SESOC

Journals Abstracts

These are short abstracts of the material printed in our journal which is published twice annually. The Journal covers items of interest to structural engineers, including but not limited to: technical papers, project reports, materials information, code reviews.

Journal: Vol 14 No. 2 2001

 

Title

Author/s


Abstract


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Title
Determination of Material Properties in Existing Reinforced Concrete Structures Vol 14, No 2 2001
Author/s

S.M. Bruce
Abstract

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.
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Title
The Influence of Precast Pre Stressed Flooring Components on the Seismic Performance of Reinforced Concrete Perimeter Frames Vol 14, No 2 2001
Author/s

David Lau and Richard Fenwick
Abstract

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.
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Title
Stiffness of Structural Walls for Seismic Design Vol 14, No 2 2001
Author/s

Richard Fenwick\ Richard Hunt and Des Bulu
Abstract

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.
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Title
The Effect of Fissuring in Auckland Residual Clays on the Capacity of Shallow Foundations Vol 14, No 2 2001
Author/s

M.J. Pender
Abstract

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.
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Title
Collapse of The World Trade Center Towers Vol 14, No 2 2001
Author/s

G. Charles Clifton
Abstract

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.
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Title
The Freedom in Choosing the Seismic Strength of Components Vol 14, No 2 2001
Author/s

Tom Paulay
Abstract

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?
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Title
Issues & Forward Directions for the New Earthquake Loadings Standard Vol 14, No 2 2001
Author/s

Andrew B. King
Abstract

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.
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Title
Who is Taking Responsibility for Performance of Steel Structures in Fire? Vol 14, No 2 2001
Author/s

Martin Feeney, Holmes Fire & Safety, Auckland
Abstract

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.
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Title
A New Pole Design Standard to Aid Innovation in Power Distribution AS/NZS 4676:2000 Vol 14, No 2 2001
Author/s

Len Mesa Veney, Market Development Manager Golden Bay Cement
Abstract

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.
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Title
Project Corner_Macau Tower Vol 14, No 2 2001
Author/s

Mark Spencer
Abstract

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.
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