Scaffolding and Temporary Works for Historic Buildings
Ian Hume
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| Temporary support designed to spread loading across vulnerable foundations |
A great deal of time and effort is put into developing schemes for the conservation of historic buildings but it sometimes seems that scaffolding and temporary works, the means by which the conservation schemes are successfully completed, receive scant attention. If not erected properly and with due care and attention to detail, these works can cause a great deal of damage to historic fabric.
The basic processes of design and erecting scaffolding and temporary works to an historic building are not greatly different from those necessary when any other existing building is affected. The purpose of this article is to highlight some of the important points which need special attention if damage to historic fabric is to be avoided.
Where non-historic buildings are concerned, damage caused by improperly erected scaffolding and temporary works, whilst being tiresome and causing unnecessary expenditure, can often be repaired without serious harm. Where historic fabric is concerned any damage is permanent and significant detail may be lost. Once scarred, an important facade is scarred forever.
All badly erected shoring work, whether or not it is to an historically important building, has the potential to allow the collapse of part of the building with disastrous and possibly fatal consequences. Experience indicates that when things go wrong it is usually due to lack of attention to seemingly minor details.
Scaffolding and temporary works must be capable of being constructed without the need for major intervention into historic fabric. This must be borne in mind by designers and erectors of scaffolding and temporary works.
ACCESS SCAFFOLDS
'Independent tied' scaffolds will normally be provided for painting, pointing or other maintenance work. They consist of two rows of standards (the vertical supports) connected by ledgers and transoms (the horizontal elements). 'Independent' scaffolds are not quite what their name suggests. They are termed 'independent' because they derive no vertical support from the building and 'tied' because they must be tied to the building for horizontal stability. Because of the need to avoid damage, tying scaffolding to the facade of historic buildings can sometimes present difficult problems. Sometimes, if the historic building is fragile, it will not be capable of providing the horizontal restraint that the scaffolding needs and this must be achieved in other ways, such as by providing external scaffold buttresses or by tying the external scaffold to an internal birdcage framework scaffold.
'Putlog scaffolds', used for the construction of brick walls, have only one row of standards which are usually erected some 900mm from the face of the wall, with the boards carried on horizontal members known as 'putlogs'. When used in new construction, the flattened ends of the putlogs are built into the bed joints as work proceeds and then withdrawn on completion, the resulting hole being pointed up. (100mm square holes used for timber putlogs can sometimes be found in medieval work.) Putlog scaffolds should not be used on historic building work as unnecessary damage is caused by cutting holes for the bearing of the putlogs.
SHORING OR SUPPORT SCAFFOLDING
Temporary works are often needed either because there is a risk that a structure might otherwise collapse or because it is necessary to remove some vital supporting member for renewal or alteration. The loads to be carried by shoring can be very great and the danger posed to passers by and the fabric by an inadequate design should never be under estimated.
The main difficulty with shoring historic buildings is to ensure that their installation does not cause damage. Shoring must be designed by a structural engineer or other competent person.
DESIGN
The design of scaffolding should not, unless it is very straightforward, be left to the scaffold erector. It is important that prior thought be given to the location of scaffold foundations, where standards can and cannot go and where boarded out decks are required to enable the work to proceed with as little difficulty and risk as possible.
All temporary works should be designed before the site staff begin erection and the level of design and drawing of scaffolding and temporary works must be commensurate with the scale of the works. A pencil sketch on a sheet of paper may well suffice, indicating that at least someone has thought about what is needed before work commences. A bigger job may well demand calculations and proper drawings.
RESPONSIBILITY
The failure of even a single telescopic prop supporting a major element of a building under repair could be fatal; therefore, as the dangers do not necessarily relate to the size of the project, the architect or engineer should examine the contractor's proposals for all scaffolding and shoring. After all, the architect and engineer will have been dealing with the building for some long time and are more likely to be aware of its weaknesses than the contractor who, however experienced, may well have only seen the building briefly before being expected to commence work. It must be ensured that schemes are erected so as to conform with previously presented proposals.
Care must be exercised to ensure that the contractor's responsibility for temporary works is eroded as little as possible. Contract documentation for works to historic buildings should always include a section concerning scaffolding and temporary works.
STATUTORY INSPECTIONS
It is a statutory requirement that all working scaffolds are inspected weekly by a suitably qualified person and that the results of these inspections are recorded in the 'Scaffold Register', an official book of forms which have to be completed weekly.
NECESSARY FEATURES AND COMMON PROBLEMS
The following features and problems are all basic and mostly fall into the category of 'common sense' rather than being highly technical requirements.
Foundations: Foundations should always be on firm, level ground and should never be undermined. Standards and props should be concentric on foundations. When scaffolding is to remain erected for six months or more, railway sleepers or similar sized treated timbers should be used as foundations.
Historic buildings often have basements outside the periphery of the ground floor which may well be incapable of supporting scaffolding, so thought needs to be given to the means of transferring loads to the ground. One site on which the author was involved had below ground storage tanks. A huge lorry-mounted crane was to be used to erect a temporary roof and it was vital to locate these tanks before the lorry arrived on site and found one accidentally!
If excavations for foundations are required, there may be a need to provide archaeological supervision of the digging operations.
If it is necessary to erect temporary structures on floors or roofs it is important to ensure that the supporting structure can safely bear the weight or that precautions are taken to ensure that the extra loads will be adequately supported.
As historic buildings often have overhanging cornices and other projections, correct setting out of the standards needs to be considered in the light of what is directly overhead.
Ties: When ties pass through sash windows, one sash can be raised to allow the tube to pass through, the resulting gap sealed and the sashes screwed to each other to prevent unauthorised entry. Casement windows are more difficult. If they carry leaded lights it may be possible to remove one small pane but casements with a single glazed sheet may need to be taken off their hinges and stored safely. Regrettably some scaffolders just smash a window (which may contain historic glass) to put their ties in place.
Fixings to masonry: Where fixings are made to stone or brickwork it is necessary to check that the masonry is adequate beforehand. Such a fixing to a facade could dislodge a stone or an area of brick thus endangering the safety of the scaffold and damaging the historic fabric. All fixings made to the wall of an historic structure must be of stainless steel. Listed building consent may be needed before permanent drilled-in fixings are installed.
Decking: Decayed, warped or split boards must never been used as they create tripping hazards. Boards that have become slippery or damaged should be discarded and precautions should be taken to hold boards down in high winds. Excessive loading on platforms should be avoided unless the scaffolding has been specifically designed to carry heavy loads. If dismantled masonry is to be stored on a scaffold platform the scaffold designer should be told of this before the design is commenced.
Scaffolding to building interfaces: However well constructed, scaffolding is always likely to move slightly and a tube end rubbing on a wall face can easily cause permanent scarring. All points of contact or near contact between scaffolding and historic buildings should be protected in some way. All tube ends that either touch a wall or are within 25mm of it should have plastic end caps. All standards should sit on timber sole plates to spread the load and floors beneath should be protected with polythene sheet, old carpet or similar materials to prevent damage. All scaffolding should be galvanised to avoid the risk of rust staining.
Sheeting: The wind loading created by sheeting, which is sometimes provided for weather protection, can be very high and special consideration needs to be given to the spacing of scaffold to building ties.
Telescopic props: These may need bracing if they are over two metres high or if they carry heavy loads. They must be plumb and be properly founded. It is common to find a missing support pin being replaced by a short piece of reinforcing bar or something even less satisfactory such as a big nail. Only the manufacturer's high tensile steel pin should be used.
Temporary roofs and temporary buildings: In relation to their area or volume temporary roofs and buildings are, by nature, light structures. As a consequence their need for lateral stability and resistance to wind uplift is a major but often ignored requirement. It is usually advisable to seek the help of a structural engineer in the erection of such structures. The contractor should always be required to provide a drawing of his proposals and in any but the smallest of cases, supporting calculations.
Earthing: All scaffolding structures which are at risk from lightning strikes should be properly earthed.
Unauthorised access to the building: Scaffolding can make buildings more vulnerable to intruders; ladders should be locked away at night and extra security precautions may be wise.
Workforce: Efforts should be made to ensure that the workforce is aware of the value of the historic fabric. It is well worth considering giving the scaffolders, and indeed other craftspersons, a 'conducted tour' of the building so that they can begin to understand its importance and not assume that is 'just another old building'. On one English Heritage site the house administrator took the scaffolders around the building before they began work. This seemed to engender some genuine enthusiasm and concern for the building and paid dividends in the extra care which they took.
FORETHOUGHT AND ATTENTION TO DETAIL
Scaffolding and temporary works are not always given the consideration that they deserve. Consequently there is risk of damage to the historic fabric either in relatively minor ways such as scarring of surface finishes or in more serious ways such as partial collapse. There is the additional risk caused to members of the workforce or to passers-by.
Documentation, both that produced by the architect or engineer and that produced by the contractor needs to be commensurate with the scale of the job, bearing in mind that failure of even a small element can cause serious problems. Even if only a single telescopic prop is proposed it is important that some proper estimate of the weight to be carried is made and reference made to literature to ensure that the prop proposed can carry the weight safely.
Architects and engineers involved in historic buildings work should have a clear understanding of the requirements of scaffolding and temporary works and be aware of the consequences if something goes wrong. The safety and success of scaffolding and temporary works in the historic building field relies heavily on two things; forethought and attention to detail.
With an historic building there will be no second chance.
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