14 August 2020

Inspecting timber bridges

TRADA image

Photo: Montmorency bridge, Quebec, Canada. Cephas.


'Timber as a material has a range of advantages for structural use. It is relatively lightweight, easy to work with, flexible in size and performance and, if sourced in the correct way, sustainable.'


Timber bridges are designed and constructed in a variety of forms for road networks, railway infrastructure, viaducts, aqueducts and pedestrian walkways. This can be a timber structure built entirely from timber or where timber is the chosen material for certain components.


Building timber bridges

Timber as a material has a range of advantages for structural use. It is relatively lightweight, easy to work with, flexible in size and performance and, if sourced in the correct way, sustainable. In addition, if designed and maintained correctly, the service life of timber components can match or exceed that of alternative materials.


A range of softwoods and more durable, stronger hardwoods are used throughout the construction of bridges. The variety of timber species, type of wood preservation and natural variability of timber can present asset managers with difficult decisions. It is this variability that can make it difficult to predict the performance of timber components.


Historically, solid timber has been the choice for the construction of timber bridges. However, in recent decades alternative ways of using timber have become increasingly available. Glued laminated timber (glulam) is an excellent way of producing large sections with longer spans of consistent quality. A range of ‘durable’ timber species can be used in conjunction with an appropriate type of structural adhesive that is suitable for exterior use. Square sections are commonly produced, however, glulam manufacture is flexible enough to produce a range of shapes and sizes and – particularly desirable for bridges – in curved lengths. Modified woods are now becoming a more popular choice of species for glulam components, with their improved durability when compared to many natural timber species, provided that research and testing has proven that they have the desired structural performance.


Bridge inspection: scope of the timber survey

There are two factors to consider in bridge inspection. These are condition and strength, and are inextricably linked. Timber and timber-based products become vulnerable to fungal decay if the moisture content exceeds the decay threshold, which is nominally 20%. The service environment of timber bridges results in periodical exposure to wetting for prolonged periods.


Timber surveys are carried out using non-destructive techniques, which include decay detection drilling with microprobes. Although core samples can be a useful tool for assessing concrete, core sampling is a far less useful tool in assessing timber structures. A timber core sample cannot provide information in the overall condition or strength of a piece. Furthermore, core sampling can actually reduce the expected service life if it breaches the treatment ‘envelope’ of preservative treated timber.


As decay develops, it usually progresses at an increasing rate with the result that the subsequent loss in strength will not be linear and cannot be predicted accurately. However, it is possible to evaluate if the extent of decay has reduced the cross-section of the member to the extent that strength has been affected. If strength has not been affected then implementing a remedial preservative treatment regime can halt further deterioration and extend service life.


A condition survey can be carried out on any timber that is accessible at the time of inspection. This may concentrate on the high-risk areas if there are access limitations, which might include timbers that are in ground or water contact, and those that have a limited provision for drainage and ventilation. In practice, the highest risk areas are usually bearing ends since these are more likely to have exposed end-grain. Focusing on those areas would be the recommendation for an initial assessment. For the same reasons, connections would be the next priority. Bolt holes, cut-outs and notches all include exposed end-grain, and, similar to bearing ends, these will usually be less free-draining than mid-span sections. Over the service life of a bridge, this would be expected to increase the risk of fungal decay, which has the effect of reducing the expected service life of the timber elements. Although they are generally lower risk areas because they are often well ventilated, mid-span sections should not be ignored, particularly where, for example, bridge deck fixings penetrate the upper surfaces, or where upper surfaces have developed drying fissures that can become water traps.


The presence or absence of water staining at mid-span should not be taken as clear evidence of a decay problem. Heavily water-stained timbers are frequently in a perfectly serviceable condition if the appearance is ignored. Water staining at or near end-grain can be a little more reliable in terms of assessing the likelihood of decay being present, but even that can be misleading. For laminated timber structures, simply assessing for fungal decay is insufficient as glueline performance must also be considered. Failure at gluelines, due either to fungal decay of adjacent wood cells, or deterioration of the adhesive bond, can be of greater overall importance than localised loss of viable cross-section.


In the vast majority of cases the strength of the timbers would presumably have been considered by a structural engineer prior to construction. If that information is difficult to obtain, or grade markings have become unreadable on site, or if faced with the task of justifying a structure where the viable cross-section had been reduced by fungal decay, in-situ grading can provide the answer. For that reason, timber surveys are often complemented with visual strength grading and species analysis. The strength characteristics of the structural timbers are determined by undertaking indicative in situ visual strength grading of the members. This in situ grading is carried out using the requirements of the relevant British and European Standards as guidance. These standards are:


  • BS 4978: 2007+A2 2017 Visual strength grading of softwood – Specification.
  • BS 5756: 2007+A2 2017 Visual strength grading of temperate hardwood – Specification.
  • BS EN 16737: 2016 Structural timber – Visual strength grading of tropical hardwood.


Small specimens (dice size) are removed to confirm species using microscopic techniques so the correct strength class for the species/ grade combination can be assigned using BS EN 1912: 2012 Structural timber. Strength classes. Assignment of visual grades and species.


Specialised timber surveys

BM TRADA provides specialised timber surveys for all timber bridges, which offer an enhanced level of detail and expertise to complement regular maintenance examinations of structures carried out by structural engineers. The service provides a level of detail that enables asset managers to co-ordinate decisions and actions across timber-containing assets to deliver enhanced service life, more efficient use of resources and more effective risk management.


The condition-based approach to supporting asset management has two purposes. The first is to detect timber defects such as decay and/or mechanical failure that could endanger safety and reliability of the bridge. The second is to carry out periodic inspections, usually at five-year intervals, which, using evidence-based knowledge on how timber deteriorates, can deliver accurate condition-based monitoring. This provides insight into the asset’s behaviour over time to enable condition forecasting and planning of maintenance or renewal.


About the author

Nick Clifford is one of BM TRADA's Technical Consultants with over 10 years' experience as a timber technologist, consultant and surveyor. Nick has advised architects, engineers, local authorities and conservation bodies, and been involved with projects at sites of historic interest. Nick is a qualified visual strength grader, and has a particular interest in wood flooring and timber species identification.


This is an extract from the Timber 2020 Industry Yearbook. Download the full article, including supporting images, references and further reading, here