
The principles of the semi-continuous/partially-restrained design approach rely upon a reserve of ductility and toughness in the material. Partial strength joints must be capable of rotating plastically to the extent necessary for compatibility with beam end (and possibly column) rotations under design load. They must be designed to exhibit ductile response through plastic deformation developing simultaneously in the web panel zone and the connection(s), in order that the intended failure mechanism of the whole structure can be formed prior to fracture of the joint. Inelastic actions should then be concentrated in those components that provide good ductility and satisfy high deformation demands. The present study is limited to the case of bare steel joints whose behaviour is governed by three possibilities:
1. joint tension capacity in the assembly end plate — bolts, which is modelled by means of equivalent T-stubs;
2. web panel zone capacity;
3. column web in compression capacity.
The research data and the key outcomes are summarized in this paper. Currently used EN 1993-1-8 design criteria for joints are revisited in the light of limited experimental and numerical evidence.
The most important conclusions are briefly summarized:
1. formulae provided in the code for prediction of the individual component resistance is generally conservative irrespective of the above governing conditions;
2. in the case of a bolted connection, although the probability of a true brittle fracture of the bolts would appear to be high, evidence suggests that the deformation capacity of the high strength steel plates is still quite significant;
3. the potentially critical issue of providing adequate deformation capacity can be resolved by setting requirements to performance indicators, such as the resistance and ductility indices that mainly depend on the geometrical and mechanical characteristics of the joints and their individual components;
4. these studies also show that there is no sudden and dramatic degradation in the extent of the ‘plateau’ in the moment-plastic rotation (or force-plastic deformation) behaviour of joints (or individual joint components) fabricated from high strength steel.