CROSS Safety Report
Structural failure of older timber connections
A reporter shares three examples of the structural failure or inadequacies of connections in existing timber structures that they have encountered.
Key Learning Outcomes
For civil and structural engineers:
When inspecting timber roofs in older buildings closely inspect all members and connections for signs of distress
Be aware that modifications may have been carried out in the past and changed the behaviour of the roof structure
The distribution of forces may not be obvious
The design of timber structures is often dictated by the connections.
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A reporter shares three examples of the structural failure or inadequacies of connections in existing timber structures that they have encountered:
Case 1: Roof spread in traditional Victorian terraced house
The reporter was asked by a builder to advise on the roof structure because they were concerned about movement which had occurred. The nailed connections of the feet of the rafters to the ceiling joists had failed allowing the rafters to push the wall plate outwards by up to 120mm. When the reporter visited site, the wall plate was about to topple off the edge of the wall which would have caused the roof to collapse.
The nailed connections of the feet of the rafters to the ceiling joists had failed allowing the rafters to push the wall plate outwards by up to 120mm
Fortunately, the builder was able to install bolts and straps very quickly and save the roof structure.
Of particular concern in this case was the fact that the building had recently been surveyed by a chartered surveyor who had noted the sag in the ridge but failed to diagnose the problem. They did not recommend that the client should seek advice from a structural engineer or indeed recommend that the client should take any urgent action.
Case 2: Inadequacies of bolted connections in hybrid 1960s trussed structure
The reporter was asked by a builder to confirm that blockwork partitions in a bungalow were non-loadbearing before he demolished them. When the reporter examined the roof structure it was apparent that the original builder had tried to modify a traditional cut rafter roof to form a number of trusses which, at first sight, appeared to span the full width of the building.
Upon closer inspection however it was clear that the eaves connections were completely inadequate to take the tie forces. Although the original builder had intended the internal 75mm thick block partitions to be non-loadbearing, they were likely to have provided some degree of support to the trusses and their removal would probably have caused the roof to collapse.
The reporter advised the builder to introduce steel straps and bolts to allow the trusses to safely span the full width of the building before demolishing internal partitions.
Case 3: Inadequate joints in early plywood I-beam
The reporter was asked by a householder to report on proposed modifications to some early plywood I-beams. The roof structure was unusual, being double pitched with four plywood I-beams spanning the full width of the house. The rafters were propped from these beams and the ceiling joists were hung from them.
Unfortunately, the construction of the joints was poor – in particular the bottom flange timbers were not continuous and had not been connected to the adjacent section (Figures 1 and 2). Remedial work was required.
In each of these cases the timber structures look as if they have been built on site and the joints have failed – highlighting the need to either use components which are assembled in the factory (e.g. modern nail plate trusses or modern timber I-beams or box beams) or ensure that, where timber frames are built onsite, they are designed by a competent engineer, built by a competent specialist contractor and checked by the engineer.
The first case also highlights the importance of chartered surveyors seeking a second opinion from a structural engineer when they spot structural movement but can’t diagnose the cause.
Expert Panel Comments
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Timber is in widespread use for domestic properties but is usually not going to be detailed by an engineer. Moreover, timber detailing is not part of a general engineer's skill set. In each case, there has been a lack of appreciation on somebody's part to grasp the forces in the structure and the role of connections.
The examples do however relate to older properties and do not reflect modern construction. Perhaps one of the great lessons here is to raise awareness of such issues amongst designers and engineers inspecting older properties, particularly the need for close inspection of all parts of the structure being assessed in order to ensure they will function as expected - i.e. not assume, for example, that connections would be adequate. The design of timber structures is often dictated by the connections. Members sized on forces alone may be too small to accept the connections.
Those inspecting roofs need to be aware of not only how they were supposed to behave but also of the effect of any modifications to the structure and deterioration in the timber. The distribution of forces may not be obvious and it is important to ensure that there is adequate bracing. Signs of decay, rot, fungus or the atmospheric condition that would enable such decay to occur may be significant.
There is a loss of traditional skills involved in timber detailing and a lack of appreciation of structural forms in timber. Fortunately, timber is very resilient to abuse and redistribution of loads, but if decay or insect attack prevails sudden failure can occur. There is therefore a need to be very careful when dealing with timber roofs and connections as often they are not properly designed or constructed.
The Institution of Structural Engineers in association with the Structural Timber Association has a series of Notebooks that provide technical information on the use of timber as a structural medium. Guidance is also available from TRADA on all aspects of timber construction.
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