|IAN HUME AND THE IHBC TECHNICAL SUB-COMMITTEE
Floor loadings in historic buildings
Careful thought about floor loadings can avoid unnecessary alterations and avoidable costs.
If a historic building is to be given a new lease of life, considerable work may be needed to make it satisfactory for its new task. It is seldom necessary to gut the building or indeed to make any major changes at all to enable it to have long and useful future. Changes in use demand that consideration be given to the loads to be carried by the building. Careful thought is needed about the loading requirements, and to avoid blanket use of design loadings taken from British Standards when something less might be equally acceptable with no increase in risk.
It is often claimed that high floor loadings are desirable to give the client the flexibility that is required and to avoid overloading, excessive deflection, or collapse. However the decision to use high floor loadings can lead to major, sometimes unnecessary, changes to the hidden but important historic fabric. Attempts to upgrade a historic building to a high load-carrying capacity will almost always result in both massive and expensive intervention into the valuable historic fabric or, in extreme cases, complete loss of the building.
Any features, visible or hidden, that contribute to a listed building's special character are important. When increasing the carrying capacity of a building it must be remembered that the beams, joists and other hidden elements of the structure of the building are also historically valuable. Listed building consent may be needed for carrying out structural alterations as well as, more obviously, alterations to the appearance of the building. Consent may also be needed for "opening up" for inspection prior to the design of remedial works.
The English Heritage leaflet Office floor loadings in historic buildings, published in 1994, debates the problem of accommodating office floor loadings in historic fabric. It suggests that very often there is no good reason for the blanket use of 5.0 kN/sqm (suggested by the British Standard Code of Practice for the "design loading for buildings" for "file rooms, filing and storage") and that 2.5 kN/sqm ("offices for general use") is usually more than adequate. 5.0 kN/ sqm = 100 lb/sqft and 2.5 kN/sqm = 50 lb/sqft.
"Do come over to the other side of the room. There's someone I'd really like you to meet "
|Ian Hume is a conservation engineering consultant and a member of the IHBC technical sub-committee. He was formerly chief engineer at English Heritage||The self weight of existing structures can be calculated with a high degree of accuracy as the floor structure and other elements of the building can be seen, measured and their weight calculated accu~ rately. So smaller than usual factors of safety can be used in structural check calculations. Those factors of safety generally recommended are intended for use in new-build work where materials may be heavier or larger than specified.
The initial and correct action when checking a floor for any loading is to look to the British Standard Code of Practice for loading. This gives advice on what loadings should be used in the design and checking of structures.
The information contained in any British Standard is only advice regarding good practice. It is not mandatory, although if a structural engineer decides to choose a different value to that proposed by the British Standard there must be good, sound evidence that the value is adequate. Use of the right designation of loading rather than some higher figure should be carefully considered.
It is often suggested that engineers and surveyors use high floor loadings in their designs because the clients demand the flexibility to use buildings in any way they wish, while clients frequently say that the higher loadings are necessary because their engineers and surveyors recommend it. There needs to be closer liaison and discussion between clients and their professional advisors on this issue.
Are high floor loadings, employed to give flexibility of use, really justified or can a lower value be used without compromising flexibility or structural safety? It is important that the use of the building is not overly restricted by the loading that the floors are capable of carrying, but equally there is no need to upgrade floors to make them capable of carrying loads which will never be imposed.
The designated loading for a library is 4.0 kN/sqm (80lb/sqft) whilst that for a reading room is only 2.5 kN/sqm (50lb/sqft). If the room in the historic building is to be a library, we need to be clear as to its precise function. Will the room under consideration be a fullyfledged library where people search for a book or is it to be a room where they sit to read and study, furnished with tables and chairs and with a few bookcases around the walls? If the latter, why does the floor need to be upgraded to carry the full library loading when the designated loading for a reading room will be more than adequate? Full bookcases can be very heavy, though, so some localised strengthening may be needed.
The floor loading designated for a museum is 4.0 kN/sqm (80lb/sqft). Museums can obviously display a wide variety of artefacts ranging from stamps, coins and other virtually weightless items displayed in relatively light flat-topped cabinets to stone sculptures and machines from the early days of mechanical power which can be extremely heavy. A blanket loading of 4.0 kN/sqm is not suitable for all cases.
Often, particularly in the case of small local museums in historic buildings, the use of each room is well known in advance, and the precise floor loading can easily and safely be calculated. The weight of the objects and their display cabinets can be worked out. An allowance must be made for the visitors, remembering that they may come in large groups. Such calculations often prove that there is no way in which the floor can be subjected to 4.0 kN/sqm, and that to upgrade the floor to carry this load is wasteful of scarce resources as well as destructive of the historic fabric of the building.
In one example, a museum building was to display a fine collection of bone china in glass cabinets that were to be fixed to the walls and floors. Even taking account of the fact that the space between cabinets could easily be crowded with visitors, it was shown that the floor loading was less than half the 4.0 kN/sqm suggested by the British Standard. A somewhat lower value was therefore used and the amount of structural intervention was dramatically reduced, without compromising the safety of the visitors or of the building.
Can heavy items be stored or displayed where they will have little effect, on floors with small spans, for example. Can filing be stored in the basement, or can racking or shelving be fixed to a wall?
In places where there are likely to be crowd loadings, it is possible to place an obstruction, such as a table or a display, in the centre of the room, to keep the heavy weight of crowds towards the edges.
This method of avoiding excessive loading is particularly useful where loading can be carefully controlled, such as for one-off events, and in museums and galleries where light display cabinets can be screwed to the centre of the floor to avoid crowd loading at the position where it will create maximum bending moments and the highest deflections in the floor.
Although not directly related to deciding what load to use in the checking procedures on an historic building, it is always wise to consider what loading the building has had to carry in its life to date, and how well has it survived its previous uses. If the building has been in use as a warehouse for the storage of grain, do the floors really need to be upgraded when the building is to become flats, even if calculations show that excessive deflections might occur? A detailed inspection of the structure of the building may be more valuable than computer analysis. It is not uncommon to encounter historic buildings where both common sense and visual inspection demonstrate that a building is sound, even though calculations indicate that it is substandard and incapable of carrying any live load at all.
Calculations of stresses and deflections often show that timber floors are capable of carrying little more than their own weight, even where the building has been carrying a substantial load for many years. Are the calculations wrong, or is the building weak? If the building has been working satisfactorily, there must be something amiss with the calculations or, more likely, the assumptions on which they were based.
Although not directly related to considerations of design loadings, the load testing of a structure or elements of that structure can prove that the building is satisfactory despite theoretical predictions to the contrary.
Upgrading a historic building to carry high superimposed loading can all too easily result in major structural intervention, sometimes virtually gutting the interior of a building. The costs, both financial and in terms of the loss of historic fabric, may not be justified by the perceived need for high floor loadings.
Unrealistic design loads should not used when it is impossible to generate that load or when, with a little forethought, high loading can be avoided.
CONTEXT 70 : JUNE 2001