The New Zealand Steel Structures Standard states that high strength structural bolts shall be supplied to AS/NZS 1252. This standard underwent a major revision and was published on 23rd December 2016. The major technical changes incorporated in the new edition relate to updated testing and conformity requirements, the inclusion of the nominated European standard EN 14399-3 8.8 HR bolt as a “Deemed to satisfy” alternative and an additional European EN 14399-3 high tensile grade 10.9 HR.
A significant change to AS/NZS 1252 has been the creation of a new Part 2, title “Verification testing for bolt assemblies’. This represents a restricted form of third party conformity assessment, to provide confidence in products manufactured to AS/NZS 1252.1.
Threaded bars are commonly used in the structural engineering industry. It is used as replacement for long bolts as well as for concrete anchors and foundation bolts. This product is not covered under New Zealand Standard AS/NZS 1252, ‘High strength steel bolts with associated nuts and washers for structural engineering’. This article is intended to provide information on the appropriate standard to specify for threaded rods used for foundation bolts and the recommended verification testing.
Structures designed to the Steel Structures Standard, NZS 3404, are required to be able to resist collapse under a maximum considered earthquake as directed by the Loadings Standard, NZS 1170.5. Brittle systems are not permitted. The nature of steel material is that it always contains some imperfections, albeit of very small size. When subject to tensile stress these imperfections (similar to very small cracks) tend to open. If the steel is insufficiently tough, the 'crack' propagates rapidly, without plastic deformation, and failure may result. This is called 'brittle fracture', and is of particular concern because of the sudden nature of failure. The toughness of the steel, and its ability to resist this behaviour, decreases as the temperature decreases. In addition, the toughness required, at any given temperature, increases with the thickness of the material. A convenient measure of toughness is the Charpy V-notch impact test. This test measures the impact energy (in Joules) required to break a small, notched specimen by a single impact blow from a pendulum. The tests are carried out with the specimens at specified (low) temperatures, and the steel material standards specify the required minimum impact energy values for different grades.
There are three common seismic frame types used in New Zealand. These are the eccentrically braced frame (EBF), concentrically braced frame (CBF) and moment resisting frame (MRF). See figure 1. All steel seismic-resisting systems are required to be classified into one of four categories for seismic design in accordance with the Steel Structures Standard, NZS 3404. The category of seismic frame designed will determine the displacement demand on an individual member of that seismic frame. Members of seismic frames are classified into 4 categories in the same manner as for the seismic resisting frame. Material requirements specified in NZS 3404 become more stringent for member categories associated with higher displacement demand.
The identification of the seismic member categories and the subsequent specification of appropriate steel grades in the contract documents, is the responsibility of the design engineer. This article identifies what the typical seismic member categories are for three common seismic frame types used in New Zealand and identifies complying material types for these seismic member categories.
Bridge construction is a demanding application typically requiring plate welded sections. Material optimisation for welded plate sections involves a careful matching of strength requirements with available plate sizes to minimise waste and welding requirements and to reduce transportation costs. This optimisation process is an iterative one. Typically a preliminary design is undertaken to determine the required plate thicknesses, dimensions and splice locations. Once material availability has been confirmed with a steel distributor, the designer continues to refine the design to suit actual plate availability and supply lead-times. To minimise the number of iterations in this process, it is helpful to have a feel for the range of plate sizes produced by steel mills. Typically local steel distributors will stock only a limited range of commonly used standard sheet sizes, however most are willing to explore stock options for special projects. 10mm loss of plate width should be allowed for in the cutting process.
The refurbishment or â€˜adaptive re-useâ€™ of existing buildings currently forms a significant part of the workload for many architects and engineers. The structural engineer will be required to make an appraisal of the existing steelwork in these buildings. This article provides sources of information for identifying the properties and making an assessment of the historical structural steelwork.
This article provides an update on the corrosion performance of the State Highway 1 Mercer weathering steel off-ramp, following an inspection by Japanese expert Dr. Makoto Ohya, two and half years after construction.
Amendment 2 of the Steel Structures Standard NZS 3404:1997 released in October 2007 (SNZ, 2007) sets new requirements for steel materials and welding of seismic resisting steelwork that is required to sustain significant plastic deformation under design earthquake events. These changes relate to the selection of steel materials in Table 12.4 and the selection of welding consumables and weld heat input in cl. 18.104.22.168. In some instances these requirements exceed the provisions in the material supply standards for steel sections to AS/NZS 3679 (SAA/SNZ, 1996), plate to AS/NZS 3678 (SAA/SNZ, 1996) and tube to AS1163 (SAA, 1991) A review of steels currently available in the New Zealand market indicates that most will comply with these requirements.
Bridge construction is a demanding application requiring heavy hot rolled and plate welded sections. These types of sections often fall outside the range of standard sections and plates held by local steel distributors. To aid bridge designers in their choice of materials, information is provided in this article for non-standard plate sizes and large hot rolled I sections, which are available to Japanese Standards.
Occasionally the question is asked as to whether a steel that has been certified to a lower steel supply grade classification eg. G250 can be accepted as complying with a higher grade such as G300 on the basis that the Mill Test Certificate associated with its batch has a higher yield and ultimate tensile strength than the minimum required for the higher grade. The answer revolves around the issue of confidence levels in the materials properties derived from large and small statistical samples.
There are three main purchase options for structural steel sections and plates in the New Zealand market depending on the project scale. The first and second ex - stock supply options are normally more costly than the third indent supply option because of overhead costs such as handling, storage and financing. Other issues should also be considered such as steel quality levels. Sometimes it is better to pay a little more for quality products from known reputable suppliers rather than risk poor end application performance. Fluctuations in demand and supply around the world can affect availability and price.