Off–the-frame brickwork - Analysis of the data from Winterton House, London

Summary

Winterton House is a 25-storey steel frame building in the East End of London that was reclad with a novel form of vertically restrained, off-the-frame brickwork during the mid 1990’s. 

An analysis of the data obtained from instrumentation installed on the building showed that, in any month, its height varied by between 5mm and 15mm in response to short-term changes in temperature. Although clay masonry is considered to undergo long-term moisture expansion, it was found that the overall height of the building decreased by approximately 10mm over the three-year monitoring period and, as a consequence, the stress in the restrained brickwork reduced. 

Modelling of the short-term and time-dependent behaviour of the building suggested that lower values for the creep and irreversible moisture movement characteristics of the clay brickwork were more appropriate than the recommended design values.  Details are also given of the brickwork productivity achieved on site.

The executive summary of the Winterton House project can be viewed below.

Executive Summary

This is the final report of the project to analyse and interpret the results from the monitoring of Winterton House, London. The project was jointly funded by the DETR, under their Partners in Innovation Programme, and the Brick Development Association Ltd. 

Winterton House is a 26 storey steel frame building in the Tower Hamlets area of London.  As part of a complete refurbishment in 1995-7, the building was stripped to the bare frame and reclad with a novel form of off the frame, loadbearing clay brickwork. To prevent differential vertical movement and to utilise the additional load carrying capacity of the freestanding brickwork cladding, the steel frame and brickwork were locked together at roof level and a prestrain (precompression) applied to the brickwork by jacking against the steel columns of the frame.  To obtain a better understanding of this type of construction, limited funding was obtained to install, and monitor, a range of instrumentation on the building.  Monitoring data was collected over a three year period, commencing March 1996. 

The analysis of these data has shown that the height of Winterton House may vary by up to 15mm during any month in response to short-term changes in temperature and/or reversible moisture movement.  The building is also slighter shorter in the winter than in summer.  In addition, the steel columns that were monitored underwent a gradual reduction in length such that, by the end of the monitoring period in 1999, they had almost returned to their original length i.e. that prior to the application of the prestrain (precompression) to the brickwork in April 1996.  This suggests that significant relaxation of the precompression force in the brickwork had taken place over the monitoring period.  A subsequent theoretical analysis of the time-dependent interaction between the brickwork and the steel frame confirmed such behaviour was possible.  Compared with the original design, this analysis used reduced values for creep and moisture movement of the brickwork.  This was to allow for the fact that it had taken approximately one year to construct the cladding, with a consequential reduction in the creep potential of the brickwork.  In addition, a proportion of the irreversible moisture expansion would also have taken place prior to locking the brickwork and steel frame together.

These findings were based upon the data obtained from a limited number of strain gauges attached to a perimeter column on each face of the building.  As such, they may not be representative of the behaviour of the columns that were not monitored.  No means of confirming this was possible, however, as the only other equipment installed on the building to measure changes in height were lasers attached to the outside of the brickwork, and these did not function as anticipated. The laser data could not, therefore, be considered in the analysis.  Similarly, the majority of the strain gauges attached to the roof transfer structure failed to function correctly.  An additional difficulty with the analysis of the results was the loss of monitoring data caused by workmen severing the power supply cables in 1996 and 1997.

The maximum and minimum temperatures recorded in the brickwork during the monitoring period were very similar to those assumed at the design stage.  At any time, the temperature of the brickwork cladding could vary both up and around the building.  After the building was occupied the temperature of the steel columns  varied.  This is contrary to the assumption made at the design stage, and may lead to  short-term reductions in the precompression in the brickwork.  The maximum temperature differential recorded between the inner and outer faces of the brickwork was 6°C.  This occurred in the 655mm brickwork at the base of the building whilst it was occupied.

Using the data from this investigation and the results of a complementary project involving the testing of seven-storey high brickwork panels on the concrete frame at Cardington, a design methodology has been developed to enable restrained brickwork cladding to be used on both steel and concrete frame buildings (Appendix C).  The analysis of the data from Winterton House has, however, shown that the choice of appropriate values for the input parameters is essential when using this design methodology, particularly when assessing the time-dependent stresses that develop in restrained brickwork.  Examples of the design of restrained brickwork cladding for both a steel frame and a concrete frame building are included in this report (Appendix E). 

The analysis of the contractor’s productivity data showed that the original pre-tender estimate for bricklaying was accurate.  In practice, the cladding at Winterton House was completed some eight weeks ahead of schedule.  This was, however, achieved primarily through increasing the number of bricklayers on site rather than through greater productivity, per se.

A detailed cost comparison carried out as part of this project has shown that off the frame restrained brickwork is cost effective in relation to cladding systems that use metalwork brackets to support the brickwork at regular storey-height intervals.  For example, for a six-storey building the initial build cost of off-the frame restrained brickwork was within 1% of the cost of cladding systems involving metalwork support brackets (Appendix D).

This overall programme of research has shown that vertically restrained brickwork can be used as an alternative to cladding systems that require metalwork brackets to support storey-height panels of brickwork.  It is considered to be particularly cost-effective for cladding tall steel and/or reinforced concrete frame structures, and is equally applicable to both new construction and the refurbishment of existing buildings.  In the latter case, the existing structural frame may not be able to support the additional loading from replacement brickwork cladding without undergoing extensive strengthening or reconstruction.

Primary author(s): Bingel, P. R., Bown A., and Brooks, J. J.

Source: Leeds Metropolitan University, Partners in Innovation Project number 39/03/603, Department of Trade and Industry.

For further information please contact Pav Bingel

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