Journals >> Abstract VOLUME 18 No. 1 (September 2005)
SESOC MANAGEMENT COMMITTEE
Outgoing President's Report (Dr. B.Davidson)
Incoming President's Report (Mr. Ashley Smith)LETTER TO THE EDITOR: re Introduction of Grade 500QT reinforcing (John Scarry)
TECHNICAL PAPERS
Theoretical Analysis and Real World Design – Jacques Heyman (reprinted from The Structural Engineer Volume 83 No.8)
Jacques Heyman shows that stresses in a real structure can never be calculated uniquely – structural response to loading is governed by small unknown movements imposed by the environment.
The response of a hyperstatic structure to given loading depends on the nature and magnitude of that loading and also on the way the structure is attached to its environment. These two actions can have comparable structural consequences but whereas the loads may be specified accurately, the boundary conditions will in general be unknown. Thus the actual load paths in the structure cannot be known but the engineer is nevertheless required to provide a safe and practical design.
Information is needed for a proposed structure so that it may be designed in accordance with accepted criteria of which the three most important are strength, stiffness and stability; it must be strong enough to resist the action of given loading it must not deflect unduly. and it must not become unstable, either locaIly or overall.
The Theory of Structures is concerned with establishing the state of a structure. Specifically. internal stress resultants (bar forces. thrusts. shears. bending moments) will be in equilibrium with the external loads. These internal stress resultants will produce internal deformations which are calculable from a knowledge of the material properties; this is a problem of stress analysis or Strength of Materials, which deals with local rather than overall behaviour. The internal deformations will lead to displacements of the structure as a whole and these large scale displacements must be compatible with specified boundary conditions. There are thus three different types of statement which can be made about a structure - equilibrium. material properties, and compatibility of deformation. If the first of these, the equations of statics of equilibrium suffice to determine the state, then the structure is statically determinate. The Theory of Structures proper is concerned with hyperstatic structures. for which the statical equations do not yield a unique state, and all three statements must be used to investigate a particular problem.
Factors to Consider in the Use of Grade 500E Longitudinal Reinforcement in the Beams of Ductile Moment Resisting Frames – Nicholas Brooke, Les Megget and Jason Ingham
This paper examines a number of issues surrounding the use of Grade 500 reinforcement in moment resisting frames. These issues include the use of this reinforcement in plastic hinges and its increased bond strength demands and higher overstrength factors relative to previously Grade 430 reinforcement. 1n particular the effect of the recent amendment to NZS 3101:1995 on the design of interior beam-column joints is assessed. It was found that for more than 60% of joints the number of Grade 500 reinforcing bars required to resist a given force was at least twice the number of Grade 300 bars. It is also shown that it is often more expensive to use Grade 500 longitudinal reinforcement than Grade 300. It is concluded that in many cases the use of Grade 500 longitudinal reinforcement in the beams of ductile moment resisting frames is not a practical design solution, but that Grade 500 reinforcement is ideal for limited ductile columns, as transverse reinforcement or in hybrid ductile jointed structures. It is important to note that all steel in this study referred to as Grade 500 is Grade 500E-MA, not QT.
Since the 1970s the strength of reinforcement used for structural purposes in New Zealand has increased. Initially two grades were commonly specified, mild (275MPa nominal yield strength) and high strength (380 MPa nominal yield strength). These strengths were minimum yield strength values. A revised specification for steel reinforcement introduced the use of lower characteristic (fifth percentile) yield strength for designating steel reinforcement. This led to Grade 275 reinforcement being re-designated as Grade 300 reinforcement. Subsequently. Grade 380 reinforcement was replaced with a new 430 MPa steel that was both stronger and more ductile. More recently a desire to standardise material properties and design standards with those used in Australia has led to the much discussed introduction of 500 MPa (Grade 500E) reinforcement in place of Grade 430.
A number of issues surrounding the use of Grade 500 reinforcement (particularly in the beams of moment resisting frames) have been raised by Paulay, Park, Bull and Allington, and Fenwick and Megget. For convenience these issues are summarised below:
- The yield displacement of members constructed using Grade 500 longitudinal reinforcement can be 67% larger than the yield displacement of otherwise identical members constructed using Grade 300 reinforcement. In situations where the member is designed to act as a yielding element in a structure, this leads directly to a 40% reduction in usable displacement ductility. and to a 40% reduction in member stiffness.
- The overstrength factor (the ratio of the maximum strength to the nominal strength of a concrete member) for beams constructed using Pacific Steel Micro-Alloy Grade 500 reinforcement is 1.4. This is higher than the overstrength factor for Grade 300 and the (previous) Grade 430 reinforcement (1.25). The required capacity of many aspects of a capacity designed structure increase in proportion to the overstrength factor of the ductile elements.
- The higher strength of Grade 500 reinforcement requires increased strength from the bond between the concrete and the reinforcement. In many situations this can be achieved by allowing for a longer development length. However. this is not normally possible within beam-column joints, where the demand on bond strength is particularly severe. Therefore the diameter of Grade 500 reinforcement passing through a ductile beam-column joint is severely restricted by the New Zealand Concrete design standard, particularly since the release of Amendment 3 to this standard.
This article focuses on the influence on design of the higher demand on bond strength caused by the use of Grade 500 longitudinal reinforcement.
Reinforcing Steel in New Zealand - Pacific Steel Future Product Range and Other Design Issues – Keith Towl and Graham Burrell
Pacific Steel are extending their product range to include Quench and Tempered (QT) product. This paper forewarns of the coming changes, as well as taking the opportunity to review and discuss various issues pertinent to structural design engineers.
The new reinforcing steel standard, AS/NZS 4671, and grade 500, are now 4 years old. However, there is still significant misunderstanding regarding the product in terms of specification and application. Some site practices which have been anecdotally reported are causing concern in the industry. Pacific Steel are extending their product range in late 2005 to provide both a Micro-alloyed (MA) and a Quench and Tempered (QT) Grade 500E product. This paper attempts to address the above overlapping and converging issues thereby providing the engineer with the knowledge necessary to design, specify and inspect safely and efficiently.
History, technical aspects of AS/NZS4671, methods of manufacture, handling issues, specifying, changes in supply and overstrength factor of these reinforcing steels are all commented on.Quenched and Tempered Reinforcing Steel – John Hare
Pacific Steel's decision to begin the manufacture of Quenched and Tempered (QT) Grade 500E reinforcing steel brings to the fore some issues that have been with us since before the introduction of Grade 500 steel and the joint standard. AS/NZS 4671. The remainder of the paper, endorsed by the SESOC Management Committee, reviews some of the facts and opinions surrounding the use of QT steel and also other grades of reinforcing. Interim recommendations are given for structural engineers to complement those in the Department of Building & Housing Practice Advisory No.7.
Among many other points, the paper notes that QT steel may not be welded, galvanised, hot bent, re-bent or threaded, without changing its mechanical properties, because the hardened outer layer is lost in these processes.Use of Weathering Steel in New Zealand Bridges – Raed Zaki and Dr. Charles Clifton
HERA has published a guideline for the use of weathering-steel in New Zealand bridges. This is HERA Report R4-97, and it covers aspects for designing, construction, inspection, maintenance and even rehabilitation of weathering-steel bridges.
Weathering-steel is a product with a limited and chequered history of use in New Zealand, principally in building cladding applications. This has made engineers wary of its use in bridges, even though, in North America and Europe, there are examples of weathering-steel bridges over 30 years old exhibiting excellent performance, in line with expectations. These examples clearly show that a well designed and detailed weathering-steel bridge in an appropriate environment, can provide an attractive, very low maintenance, economic solution and therefore extend the scope of cost-effective steel use in bridges. The paper offers a summary of the important issues relating to the use of weathering-steel in New Zealand bridges, starting with a brief introduction to the material and what makes it different to conventional constructional steel.
Reinforced Concrete Seating Details of Hollowcore Floor Systems – C.J. MacPherson, J.B. Mander and D.K. Bull (by kind permission of the N.Z. Earthquake Engineering Society)
Recent earthquake engineering research has raised concerns of the seismic performance of precast prestressed concrete hollowcore floor systems. Experimental research showed that with simple detailing enhancements, significant improvement in the seismic performance of hollowcore floor systems can be expected. The present experimental research aims at validating several new detailing enhancements. Based on previous research findings, the present super-assemblage experiment included the following details: (i) a reinforced connection that rigidly ties the floor into the supporting beam, (ii) an articulated topping slab portion cast onto a timber infill solution that runs parallel to the hollowcore units and edge beams, (iii) specially detailed supporting beam plastic hinge zones reducing potential damage to the hollowcore units, (iv) Grade 500E reinforcing steel used in the main frame elements; and (v) mild steel deformed bars in the concrete topping in lieu of the customary welded wire mesh. The full-scale structure was cyclically tested in both the longitudinal and transverse directions to inter-storey drifts of +/-5%. Observations show extremely positive results with minor damage incurred by the hollowcore flooring and the overall performance dictated by the performance of the moment resisting frame. Recommendations for the forthcoming revision of the New Zealand Concrete Standard, NZS 3101 are also made.
Structural Steel Design for Seismic Performance – Clark Hyland, W. George Ferguson and John Butterworth
Structural design engineers require steels forming the primary structural lateral load resisting system of a building to be able to sustain high levels of plastic strain without suffering brittle fracture. The ability to focus plastic deformation into designated hinge zones of the structure allows kinetic energy developed by the structure during an earthquake to be dissipated in a way that minimises the likelihood of sudden collapse. This reliance on the plastic deformation characteristics of steel indicates the need for suitable material performance criteria that can be clearly understood and communicated between the structural design engineer and the steel producer. Most structures fabricated from structural steel do not need to sustain the potentially large cyclic plastic strain levels imposed on earthquake resisting structures. Consequently materials research and development tends to focus on the properties of steels operating well within the elastic stress range of the material. This paper presents some interim results of research being undertaken on the effect of plastic strain and aging on the characteristics of structural steel used in New Zealand seismic resisting structures. Of particular interest is the effect of plastic strain and ageing on the ability of the steel to develop ductile fracture in the presence of sharp notches and cracks, as often occur in fabricated steelwork. To investigate this characteristic, Charpy V-Notch (CVN) and Crack Tip Opening Displacement (CTOD) tests, were undertaken at the University of Auckland on steel taken from the flange of a 310UC158 Grade 300Plus section produced in Australia. The tests were made on the steel after it had been subjected to a range of plastic pre-strain levels and ageing.
Fire Safety and Steel Construction – Issues and Future Developments – Dr. Ian Bennetts
The direct and indirect costs associated with the fire protection of structural steelwork has often been seen as an impediment to the construction of structural steel buildings. As a result the steel industry worldwide has undertaken research into aspects of fire safety in an attempt to develop more cost-effective solutions for fire safety and reduce the cost of fire protection. The development of fire-safety engineering as a discipline has assisted this process and there are now many examples of fire-engineered steel framed buildings with reduced levels of fire protection applied to structural steelwork. This paper describes some of these examples and discusses some of the approaches that have been adopted in justifying these designs. Important issues, particularly in relation to high-rise buildings, are noted. These issues are the subject - or need to be the subject, of further research and investigation. In particular, a better way of quantifying fire severity on large area floors and the development of' appropriate details and measures are needed, to avoid catastrophic building failure. There is an on- going need for cost-effective fire protection measures for structural steelwork to enable solutions that combine protected and unprotected steelwork.
PROJECT CORNER
Christchurch women’s Hospital – Gary Haverland
The new $80m Christchurch Women’s Hospital has been in the planning for some time, and was opened on the 30th March 2005.
This article provides the background to the project and describes aspects of the design and construction of this 9-storey base isolated hospital building. Aspects of seismicity, ground conditions, base isolation and construction method and sequences are covered.Test Your Skill – Solution to Structural Checking Test – Richard Fenwick
(Sesoc Journal Vol 17,No.2 Page 70
NEWS FROM THE REGIONAL STRUCTURAL GROUPS
Auckland Structural Group – Paul Campbell
Canterbury Structural Group – Dene Cook
Waikato Structural Group – Gordon Hughes
Wellington Structural Group – Graeme BeattieThese consist of reports of past meetings, and planned future meetings.
