Charring Rate of Timber

14thApril 2021
By Long Le

Welcome back to the Permax Blog, a space where we try to give you a layman’s insight into the world of Fire Rating and passive fire protection. This week we wanted to take a deeper dive into fire protection for timber (we have covered off sustainable building construction before). Why you ask? Well simply put, Timber based materials are increasingly being utilised for structural engineering and design applications in structures due to the material strength, architectural/aesthetic purposes and the environmental advantages it offers. In other words, It offers architects and those in construction a way to make sustainable, affordable and environmentally friendly buildings with cost-efficient design solutions.

As the world moves on from the Coronavirus pandemic of the last year and focusses forward, the conversation is again shifting to predicted difficulties ahead with regard to housing shortages, climate change, and personal wellbeing. Developers need to keep abreast of the conversation in terms of how to best address the challenges and present workable, affordable solutions. With a green profile, timber – once shunned for steel – is again rising in popularity as it continues to offer a sound alternative to steel possibly offering incredible advantages moving into a sustainable future. This has created immense interest in it as a building material from government officials, planners, architects, designers and business visionaries.

The catch in using Timber

Timber is combustible by its very nature thus making timber structures vulnerable to fire. Indeed, The fire performance of timber relies upon the thermophysical properties of its components at raised temperatures. As such, a proper fire design approach for timber members alone is not adequate if the joints are not equipped for supporting the applied load in a situation of fire.

Although the burning behaviour of wood is complex, it is well documented and known. When there is application of sufficient heat to wood, it leads to a process of thermochemical decomposition; pyrolysis, ignition, reradiation, char formation which results in alteration of the chemical composition and physical appearance (formation of char) on timber. This phenomenon is also accompanied by mass loss and charring appears in all exposed surfaces in the form a layer which increases in depth with the progression of fire. As the insulating temperature rises in the core of timber, the charred layer loses all strength. The charring rate is pretty much consistent and fundamentally relies upon the density and moisture content of the wood.

Since wood is combustible, designers consistently have significant concerns about fire safety in timber construction. When timber materials are utilised, a non-combustible fire protective covering is required (for timber linings, materials and assemblies) and the protection required can be determined. The NCC and Building regulations apply different requirements for the fire protection of wood products. The fire resistance rating calculation is based on standard fire test methodology and specifies the identification of the amount of time for which the structure should adequately carry load.

Factors to consider in Fire Design for Timber

In layman’s terms the Charring rate of timber will be determined by considering the following factors:

  • Level of radiant heat exposure;
  • Moisture content;
  • Formation of char;
  • Timber species; and
  • Dimensions of the wood.

However, more technically correct, the The charring rate of timber will be determined by considering the timber species-densities at a moisture content of 12% in Kg/m3 as defined in AS/NZS 1720.4:2019.

When exposed to high temperatures as experienced in a building fire, timber structure design models need to consider the loss in the cross section due to the charring layer and the temperature dependent reduction of strength and stiffness of the uncharred residual cross section.

The stiffness and strength of timber significantly decreases with increasing temperature. The interface between charred and non-charred timber is the demarcation plane between black and brown material (see image below). The charring rate for different types of species exposed to the standard time-temperature curve has been studied in different countries by many researchers. The charring rate determination for timber is an important factor in fire safety design because it determines how quickly the size of the load-bearing section decreases to a critical level.

Loss of section due to charring in Timber - Figure from Permax

What is the accepted charring rate of timber?

Restrictions are ordinarily placed to decrease the risk of an unacceptable fire hazard while considering the fire performance of wood structures in construction. This apparent risk can be extensively characterized as the danger of catastrophic collapse or fire spread throughout the building and to other buildings, posing a threat to both the general public and property alike.

Because timber has traditionally been perceived as a fire risk, a Deemed-to-Satisfy solution was introduced to the NCC permitting, allowing construction in fire-protected timber building systems to an effective height of 25 metres in schools, retail premises, hospitals and aged care facilities. In reality, massive timber structures often exhibit excellent fire performance due to the inherent strength and stiffness properties. During periods of prolonged fire exposure, timber assemblies have in the past be used to separate compartments and pre-manufactured CLT cores to provide loadbearing and lateral-resistant functions.

There has been a lot of experimental testing and numerical modelling of timber systems under fire conditions, culminating in a simplified charring method derived from the results of the research and grounded in generally accepted calculation methodologies from around the world.

A number of simplified design methods exist for the prediction of timber member capacity in fire conditions, such as the reduced cross-section method or the reduced properties method. Although these methods have been developed to suit local design practices, there is consensus that charring rate methods are simpler to understand and apply as they reflect the physical changes a burning timber element and hence represent member design capacity in different degrees.

Charring rate determination for timber

Typically, acceptable char rates for timber are about 0.5 mm/min for hardwoods and 0.65 mm/min for softwoods. In addition to the loss of section due to charring, AS/NZS 1720.4:2019 applies an additional envelope reduction of 7mm (per AS1720.4) to cater for heat-affected timber. Most timber constructions that use charring to achieve BCA-specified fire ratings are made from large sections. Oftentimes these are commonly available as glulam or CLT sections. Fire tests have shown that for the application of AS 1720.4 glulam or LVL members can be regarded as solid timber.

We have previously spoken about CLT and the value it offers to construction. If you are looking for more information on CLT, please check out that blog.

Otherwise, if you need assistance when it comes to timber charring and fire protection for timber, ask one of our team at Permax for help.

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Permax constantly update the documentations based on the new fire testing outcomes and change of standards and regulations. To ensure the documents you read are up-to-date, please contact the Permax technical team.