Journals >> Abstract VOLUME 14 No. 2 (September 2001)
SESOC INFORMATION
SESOC MANAGEMENT COMMITTEE – PRESIDENT’S REPORT (Dr. B. Davidson)
GUEST EDITORIAL: INTERNATIONAL RELATIONSHIPS (Richard Aitken)
SESOC AWARD FOR EXCELLENCE IN THE YEAR 2001 - To Albert SmithLETTERS TO THE EDITOR:
1. Response to Letter from Mr. P.C. Smith re NZRMCA – D.P. Barnard.
2. Revision of NZS4230 – Masonry Design – D.P. Barnard.PROFESSOR R.PARK AWARDED DOCTORATE OF ENGINEERING BY UNIVERSITY OF CANTERBURY – An Apology and Correction
REPORT ON ATTENDANCE AT ISO TC98 ‘BASES FOR ACTIONS ON STRUCTURES’ Held in Washington DC, 14th to 19th May 2001 – Andrew King
TECHNICAL PAPERS
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 Prestressed 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 prestressed 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. This arrangement is typical of ductile perimeter frame structures where corner 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 percent. Some of the increase in strength arose from the stiffness of the slab allowing bending moments to be resisted by the cantilever spans. If 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 percent. 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 Bull
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 percent 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 but usually less, is fissured. Photographs from a site on the North Shore, kindly supplied by Mr. Bill Thompson, are shown. 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. 1 and 2 are produced. To my knowledge this mechanism was first proposed by Terzaghi (1929) for explaining large pressures against walls retaining clay, it was also offered by Tschebotarioff (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 Ko triaxial cell; a test intended to replicate the mechanism proposed by Terzaghi and Tschebotarioff for the formation of the low angle fissures such as those in Figs. 1 and 2.
The purpose of the paper is to consider the implications of fissures in the upper part of the Auckland clay profile for the design of shallow footings and pole wall foundations. Prior to shallow footing construction, the fissured zone, or the most severely fissured part, is likely to be removed as part of the site preparation. Thus foundations for pole walls are the shallow foundation situation for which the presence of fissures might be most significant.
COLLAPSE OF THE WORLD TRADE CENTER TOWERS – G.C. Clifton
Construction of the World Trade Center Towers began on August 5 1966 and they were officially opened on April 4 1973. Fig 1. 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 ¾ hours after the impact. The second tower was hit at 9.03 am but collapsed more quickly, at 10.05 am.
This article gives the writer’s thoughts on the possible sequence of damage and collapse. It is written 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 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 his 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 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 it’s role in the collapses. On the basis of what has been seen and heard to date, it is the writer’s 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 – Prof. Tom. Paulay
In our existing seismic design procedures for buildings, generally we employ analysis 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.
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 is to address the many comments received. It 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.
ARTICLES FOR DISCUSSION
Who is Taking Responsibility for Performance of Steel Structures in Fire – Martin Feeney
A New Pole Design Standard to Aid Innovation in Power Distribution AS/NZS 4676:2000 – Len McSaveney
Why Masons’ Registration – D.P. Barnard
PROJECT CORNER
Macau Tower – Mark Spencer, Beca Carter Hollings & Ferner Ltd.
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.
The HK$1 billion development comprises a 338 m tall observation and communication tower (10th tallest tower in the world), an entertainment centre with conference facilities for up to 2,200 delegates, a 500 seat theatre, numerous restaurants, retail and entertainment facilities, as well as underground car parking. Located on the outdoor plaza are large artwork sculptures, interactive fountains, electronic billboards and video screens for outdoor events, water features and an adventure playground complete with a carousel and even an almost full-size galleon. Computer-controlled colour floodlights illuminate the metal facade of the entertainment centre at night-time.
Conceptually Sky Tower and Macau Tower differ in a number of aspects. The form of Sky Tower's pod and mast was, to a large extent, dictated by telecommunications requirements. In addition to antennae located on the mast, various levels of the pod house microwave dishes are clad with materials transparent to radio and microwaves. Conversely, Macau Tower is primarily a tourist attraction. Income derived from broadcast antennae and microwave dishes located on the outside of the upper pod levels and mast are a secondary consideration. In recognition of the greater visitor numbers anticipated in Macau, the public levels in the pod are considerably larger, with a capacity for 1,000 visitors, compared to Sky Tower's limit of 850.
The two towers have a markedly different appearance. Sky Tower is referred to as the clutch pencil, with the Macau Tower likened to a jewel setting. Macau Tower’s higher pod, with exposed fin columns and mast lattice, combined with very high typhoon wind loadings resulted in overturning moments at the base approximately 2.5 times those of Sky Tower. The broader spread of the legs responds to these higher forces and suits the open nature of the spectacular waterfront site.
COMPUTER CORNER
Use of FEM Software for the Analysis and Design of Ground Floor Slabs & Pavements – Darrin K. Bell
The Cement and Concrete Association of New Zealand (CCANZ) have recently released the design guide "Concrete Ground Floors & Pavements for Commercial & Industrial Use Publication Part 2 - Specific Design". The design guide includes a section on Computer Design Software for slab analysis and design. This paper presents guidelines provided by Compusoft Engineering for the CCANZ on the use of Finite Element (FEM) Software.
Ground Slab systems are a special class of structures. They are effectively horizontal plates with uniform support subject to a series of concentrated loads. Ground Slab systems may be analysed using Finite Element (FEM) software employing shell elements to model the concrete slab.
Most commercially available finite element programs with shell elements have the capacity to model ground slabs. However general software packages may not recognize the particular characteristics of ground slab systems, resulting in excessive modelling, computation and post processing effort. It Is therefore preferable to use a software package that specifically caters for slab systems.
Specialist FEM packages would utilise data management systems and equation solvers which would efficiently handle the relatively large computational problem. Additionally they would provide pre- and post- processors for speedy modelling and interpretation of results. The development of these packages, along with the increases in computer hardware capabilities mean that computer analysis is now a practical option for ground slab design.
This paper provides an outline of the application of specialist FEM software in ground slab design. Illustrative examples are included using "SAFE", software for the integrated analysis and design of slab systems. SAFE is a product that is widely used internationally and is available in New Zealand.
Behaviour of Simple Structures: Challenges – Part 5 - G. Bird.
Test your analytical ability.
STANDARDS NEW ZEALAND
Standards for Structural Engineers – Ian Brewer.
Amendment to NZS 3603: The Earthquake Loading Standard: AS/NZS 4673:2001: Review of NZS4230: 1990: Design of Masonry Structures”: Review of NZS3101”Concrete Structures” (feedback requested).
JOINT SESOC / IPENZ / STRUCT.E COMMITTEE NEWSLETTER – R. Aitken
“The Structural Engineer”: Part 3 Examination: Current President of I.Struct.E.
NEWS FROM THE REGIONAL STRUCTURAL GROUPS
News of Projects, Structural Group Meetings and Visits, from Auckland (Andrew Simpson), Wellington (Graeme Beattie) and Christchurch (Dene Cook).
The Auckland Committee is close to signing off a final approval of the Standard Piling Specification.
