UK FOREWORD

INTRODUCTION

This book is the second in a series of joint publications of the European Convention for Constructional Steelwork (ECCS) and The Steel Construction Institute (SCI). It will help provide UK engineers and designers with a better understanding of the design principles in the European Code of Practice for the Design of Steel Structures (EN 1993 - Eurocode 3), and the individual Part 1.8 (EN 1993-1-8) for the design of steel joints, in particular. Reference will also be made to the associated Eurocodes for concrete (EN 1992 - Eurocode 2), and earthquake engineering (EN 1998 - Eurocode 8). The book complements other SCI design guides, e.g. SCI (2011 and 2013a).

The current publication is a first UK edition based on the first edition of the ECCS manual for the Design of Joints in Steel and Composite Structures, which was published in 2016. The scope of this UK edition is narrower than the original publication in two specific areas that are felt to be of limited interest to current practice in the UK; there is no content covering composite joints, and chapter 9 (covering frame design strategies) has been shortened to simply give an overview of strategies. In this edition, the reader will find information that is either of a general nature, or relevant to specific sections of the publication, to facilitate its application in a UK context.

The content includes useful background to the code rules, and a reminder of some engineering principles. Information is presented in the context of the Eurocodes terminology and notation, and with reference to clause numbers, etc. to help the reader's familiarity with EN 1993-1-8. The document that serves as the basis for content and numbering of clauses is the UK implementation of EN 1993-1-8:2005 incorporating corrigenda December 2005 and July 2009. At the time of writing (Autumn 2016), part 1-8 of EN 1993 is being revised with publication anticipated after 2020.

Finally it is noted and should be accepted that there will inevitably be some differences of interpretation between the ECCS recommendations and those previously published by the SCI.

GENERAL COMMENTS

Design of joints (connections) in steel structures in the UK is covered by BS EN 1993-1-8 and its national Annex (NA) (BSI, 2010). BS EN 1993-1-8 has clear definitions for connections and joints. The term joint refers to the zone where members are interconnected, and a connection is the location where elements meet. The distinction in terminology is usually emphasised in Continental Europe but traditional practice in the UK is to use the generic term connection.

The following partial safety factors are defined in the UK NA.

Specific reference to the UK Building Regulations is also made in this UK edition. The UK Building Regulations code has structural integrity requirements in order to avoid disproportionate collapse, and ensure robustness of the complete structure. Disproportionate collapse can be defined as a region of structural collapse out of proportion to the initiating cause of failure. The most relevant to connection/joint design is that the detailing for every beam member must be adequate to resist a minimum horizontal force at ultimate failure (or tying resistance). Guidance on the design values of tying forces can be found in BS EN 1991-1-7 Annex A and its UK annex (BSI, 2014).

SECTION SPECIFIC COMMENTS

Section 1.1.1

UK practice normally considers only two main forms of joint modelling, simple and continuous. Ideally frames designed on the basis of simple joint modelling should use joints between members that possess negligible rotational stiffness, and transmit the beam reactions in shear into the columns without developing significant moments. These joints may be treated as perfect pins. Members can then be designed in isolation, either as (predominantly) axially loaded columns, or as simply supported beams. Joints designed within the principles of continuous modelling are capable of transmitting significant moments and are able to maintain the original angle between adjacent members virtually unchanged. This form of construction of steel frames is particularly advantageous when beam deflections are critical or if bracing systems are not possible.

To comply with one or the other design assumption, steelwork connections are detailed as simple or moment-resisting joints. Because joints in frame structures are often treated by adopting a degree of standardisation, the designer usually details the connections in accordance with the principles in the series of SCI Green Books, which present this material in the form of step-by-step design procedures and tables covering standard arrangements.

Other approximate analysis methods continue to be used among British designers, the most representative being the wind-moment method for the design of unbraced multi-storey steel frames. The method assumes that (i) under gravity loads the beam-to-column joints act as pinned connections, and (ii) under horizontal wind loads these joints are rigid, although it lacks transparency. This method can be seen as a manifestation of the semi-continuous principles. Wind moment connection details can be found in SCI (1995a). With the now widespread use of software in design offices for rigorous second-order analysis, it is doubtful if the use of the wind-moment method should be advocated any longer, other than for initial sizing.

Section 1.3

Although S460 is attracting growing interest, S355 is still the dominant grade of steel used in the UK.

Section 1.6.1

With reference to Figure 1.20 it is noted that (site) welded joints and angle flange cleats are rarely, if ever, used in the UK.

Section 1.6.2

Hybrid connections are rarely used in the UK for the reasons noted.

Section 1.7.2

There is not much use of CoP© in the UK. There are a number of other software tools on the market, some more comprehensive than others.

Section 2.2

Although in UK design practice most aspects related to joint modelling have developed to a high degree of sophistication, proper consideration of the structural interaction between the joints and members is rarely fully taken into account. It is indeed very difficult to predict and model the complex behaviour of a joint (strength, stiffness and rotation capacity). Traditional UK practice is to predict joint behaviour on the basis of past experience.

BSI (2010) separates the modelling of the rotational behaviour of the connection elements (that is, the bolts, the welds, the plates, etc.) and that of the column web panel in shear to characterise the overall joint behaviour. As a simplification, the two sources of joint deformation are combined by means of the so-called transformation parameter ß. This coefficient is related to the web panel internal actions and can be calculated from the moments at each side of the web panel. The ß parameter is currently only defined for same-depth beams which meet at the same level. In many practical double-sided joints, beams will not be of the same depth, or at the same height. BS EN 1993-1-8 is silent on such joints.

Section 2.3

As explained above, typical UK design practice does not consider this sophisticated procedure for idealisation of joints.

Section 2.4

The NA to BS EN 1993-1-8 gives practical guidance to classify joints. It also states the following:

"Until experience is gained with the numerical method of calculating rotational stiffness given in BS EN 1993-1-8:2005, 6.3 and the classification by stiffness method given in BS EN 1993-1-8:2005, 5.2.2, semi-continuous elastic design should only be used where either it is supported by test evidence according to BS EN 1993-1-8:2005, 5.2.2.1(2) or where it is based on satisfactorily performance in a similar situation."

Evidence on values of column base stiffness for practical global analysis is given in SCI (2006).

Section 2.5

The concept of ductility classes is not widely applied in the UK. It is generally assumed that standard connection details to the SCI Green Books provide enough rotation capacity and so behave in a ductile manner, as...