Comparison of Current Timber Design Standards
Timber has been a staple building material in construction for centuries, only losing prominence upon the emergence of concrete in the mid-1800s. Since then, concrete and steel have replaced timber as the primary resources for building residential, commercial and civil structures. Consequently, the majority of design codes and standards used in the industry have been developed with reinforced concrete and steel structures in mind, with little attention given to timber. However, as environmental issues start to bite, structural mass timber products are becoming increasingly popular. So revision and expansion of current timber design procedures are becoming necessary. Read more on our thoughts on the growing appeal of mass timber projects in the construction industry here.
For the time being, mass timber building codes have not kept up with the rate of timber usage. Leaving engineers with a massive challenge whenever they design mass timber. Mainly, what rules and guidelines should they adhere to in unique scenarios or when dealing with new timber products? As a result, engineering teams are forced to scramble together guidance from multiple code sets, with misaligned clauses and formulae. Costing time, money and a lot of stress!
We’re sure that building code writers will catch up eventually, but for the time being, it’s critical that engineers understand the main building codes available. As such, we’ve written this article to compare the varying timber design codes to help streamline your decision-making when designing mass timber!
Australia- AS1720 and AS1684
They provide calculation procedures for design properties and capacities of timber elements, including beams, columns and connections. It includes formulae and guidelines for determining load and serviceability limit states for structural elements produced using sawn timber, laminated timber, round timber, plywood and laminated veneer lumber (LVL). Design of joints in solid timber fabricated with mechanical fasteners including nails, wood screws, bolts, coach screws, split-ring fasteners and shear-plate fasteners are also covered.
AS1684 and its parts are an extension of AS1720.3, focusing on the design of residential timber buildings. Parts 2, 3 and 4 of AS1684 focus on the variance of design methods and values of parameters appropriate to cyclonic/non-cyclonic regions.
Europe- Eurocode 5
The Eurocodes are a series of standards developed to harmonise technical design specifications across the European Economic Area (EEA). It includes European Union (EU) nations, Iceland, Liechtenstein and Norway. Eurocode 5 or EC5 is the standard for structural timber and consists of three parts:
In comparison to national standards like AS/NZS, which are tailored specifically to particular jurisdictions, EC5 contains only the essential principles and formulae for design. It serves as the ‘backbone’ for designing mass timber structures in the EEA. The standardisation authority in constituent nations is permitted to add country-specific data as a “national annex” to EC5. This includes loading conditions like snowfall depths, wind speeds, earthquakes, etc. and material properties specific to the type of timber used in each country.
USA- 2018 NDS
National Design Specifications (NDS) for Wood Construction is the American National Standard for timber design, developed by the American Wood Council (AWC). As the governing design guideline for timber elements in the USA, it provides methods for visually graded lumber, mechanically graded lumber, structural glued laminated timber, timber piles, timber poles, prefabricated wood I-joists, structural composite lumber, wood structural panels and cross-laminated timber. A distinguishing feature in 2018 NDS is the use of “Allowable Stress Design” (ASD) and “Load and Resistance Factor Design” (LRFD), whilst the Australian standards and Eurocode 5 use limit state design (LSD) methods. What are the differences?
AS1720 and AS1684 are considered subpar compared to other timber standards due to their limited coverage. Whilst they are excellent for residential timber elements (such as trusses, struts, roof purlins and rafters), the standards aren’t suitable for larger timber structures. The guidelines for deflections are only applicable to residential buildings, and vibration control is practically untouched by the standards. Furthermore, these standards do not accommodate designs using cross-laminated timber or timber-composite materials.
Like the Australian standards, Eurocode 5 uses limit state design approach and modification factors in its design procedures. A national annex normally accompanies EC5; therefore, it doesn’t include specific values for factors that may vary from country to country (such as the effect of temperature, loads, etc.). Eurocode 5 accounts for timber cracking and vibrational performance, unlike the Australian and American standards. It considers the influence of cracks in the shear design of members and sets a limit for the vibration frequency of residential timber floors.
2018 NDS offers two design approaches (ASD and LRFD). Different factors and numerical values are used depending on the chosen approach, unlike the Australian standards and Eurocode 5, which are solely based on limit state design. NDS is the only standard to include a design section for cross-laminated timber. Since its introduction, changes have been made for design using CLT, and recently the deflection calculation methods of CLT elements were revised. However, NDS has yet to account for cracking and vibration control.
Some commercial software like Timber Solutions (by Wood Solutions), RFEM 6, RSTAB 9 and SkyCiv are available for structural analysis of timber structures. However, these are also premature and mainly oriented towards smaller commercial and residential structures. It is evident that more attention must be paid to the technological development of timber structural analysis as well.
It’s clear that each standard has its high and low points. As the demand for sustainable structures increases, broad and reliable timber design guides are becoming more necessary. Current standards are good, but to truly take advantage of the advancing development of timber engineering, they must be improved to accommodate new materials (such as CLT and timber-composite materials) and be adaptable to different environments.
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