edition of Mitchell (which is the first one I will admit to having bought new!) uses the standard working stress method which would not be out of place in an office today. THE MODERN BRITISH STANDARD Working stress methods are used today when using the latest British Standard BS 5268: 1991. Now we have a wide range of allowable stresses and moduli of elasticity for large number of different species of timber. We are permitted to consider load sharing where there are a number of members side by side, as with floor joists, and deflection considerations are important. Also included in che design process are a variety of modification factors for matters such as dry/wet stresses, duration of loading, notches at bearings etc. le is possible to do a quick calculation and conclude that a particular size is the correct one but a close reading of the British Standard with careful use of the modification factors can result in a smaller size being permitted. This 'pulling out of all the stops' approach is very necessary when checking sizes of members in historic buildings. Examination of the table for the sizes of 'bridging joists' verifies my experience. Neve suggests 8" X 3" for 12 foot span whilst for a slightly longer span of 15 feet Newland's calculations indicate that a 10. 5" X 2" joist is required. BS 5268 calculations result in an 8" X 2". The 'binding joist' sizes in a double floor suggest that a builder referring to Neve in 1726 would have used a 13" X 9" oak for an 18 foot span; his counterpart about I 00 years lacer, if he had adhered to the chen current information would have used an 11" X 10" fir, whereas a modern day engineer using SC3, a reasonably good softwood, would have concluded that 12" X !0"would be right. Not a great deal of difference. The girders of a framed floor put in by a builder referring to Newland (or T redgold) would have been considerably weaker than BS 5268 requires. Newland suggests a 15" X 12" for a 24 foot span whilst BS 5268 dictates chat the size should be I7" X 14", some 50% stronger in stress terms and about 70% less stiff in deflection. However, old timbers are very often better than timber that can be bought today and consequently these apparently considerable reductions in strength and stiffness are less significant. Someone using the 1911 version of Mitchell would have used sizes considered satisfactory by today's standards. The firststep in manyconservationprojects...... . A Consultancy service to help you obtain WHAT DOES THIS MEAN FOR CONSERVATION? I believe that it means that, provided that the timber is in good condition and chat there is no change in the loading applied to the building, the joists are going to be perfectly satisfactory and that, if there are no signs of problems owing to excessive deflections, then the beams are also likely to be satisfactory but perhaps with rather more deflection than we might accept today. Deflection will be no great problem if there are no finishes, and of course older and more flexible finishes are more able to sustain higher deflections than modern, more brittle, finishes. I am sure the foregoing means that we should not lee floors be taken out or suffer massive interventions for strengthening reasons without a great deal of consideration. We must look very closely at proposals to strengthen or remove; if calculations are to be done then they must be stretched to the limits. Most importantly we must look to the history of the behaviour of the floor which is, after all, a full scale, long term load test. Most importantly of all "if it ain't bust, don't mend it". fan Hume is Head of the Conservation Engineering Branch at English Heritage. GENUINEPUDDLED WROUGHTIRON BARANDSHEET. CHARCOAILRONFOR REPOUSEE SPECIALSECTIONS MEASURED SURVEYS of HISTORIC BUILDINGS AND SITES SPECIALISTISN TRADITIONAL WROUGHTIRONWORK The Conservation Officer's first step with a new project is often to recommend the measured survey and recording of a property. However, the preparation of measured surveys can nowadays be a complex matter, involving choice of method, careful specification and choice of contractor. With over 20 year's experience in the Public Sector including workingfor English Heritage, using my services will ensure that you obtain the best advice and the best products tailored to your requirements. For infonnation, contact ROSS WA DALLAS BSc FR/CS Chartered Land Surveyor and Architectural Photogrammetry Consultant 23 East Mount Rd The Mount York YO2 2BD Telephone/Fax 01904 652408 CONTEXT 43 RESTORATIONS BLACKSMITWH ORK FORTHECONSERVATION INDUSTRY ~c~ ~~ ~ Carlton Husthwaite, ' Thirsk North Yorkshire Phone/Fax (01845) 501415 27
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