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Practical Application of Best Practice in Concrete Frame Construction: St George Wharf Case Study


 
Project Illustration
Project Objectives
Approach
Innovations being applied
Steering Group
Planned Outputs
Project Team
Project Gantt Chart
Sources of Funding
Best Practice Guides for In-Situ Concrete Frame Buildings
Flat slabs for efficient concrete construction
Concreting for Improved Speed and Efficiency
Early Striking and Improved Backpropping for Efficient Flat Slab Construction
Early Age Strength Assessment of Concrete on Site
Improving Concrete Frame Construction
Improving Rebar Information and Supply
Rationalisation of Flat Slab Reinforcement
Prefabricated Punching Shear Reinforcement for Reinforced Concrete Flat Slabs

Case Studies
Project Overview
Slab Deflections
Construction Loading
Strength Assessment
Reinforcement
Special Concretes



Project Illustration

St George Wharf Completed Phase
Figure 1: St George Wharf Completed Phase

Project Objectives

The principal objective of the project is to demonstrate the practical benefits of adopting many of the innovative features and techniques used in the design and construction of the in situ concrete building at Cardington. This will be achieved by applying the innovations to a live case study involving the construction of a series of flat slab frame structures in a large residential and commercial development (Figure 1) to demonstrate continuous improvement, and establishing this as a demonstration project in its own right.

By demonstrating these benefits under commercial conditions the other principal objective is to further persuade the wider industry of the quantifiable value to them of taking up these innovations and approaches, to improve their efficiency and profitability.

The intended long-term impact is the more widespread adoption of new techniques and approaches, which will benefit the wider industry.

In line with Latham and Egan imperatives the intention is to establish a culture of continuous improvement in the Concrete frame industry, on the part of all those involved in the process of designing and constructing in situ concrete frame buildings. This will help improve efficiency and profitability of all those involved in the supply chains for the construction of such buildings, and increase the potential market share for concrete frames within the building frame market.

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Approach

The innovations to be adopted in the context of the specific St.George Wharf development, and the parameters to be measured have been defined so that an accurate picture of the benefits achieved can be obtained. The St.George Wharf development in Vauxhall, South London represents an ideal opportunity for a number of reasons not least the nature of the blocks being built in discrete stages and the opportunities this provides for continuous improvement. The development is very large comprising 100,000sq. m of mixed-use accommodation including 750 homes and is very high profile occupying as it does 275m of frontage on the River Thames. Further details can be found in the RCC's Project Profile on St George Wharf, which can be found at www.rcc-info.org.uk.

BRE are working directly with St.George and their engineers and contractors to develop and implement possible solutions and improvements tailored to the St. George Wharf development. This approach is being followed so that the benefits though specific to a particular project are more clearly visible and measurable. The St.George Wharf development offers the advantage that it is being taken forward in a series of repetitive phases enabling benchmarking and measurement of performance improvements, as a result of implementing the proposed innovations.

The other principal advantage is that because of the nature of St George themselves being a developer/contractor, they effectively have control over all phases of the project, enabling the pushing through of new ideas and innovations which would be more difficult in a more conventional contractual arrangement. BRE have become involved from the outset in plans to construct two particular blocks of this phased development. The engineer for these phases is White Young Green and the frame contractor is Stephensons.

Lessons learnt during the construction of successive blocks will be carried forward on to the next block so that a process of continuous improvement can be established. A team-based approach will be favoured working closely with the frame contractor so that maximum benefit can be achieved. This will have the added benefit of exchanging site-based knowledge with the lessons learnt from Cardington.

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Innovations being applied

Electronic Exchange of rebar information

The use of electronic exchange of rebar information should introduce considerable efficiencies in the overall rebar supply chain by the removal of the need to re-key in the information by different parties. Proprietary software known as SteelPac is being used, further information about which is available at www.steelpac.co.uk. This is providing additional potential benefits in terms of ordering, revision control and updates for the contractor.

The intention will be to gather information on the time and cost of preparing schedules and processing this information by the rebar supplier, the contractor and St George.

 Figure 2 illustrates a completed schedule produced using SteelPac.


Figure 2: Electronic schedule produced using SteelPac

Use of National Structural Concrete Specification (NSCS)

The intention of the National Structural Concrete Specification is to have an agreed common specification for the majority of building structures. This is seen of particular value to the contractor in knowing what is required of him at tender stage. In the context of St George Wharf the contractor already has a good understanding of what is expected of him, so that the benefits of adopting the NSCS may be limited. Nevertheless some useful feedback has already been obtained as a result of applying the document.

Rationalisation of reinforcement

The basic concept of rationalising the reinforcement is reducing unnecessary variation in bar sizes and spacing, making the detailing, scheduling, supply, call-off and fixing of the reinforcement more straightforward. Although material costs will be increased as a result this will be more than offset by the savings in time and labour costs.

In the context of St George the reinforcement is already highly rationalised. However historical information is available for a non-rationalised solution on earlier phases against which comparisons can be made.

Use of prefabricated punching shear reinforcement

This is a specific form of reinforcement rationalisation relating to the provision of reinforcement to resist punching shear. The same principles as for reinforcement rationalisation in general apply, but the benefits can be even more dramatic because of the huge time savings which can result compared with fixing many thousands of individual shear links.

The intention will be to directly compare the fixing time and costs of a number of proprietary systems both with each other on a floor to floor basis, and within a given floor compare different methods of providing shear links on a column to column basis.

One such proprietary system is illustrated in Figure 3.


Figure 3: Proprietary punching shear reinforcement system

Accurate prediction of deflections

Prediction of deflections can be a specific requirement to meet clients' requirements and those of follow-on trades such as cladding and internal finishes. At St George Wharf a complicated fixing detail has had to be adopted to accommodate movements in internal finishes which it is suspected is unnecessary.

Measurement of the deflections actually occurring is providing valuable data for calibration of theoretical models and justification for simpler and cheaper architectural details on future blocks.

Early age strength assessment using Lok tests

The practical benefits of using LOK tests (Figure 4) for determining the strength at which the slabs can be struck are being investigated. Initially the carrying out of LOK tests has been run in parallel with the making and testing of cubes, so that confidence can be gained in their use and comparison made with cube test results.

The costs and convenience of carrying out LOK tests has been compared with that of making and testing cubes, and found to be broadly comparable. Their principle advantage however is where the results are required quickly.

 


Figure 4: LOK test being carried out

Specification of 'superstriker' concrete

There may be advantages in specifying a higher grade of concrete to enable required early age strengths for striking to be achieved, especially in cold conditions. The additional cost associated with this will be weighed up against the benefits that accrue if this option is pursued.

Revised striking criteria

As a result from the work at Cardington new striking criteria have been proposed taking serviceability criteria as those which are critical. The opportunity will be taken at St George Wharf to assess the practical implications of the new criteria in terms of promoting early striking, and the benefits which result from it in terms of speeding up the floor cycle.

New criteria for design of backpropping

Again as a result from the work at Cardington improved understanding of the true distribution of loads through backprops and supporting slabs has been gained. This potentially will enable the numbers of levels of backpropping and total amount of backpropping to be reduced.

Use of CRC Jointcast

Measurement of the time and resources associated with constructing the vertical elements has highlighted the potential benefits that might be achieved by considering alternative approaches to the construction of these elements. One approach, which is being considered, is the use of precast elements joined together with CRC Jointcast. This is a cementitious based material used with a high proportion of steel fibres which enables monolithic construction by the lapping of bars over very short distances. Further information about the material can be obtained at www.crc-tech.com.

Use of self-compacting concrete

Self-compacting concrete offers potential advantages in terms of reduced noise and improved health and safety. The opportunity will be taken to use it in limited areas to compare costs and the quality of finish achieved, and the ease of specifying and obtaining the material.

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Steering Group

A steering group has been set up to oversee the project. The role of the steering group is principally be to oversee the production and dissemination of the findings from the project as they emerge.

The current membership of the Steering group is as follows.
Marcus Blake/David Watson (St George)
Hardev Soorae/Tim Dodd (White Young Green)
Dave Gebler (Stephensons)
John Mason/Patrick Mahon (Alan Baxter and Associates)
William Mackenzie (DTI)
Peter Campbell (Consultant/Construct)
Julian Maw (Construct)
Martin Southcott (RCC)
Pal Chana (BCA)
Martin Hopkins (M4I)
Robert Vollum (Imperial College)
Bendt Aarup (CRC Jointcast)
Vic Dunham (Peter Brett and Associates)
Tony Jones (Ove Arup and Partners)
Andy Lyle (NRM Consultants)
Bjorn Watson (Anthony Hunt Associates)

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Planned outputs

Case Studies

Case studies will be prepared for each of the innovations adopted on the project. The objective of the Case studies will be to promote awareness and stimulate interest on the part of the wider industry of the business benefits in terms of profitability and predictability that can be achieved as a result of adopting that particular innovation. Five such case studies are anticipated.

The Case studies will thus be high profile short punchy documents summarising the details of the particular innovation, the rationale behind the choice of the particular innovation on this project, and the actual benefits achieved as a result of adopting the innovation compared with the expected benefit at the outset.

An overall Case Study will be prepared for the project as a whole, summarising the overall benefits.

The target audience will be all of those involved in the design and construction of in situ concrete frame buildings including the material supply chains where relevant. However particular effort will be made to raise awareness of the possible benefits amongst major construction clients, since they have potentially the most to gain and in many cases are in the strongest position to push through changes in current practice.

Case Studies
Project Overview
Slab Deflections
Construction Loading
Strength Assessment
Reinforcement
Special Concretes

Full report

A full report will be prepared by BRE at the end of the project documenting the work completed.

The Case studies will make reference to and be underpinned by this full report which subject to the agreement of BRE's publisher will be formally published, e.g. as a short-run BRE report, giving the background and further information on the innovations. This report will also document the methodology used during the project, particularly in relation to the manner in which judgements were made and measurements were undertaken. This will be very valuable to all construction professionals wanting to explore construction process optimisation further, and not necessarily just for concrete frames.

Workshops and seminars

Towards the end of the project, when the Case Studies become available, promotional events are planned to promote awareness and uptake of the information contained within them. If possible this will be in conjunction with professional bodies such as the ICE, ISE and Concrete Society. At least four such events will take place, some at the St. George Wharf site itself. It not planned to charge for these events.

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Project Team

The consortium coming together for this project is new. However it is a natural extension of the existing European Concrete Building Project involving team members who have worked together on a range of projects over a number of years. These include the Cardington initiative, the National Structural Concrete Specification, and many of the initiatives from the RCC and BCA such as Spreadsheets for Concrete Design.

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Project Gantt Chart (please click here to view)

Sources of Funding

Partner Name Contributions in cash Contributions in kind Reference of letter of support
    Describe Value  
BRE    
St. George Plc 120 man-days 54k 12/01/01
White Young Green 30 man-days 13.5k DCD/dd/A007734/0007
Imperial College 10 man-days 5k 20/12/00
CRC Jointcast 12 man-days 6k BA/CRC
RCC 10 man-days 5k 5245
Construct 10 man-days 5k ctc/005
BCA 10 man-days 5k MAC/EF
 
Sub-totals -   93.5k  
 
DETR contribution 71.5k - - -
 
Total Contributions 165k


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