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National Design Specification For Wood Construction 2018

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 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, most 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, structural mass timber products are becoming increasingly popular as environmental issues start to bite. So revision and expansion of current timber design procedures are becoming necessary. Read more about our thoughts on the growing appeal of mass timber projects in the construction industry 

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, engineers must 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!

Australian Standards (AS) present two series of standards for timber structural design, 

They provide calculation procedures for the 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. 

The Eurocodes are a series of standards developed to harmonize technical design specifications across the European Economic Area (EEA). It includes European Union (EU) nations, Iceland, Liechtenstein, and Norway. 

5 or EC5 is the standard for structural timber and consists of three parts:

Compared 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 standardization authority in constituent nations can 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 timber used in each country.

) for Wood Construction is the American National Standard for timber design, developed by the American Wood Council (AWC). As the USA's governing design guideline for timber elements, 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 of the 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 cross-laminated timber or timber-composite materials designs.

Like the Australian standards, Eurocode 5 uses a 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 depend 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 to 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. 

are available for structural analysis of timber structures. However, these are also premature and mainly oriented toward smaller commercial and residential structures. More attention must be paid to the technological development of timber structural analysis as well.

Each standard has its high and low points. As the demand for sustainable structures increases, comprehensive 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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Comparison of Eurocode 5 and other Timber Design Standards - CalcTree

A common problem faced by engineers when designing structures is identifying relevant information from standards and applying them to their projects. Each design standard contains a vast amount of information; therefore, time and resources may be wasted in searching and selecting bits that are useful for design. This article aims to aid engineers in such a situation by providing a guideline on the 

 according to the American standard 2018 NDS (The national design specification for wood construction). Fundamental equations, 

calculation methods for different timber materials

, and important design parameters are explained in the following sections. Ultimate strength and serviceability limit state designs are extensively discussed, and any shortfalls in the standards that need improvement are pointed out. 

LRFD (load and resistance factor design) or limit state design is the dominant design method for structural design in the Australian standards and Eurocode 5. LRFD is used in the American standards alongside ASD (allowable stress design), which is a method unique to American Standards. 

The general equation for LRFD/LSD and ASD can be expressed as:

Loads acting on a structure do not act in isolation; they usually act in combination (for example, a skyscraper experiencing live loads from human activities and wind loads at the same time). Also, these loads usually do not act on the structure at their maximum values. Hence, load combinations are provided to account for the most critical loading case by amplifying the applied loads by “load factors". Loads at their actual magnitude are referred to as “service loads”, and their values in the most critical case are the “design loads”. Load combinations are calculated according to ASCE 7 in the American standards.

In LRFD, there are essentially two “layers’ of safety produced by the resistance and load factors. The resistance factor reduces the theoretical capacity of the structure, and the load factor increases effects due to design loads such that the member is adequate at the ultimate limit state. In contrast, service loads are used instead of design loads in ASD. The rationale behind this is that load factors can be neglected if the applied loads are predictable and vary insignificantly.

What LRFD/LSD provides through the use of load factors (which ASD doesn’t) is the consideration for the variability of the applied loads. LRFD tests statistically consistent structural reliability by requiring higher load factors for loads with greater variance. Live loads like vehicles and occupants, snow, loads and wind loads are examples of varying loads, and their variance is treated with more importance in LRFD than ASD calculations. This can be clearly seen in the load combinations provided in ASCE 7 shown above.

Traditionally, steel and structures were designed using ASD until the introduction of LRFD in 1986 and 2005 to respective design standards. Due to the cobehavioraviour and difficulty of stress analysis, concrete structures are designed using LRFD only.

2018 NDS uses the load duration factor to convert the stresses for a “normal duration” to design stress values for other load durations. Clause 2.3.2 defines a normal load duration as 10 years, which means that the load is applied to the structure for a cumulative maximum duration of 10 years. Timber has a unique characteristic of carrying substantially higher loads for short durations than long durations. In other words, the strength of a timber member is the function of load duration. Therefore, the load duration factor upscales/downscales the strengths of materials based on how long the load lasts. For short-duration loads, timber performs relatively well thus higher CD is used; converthe sely, lower CD is used for high-duration loads like dead loads.

It should be noted that the shortest duration load is used for load combinations. For example, if the load combination is D + L, then 1.0 is used as it corresponds to the load duration for a live load, which is shorter than the load duration of a dead load. 

CM accounts for the variance in strength due to moisture contenthe t. In 2018 NDS, the maximum allowable moisture content for all timber materials except for sawn lumber is 16%. For sawn lumber, the maximum moisture content is 19%. At the maximum moisture content, CM = 1; if any higher, engineers must refer to tables in the Appendix for specific values.

CF accounts for the variann the strength due to size. For some timber materials, as the dimensions of the timber member increase, its strength (per unit area) changes. The size fappliesicable to some types of timber, not all.

Under special circumstances, a failed member can transfer loads to adjacent members and the structure may function without any trouble. Cr accounts for this redistribution of stresses between timber members and is only applicable to members deemed repetitive. A member is repetitive if:

• Minimum three adjacent parallel members

• Member spacing is less than 24 inches between the centroid of the members

• The members are connether by a sheathing

For repetitive members, Cr = 1.15; for all other cases, Cr = 1.

Cfu accounts for members stressed in their minor axis. Most members in design bend about their major axis, however, in rare situations like stair treads they may bend about the minor axis.

For members bending about the strong axis, Cfu = 1; for bending about the weak axis, engineers must refer to the NDS supplement.

To ensure that the timber does not rot and weaken during its service life, chemical preservatives can be added. This process is known as pressure treatment, and some incisions parallel to the grain may be made in the timber to increase the effectiveness of the treatment. The incising factor is only applicable for sawn lumber, and its design values are provided in Table 4.3.8:

For timber, temperature increases beyond 100 degrees Fahrenheit (~37.8 degrees Celsius) lead to a decrease in strength. Ct accounts for this effect using values in Table 2.3.3:

CL accounts for the effect of lateral-torsional buckling, the twisting of beamsoccurs occur due to the movement of the compressive and tensile edges in opposite directions. Lateral-torsional buckling only occurs in members that are restrained at ends. For members that are fully braced by decking or sheathing, lateral-torsional buckling will not, occur and CL = 1. For other cases, Table 3.3.3 and the equations given in Clause 3.3.3 must be followed.

Similar to CL, Cp accounts for the buckling stability of columns which is a function of the column stiffness, fixity (e.g. are the ends pinned, fixed, etc.), sectio,n size and effective length. For members that are braced or restrained about their buckling axis by sheathing or walls, Cp = 1. For other cases, engineers must refer to equations given in Clause 3.7.1.

All reference design values for different timber materials such as material strengths and moduli in 2018 NDS and its supplementary data are only applicable to ASD calculations, as they are based on a normal load duration of 10 years. KF converts these values for LRFD calculations and is provided in Table 2.3.5:

Lambda accounts for the effect of load duration and is dependent on the load combination applied to the timber member. These factors have been derived based on a probability-based approach and are provided in Table N3:

Chapters 2 and 3 provide design provisions and equations for flexure, shear, and deflection of bending members, followed by members in compression, tension, and combined actions of bending and axial loading. Guidelines for solid columns and design for bearing are also included. 

Chapters 4 to 10 focus on different timber materials and provide material-specific design provisions and equations. The selection of adjustment factors is also discussed. Chapters 11 to 14 are dedicated to timber connections and follow the same format as previous chapters.

In each of the chapters between 4 and 14, the standard includes a table that outlines which material strengths are to be considered in calculation and the consequent applicability of adjustment factors. Examples are shown below:

The leftmost columns include the strengths that are used to calculate the capacity of members and the first row shows the adjustment factors that are considered. These vary depending on the material type.

Explore the national design specification for wood construction 2018 NDS. Enhance your timber design skills. Get started today!

National Design Specification For Wood Construction 2018 - CalcTree

This article discusses and summarizes the basic design procedure for timber elements as outlined in EN Eurocode 5, which covers the design approaches, calculation, and analysis of structural strengths and material properties of timber materials. Every country, sometimes even states, has unique design methods, which often leads to scattered calculations, incoherency in design, and wasted time. As such, this piece was written to help engineers who may need to use Eurocode 5 and wish to develop a basic understanding of its concepts without reading the entire document.  

EC5 [1] uses a similar approach to AS1684 and AS1720 for ultimate strength limit state design. The general equation for calculating design capacities is:

Rk is the characteristic design capacity and γM is the “partial factor”. This parameter accounts for differences due to material type in design value calculation:

The modification factor kmod in EC5 accounts for the effect of moisture content and load duration using different categories of ‘service class’ and ‘load-duration class’. There are 3 service classes and 5 load-duration classes; climatic loads like snow and wind vary between countries therefore, EC5 allows nation annexes to assign country-specific load-duration classes in the national annexes:

Effects of other parameters are not accounted for by kmod in EC5. Instead, each expression to be satisfied supplements its own modification factor. For example, for design compression capacity parallel to the grain, AS1720 uses Nd,c > ϕ k1 k4 k6 k12 f’c A­c where k1,k4, k6, and k12 are modification factors; EC5 uses σc,90,d < kc,90fc,90,d where kc,90 is the modification factor accounting for the effect of load configuration, splitting, and deformation. The theory behind these equations is the same (as they both use elastic analysis), but the selection criteria for modification factors and their numerical values are different.

A component covered in EC5 but not in AS1684 or AS1720 is the cracking of timber. The strength performance of timber systems mainly depends on shear connectors (joints and fasteners) that create combined actions between adjacent members. These connections are often the first to fail upon reaching the ultimate strength limit. Clause 6.1.7 of EC5 accounts for the influence of cracks for shear resistance of members in bending using the factor k­cr, which varies depending on the type of timber material.

Deflection limits in EC5 are also based on elastic analysis methods, similar to Australian standards. It recommends the use of deflection limits relative to the beam span and directs engineers to refer to national annexes for specific values.

It does not offer any structural models like AS1684 and instead provides formulae for modulus of elasticity, slip moduli, and shear moduli to be used to calculate deflections. The most significant difference between EC5 and the Australian standards is the inclusion of vibration control for timber flooring systems.

Fundamental frequency, or natural frequency, is an inherent property of all materials; it is the frequency at which objects vibrate in free, unrestrained conditions. When the frequency of external forces (including human activities, such as walking or running, machinery, wind, seismic activity, etc.) is equal to the fundamental frequency, resonance occurs.

In structures, resonance must be avoided at all costs. It leads to amplified deflections and perceivable oscillations that cause annoyance and discomfort. In extreme cases, total structural collapse may occur. The Tacoma Bridge incident is an example of structural failure due to resonance: the frequency of the wind loads caused vibrations equal to the fundamental frequency of the bridge. It led to uncontrollable deflections and the bridge eventually collapsed. EC5 provides a formula for estimating the fundamental frequency using the floor’s material properties (bending stiffness, mass, and span). However, studies have shown differences when compared to experimentally obtained values. Engineers should use experimentally verified values if they are available.

To prevent such phenomena, EC5 suggests that the fundamental frequency of residential flooring systems should be above 8 Hz. Studies show that the frequency of day-to-day human activities is between 1 - 8 Hz [2,3], hence the flooring systems are at risk if their fundamental frequency is within this range.

If the fundamental frequency is lower than 8 Hz, “special investigation” is required and additional limits must be satisfied. These limits are:

w is the maximum instantaneous deflection caused by a concentrated vertical static load F (summation of forces acting on the floor represented as a single point load).  represents the damping ratio, the measure of vibration absorbance of a material (obtained experimentally). v is the velocity of the floor’s vibration under a unit load and it is calculated using the following equation found in EC5 Clause 7.1.3:

Where m, b, l represent the mass, width, and length of the floor respectively and n40 is the number of fundamental frequencies under 40Hz (obtained experimentally or approximated using the formula in EC5 Clause 7.1.3). Additionally, EC5 encourages engineers to follow the provided figure of the recommended relationship between parameters a and b:

This article discusses and summarizes the basic design procedure for timber elements as outlined in EN Eurocode 5, which covers the design approaches,

A common problem faced by engineers when designing structures is identifying relevant information from standards and applying them to their projects. Each design standard contains a vast amount of information; therefore, time and resources may be wasted in searching and selecting bits that are useful for design. This article aims to aid engineers in such a situation by providing a guideline on the design procedure of 

 according to the Australian Standards, namely AS1648, and AS1720. Fundamental equations, calculation methods for different timber materials, and important design parameters are explained in the following sections. Ultimate strength and serviceability limit state designs are extensively discussed, and any shortfalls in the standards that need improvement are pointed out. 

Australian Standards also present guidelines to be followed during the manufacture of timber materials. These documents set standardized testing and grading methods and outline minimum requirements for material properties (such as moisture content, modulus of elasticity, etc.). All timber materials manufactured in Australia must satisfy conditions outlined in the standards and be appropriately graded before they apply to the actual design of timber structures:

AS 1684 and AS 1720 provide detailed, comprehensive guidelines for ULS design. The calculation methods for design bending moment, shear, and bearing capacities of structural timber members, joints, and fasteners are robust enough to account for different scenarios and environmental factors. According to 

 2.1.2 and 2.1.3, the design capacities of members and joints must satisfy the following limit:

Rd is the design capacity of the timber member/joint, or the maximum amount of load that a particular element can take before failure. Rd must be larger than R*, the design action, or the maximum load that the element is expected to experience during its usage. The design action R* is calculated following load combinations outlined in AS1170, which accounts for 

 acting on the structure, such as wind, snow, live and dead loads.

Capacity factors are used to upscale or downscale the design capacity of structural elements. In AS1684 and AS1720, the ϕ values for timber members/joints are selected based on their application. Higher values are used for structural elements deemed to cause less danger upon failure.

the mod is the product of a combination of modification factors (k1 x k2 x … kn) that affect the capacity of the member/joint. These various factors depend on the placement and orientation of the element, loading conditions, and material properties. The engineer should select and change k values as necessary for their design. 

Modification factors in AS1684 and AS1720 account for a large range of variables and scenarios, which allows flexible adaptation of parameters. 

f’oX is the characteristic capacity for the relevant design action in question (bending, tension, compression, shear, etc.). For example, when designing for bending moment capacity, f’o = f’b (characteristic value in bending for the section size) and X = Z (section modulus of the beam about the bending axis).

 is defined as the strength below which no more than 5% of the results are expected to fall, as assessed by a standardized test. When timber is manufactured, it undergoes tests, and grades are allocated according to the results. In Australian standards, timber is graded by its strength in shear, bending and tension, and compression parallel to the grain.

A unique property of timber is that strength varies depending on the direction of the applied load. Due to the cellular structure of wood, engineered wood products are stronger when loaded parallel to the grain compared to perpendicular. AS1720 Section 3 accounts for this phenomenon through the provision of design tension, compression, and torsion capacity formulae in both directions.

The major shortfall in AS1684 and AS1720 is the lack of comprehensive consideration for deflection and vibration control. Timber is lighter and proportionally less stiff compared to reinforced concrete and steel; hence, it is extremely prone to deflections and vibrations when subject to similar load patterns. Timber’s lighter weight leads to oscillations that can cause discomfort to occupants, and lower stiffness leads to larger deflections. Additionally, timber is an environment-sensitive material that is heavily influenced by temperature and moisture content in the atmosphere. Therefore, timber structures must be constructed to ensure satisfactory performance and safety throughout their service life, accounting for variance in material properties over time. In fact, serviceability is often the governing limit state for timber elements rather than ultimate strength.

For the calculation of deflections, AS1720 suggests the use of upper-bound estimates obtained through elastic analysis methods. The upper-bound deflection limit is found using the lower 5th percentile estimate elasticity of modulus, E0.05 (generally, a range of E values are attained whilst grading through tensile tests). In cases where this information may be unavailable, the standard recommends calculating it as a proportion of the average value of the modulus of elasticity:

While these estimates may be useful for smaller projects like residential complexes, they are not appropriate for large-mass timber structures. Upper-bound estimates tend to greatly under-estimate the limits, and engineers may design conservatively, leading to unnecessarily thicker and stronger elements. This can result in dramatically increased cost and time demands on a project.…even more than usual!

In contrast, AS1684 provides tables with “structural models” that are used to determine span-relative deflection limits. The structural models are categorized according to the load type (e.g. distributed or concentrated, live or permanent, etc.), and then modification factors and deflection limits are selected based on the load category. Section 2 of AS1684.1 covers deflection limits for the design of timber roof members in residential buildings, which includes roof battens, rafters, underpurlins, strutting beams, ceiling battens, and joints, as well as hanging, counter, and verandah beams. An example of structural models and load categories for serviceability design from AS1684 Clause 2.1.3.2 is shown below:

Vibration control measures are covered briefly in AS1684 and AS1720. Vibrations induced by machinery and human activities (walking, running, dancing, etc.) cause the structure to sway and oscillate, which may be fatal if the frequency of this oscillation matches the natural frequency of the structure. The standards state that ‘the dynamic response of floor systems, including frequency of vibration, should be considered…’ but no equations or guidelines are provided.

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