Assessment, Reinforcement and Monitoring of Timber Structures...

In recent years, the use of timber in structures has become particularly important, considering that it is the only truly renewable building material and carbon storage. Timber has been used as structural material for centuries and numerous examples demonstrate its durability if properly designed and built and when adequate assessment and monitoring has been applied. The objective of the Action is to increase the acceptance of timber in the design of new structures and in the repair of old buildings by developing and disseminating methods to assess, reinforce and monitor them. The need for assessment, reinforcement and monitoring of timber structures can arise from multiple motivations such as the expiration of the planned lifetime, materials aging, exceptional incidents, and ever more important, a change of use. The Action will benefit from multidisciplinary views of the problems and followed innovative solutions by the involved stakeholders, enable synergies between them and provide an effective way of discussing and disseminating the results from ongoing projects within this research area to the European industry. The Action will increase the confidence of designers, authorities and end-users in the safe, durable and efficient use of timber and consequently increase its use in construction.

Current state of knowledge

1. Assessment:

The last decades were marked by a significant widening in the range of application of timber in structures and consequently a growing importance of the assessment of these structures. The time and cost of structural assessment are justified by ensuring the safety, protecting of capital investments and cultural heritage. A wide variety of methods exist to assess timber structures, however, their frequency and scope, the decision making approach concerning safety and the necessary interventions are far from being agreed upon.

The assessment of structural members probably is as old as building with timber beginning with the visual appraisal of trees and pieces of timber. In the last decades, much research has been conducted to grade timber according to physical and mechanical properties in the timber products processing industry. The focus is now shifting towards research to improve the assessment of in situ timber members to both estimate individual member strengths as well as obtain accurate quantification of deterioration. Most assessment methods utilized today can give qualitative information but only few can give reliable quantitative information. Methods can be non-destructive (NDT); which are useful for the screening for potential problem areas for the qualitative assessment of structures. But a drawback of NDT is the relatively poor correlation between the measured quantity and material strength. Semi-destructive techniques (SDT) bridge the gap between NDT and fully destructive methods; they often require the extraction of samples for subsequent testing to determine elastic and strength parameters while preserving the member's integrity; one problem of SDT is the high variability in test observations.

The current practice cannot be considered suitable to ensure confident decisions due to the low correlation between non- destructive on-site measurements and physical material parameters. The deepening of the scientific foundations within this Action will have a direct effect on the competitiveness of timber and societal confidence in it as a structural material.

2. Reinforcement:

The need for structural reinforcement of timber buildings may become necessary from motivations such as change of use, deterioration due to a lack of monitoring and maintenance, exceptional damaging incidents, after changes in regulatory specifications, or interventions to increase seismic resistance. Over 80% of European buildings are over 50 years old; they need to be adapted for uses that are more sophisticated for the present and future than was acceptable in the past. About 50% of all construction in Europe is connected to the repair, maintenance and improvement of existing buildings. While change of use usually requires an increased load-bearing capacity of floors, reinforcement of damaged elements most often involves beams and posts. Recent developments related to structural reinforcement can be grouped into three categories:

In the latter category, the options range from mechanical fasteners like glued-in rods and self-tapping screws, adhesive systems, steel straps and plates, ever more widely fibre-reinforced polymers (FRP), and most recently nanotechnology These methodologies, however, are not harmonised for their use in-situ nor adapted depending on whether the structure is part of the regular building stock or belongs to cultural heritage. The Action will harmonize developments related to reinforcements of timber structures.

3. Monitoring:

The monitoring of timber structures received special attention after the collapse of the ice rink in Bad Reichenhall, Germany, which only stands as one example of a series of other structural failures: e.g. those of over 50 timber structures in Sweden caused by poor control and communication between various stakeholders.

The monitoring of timber structures lacks a consensual approach. Mostly, monitoring consists of regular on-site visits that only give qualitative answers to whether a structure still conforms with regulations or not. Monitoring is increasingly being used during structural renovations where the acquired data at each stage is used to provide the basis for further action. Structures are being monitored to acquire useful information when progressive phenomena are suspected, as an essential part of building maintenance to prevent or reduce the cost of interventions, and to evaluate the long-term effectiveness of interventions. Recent developments have focused on developing continuous monitoring systems such as load sensors, dynamic analysis or deflection measurement through digital image processing. These approaches however do not provide information about the location of failure which is crucial for fast events. For such cases, damage identification in form of acoustic emission analysis (well established technique for other materials, to timber only applied on laboratory scale and not structures) might be more appropriate. Further research is also needed to apply fibre optics to monitoring timber structures.

The Action will provide a platform where the aspects of assessing, reinforcing and monitoring timber structures will be linked and dealt with in a holistic approach. Simple, robust and redundant monitoring systems can be applied to timber structures and, to a certain extent, replace expensive interventions to the building. COST offers the appropriate framework by funding capacity -building activities that will allow disseminating the information to the European industry.

This Action will advance and disseminate the knowledge regarding the assessment, reinforcement and monitoring of timber structures, specifically through: