When buildings are too safe
A study into the strength of clay brick masonry suggests that current design standards in Australia are overly conservative, and that substantial efficiency and cost savings could be achieved if these are relaxed. Professor Mark G Stewart, from the Centre for Infrastructure Performance and Reliability at the University of Newcastle, Australia, reports.
There is a general feeling in the construction industry in Australia that design specifications for masonry walls are too conservative. While we want our designs to err on the safe side, we also need to ensure that they do not, in turn, increase construction costs and reduce the competitiveness of masonry.
Approximately $4bln is spent on masonry construction in Australia each year. If research can reduce the conservatism of design methods, this will have a significant impact on the Australian construction industry and economy.
Modern codes of practice for structural engineering design using materials such as steel, reinforced concrete, timber and masonry are almost exclusively based on ‘limit states’. The basis of this approach is that various limit states, including excessive deflection, cracking, stability and structural collapse, can be defined. The structure is designed with the intention that it will have an acceptably low probability of exceeding the various limits during its service life. This is achieved by using the characteristic values of material strengths, and accounting for partial load and resistance factors.
Load and resistance factors must be determined using reliability-based calibration methods. Such approaches make use of probability distributions of loads and resistances to calculate the probabilities of failure – the probability that load exceeds resistance:
Limit state specifications for structural steel and reinforced concrete design have been successfully developed over the past 25 years from reliability-based methods. Unfortunately, this has not happened for masonry design.
A typical limit state equation in the Australian masonry design code (AS3700-2001) is φRn≥ΣγiQi where φ is the capacity reduction factor, Rn is the design strength, Qi is the design load effect for load i and γi is a load factor. The load factors are identical for all structural materials, whereas the capacity reduction factor depends on the material. Nearly all research into structural masonry has focused on how to predict the design strength more accurately. Surprisingly, though, the value of the capacity reduction factor (φ), which provides a margin of safety allowing for structural deficiencies, has received almost no attention. Yet its importance on design outcomes is as relevant as design strength.
The Australian masonry design code has been in a limit states format since 1988. However, limit states specifications for masonry in Australia, the USA, Canada and Europe have not been developed from reliability-based calibration methods, but rather calibrated to past practice. The result (in the Australian code) is a situation where the bending strengths are specified, which are already five per cent lower than characteristic values, and are further discounted by a factor of φ =0.6 for bending, and compressive strengths are discounted by a factor of φ =0.45. The resulting strength specifications seem low, even for masonry, but additionally, intuition would suggest that the reduction factor for compressive strength should be less onerous than that for bending strength. The adoption by code AS3700 of the opposite has a severe effect on the specifications for load-bearing masonry walls, such as those used in low-rise residential units.
The φ factors have never been rationally calculated and the true safety levels for masonry structures are unknown. It has been suggested that the rare occurrence of any form of structural failure in masonry could be an indicator that the safety levels are too high. This would mean that the full potential of masonry materials is not being exploited. In particular, it is not known how the safety levels for masonry structures compare with those for other materials in common use, such as steel, concrete and timber.
Researchers at the University of Newcastle in Australia have been developing new probabilistic analyses to calculate the probability of failure of clay brick masonry walls under various loading conditions. A comparison of reliabilities will show which design criteria are inconsistent and whether changes to φ can be suggested to ensure adequate and consistent levels of safety.
Failure of a structural element occurs when the load effect (S) exceeds the resistance (R). Structural reliability may then be expressed as a probability of failure or ‘reliability index’ (ß). A reliability index of 3.8 indicates a probability of failure during the structure’s 50-year design life of about one in 15,000 and a value of ß = 4.4 indicates a probability of failure of about one in 200,000. Typical target values of ß for ordinary structures are 3.5-4.0. These target values are specified by ISO 2394:1998 General Principles on Reliability for Structures, which recommends ßT = 3.8 for design based on ultimate strength limit states where the consequences of failure are great and the relative costs of safety measures are moderate. This is the same target reliability adopted for reinforced concrete and steel design.
A recently completed research project funded by Think Brick Australia and conducted at the University of Newcastle has for the first time compared design strengths with actual wall test data and used this information to calculate the structural reliability of clay brick masonry walls in compression. The analysis was done in accordance with ISO 2394:1998.
Equation 1 shows that, as the capacity reduction factor increases, design resistance Rn is reduced while resisting the same loads. In practical terms, this means that the design thickness of masonry reduces as the φ factor increases. With reference to the chart below, it is seen that when the current value of φ = 0.45 is used, the reliability index is 4.4, which is far higher than the ISO 2394:1998 failure target of ßT = 3.8. Therefore, masonry designed according to this φ factor is overly safe. A reliability equalling the target value is reached when φ = 0.72.
The existing Australian safety levels for masonry have therefore been found to be much higher than those accepted for other materials. Based on this rational analysis, the study has recommended increasing φ for compression loading from 0.45 to 0.75, resulting in a 66% increase in the compressive capacity of structural masonry. This increase in φ is included in the draft 2011 edition of the Australian Masonry Code. Work is now progressing on assessing the reliability of masonry walls in bending, with a view to proposing revised φ factors to the Australian Masonry Code committee within the next 12 months.
For new construction, a more efficient use of structural masonry will mean using less material. This will result in lower construction costs and could help contribute to an increase in building approvals and the competitiveness of masonry with other materials. For example, a 66% improvement in the efficiency in using masonry walls used by five per cent of the market produces ongoing savings to the Australian economy of $132m (£84m) per annum.
This research was completed with co-researcher Dr Stephen Lawrence and funded by Think Brick Australia. Professor Mark G Stewart, Centre for Infrastructure Performance and Reliability, The University of Newcastle, NSW, 2308, Australia. Tel: +61 2 49216027. Email: firstname.lastname@example.org Website: www.newcastle.edu.au/research-centre/cipar