Structural design considerations and testing
Structural design considerations and testing
The eighth episode of Sawmilling South Africa's (SSA's) Talking Timber webinar series featured the University of Pretoria's structural engineer, Dr Johann van der Merwe, and Stellenbosch University's wood scientist, Prof Brand Wessels, explaining why the mechanical properties of wood must be considered when designing timber structures.
Wood is a natural material with enormous variations in properties, which impact its structural applications. Timber grading is categorising wood based on its quality and strength. This classification system ensures consistency and reliability and helps practitioners understand the potential applications and limitations of different types of timber.
Uncertainty and risk
Van der Merwe and Wessels said it is vital for architects and engineers designing timber structures to have a thorough knowledge and understanding of the physical and mechanical properties of wood and the timber grading system. The properties dictate the suitability of wood for specific applications and are elementary for making informed decisions.
Wessels said the answer to the question, "How do we know if a timber product is fit for purpose?" is to assess the uncertainty and risk of the product.
It may be a new product on the market or an existing product used in a different application with insufficient information on its strength. Or the engineer is not prepared to take the risk that the timber may fail under certain applications, for example, load-bearing beams in a public building.
Consider the loads
Van der Merwe said the starting point for designing a timber structure is to consider the loads the building's components will be exposed to during use.
"The loads will never have one specific value but will have some statistical probability distribution. So, in other words, there might be a low number of very low loads that the building will be exposed to, a few instances of very high loads, and a high number of loads that might be somewhere in between", explained Van der Merwe.
Wessels said if the project designer has not used the product before, there are avenues to assess whether a product is suitable:
- Has it been certified by an auditing body like the SABS and SATAS?
- Are independent test results available from reputable organisations like universities, the CSIR, the SABS or SATAS?
- Are there experienced practitioners or technical experts to consult?
- Are peer-reviewed research results available?
- Test the products.
Mechanical properties
The costliest and most accurate option is to test the timber to see if it is fit for purpose. The speakers said there are seven mechanical properties engineers need to know when designing timber structures:
- Tension parallel to the grain of the wood
- Compression parallel to the grain
- Tension perpendicular to the grain
- Shear parallel to the grain
- Compression perpendicular to the grain
- Bending strength
- Stiffness.
"It is very important to realise that timber is highly variable. The characteristic strength and stiffness values used in designing timber structures are determined by testing large numbers of full-sized structural grade wood, a process referred to as in-grade testing", explained Wessels.
"After testing, the strength distribution curve depicts the characteristic or fifth percentile value. It means that, on average, while 5% of the timber is weaker than the safe design value, 95% will be stronger than that value".
SA pine's characteristic value and grade classification table is published in the South African National Standard (SANS) 10163-1:2003.
Specifying
The architect or engineer has three options when specifying structural timber:
- Local structural or test-graded timber (SANS or SATAS) with the producer's name, the grade number and the auditor's mark. The wood does not need more testing.
- Structurally graded timber (other), such as imported timber graded to an EN standard.
- Ungraded timber or timber that is not structurally graded. The specifier has three options: Use an accredited testing centre to test the wood to get the characteristic values, use proof-grading or use other criteria to estimate the grade.
"The advantage of proof-grading is that it can be used for any timber. The timber is stress tested to the grade characteristic value; if it doesn't break, it is fit for purpose", said Wessels.
Van der Merwe explained how timber material properties are used in structural design calculations. The distribution curve of the characteristic strength values is used to determine a reasonably safe approximation for the load it is designed for.
The distribution can be narrowed by stress grading the timber. Another way of narrowing the distribution of characteristic values is to specify engineered timber products because they improve dimensional stability and reduce warping, twisting, andshrinkage. The load shearing in the different pieces of timber have different strengths and stiffnesses.
Cross-laminated timber
Engineered timber products, like cross-laminated timber (CLT), are large panels comprising multiple layers of wood laminated with a strong adhesive. CLT is dimensionally stable and has inherent load bearing and shear strength that lets it serve as a vertical and horizontal assembly material.
Grading of engineered timber products focuses more on the manufacturing process and the quality of the adhesives rather than the wood's natural characteristics. Wessels said an MSc Wood Science graduate, MJ Jacobs, developed the strength values of SA pine that local engineers can use when designing CLT structures.
Commenting on the cost of building with CLT, Wessels said CLT is a manufactured product heavily dependent on volumes. When CLT is used at scale in South Africa, the price should decrease because timber and labour costs are less expensive than overseas." South Africa could, theoretically, be a lower cost producer than, for instance, Europe and the USA".
Connections
Connections in timber construction, including those built with CLT, play an important role in maintaining the integrity of the timber structure and providing strength, stiffness, stability and ductility. Consequently, the types and placement of connections require thorough attention.
Writen By: Joy Crane
