Journals >> Abstract VOLUME 15 No. 2 (September 2002)
SESOC INFORMATION
SESOC MANAGEMENT COMMITTEE - President's Report (Dr B Davidson)Guest Editorial: Some thoughts on Education and the Health of Structural Engineering in New Zealand (Richard Fenwick)
TECHNICAL PAPERS
Minimum Specifications for Concrete Durability – J.R. Mackechnie
Modern Portland cements are well ground and carefully selected materials that give good early strength development and consistent performance. 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 readymix 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 the closed microstructure needed for durability. Given the high strength performance of New Zealand cements, the use of low structural grades with high w/c ratios needs 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 and 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 slabs and three composite steel-concrete slabs. The slabs measured 3.3m by 4.3m and had thicknesses ranging from 90mm to 130mm. The slabs were simply supported at all four edges on a 3m x 4m furnace and were horizontally unrestrained. The slabs were subjected to a live load of 3 kPa and heated with the ISO standard fire. All the slabs performed very well as they supported the full design loads for three hours in the ISO fire without collapse; despite suffering significant deflections and loss of flexural strength. The fire tests illustrated the significant effect of tensile membrane action in the slabs. Finite element analyses of the slabs with the SAFIR program showed good agreement with the test results.
A Calculation Method for Plastic Analysis - Esli Forrest
The 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 bi-linear or even tri-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 section, in analysis, at the yield point is considered to be a "hinge" for post elastic rotations. The second part of the bi-linear diagram is therefore flat. With large rotations and cyclic behaviour this assumption is not always valid.
Traditionally, in earthquake design we analyse everything as elastic and then adjust displacements by a pre-guessed ductility factor designated as µ This factor has also dictated the load level we applied elastically as it dictated the structure's assumed response. To refine the design we then work backwards and check the structure's actual limit displacement and hence ductility, to make sure it is not less than what we assumed in the response. Would it not have been better to consider the structure as yielded, and consider pre-yield and post yield displacements first off? We would then be able to compare load capacity against response demand.
The aim of this paper is to present a simple calculation method for doing this. It demonstrates that all rotation-displacement analysis is a simple function of yield strain and member depth and length. It is concentrated on steel but with minor adjustments the methods can apply to reinforced concrete.
The Seismic Performance of flooring Systems – Special Research Report – Interim Executive Summary
By the Technical Advisory Group of Precast Flooring SystemsThis report has been prepared by the Technical Advisory' Group on precast flooring systems. The group includes representatives from:-
Universities of Canterbury and Auckland ~ The NZ Society of Earthquake Engineering ~ Structural Engineering Society NZ (Inc) ~ NZ Concrete Society Inc. ~ Precast NZ Inc ~ Precast floor manufacturers ~ Cement and Concrete Association of NZ
Technical Advisory Group Members:
Dene Cook (Chairman), Bob Park, Craig Stevenson, Des Bull, Geoff Banks, Jeff Mathews, John Mander, John Marshall, Keith Norgate, Len McSaveney, Richard Fenwick, Rod Fulford, Ross Cato
The group has been formed to disseminate the results from recent research to the industry, and provide input 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: 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 (500-800mm) and linked to the beams by the concrete topping only
- Exterior columns should be tied back into the structure either by transverse beams, 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.
The sections provide information on:-.
- The reasoning behind the above recommendations.
- Interpretation of recently completed research on flooring systems.
- Aids to interpreting the results for structures with a different structural form to those tested
- The direction of future testing
The paper is supplemented with illustrative diagrams.
Note that this is an interim report, to assist in interpreting the results of a research project that is still in progress. Further research programmed in the near future may result in modifications to the recommendations.
Design of Multi-Storey Buildings for Satisfactory In-Service Response to Wind Induced Vibrations – T. Mahoney and G.Charles 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 selfweight 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.
These accelerations are caused by the movement of the building due to the wind flowing around it. The nature of the wind flow is complex, as is the building's response to it. Generally, for buildings that are torsionally regular and torsionally stiff relative to their translational stiffness, the flow of wind past the building will generate both an along-wind response and an across-wind response, with the latter typically governing.
The design of buildings for wind-induced vibration serviceability criteria is not well covered by NZS 4203:1992 [4], with this coverage being restricted to a simple threshold limit check given in Commentary Clause C5.2.2.3. This check is a function of the building's height and mass and takes no account of wind speed.
If the proposed building fails this check, then a designer using either the current loadings standard [4] or the proposed replacement standard (to be AS/NZS 1170.2 [2]) has to attempt a full dynamic design. However, the design procedure presented in [2] or referenced from [4] is very complex, is difficult to apply and gives unreliable results.
In an attempt to try and help designers out of this unsatisfactory situation, HERA commissioned research into this topic which led to a full-scale experiment being conducted at the University of Auckland during 2001. One of the objectives of that experiment was to test the accuracy of a preliminary design technique developed by Cenek et al. [3]. which appears to offer designers a method for establishing a much more accurate threshold limit for assessing a building's adequacy in this area.
The aim of the experiment was to record both wind flow data and building along-wind and across-wind accelerations for a given test building. The accelerations were recorded at the top of the building. The wind flow was also recorded at the top of the building, sufficiently far above roof level to be effectively free from the local effects of turbulence generated by the building itself.
This generated data sets of wind flow and building acceleration that could be used directly to compare the accuracy of the full dynamic design procedure from [2 or 5] and the much simpler Cenek et al. recommendations [3], with a view to making improved design recommendations. Ref. [5] is AS 1170.2:1989.
Some Considerations in the 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 USA, Japan and Europe vary significantly, mainly due to different interpretations of test data, different models of behaviour, and different performance criteria.
Shear Strength: There are significant differences between the approaches for the design of beam-column joints for shear strength between major codes, particularly between the New Zealand concrete design standard and the building code of the American Concrete Institute. The New Zealand approach is based on models of shear transfer across the joint core involving a diagonal compression strut and a truss mechanism of shear reinforcement and a diagonal compression field. There is evidence that in future revisions of the New Zealand concrete design standard the design equations could be simplified and the amount of joint shear reinforcement eased. The approach of the American Concrete Institute is an empirical approach which is not based on a model of joint shear behaviour. It ignores a number of variables such as the effect of column axial load level, the deterioration of joint confinement by beam plastic hinging due to bi-directional seismic loading and the need for vertical shear reinforcement.
Bond Strength: There are also significant differences between the design approaches of the major codes for the permitted diameters of longitudinal beam bars passing through the joint for anchorage. Design standards normally limit the bond stress by specifying maximum permitted values for the ratio of bar diameter to column depth.
When beam bars of relatively large diameter pass through a column of relatively small depth at an interior beam-column joint, during severe seismic loading the “compression” reinforcement in the bar on one side of the column may actually be in tension due to bond deterioration within the joint. Analysis taking into account the effect of the actual stress in the “compression” reinforcement demonstrates that the flexural strength and the available curvature ductility factors of a beam will be reduced as a result of increasing tensile stress in the “compression” reinforcement leading to larger neutral axis depths. The flexural strength of the beams is not significantly effected by bond deterioration, perhaps reducing by 5-10%. However, the available curvature ductility factor Ф u/Ф y of the plastic hinge in the beam, before crushing of compressed concrete occurs, is significantly reduced. This outcome should be considered when specifying the maximum permitted ratios of diameter of longitudinal beam bar to column depth in seismic design standards and codes.
It is evident that the maximum d b/h C values specified for seismic design by standards and codes is a matter of judgement. Some bond deterioration is inevitable, and should be accepted. The considerations are that, on the one hand, too small a specified d b/h C ratio will lead to the necessity for small diameter bars and/or large columns which results in design and construction difficulties. On the other hand, too large a specified d b/h C ratio will lead to significant bond deterioration during a severe earthquake, resulting in a reduction in stiffness of the frame which is residual. Also, bond deterioration is difficult to repair by epoxy resin injection and, as demonstrated in this paper, leads to a reduction in the available flexural strength and curvature ductility factor of the adjacent plastic hinges in the beams. These issues need to be weighed up. Hence differences in the d b/h C ratios specified in standards and codes are understandable.
Possible Future Harmonisation: It is frustrating that after so many years of research and discussion that international harmonization has not been achieved on the issues of the design of reinforced concrete beam-column joints. It is likely that a further attempt at the harmonization of a number of differences in international seismic codes will be undertaken by a task group of Commission 7: Seismic Design of the International Federation of Structural Concrete (fib) in the future.
ARTICLES FOR DISCUSSION
To Every Action There Is ???? – Esli Forrest.
This is a thoughtful article demanding a response from the profession. One of the most important points is made in the following:-
Professional colleagues should not be made to compete through fees charged but rather through work quality. The lowest cost in design will not produce the most economic and best project. It is inherently unethical to compete in fees charged. If an innovative design requires a certain standard of site supervision, then no client should be able to go down the road to another professional and get that design without that site supervision.
In the present climate, fees are being cut to a point where proper levels of work in design detailing and site supervision cannot be performed. The legal and medical professions do not compete this way. The commerce act does not make them blush about charging for their services. It is time we had more true business sense and looked after, not only ourselves, but also our professional colleagues, and our professional standing and quality of work. Our focus must shift from getting the job to doing the job that is in the best interest of our client.
PROJECT CORNER
Nam Cheong Station – Philip Yong, 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.
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 DesignTeam led by Robert Benaim & Associates, working with Contractor Balfour Beatty Zen Pacific Joint Venture (BBZP JV).
MASS[VE INTERCHANGE: Nam Cheong Station is a massive interchange which forms part of the HK$46.5b (NZ$13b) West Rail project, currently under construction. West Rail is part of the ambitious portfolio of work currently being undertaken by the Kowloon Canton Railway Corporation (KCRC) in Hong Kong.
Construction of the line is extremely complex with much of it passing through mountainous terrain or built-up areas. The whole of the 30.5 km of rail track is in tunnels, on viaducts or in enclosed structures. Procured as one of a number of station contracts on the West Rail Line, work on the HK$2.2b (NZ$600m) Nam Cheong Station & Tunnels commenced on site in November 1999.
The overall plan area of the station is 350 m x 80 m, with an average depth of concourse construction up to 15 m below ground level. This represents 400,000 m 3 of excavation. The final station will have a concourse slab at basement level, constructed either side of the Airport Express Line (AEL), the east and west boxes, with tunnel links beneath the AEL linking the two halves of the concourse together. The building podium then rises to 21 m above ground on either side of the West Kowloon Expressway. In addition the substantial development above had to make provision for a further 10-storey, 300m long 'screen' building. The intention is that this office building will act as a noise screen for the residential developments in the vicinity. A figure shows a typical cross section through the station.
COMPUTER CORNER
Behaviour of Simple Structures: More Puzzles - Geoff Bird
Test your analytical ability.
JOINT SESOC / IPENZ / STRUCT.E COMMITTEE NEWSLETTER
Tall Buildings – Thinking the Impossible – The I.Struct.E Report on Safety in Tall Buildings, as abstracted from the Structural Engineer V80: No. 15, by Kathy Stansfield.
The Report is about what we can learn from the collapse of the WTC and how we can move forward. In dealing with unanticipated events we have to think the impossible! The international team which compiled the Report has produced guidelines for design and research which could now be taken forward. Key issues covered were vulnerability to progressive collapse, passive and active fire protection, and escape and management of emergency services.
Many questions are raised about the performance of tall buildings in fire. At the twin towers all the main defences against fire failed.
A lot of reports show how many of the WTC other buildings had no fire proofing, or that it fell off early in the disaster. A lot more needs to be researched about what happens in a real fire, not just in a test. The question of the evacuation of people was also looked into.
The Report “Safety in tall buildings and other buildings with large occupancy” is available from I.StructE. Fax 020 7235 4294. £30
NEWS FROM THE REGIONAL STRUCTURAL GROUPS
Auckland Structural Group – Ashley Smith
Canterbury Structural Group – Dene Cook
Waikato Structural Group – Gordon Hughes
Wellington Structural Group – Graeme BeattieThese consist of reports of past meetings, and planned future meetings.
