Keynote Speakers

Reidar Bjorhovde

President of The Bjorhovde Group

Tucson, Arizona, USA

STEEL FOR FUTURE STRUCTURES AND DESIGN CODES

Over the past twenty years structures have become increasingly complex, with corresponding performance needs that must reflect the design and the behavior of the structure. Long spans and large heights are combined with high demands for strength, ductility and service behavior. As a result, engineers and architects continue to look for solutions that may offer higher strength materials and structural systems. Expanded design codes is one option, but due to issues of reliability and general acceptance within the construction industry, this approach will require significant time and especially additional and costly research.

Higher strength steel is certainly one option, but such materials typically may mean higher cost but also the benefit of a lower steel mass. The very high strength steels that are used for cranes and similar structures may not be suitable for buildings and bridges, due to limited ductility, especially for connections. It is anticipated that significant research work on structural systems and especially connections will be needed. Economically, the size of the ultra-high strength (UHS) steel market is likely to be small, making practical inroads difficult if not impossible. As a result, the practicality of ultra-high strength steels is a challenging concept.

However, for most structures the governing performance criterion is not strength, but serviceability. In general terms, this means structural stiffness and limited deformations. Steel that is currently classified as high strength, such as material with yield stress of 350 to 450 MPa, has excellent deformation capacity and proven ductility, especially when used for all types of connections. And there is also the issue that stiffness is heavily governed by the magnitude of the modulus of elasticity (E), which is the same value for all strength grades of structural steel.

The best solutions therefore may involve the use of the mid-strength (MSS) grades of steel, along with different but especially highly efficient structural systems that can make full use of the governing criteria such as strength, stiffness and energy absorption.

Dennis Lam

Professor

School of Engineering

University of Bradford

UK

BEHAVIOUR OF DEMOUNTABLE SHEAR CONNECTORS IN COMPOSITE BEAMS WITH PROFILED DECK FLOORING

The use of sustainable materials for infrastructure is a global issue and reducing material use is a top priority for the construction industry throughout the world. Much of the environmental impact from the construction industry is associated with the consumption of resources and generation of waste. The construction industry in Europe consumes over 70,000 million tonnes of materials each year and generates over 250 million tonnes of waste. Composite flooring formed by connecting the concrete slabs to the supporting steel beams has been widely used for many years and is well established as one of the most cost-effective floor systems in multi-storey steel frame building structures. Composite action between steel beams and concrete slabs through the use of shear connectors are responsible for a considerable increase in the load-bearing capacity and stiffness of the steel beams, which in turn results in significant savings in steel weight and construction cost. However, shear connectors are welded through the steel decking to the steel beams and cast into the concrete; this made deconstruction and reuse of the steel components almost impossible. A demountable shear connector has been developed and tested to assess its potential and suitability in terms of replacing the traditional welded shear connectors. A series of push tests were carried out to investigate the load-slip behaviour of the demountable shear connectors in composite floor with profiled decking. Test results have shown that these shear connectors can be easily demounted after test and have a similar capacity and behaviour of the welded shear connectors. In addition, test results showed that these demountable shear connectors process high ductility in comparison with the equivalent welded shear connectors.

J.Y. Richard Liew

Professor

Dept. of Civil and Environmental Engineering

National University of Singapore

Singapore

DESIGN OF COMPOSITE COLUMNS WITH HIGH STRENGTH STEEL AND CONCRETE

High strength concrete and high tensile steel are becoming very attractive materials for high-rise buildings because of the need to reduce member size and structural self-weight. However, limited test data and design guidelines are available to support the applications of high strength materials for building constructions. This paper presents significant findings from comprehensive experimental investigations on the behaviour of tubular columns in-filled with ultra-high strength concrete at ambient and elevated temperatures. A series of tests was conducted to investigate the basic mechanical properties of the high strength materials, and structural behaviour of composite columns under compression and bending. High tensile steel with yield strength up to 780MPa and ultra-high strength concrete with compressive cylinder strength up to 180MPa were used to construct the concrete filled specimens for testing at ambient and elevated temperature. The test results were compared with the predictions using a modified Eurocode 4 approach. In addition, more than 2000 test data samples collected from literature on concrete filled steel tubes with normal and high strength materials were also analysed to formulate the design guide for implementation in practice.

David Nethercot

Emeritus Professor

Imperial College

London, UK

THE CHANGING BASIS FOR THE DESIGN OF COLD FORMED PURLIN SYSTEMS

Cold formed purlins, when used to support the roof cladding on industrial buildings, constitute one component in a complex structural system. Their shape, that has evolved through a mix of forming possibilities and structural efficiency, is often complex. Thus their resulting structural behaviour, involving combinations of bending, various forms of buckling and twisting, requires sophisticated treatment if all important features are to be included. If the interactions with their adjacent components, including provision for bolted connections, is also to be treated, then a formidable structural challenge results. Design has, however, generally relied on simplified treatments supported by full scale testing. More recently, numerical analysis has made it possible to model all important features, with the result that new insights into behaviour have emerged. The paper will reflect on the limitations of early approaches, will demonstrate how numerical analysis can be employed and will show how the results can form the basis for generally applicable yet simple to use design methods that reflect all important facets of behaviour.

Kim J.R. Rasmussen

Professor

School of Civil Engineering

University of Sydney

Australia

SECOND ORDER EFFECTS IN LOCALLY UNSTABLE STEEL STORAGE RACK FRAMES

Local and distortional buckling reduce the flexural and warping rigidities of steel frames. As a result, the sway buckling load of locally unstable unbraced frames is reduced and sway deflections increase at a faster rate than corresponding locally stable unbraced frames. This leads to greater second order moments and potentially premature collapse, since commonly, unbraced steel storage racks are designed using elastic analyses (2^nd order or 1^st order with moment amplification) which assume unreduced values of the flexural and warping rigidities.

The paper investigates the effects of second order moments induced by local and/or distortional buckling of the uprights of steel storage frames. First, an experimental program is presented featuring 5m tall single-bay rack frames with ultra-thin perforated uprights. The tests were conducted using a dual-actuator set-up to simulate vertical gravity loads. Full details of the rack frames were obtained prior to testing, including material properties and measurements of local and member imperfections as well as frame out-of-plumb. Subsequently, the paper presents the calibration of finite element models against the experimental tests and the use of advanced FE analyses to predict the strength of steel storage rack frames. The FE strengths are compared to design strengths predicted by current specifications for steel storage racks and conclusions are drawn about the extent to which current specifications are able to accommodate second order moments generated by local and/or distortional buckling.

Benjamin W. Schafer

Professor

Department of Civil Engineering

Johns Hopkins University

Baltimore, USA

SEISMIC RESPONSE AND ENGINEERING OF COLD-FORMED STEEL FRAMED BUILDINGS

Buildings framed from thin-walled cold-formed steel members are becoming increasingly common. A great deal is known about how individual members behave, but little is known about full structural systems. Testing on individual shear walls has been used in the past to provide insights and create safe designs, but modeling of these sub-systems or the building as a whole has remained out of reach. As a result, seismic performance-based design, or other design methodologies that require a robust understanding of response have remained out of reach for cold-formed steel framed buildings. Recently, a team lead by Professor Schafer completed a multiple year U.S. National Science Foundation funded project and companion industry-funded projects taken together under the abbreviated name: CFS-NEES. Major deliverables in the CFS-NEES effort include: experimental shear wall testing, characterization, and modeling; experimental cyclic member testing, characterization, modeling, and design; and, complete building design, modeling, and shake table testing. The research enables performance-based design by providing the necessary building blocks for developing nonlinear time history models of buildings framed from cold-formed steel. In addition, the experiments demonstrate the large difference between idealized engineering models of the seismic lateral force resisting system and the superior performance of the full building system. The full-scale testing and modeling work will be emphasized in this talk. Significant work remains to bring the findings to design practice, and this effort is both ongoing and an area of future need.

Luís Simões da Silva

Professor

Faculdade de Ciências e Tecnologia

University of Coimbra

Coimbra, Portugal

ON THE SAFETY OF STABILITY DESIGN RULES FOR STEEL MEMBERS

The stability design rules for steel columns, beams and beam-columns are classically based on Ayrton-Perry formulations that rely on the calibration of imperfection factors in order to estimate the maximum resistance. More recently, "general approaches" have been proposed that combine in-plane and out-of-plane behavior based on a single reference length.

The safety of design rules in modern codes of practice is based on the use of partial safety factors and the separation of the uncertainty related to loading and resistance. EN 1990 - Annex D, for example, contains a procedure for the safety analysis of resistance functions, based on First-Order Reliability Methods. However, its application to stability design rules is not straightforward and several additional assumptions are necessary to ensure that a target probability failure is achieved.

Finally, design rules and its accuracy depend on the accuracy of the relevant basic variables such as material properties, geometric properties and imperfections. It is therefore required to appropriately characterize the statistical distributions of these basic variables in order to comply with the (semi-) probabilistic safety level assessment of design rules.

This paper discusses the different approaches for the derivation of design rules for columns, beams and beam-columns and proposes a mechanical consistent general approach. Secondly, a consistent procedure for the probabilistic assessment of the safety level of stability design rules is presented that simplifies the way material and geometrical properties are considered while maintaining its statistical relevance. Finally, the "European database of steel properties" that was developed in the framework of the European RFCS project SAFEBRICTILE is presented and discussed.

Ben Young

Professor

The University of Hong Kong

Hong Kong, China

COLD-FORMED HIGH STRENGTH TUBULAR SECTIONS OF STEEL GRADE
UP TO 1100MPa

This paper presents the experimental investigation of cold-formed high strength carbon steel tubular stub columns and beams. The nominal 0.2% proof stress of the specimens ranged from 700 to 1100 MPa. The experimental program focuses on the square hollow sections (SHS), rectangular hollow sections (RHS) and circular hollow sections (CHS). The material properties and geometric imperfections of the specimens were first measured. The compressive behaviour at cross-section level was studied through testing 25 stub columns. The test results including modulus of elasticity, 0.2% proof stress and ultimate strength for the complete sections were also determined. The cross-sectional flexural behaviour were investigated through 26 four-point bending tests.

The load-deformation histories and failure modes of the stub columns and beams were analysed. The experimental results were compared against design values calculated from the European, Australian and North American standards. The compactness criteria of tubular sections were assessed by comparing the section slenderness to the slenderness limits in the standards. Improved section slenderness limits and design recommendations are proposed in this study.

Riccardo Zandonini

Professor

University of Trento

Trento, Italy

PROGRESSIVE COLLAPSE: THE CASE OF COMPOSITE STEEL-CONCRETE FRAMES

Vulnerability of structures to progressive collapse and mitigation of the effects of local damages are topics widely discussed inside the scientific community. Recent terrorist attacks, and the consequent huge loss of human lives, pointed out the need of new design criteria with the aim to design robust structures, i.e. structures able to withstand exceptional loads without being damaged to an extent disproportionate to the original cause. Event control, specific local resistance, redundancy, design of key elements, compartmentalisation and detailing rules are some of the preventive strategies proposed by the more recent studies and by design standards.

In this lecture the main facets of this problem are first briefly illustrated, the approaches recommended by the standards, and in particular by the EN 1991-1-7, are also reviewed. The key outcomes of some important studies related to composite steel-concrete frames are then presented, and potential practical implications are underlined.

Finally, the work carried on within European research projects to which the University of Trento has being contributing is discussed. In particular, the ongoing project comprises of full scale testing of composite sub-frames, and the related results provide an insight into the 3D action as an important factor in preventing progressive collapse by activating alternate path of resistance.