Vol. 11 – 15

Vol.11 No.1 1998.pdf



Vol.11 No.2 1998.pdf



Vol.12 No.1 1999.pdf



Vol.12 No.2 1999.pdf



Vol.13 No.1 2000.pdf



Vol.13 No.2 2000.pdf



Vol.14 No.1 2001.pdf



Vol.14 No.2 2001.pdf



Determination of Material Properties in Existing Reinforced Concrete Structures

S.M. Bruce

Measurement of actual material properties in existing reinforced concrete structures can improve the accuracy of structural analysis and may increase the calculated capacity. This paper considers and evaluates the destructive and non-destructive test methods available for determining the strength of concrete and the location, size and strength of reinforcing steel.

The Influence of Precast Pre Stressed Flooring Components on the Seismic Performance of Reinforced Concrete Perimeter Frames

David Lau and Richard Fenwick

This paper is a preliminary report on two tests that have been made to assess the influence that floors constructed with precast pre-stressed components have on the seismic performance of ductile perimeter reinforced concrete frames. The first test unit represented a bent of a frame with two internal bays and two cantilever spans. 1bis arrangement is typical of ductile perimeter frame structures where comer columns have not been used. The second test had a near identical frame but with the addition of a floor slab constructed from precast units on one side. The addition of the precast floor was found to increase the lateral strength of the bent by a factor of about 2.5 for inter-storey drifts of between 1 and 3 per cent. Some of the increase in strength arose from the stiffness of the slab allowing bending moments to be resisted by the cantilever spans. 1f allowance is made for this effect the average flexural strength increase of each plastic hinge zone due to the addition of the floor was 70 per cent. The elongation measurements in the units indicated that the slab initially restrained elongation of the beams and it was this that increased the flexural strength. However, extensive damage to the interface between the beam and slab allowed elongation to increase in the latter displacement stages and the strength enhancement to decrease. The damage in the zone connecting the floor to the beams arose from the vertical movement of the floor relative to the beam and from the shear action along this interface.

Stiffness of Structural Walls for Seismic Design

Richard Fenwick\ Richard Hunt and Des Bulu

The flexural stiffness of reinforced concrete structural walls for seismic design is assessed in an analytical study. The analyses are limited to rectangular walls in which the longitudinal reinforcement is equally spaced along the length. It is shown that for regular structural walls of up to 6 storeys in height the proportion of longitudinal reinforcement, up to a value of 0.015, has little influence on the overall flexural stiffness. It is also found that allowing for typical values of creep and shrinkage in the concrete reduces the stiffness of the walls to a value that is typically 75 per cent of that found neglecting these material properties. A number of simple equations are presented for assessing the appropriate flexural stiffness of walls and these are compared with current code recommendations.

The Effect of Fissuring in Auckland Residual Clays on the Capacity of Shallow Foundations

M.J. Pender

Excavations in Auckland clays reveal that the upper part of the soil profile, up to depths of a metre or so hut usually less, is fissured. Photographs from a site on the North Shore, kindly supplied by Mr Bill Thompson, are shown in Figures I and 2. One possible explanation for the fissures is the cracking of the ground surface that occurs in the summer. This is a feasible explanation for the vertical and near vertical fissures in the photographs but does not explain the presence of the low angle fissures also apparent. Swelling in wet periods following the cracking has been suggested as a possible explanation. After the cracks are formed, debris falls into the cracks, or rootlets intrude into them. In the wet season the clay absorbs water and swells, but the swelling is restrained in those cracks which now contain debris. This process can produce passive failure of the clay with consequent low angle failure surfaces. The process will be repeated from year to year and over a period of time clay structures such as those shown in Figs. I and 2 are produced. To my knowledge this mechanism was first proposed by Terzaghi (1929) for explaining large pressures against walls retaining clay, it is also offered by Tschebotatioff (1973). Pender (1996) presents data showing extension failure of Auckland clay on a low angle failure surface induced during one-dimensional swelling in a laboratory K triaxial cell; a test intended to replicate the mechanism proposed by Terzaghi and Tschebotatioff for the formation of the low angle fissures such as those in Figs. 1 and 2.

Collapse of The World Trade Center Towers

G. Charles Clifton

Construction of the World Trade Center Towers began on August 5 1966 and they were officially opened on April 4 1973. Fig I. shows the two towers prior to the attack. The towers, which are forever seared on the memory of all readers, were destroyed in a terrorist attack on 11 September 2001. The method of destruction was simple and devastating, namely suicide attack by aircraft. The resulting images of the towers burning and collapsing were ones no one ever expected to see.

The first airplane hit the North Tower at 8.45am local time and that tower collapsed at 10.28 am or 1 ½ hours after the impact. The second tower was hit at 9.03 am but collapsed more quickly, at 10.05 am. This article has been written by Charles Clifton, HERA Structural Engineer and gives my thoughts on the possible sequence of damage and collapse. I am writing this from 17 years of experience in the research, design and construction of steel framed buildings. A significant part of the research has been determining the behaviour of steel framed buildings under the extreme events of severe earthquake or severe fire. This has given me some insight into what may have happened to these towers under the much more severe event of a direct hit from a near fully loaded large modern airplane. It is important to note that the explanation given is only my considered opinion, based on the information available six to eight days after the event. Before presenting those details, some details of the building are given, followed by brief details of the impact. The effect of the impacts can only be assessed in light of these details, in particular the devastatingly high local impact force on the buildings from the planes. This is followed by my assessments of the effects of this impact on each of the two towers, which showed some significant differences. There has already been considerable speculation on the severity of the fire and its role in the collapses. On the basis of what I have seen and heard reported to date, it is my opinion that the effect of the fire was of much less importance than the effect of the initial impact, especially on the first tower to be hit (the North Tower). The reasons behind this opinion follow details of the effects of the impacts on each tower and the article ends with a personal footnote on the tragedy and a reference.

The Freedom in Choosing the Seismic Strength of Components

Tom Paulay

In our existing seismic design procedures for buildings, generally we employ analyses techniques that are applicable to elastic systems. These are based on initial assumptions with respect to component sizes of the chosen contemplated systems. Subsequent assumptions for the flexural rigidity of components enable stiffness for given boundary conditions to be processed. For a given mass and the assumed system stiffness, the lateral design forces are adjusted according to both codified response spectra and global displacement ductility capacity of the structure. This traditional analysis process then assigns strengths, associated with lateral design forces, to components in the proportion of their stiffness. Within prescribed limits, subsequent adjustments, based on strength redistribution between components, may then be utilized, if desired. With the increased awareness of the importance of earthquake-induced displacements, particularly relevant to ductile reinforced concrete buildings, a number of questions may be posed:

(I) How reliable are our stiffness assumptions, used in the estimation of displacements?

(2) How important is the distribution of the required total seismic strength to constituent components in proportion to their stiffness? What could be the consequences of radical departures from strength distribution implied by elastic behaviour?

(3) How reliable are our current estimates, if any, of the yield displacement of the system when quantifying the displacement ductility capacity of structures, particularly those comprising very different components?

(4) Should displacements, corresponding with perceived performance criteria, be checked? Could displacements be estimated at the stage of the preliminary design, when the seismic design strength of the structure is yet unknown?

(5) It has been recently suggested that, with disregard for established criteria for elastic systems, strength to components may be assigned arbitrarily. What are the limits on this arbitrariness? What are the possible benefits, if any, which the designer could advantageously exploit?

Issues & Forward Directions for the New Earthquake Loadings Standard

Andrew B. King

The development of a common earthquake standard was expected to be challenging since it is required to cover both the intraplate Australian and interplate New Zealand seismic environment. So it proved to be with the standards review committee now heavily embroiled in developing a standard which can be used within the two subtly different regulatory environments and by practitioners with significantly different operational procedures, all of whom have disparate expectations as to the importance of earthquake design for their buildings.

This paper outlines the essential features contained in the public comment draft. The strategy the review committee is following to address the many comments received is discussed along with the proposed means by which guidance is to be given to the related material standards committees so that they can develop the detailing and design requirements necessary to achieve the levels of structural deformation ductility assumed during the earthquake design. Other issues such as the linkages with other parts of the loading standard, the new robustness provisions of the General Design Requirements and the placement of societal value goals will also be discussed.

Who is Taking Responsibility for Performance of Steel Structures in Fire?

Martin Feeney, Holmes Fire & Safety, Auckland

The performance of a steel structure when exposed to fire is referred to in two clauses of the New Zealand Building Code. In Building Code Clause C4 Structural Stability During Fire the functional requirements are: that stability during fire be maintained to safeguard occupants and fire fighters from injury, and avoid collapse and consequential damage to other property. Structural fire resistance is required to be appropriate to the function of the structural elements. The free load and intensity, height of the buildings and the fire control facilities available.

A New Pole Design Standard to Aid Innovation in Power Distribution AS/NZS 4676:2000

Len Mesa Veney, Market Development Manager Golden Bay Cement

This joint Australia – New Zealand Standard covering the Structural Design of Utility Services Poles, can be used to evaluate competitive bids between poles made of any material, in any of the common utility services applications. The Standard gives the purchasers of poles, and their consultants, a rational method to compare the value proposition, for example, of distribution and transmission poles of competing materials. It enables each prospective pole supplier to provide innovative solutions to meet the limit state performance criteria set out in the document.

Project Corner_Macau Tower

Mark Spencer

Macau Tower forms the centrepiece of a new integrated convention, tourist entertainment and amusement centre being built on the Nam Van lake reclamation in Macau ( approx. 65 km west of Hong Kong). The tower affords panoramic views of the Macau cityscape, neighbouring China and the Pearl River, and even the islands of Hong Kong on a clear day. The success of Auckland’s Sky Tower led Hong Kong investor and developer, Dr Stanley Ho to approach the same team. Design was undertaken on a fast -track basis, with construction of the foundations, basement excavation and ground retention works starting four months after commencement of the design. The project is currently nearing completion, with a formal opening ceremony scheduled for December this year.

Vol.15 No.1 2002.pdf



Guest Editorial Re: Current state of the New Zealand construction industry and role of structural designer within it.

Barry Brown

Since June 2001 when I was appointed as Presiding Member (Chair) of the Building Industry Authority (BIA) by the Internal Affairs Minister, George Hawkins, I have had a unique opportunity as a structural engineering practitioner to look at the nation’s building control industry “from the inside”. Some comments from me on two topical issues are therefore appropriate.

Influence of Hysteretic Form on the Basic Seismic Hazard Coefficients

Judi Hayder, Richard Fenwick and Barry Davidson

The hysteretic behaviour of different structural forms and materials varies widely. However codes of practice generally only give one set of basic seismic hazard coefficients (response spectra) to cover all structural types. The analyses indicate that the form of hysteretic response has only a relatively minor influence on the maximum displacement that is sustained. Varying the viscous damping level was found to make a significant difference to elastically responding structures but it had less effect on ductile structures. Changing the strain-hardening ratio was found to have only a small influence on behaviour.

To assess the influence of different hysteretic forms on seismic response a large number of time history analyses are made using a number of different earthquake records and hysteretic models. In addition the influence of changing both the level of damping and the rate of strain hardening are examined.

L, N and E Grade 500 Reinforcing Steel

D. Bull, C. Allington

The advent of the Joint Australian/New Zealand Standard ASINZS 4671:2001, “Steel Reinforcing Materials” has resulted in the introduction of three classes of Grade 500 reinforcement into the New Zealand marketplace. A number of issues have been raised. This paper discusses implications for the classes of Grade 500: L, N and E, with respect to elongation capacities of each, as well as: bond performance, stiffness of members, flexural over strength, fatigue resistance and site issues. A number of recommendations are made.

Development of Techniques to Maximise Benefits of Post-Tensioned Slabs on Grade

Jeff Marchant

Although PT slabs were introduced into New Zealand over 30 years ago, it is only in the last couple of years that they have been constructed in significant numbers. To illustrate this point, whereas only a dozen PT slabs were constructed prior to January 2000, at least 50 have been completed since then with many more under consideration. The reason for this increase has been a determined effort to optimise the design process, minimise component costs and streamline construction methods to provide cost savings. The main advantage of PT slabs is the ability to construct vast areas of floor with no joints or sawcuts. This feature is particularly desirable for applications such as large distribution warehouses with high racks serviced by solid-wheeled high-reach fork trucks which have a low tolerance to discontinuities in the floor surface. Although it is easy to design PT slabs of over I hectare for a single pour, the logistics of constructing such a floor are daunting and hence it is unlikely to occur, unfortunately. Hence the full advantage of PT slabs has often been forsaken in favour of the limited available concrete supply and delivery resources. This paper outlines the recent development of design and construction techniques to overcome this challenge.

What is the Stiffness of Reinforced Concrete Walls? Discussion of the Paper by Richard Fenwick and Des Bull

Nigel Priestley and Tom Paulay

We read the paper (1) with interest, particularly as it raises questions about a key point adopted in several of our recent papers, namely that, for a given structural member with different amounts of reinforcement and/or axial load, strength and stiffness are essentially proportional. Since we find ourselves unable to agree with many of their conclusions we provide the following discussion of the paper.

The authors restrict their comments to a comparatively small subset of structural elements – namely slender cantilever walls in low rise buildings – and specifically exclude discussion of the influence of shear deformation or for example additional deformation resulting from strain penetration of vertical reinforcement into foundation members. A range of axial compression loads of 0 to 0.2 ( Ag, and a range of reinforcement ratios of 0.0025 -D.02 are considered, though the authors state that reinforcement ratios above 0.0125 are not practical. Although it appears that the results presented in the paper were based on analyses of specific wall details (see Fig. 4, in (1)), relevant details are not provided. This makes direct checking of their results difficult. However, we feel that first we must question the range of parameters considered by the authors.

What is the Stiffness of Reinforced Concrete Walls? - Response to Discussion from Priestley and Paulay

Richard Fenwick and Des Bull

The writers (Fenwick and Bull) thank the authors (Priestley and Paulay) for their comments on our paper on the stiffness of concrete walls in the SESOC Journal Vol. 13, No.2, Sept. 2000, which we subsequently referred to as “our paper”. We hope that the authors’ comments and our response will be of interest and value to practicing structural designers. To give the background to some of the analytical research that has been carried out in connection with assessing appropriate stiffness values is described. This is followed by detailed discussion of the points that the authors have raised.

The Challenge of Grade 500 Steel

Carl R. O’Grady

With the introduction of Grade 500 steel, it would not be unreasonable to pre-suppose that, before embarking on this major change, some actual bond testing had been carried out, to determine adequate lap lengths. It would seem an ideal opportunity, to compare the results of actual tests, with existing code formulas, using realistically higher strength concrete, to compliment the higher strength re-bars. The relevance of need for such tests is augmented by the concurrent change in bar


It is also imperative, that the whole question of beam/ column joint design be re-addressed, as this area, presently, is problematical, not only to the designer, but, even more importantly, to the builder, as the steel congestion militates against the ease of concrete placement. Obviously, with the introduction of higher strength reinforcing steel, the present problems can only get worse. Will over strength factors increase? Unless these problems are immediately addressed by a realistic testing program, the economic viability of reinforced concrete will suffer, in relation to other forms of construction. Surely, this should be of paramount concern to the industry!

Some Considerations on Education and the Health of Structural Engineering in NZ

Dr Richard Fenwick

Of the 40 years that have passed since I graduated, one third has been spent as a practising structural engineer and the remainder as an academic. This career history in itself, highlights one of the major changes that has occurred in engineering education.

When I started at Auckland University in 1975 practical experience was highly valued and the majority of academic staff employed at that time had a similar level of practical experience. However, over the years the emphasis has changed.

Minimum Specifications for Concrete Durability

J R Mackechnie

Modern Portland cements are well ground and careful& selected materials that give good early strength development and comisfentpeif2ormance. New Zealand is blessed with high performing cements in terms of strength and good quality aggregates for concrete production. This has lead to a very competitive ready mix industry where cement contents are kept to the minimum. The quality of some of these concrete mixes was investigated using compressive strength and permeability testing. Results from the study indicate that a minimum threshold level of cement is required in concrete to provide a closed microstructure needed for durability. Given the high strength performance of New Zealand cement, the use of low structural grades with high w/c ratios need to be reviewed due to their relatively open microstructure.

Experimental Testing and Numerical Modelling of Two-Way Concrete Slabs under Fire Conditions

Linus Lim, Andrew Buchanan, Peter Moss

This paper describes the tests and computer modelling of two-way concrete slabs exposed to fire. The fire tests were conducted to investigate the influence of tensile membrane action in concrete slabs at elevated temperatures. Six slabs were tested comprising three reinforced concrete plain flat slabs and three composite steel-concrete slabs. The slabs measured 3.3 m by 4.3 m and had thicknesses ranging from 90 mm to I30 mm. The slabs were simply& supported at all four edges on a 3 m x 4 mj5mace and were horizontally unrestrained. The slabs were subjected to a live Load of 3.0kPa and heated with the 1SOstandard fire. All the slabs performed very well as they supported the full design loads for three hours in the IS0 fire withgout collapse, despite suffering significant deflections and loss of flexural strength. The fire tests illustrates the significant effect of tensile membrane action in the slabs. Finite element analyses of the slabs with the S4FIRprogram showed good agreement with the test results.

A Calculation Method for Plastic Analysis

Esli Forrest

Our traditional approach to moment-force analysis is based on pre-yield elastic concepts. It centres on a concept of increasing load with linear elastic stress response from zero to yield point. In earthquake design however, we are forced to think beyond the yield point. This applies to both steel and concrete. With timber, failure occurs without a very long curvature increase beyond the yield point and so a single linear approach is acceptable. With steel and reinforced concrete however, a bilinear or events-linear system of analysis for bending moment and displacement is necessary. Due to the yield plateau that occurs in structural grade steels, with I sections, the whole section for practical purposes develops plasticity. The stress and strength increase after yield, due to strain hardening, is generally not considered in design. However, in reality it is important as it allows the plastic (hinge) zone to spread, and hence sustain high rotations before a failure strain is reached.

The Seismic Performance of Flooring Systems Executive Summary

Technical Advisory Group of Precast Flooring Systems

This report has been prepared by the Technical Adv3ory Group on precast flooring Systems. The group has been formed to disseminate the results of recent research to the industry and provide

info into the direction for future testing. The fundamental messages the group wishes to take to the

industry are

• The preferred seating arrangement for hollowcore units supported on concrete beams is shown below/ It is considered that using this seating detail will ensure improved seismic performance above that of the commonly used detail of providing plastic cut-offs in the cores to prevent infiltration of the topping concrete. The proposed detail has no cost penalty over the existing practice.

• Hollowcore units& should not be positioned parallel and immediately adjacent to beams. They should be located a distance away (500-800 mm) and linked to the beams by the concrete topping only

• Exterior column should be tied back into the structure either by transverse beam, or by ductile reinforcement in the floor slab. The reinforcement shall be capable of resisting a force equal to 5% of the gravity axial load in the column.

Design of Multi-Story Buildings for Satisfactory In-Service Response to Wind Induced Vibrations

Thomas Mahoney, G Chares Clifton

Steel framed multi-storey buildings are generally lighter in weight than reinforced concrete framed multi-storey buildings. The principal reason for this lies in the self weight of the flooring systems used in each instance.

The lighter weight of steel framed buildings makes them potentially more susceptible to unacceptable levels of

acceleration generated by wind-induced vibration under serviceability limit state conditions.

Some Considerations In Design Of Reinforced Concrete Interior Beam-Column Joints Of Moment Resisting Frames

Prof Robert Park

During the past forty years a great deal of research on the behaviour of beam-column joints of reinforced concrete moment resisting frames subjected to seismic loading has been conducted in structural testing laboratories all over the world. Based on the results of these tests design recommendations have been developed and incorporated in the seismic design codes of the different countries. It is of concern that the recommended approaches for the design of reinforced concrete beam-column joints in New Zealand’, the USZ, Japan3 and

Europe4 vary significantly mainly due to different interpretations of test data, different models of behaviour, and different performance criteria.

Articles for Discussion_To every Action there is ???

Esli Forrest

We have got what we wanted – a completely deregulated construction industry. Everyone can compete with everyone in every aspect. Nothing is sacrosanct. Design, supervision, new ideas, standards of performance, professional ethics, and the fees that are charged to execute them, all are subject to the driving force of the dollar. The only yard-stick is that the minimum requirements in performance of a totally non prescriptive building code are met. It will, we are told, encourage innovation.

Project Corner_Nam Cheong Station Hong Kong

Philip Young, Robert Cook, Rohit Patel

The construction of buildings below ground can present a number of high risk problems. The designer is faced with the inexact science of soils, all too often coupled with an inadequate site investigation. The integration of geotechnical engineering with civil and structural engineering, and of permanent with temporary works, combined with varying sensitivities and constraints of adjacent development and the tight construction programme, present both challenges and opportunities. The premium placed on land values, particularly in a thriving city such as Hong Kong, means that the easy sites have all been used up and that there is pressure to build down as well as up. Nam Cheong Station on the West Rail Project, was a project that involves both above and below ground structures, with very challenging site constraints. Knowledge of practical methods and sequence of construction is of vital importance to the success of such a construction project. This article describes the key design issues that had to be dealt with and the approach that was taken by the Alternative Design Team led by Robert Benaim & Associates, working with Contractor Balfour Beatty Zen Pacific Joint Venture (BBZP JV).