beaIn older buildings beams over openings were not designed mathematically, as would be the case today, but sized by experience or followed rules of thumb. As a result, they are quite commonly overloaded to some degree. A smaller beam is generally cheaper than a large one as there has always been an economic incentive to design and build to the minimum practical. Designing to a practical minimum size is arguably a good thing. Designing to the smallest practical minimum size is the most efficient solution as it avoids waste and is therefore more environmentally sustainable.
Deflected beam or lintel – is it something to worry about?
All materials will deflect to some degree under load, even when they are nowhere near their maximum load bearing capacity. Over time and under load all materials will creep. Timber will bend a lot over time. Steel and concrete will also deflect but to a lesser extend. Beam that is perfectly capable of supporting a load without collapse will eventually experience some degree of sagging over the years, decades or centuries.
Even in modern buildings where beams have been mathematically designed with generous factors of safety there is some allowance for acceptable deflection and again long-term creep is likely to occur. Historically people were willing to accept a much greater degree of deflection than we are today. A good example is an old roof which often is visibly sagged. Morden roofs are expected to be straight to the naked eye. The result is that timbers, designed in accordance with modern regulations, have to be of a much bigger size than Georgians or the Victorians would have used; even allowing for the relative strength of the timber available at the time.
Lintel or beam cracks – what you should know and do
The amount of deflection related to this type of movement will vary depending on the degree of overloading. In most cases cracking will be hairline to a couple of millimetres and no remedial action needed. The appearance of cracks will likely be of a stepped pattern. The magnitude of displacement will be similar both horizontally and vertically.
Cracking can be much more pronounced than a couple of millimetres before there is any great structural concern. Timber (depending on species) will deflect considerably before it is anywhere near collapse point. However, when deflection is clearly visible and cracks are 3 to 5mm or more, there is a tendency for people to become concerned. It is not just a risk of failure but also how safe people feel within a building that is important. When cracks develop to this degree it would be worth having design check calculations carried out by our structural engineer Croydon to establish root cause of the problem and advise on appropriate strengthening measures.
New design criteria vs old building structures
On the point of checking calculations from an engineer using modern design criteria, it should be recognized that almost all old beams would fail the mathematical calculations. This applies even where they have been perfectly adequate throughout their design lives and would continue to do so. This is purely due to limitations of the assumptions that a designer can reasonably take into account factors of safety involved in the design. For example, our structural engineer Croydon cannot know how strong and old piece of timber is without testing it to destruction, which would negate the whole point of the exercise. The engineer has to make an assumption and it has to be on the safe side of the possible range of strength that the timber could have.
This is a limitation of the mathematical approach that cannot be avoided, which is why most of the buildings are not assessed in this way. In practice they are assessed visually, using different available techniques and quite often calling on experience and personal judgement.
A common example of this type of deflection is found in houses with single-storey bay windows of the late Victorian period. The supporting beam above bay windows is usually built of timber and known as a bressummer beam which deflects over time. The cracking is often hidden behind the roof of the bay window externally. Internally it is likely to have been decorated over. The deflection can, however, be seen in the first window sills above the bay. This quite often is associated with cracked in the ceiling across the bay window opening.