Structural Performance And Behavior of Timber

The use of timber as a structural material is not new. It stretches back many centuries and predates the invention of concrete and steel as structural elements. Modern advanced timber products are now readily available, enabling structural engineers to achieve the performance and efficiency required in the 21st century construction industry. With the passage of time, advancements in the various types of timber elements available and their use in different structural forms have taken place.

Mass Timber

Timber is a material that can be used for many different structural forms, including beams, columns, trusses, and girders. It can also be used for piles, deck members, railway sleepers, and concrete formwork.

Timber is an ideal building material due to a number of its inherent qualities. They include its remarkable durability and performance record, high strength to weight ratio, and good sound and heat insulation qualities. The natural development properties of timber, such as grain patterns, colors, and availability in a wide range of species, sizes, and shapes, make it a material that is remarkably adaptable and visually beautiful. Wood may be easily formed and joined together with the help of bolts, screws, dowels, and adhesives.

When constructed, treated, and detailed properly, wood constructions can be very long-lasting. Historic structures all across the world have examples of this. Wooden constructions are easily reconfigured or modified, and they may be fixed if they are damaged. Comprehensive information on the material qualities of timber, its reconstituted products, and engineered goods, as well as their implications on structural design and service performance, has been developed as a consequence of extensive research over the past few decades. Our knowledge of safe construction practices, connecting details, and design restrictions is based on much experience and research on using timber in buildings.

Types of Timber And It’s Applications

When referring to the wood-based structural products, the word “timber” is frequently employed. Timber is divided into two categories: “softwood” and “hardwood.” Coniferous trees provide softwood, whereas broad-leaved trees provide hardwood. The words “softwood” and “hardwood” are botanical in nature and may not always indicate how dense or hard the wood is. For instance, Douglas Fir is a softwood with strong durability and high strength properties, whereas Balsa, which is known to be soft and utilized for making lightweight models, is a hardwood.

Softwood is commonly used for wooden buildings because it is easily available, well utilized, reasonably inexpensive, and because of its high growth rate, which ensures a steady supply from regenerated forest areas. When strength and specific aesthetics, like as color or grain pattern, are required, hardwoods are typically employed in exposed frameworks and cladding.

Structural Properties And Performance of Mass Timber

The structural performance and behavior of mass timber is a function of several properties including its strength, moisture content, creep, durability and fire resistance.

Strength

The intrinsic qualities of the wood and the way the tree grows give mass timber its strength. The roots of trees are sawn to create typical wooden joists. The roots are surrounded by cells that make up the trunk; these cells are long relative to their breadth. These cells, which provide axial and flexural support, are parallel to the circumference of the timber joists and beams that are sawed from the tree stem. Timber that is parallel to the grain has an even higher stress and strain capacity than timber that is perpendicular to the grain because of the inherent characteristics of these cell arrangements.

The strength of sawn wood depends on its species, width, size, and member shape, as well as on the moisture content, length of loading, and strength-eroding traits such as grain slope, knots, fissures, and wane. Strength grading techniques were established to differentiate timber using either machine strength grading techniques or visual force grading techniques.

According to ANSI/AWC NDS 2016, a set of resistance groups is formed from timbers with similar strength characteristics. This streamlines the design and development process by enabling projects to be constructed within defined strength class boundaries without the need to categorize and procure a specific combination of species and grades. Coniferous prefixed softwoods and deciduous prefixed hardwoods are the terms used to describe the strength classes.

Moisture Content

The water from the tree sits inside the cell walls and voids immediately after it has been felled. The moisture content (m/c, the weight of water to oven-dry timber) decreases as the wood dries, and water is then removed from inside the cell voids without dimensional adjustment. This process continues until the “fibre saturation stage” (about 25% m/c), at which point water begins to be removed from the cell walls. The next step is the beginning of shrinkage, often known as dimension change.

The strength and rigidity of a piece of wood can be affected by its moisture level, which normally increases the strength of the wood as the moisture content decreases. The level of construction quality has to do with the wood’s natural strength at a moisture content that complies with the requirements of Service Classes 1 and 2. Timber is considered wet at a moisture content of 20% or more (Service Class 3), and stresses decrease as moisture content rises to 30%. According to standard calculations, the dimensions of wood perpendicular to the direction of the grain change by 1% for every 4% change in moisture content.

Creep

“Creep” is a significant characteristic of timber that has an impact on its effectiveness and serviceability. When originally loaded, wood deforms elastically; nevertheless, as time passes, more deformation takes place.

As the moisture content of the timber increases and the load is applied for longer periods of time, the quantity of creep deformation would increase. For instance, the creep can increase the initial deflection of a Service Class 2 sawn timber joist by up to 80%.

Durability

Depending on the plant, several levels of inherent durability exist for timber materials. When moisture builds up to this level due to structural flaws in a building or poor maintenance, timber rot can often occur if the moisture content is over 22% over an extended period of time.

Chemical preservation treatments may be used when a non-sustainable specie needs durability against the likelihood of degeneration or insect attack. Preservatives are also employed as a secondary line of “insurance” against production or design flaws that can cause moisture contents above 22%.

For projects requiring natural durability, such as exposed timber columns, bridges, and water-holding structures, a structural engineer may instead choose to work only with a particular species of wood.

Fire Resistance

As a flammable substance, wood has the potential to ignite and quickly spread flame across its surface. However, carbon buildup on the top (in the form of charred wood) restricts the oxygen supply to the underside wood and serves as an insulator, keeping the wood below the charred level relatively cool and preserving its structural integrity.

By taking into account the characteristics of the deteriorated section beneath the charred timber, solid timber of broad cross section can therefore be manufactured without additional fire protection. As an alternative, plasterboard or other fire-resistant linings can be used to cover the wood. By applying a surface coating or chemically infusing exposed timber surfaces, the ignitability and surface spread of flame can be decreased.

Factors That Affect The Structural Behavior of Timber

Timber shouldn’t degrade under ideal circumstances, however when utilized outdoors or in exposed environments, it can degrade for a number of different natural reasons. Some of them are:

Timber Defects

The structure of naturally grown wood is distorted by the process of converting logs into structural timber. This wood is known as in-grade wood, because it has less desirable qualities than clear wood. This is because there are traits or flaws that weaken the wood, including knots, grain slopes, gum veins, reaction wood, etc. As a result, it is necessary to test in-grade specimens in accordance with defined techniques in order to ascertain the dependable mechanical properties of the in-grade timber.

Moisture Content

The influence of moisture content on the qualities of timber varies with relation to the property being evaluated. The failure mechanism in bending is moisture dependent, and the bending strength decreases as the moisture content rises.

Bending failures typically happen in the tensile zone with low moisture contents, while at high moisture contents, failures typically happen in the compression zone. Stress modification variables are included in design codes all around the world to take moisture influence into account.

Loading Duration

Strength and stiffness are both significantly influenced by the duration of the load. The strength of a timber member decreases with increasing load duration for a fixed load size. In other words, the long-term strength for constant loads like self-weight or dead loads is only about 60% of that for the timber when it is first loaded in a structure. This loss of strength may be as high as 40%. On the other hand, for members subjected to rapidly applied and extremely short term loading, such as peak wind occurrences, the period of load effect on strength is smaller and the load carrying capacity is larger.

Fire And Temperature

Wood is a flammable material that will catch fire if exposed to it. Plasterboard, for example, is typically used as a fire-resistant covering to shield light structural members from flames. The strength of wood is constant within the normal range of ambient temperatures, however at higher temperatures, strength levels are often decreased. Strength recovery is feasible if exposure to higher temperatures is brief. However, depending on the temperature and length of exposure, prolonged exposures typically cause severe damages. Using intumescent coatings or soaking wood in salts that are flame or fire retardant can protect it against fire.

Structural Benefits of Timber

Timber is a sustainable and environmentally friendly building material with exceptional ecological features. It has low embodied energy and serves as a carbon sink. Hence, compared to other building materials like steel and concrete, the energy needed to transform trees into wood and subsequently structural timber is far lower.

Although timber can come in a wide variety of species, it can also have high heat and sound insulation qualities, as well as good durability features. Timber has a very high strength-to-weight ratio. Softwood timber is also less dense than other building materials. With smaller foundation loads and easier lifting of prefabricated components during transit and assembly, it can therefore offer lightweight structural solutions.

Prefabrication is encouraged by the use of timber as a structural material. Prefabricated off-site materials are used to build many modern constructions. There is therefore a general requirement for less on-site activity and a shorter service period for on-site work. Prefabrication, in turn, provides a purpose for quality control and eliminates the whims of weather and site conditions.

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