Great slabs of granite and sandstone cladding are a common sight on tall office buildings in modern cities across the world. But recent research has revealed that some of this high rise masonry is weakening at an alarming rate and there is a real danger of it failing and plummeting to earth.
Changes are needed to the British Standards for stone cladding, says Messrs Sandberg consultant Roy Kinnear. And contractors could soon face calls for extended warranties on the cladding they install.
Cracks are starting to appear in many cladding systems. And while this is usually attributed to poor workmanship during construction, there are indications that engineers may be overlooking a more fundamental problem, says Kinnear.
Fractures and other discontinuities such as bedding are inherent in all rock types. One of the main causes of fracturing is stress relief. When rock is exposed or extracted from the ground, the earth pressure it has been subjected to is suddenly removed, causing expansion and cracking. And once exposed, the rock is affected by climatic changes, which can cause fractures to propagate and expand with time. The rock is weakened by fatigue.
'What has happened is that no-one thought to translate the natural behaviour of rock to thin stone cladding,' says Kinnear. Engineers, he adds, have generally misinterpreted cracking as a problem caused by poor fitting and fixing, but in fact the cladding is undergoing a loss of strength through long term fatigue.
In stone cladding, fatigue is mainly caused by changes in temperature during the course of the day and with changes in the weather. As the stone expands and contracts, cyclic stress loading occurs. Kinnear says that fatigue starts to occur if the cladding is 'locked-up' - restricted by abuting with stronger material, or prevented from moving by the mortar between slabs.
As movement is restricted by the fixings holding the cladding to the main structure, these points come under increasing stress. 'Drilling holes or cutting slots in the stone for fixings forms microcracks, however carefully this is carried out,' says Kinnear. So not only are fixing points under the most stress but they are also the most vulnerable to fatigue. The concentration of stress causes the cracks to enlarge. Entire sections of the cladding around the fixing points can then separate from the main part of the slab, leaving it unsupported, with the risk of catastrophic failure.
Kinnear says that the fatigue is also caused by the ageing of concrete structures, with shrinkage and creep of concrete setting up further stress in the stone as the fixings move.
Kinnear first struck upon the concept of long term fatigue in the late 1970s, when he was involved in assessing the unexplained progressive failure of granite cladding on the New Scotland Yard building in London. As part of the investigation, Sandberg carried out limited laboratory testing on samples of the cladding. 'The cracking seemed to be explained by long term fatiguing action,' he says.
It was not until recently that any further work was done. Kinnear was asked to act as an expert witness for a legal case in Hong Kong at the beginning of the 1990s. Inspections of a large building showed that the cladding was failing and he realised that this was another example of long term fatigue. Kinnear then persuaded the client that testing was needed to prove that the problem was one of fatigue rather than poor workmanship.
Testing by Sandberg confirmed his theory. 'Not only did we demonstrate the fatigue but for the first time we were able to put some numbers to the problem,' explains Kinnear. Cyclic loading was used to represent changes of temperature-induced stress over time. Testing of samples revealed that after 1,000 cycles, the stone was showing signs of failure, its ultimate strength reducing to around 80% of the original design value.
After 10,000 cycles the stone was at around 75% of its design strength. Further tests on other stone used for cladding demonstrated that this was a problem not restricted to one rock type with similar patterns of strength loss. Furthermore, the direction that stone is cut with regard to bedding and grain orientation was shown to influence the slabs' resistance to fatigue.
Kinnear's main concern is that the design values for cladding strength and the factor of safety used in calculations are too high. He believes that changes in liability insurance will have to be made by contractors to ensure that they are covered in the event of failure. He points out that 5,000 loading cycles is roughly equivalent to 10 years or so, a period in which contractors might clearly be responsible for the structure, and 50,000 is approximately the working life of a structure. Building owners will also start calling for extended warranties for cladding, he says.
'It is clear that this is a serious problem,' he says. 'We are now at the stage where further research is required.' Engineers must be made aware of the danger, and to this end Kinnear says that changes to the British Standards for thin natural stone cladding (BS 8297 and 8298) are needed. Currently they do not assess the problem of long term fatigue.
'Any further research should provide a basis for assessing the quality of stones supplied,' says Kinnear. He is also calling for a database that lists the vulnerability to fatigue of different types of stone cladding. And although the problem is becoming more known in the industry, he says that there is antipathy among the industry, as many engineers believe that testing will require more resources. But he explains research can be carried out on conventional load testing equipment.
Kinnear says that one way of reducing the chances of fatigue is quality control at the source - the quarry. The first stage is to avoid material that is already has cracks and to ensure that slabs are cut in the orientation that gives the most resistance to cracking. 'Identification of microcracks is not easy, as many are not visible to the naked eye,' he says. In Italy, a special dye is sprayed on to the large slabs that the cladding is cut from, revealing the smallest cracks and allowing the most damaged areas to be avoided.
Kinnear stresses that this is not scaremongering. 'While incidents of cladding actually falling off buildings are rare, the risk is still there. Hidden deficiencies increase the vulnerability of stone to fatigue which can cause failure, say within 10 to 12 years of construction, and when failure occurs, it will be sudden and without warning.'