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.

The Steel Construction New Zealand publication Steel Connect (SCNZ 14.1 and SCNZ 14.2) provides structural engineers with a rapid and cost-effective way to specify the majority of structural steelwork connections, in accordance with accepted fabrication industry norms. Specification of these connections also facilitates the development of reliable cost estimates by designers, fabricators, consulting quantity surveyors and constructors.

Eccentrically braced frames are required to be laterally restrained at both the top and bottom of the active link member ends to ensure reliable performance in a seismic event. There are occasions when direct lateral restraint to the bottom flange is not permitted. In this instance the lateral restraint of the bottom flange of the active link end can be achieved from minor axis bending of the brace. This article presents a simplified procedure for this approach by a way of a design example.

The Link Column Frame (LCF) system is a brace free hybrid system combining proven seismic load resisting technology; eccentrically braced frames (EBF) with removable links and moment resisting frames (MRF). It was developed to meet the requirement for continued occupancy, or at least rapid return to occupancy after a severe earthquake. This is a departure from the prevailing New Zealand seismic design approach pre the 2010/2011 Christchurch earthquake series which involves designing for controlled damage (energy dissipation) in selected elements of the seismic load resisting system which are typically not rapidly or cost effectively repaired. Engineers familiar with the design of EBF and MRF systems will readily understand and be able to apply the LCF frame structural system design concepts. The only departure from standard office practice is the requirement to undertake a non–linear push over analysis. As a result, practicing engineers will likely find the design methodology for this system easier to implement than those for some other low damage seismic load resisting solutions. Useful background information to the LCF system is found in the paper of Dusicka et. al.[1]. It is recommended this is read in conjunction with the present paper which is intended to illustrate the application of the capacity design principles of the Steel Structures Standard (NZS 3404 [2]) to this new system.

Floors consisting of precast ribs supported on structural steel beams and in situ structural topping are a common form of composite floor construction. In this article guidance is given on the modifications required to previously published design guidance for computing shear stud capacities to adapt them for use with precast rib flooring systems. In addition consideration will be given to steel beam top flange stability during construction using this type of flooring system. COBENZ 97 is a popular spreadsheet based programme used for steel composite beam design. Explanation is given to allow designers to use this programme to design composite floor systems utilising precast ribbed floor systems

Steel eccentrically braced frames are expected to sustain damage during a design level earthquake through repeated inelastic deformation of the active link. A method for assessment of the extent of inelastic strain in the active link is currently being developed. Repair is therefore expected to be costly and disruptive, even if the structure has met its goal of providing life safety during an earthquake. The replaceable or removable active link concept addresses these drawbacks. It allows for quick inspection and replacement of damaged links following a major earthquake, significantly minimising the disruption of the structure. The development, research, design methodology and detailing considerations of eccentrically braced frames with removable links are covered in Steel Advisor articles EQK1005 and EQK1006 (the former is in preparation at the time of writing this article). This article illustrates the application of the proposed eccentrically braced frame with removable link design procedure by way of a design example. Understanding of the design of conventional eccentrically braced frames has been assumed. For those not familiar with the seismic behaviour and design of eccentrically braced frames reference should be made to HERA Report R4-76.

Steel eccentrically braced frames are expected to sustain damage during a design level earthquake through repeated inelastic deformation of the active link. Repair is therefore expected to be costly and disruptive, even if the structure has met its goal of providing life safety during an earthquake. The replaceable or removable active link concept addresses these drawbacks as it allows for quick inspection and replacement of damaged links following a major earthquake, significantly minimising the time to reoccupy the building. The development and research of eccentrically braced frames with removable links is covered in Steel Advisor article EQK1005 (in preparation at the time of writing this article). This article presents the design methodology for eccentrically braced frames with removable links. It focuses on the design and detailing of the link, and where necessary, modifications to the design procedure for conventional EBF’s. Readers not familiar with the global seismic behaviour and design of eccentrically braced frames should consult HERA report R4-76. Steel Advisor article EQK1007 provides a worked design example to illustrate the application of the proposed eccentrically braced frame with removable link design procedure.

The Steel Construction New Zealand publication Steel Connect (SCNZ 14.1 and SCNZ 14.2) provides structural engineers with a rapid and cost-effective way to specify the majority of structural steelwork connections, in accordance with accepted fabrication industry norms. Specification of these connections also facilitates the development of reliable cost estimates by designers, fabricators, consulting quantity surveyors and constructors.

It is common and cost effective to separate the building gravity only steel structure from seismic/wind lateral bracing frames. The gravity structure can then be designed as simple construction in accordance with NZS 3404 Steel Structures Standard. In simple construction the bending members may be assumed to have their ends connected for shear only and to be free to rotate. Examples of such connections are shown in figure 1. In beams and column frames for which simple construction is assumed, there will none the less be bending moments acting on the columns which are caused by eccentricity of the beam reactions.

It is common and cost effective to separate the building gravity only steel structure from seismic/wind lateral bracing frames. The gravity structure can then be designed as simple construction in accordance with NZS 3404 Steel Structures Standard. In simple construction the bending members may be assumed to have their ends connected for shear only and to be free to rotate. Examples of such connections are shown in figure 1. In beams and column frames for which simple construction is assumed, there will none the less be bending moments acting on the columns which are caused by eccentricity of the beam reactions.

Angle sections are commonly used as members in light weight triangulated structures such as trusses and towers. The attraction of angle section is their ease of fabrication, they are however more complex to design than other types of structural sections. The reasons for this is the eccentricity of the common forms of end connections (figure 1) and also because the principal axes of a single angle do not coincide with the axis of the frame or truss of which the angle is a part (Temple et al, 1995).

When a composite beam is constructed using a profiled steel deck, a void is created between the deck and the top flange of the beam. With open trapezoidal steel decks this void is comparatively large while with a 'closed dovetail'™ deck the void is much smaller. In the case of the open trapezoidal deck the fire resistance of the composite beam may be reduced because of the effects of additional heat entering the steel beam through the top surface of the flange. This article presents the background to determining the fire resistance of composite beams with profiled steel decking and recommendations for how to modify fire protection of beams supporting steel decking where voids occur.

This worked example illustrates the ductile design of brace/beam/column gusset plate connection. The example is based on the design approach presented in Steel Advisor CON1301.

This article is written in response to frequently asked questions regarding the performance of composite floors soon after concreting. This Steel Advisor article clarifies the basis for some of the requirements given in national and international good practice guides, and provides supplementary information to assist designers when considering this important load case.

Heavy brace gusset connections for the purposes of this paper are defined as gusset plates that connect large tubes (circular and square) or buckling restrained braces used in braced frames (concentric and eccentric) for medium and high rise applications. This includes both corner and midspan gusset plate connections (figure 1). While the guidance in this paper pertains principally to gusset plate connections, the principles will also be applicable to UC brace/beam/ column connections. Reference is made to HERA Design and Construction Bulletins (Clifton 1998, 2000, and 2001) that cover this situation.