Timber

BCRC announces their association with the UK’s TRADA Technology Limited -Timber Technology is the UK’s premier timber research consultancy organization. Previously a subsidiary of TRADA (The Timber Research and Development association), it is now in private ownership.

A.Abbott, TRADA’s CEO comments “We are pleased to associate ourselves with BCRC in Australia. The combination of BCRC’s expertise in structural surveys and TRADA’s internationally recognised experience in assessment of timber structures will provide Australia with a world class team for survey and assessment of timber bridges, buildings and wharves.”

Timber Condition Surveys

As with other forms of structural investigation, a timber condition survey has two parts. The objective of the initial condition survey is to identify and map the extent of fungal decay and/or insect attack, moisture ingress and to undertake a risk assessment of these factors. The second part of the survey requires skill and expertise with timber to interpret the survey data and to establish the structural performance and serviceability of the structure and to recommend/specify remedial and strengthening works if deemed necessary.

Whether the inspection is on modern buildings, civil structures or historic buildings non-destructive tests cause minimal disruption and form the basis for selecting areas to opening up works where decay is a risk. They are a vital supplement to visual inspection in selecting the sampling locations. Timber condition surveys should consider the condition of other materials in contact with the timber. All potential sources of moisture and damp penetration must be identified and these areas thoroughly investigated.

Where load capacity is required opening up should be limited to representative areas. The structural members must be exposed and, if consistent NDT results are obtained, opening up can be kept to a minimum.

Buildings

The detailed structural survey records the general arrangement of the structural timber members and components. These include frames, floor and roof beams and joists, columns and posts, trusses, bracing and rafters. The survey identifies significant defects and signs of deterioration and links them to their true causes. The timber species is generally identified and grading features such as knots, slope of grain, and other aspects registered. These allow strength and stiffness to be assessed. Moisture contents are recorded.

Strength and stability assessment determines the overall ability of frame members and joints to support loading, and to remain in a stable condition. Deflection is what modern codes term a ‘serviceability limit state’. Although codes give deflection limits these should be used as a guide, the main criterion, particularly for a built structure, is appropriateness. Creep deflections may well be present in older structures and may be regarded as part of the characteristic nature of the building.

When designing repairs traditional methods (scarfs, tenons dovetails, pegs), mechanical fasteners and adhesives are considered. Traditional methods may maintain the buildings visual image but may be difficult to assure structurally. Mechanical fasteners give a higher degree of assurance of connection load capacity but may be visually unattractive. Adhesives are particularly useful where access to the timber is limited and structural disturbance must be minimized.

Civil Structures

Timber piles and girders are widely used for bridges, piers, and wharves. As they age, it becomes critical that their in situ condition be assessed so their remaining service life can be evaluated. Fungal attack often initiates damage with decay then accelerated by termites. Visual examinations and sounding tests may be unable to disclose internal damage, degree of deterioration, depth of penetration, or remaining load-bearing capacity and drillings may miss critical defects.

A range of NDT’s are used to supplement the traditional inspection methods. Ground penetrating radar (GPR) with a high frequency antenna is used to detect internal defects (rotting and piping) in dry girders while ultrasonic dispersive wave propagation tests provide an excellent means for evaluating the degree of hollowness and borer damage on piles below water. Buried pile length is measured using longitudinal stress wave propagation.

Load testing and vibration analysis of elements of the complete structure can also be undertaken in critical cases.

X -rays and γ -rays	Endoscopy	Indicator dyes
Both x-rays & γ -rays can be used to inspect timbers to detect decayed areas, splits, checks, knots etc.	Rigid borescopes are used to inspect timbers inside cavities and behind claddings.	Indicator dyes, which change colour with changes in pH value, are used to detect the presence of decay fungi.
Mechanical stiffness / load testing	Hardness	Photogrammetry
There are two forms, the first uses low loads to determine the load deflection behaviour, the second uses higher loads to prove that the design load can safely be carried by the structure.	Hardness test are carried out to determine the surface condition. It involves measuring the indentation caused by a spherical object pressed into the surface (Janka/Brinnell test).	Photogrammetry is used for producing three-dimensional models of any building or structure. It has been used for load testing of bridges under a known traffic load.
Pilodyn	Moisture Meters	Hammer sounding
The Pilodyn operates by firing a blunt pin, with a known energy, into the wood surface and measuring the pin’s depth of penetration. The instrument provides an indication of fibre strength and the extent of external decay, but is sensitive to other factors, such as density, moisture content & angle to the grain.	Electrical resistance has been used as the basis for most timber moisture meters. These measurements rely on a probe of sufficient length for the timber being studied. Electrical capacitance has also been used as the basis of commercial moisture meters, with the advantage of not requiring pins to penetrate the surface.	This involves repeated impacts of the timber under test with a hammer. The operator judges the condition of the timber by the sound and feel of the impact. Although the technique is thought of as being subjective, in experienced hands it can become quantitative and is still the mainstay for wood pole inspections.
Impulse Response	Drills	Bioassay
When a body is vibrated, eg by an impact, the frequency at which the highest amplitude of vibration occurs is normally the body’s first resonant frequency. A resonance test may be carried out to determine the modulus of elasticity of simple members.	The Decay Detecting Drill and Densitomat use the technique of resistance drilling. Both devices detect density changes, other effects, such as changes in moisture content, angle to the grain, splits and checks can give spurious results.	Bioassay techniques determine the presence of fungal decay & the effectiveness of preservative treatments. A small sample of material is removed & incubated to monitor the development of any fungal growth.
Ground Penetrating Radar	Ultrasonics	Impact echo
Microwave reflection is used to detect fungal decay and termite attack in dry timber girders. BCRC use Sensors and Software equipment.	Ultrasonics use 25 to 100 kHz transducers to generated sound waves The wave velocity or the signal attenuation is the measured parameter. BCRC use the Tico.	Stress waves are impact-induced and of low frequency. BCRC use a simple hand held device by Olson instruments.

Historic Structures

If the timber has not been and is unlikely to be affected by fungal decay, and it can still function structurally, it should be left undisturbed. The necessary knowledge to make such decisions can be gained by a close inspection of the building using a range of inspection techniques.

The principles laid down in modern strength grading standards can form the basis for the skilled surveyor to apply a strength class to historic timber. Furthermore, the experienced surveyor can consider the actual effect of strength-reducing features. For example, are knots above or below the neutral axis of a beam and how does their position affect strength? Where are the points of maximum stress and what is the residual sound cross section of the element at these points?

This expertise can also be supplemented with the very latest ultrasound non-destructive testing technology. Specialist equipment provides direct readings of member strength and stiffness, which can often result in engineers being able to justify the use of enhanced design values.

In almost all cases, significant savings in construction time, materials and costs can be achieved by a detailed condition survey of historic timbers.

Civil Structure Construction Advice

In comparison with other common structural materials, including steel and reinforced concrete, timber generally performs well in adverse environments. With suitable precautions, hardwoods are often selected for exterior and exposed situations.

Although hardwoods are not subject to corrosion in the same way as metallic substances, continuous levels of high moisture, and chemical substances in particular environments do have an effect upon them. Adverse situations may be classified as follows:

Full exposure to the weather, or to continuously damp atmospheres Exposure to chemical fumes, or contact with chemical solutions Immersion in water, particularly sea water, and contact with sea water spray.

BCRC can advise on:

• Correct selection of timber species
  • naturally durable species
  • specification of preservative treatment
  • abrasion resistant species for bridge decking & flooring
  • splinter-free material for handrails
  • for small movement characteristics where dimensional tolerances are critical.
• Correct selection of wood-based panels
• Specification of moisture content
• Correct selection of other materials, including  adhesives, seals, fixings, flashings and surface coatings, to ensure that they perform adequately and are not deleterious to the timber component design.

BCRC and TRADA have already consummated their cooperation with the preparation of a durability plan for the Prison and Forensic Hospitals at Long Bay Correctional Complex for Multiplex Constructions. The environment for the timber piling was generally defined by reference to AS 1604, Appendix D. However, local exposures were also considered. Advice on appropriate species, preservatives and splice location was given.

Timber Team

The Timber team comprises Marton Marosszeky on design, Robert Munn on Building, Reuben Barnes on Site Testing and Tony Song for laboratory testing.