Timber structures

In old buildings timber often fails where it enters a masonry wall or in roofs at the junction of rafter and tie beam where the truss is supported on the wall plate. In both cases leaking water over a long period has resulted in wet rot. The repair process consists of replacing the decayed timber with reinforced epoxy mortar. The reinforcement may consist of steel plate or bars which are drilled or set deep into the sound portion of the timber to ensure a good bond and load transfer. Fig. 6.23 shows bonded dowel bars used to repair the end section of a column or beam while Fig. 6.24 illustrates a typical repair to a roof truss. 

Deteriorated or worn timber floors can be upgraded by bonding new timber to old with epoxy resin. The procedure involves removing all deteriorated timber back to a sound surface prior to the application of a penetrating epoxy sealer. The new timber is then bonded to the old and composite action ensured by drilling and inserting dowel pins set in resin as shown in Fig. 6.25. 

New timber bonded to original with epoxy resin 

WOOD-BASED COMPOSITES PLYWOOD AND VENEER-BASED PRODUCTS 

A wood-based composite can be defined as a composite material mainly composed of wood elements. These wood elements are usually bonded together by a thermosetting adhesive (wood truss products could also be regarded as wood-based composites, but connected by metal connectors). The commonly used adhesives include urea-based adhesive (such as urea formaldehyde resin), phenolic-based adhesive (including phenol resorcinol adhesives), isocyanate-based adhesive, and adhesives from renewable resources (like soybean, lignin etc). The wood elements in wood composites can be in many different forms such as  

Traditional composite panels are made from veneers and from mat-formed eomposites bonded by adhesive. More recently wood has also been combined (eompression moulded or extruded) with synthetic polymers, e.g. thermoplastic polymers, to make wood-polymer composites (WPC). WPC products have been growing very rapidly in the recent years, especially in the deeking market, where Woleott (2004) observed that their market share has grown from 2% in 1997 to 14% in 2003. Further, much research work has explored the use of fibre-reinforced polymers (FRP) to enhance the structural performance of engineered wood eomposites, ealled FRP-wood hybrid composites (Dagher et ai, 1998 Shi, 2002). 

Some engineered wood composite products are made from combinations of other wood eomposites, such as wooden I-joists that can have flanges of sawn lumber, LVL, or other structural composite lumbers together with webs of OSB or plywood. 

In North America, all plywood manufactures used to follow the prescriptive standard PSl (APA, 1995). However, since the introduction of performance-based standards such as PS2 (NIST, 2004) and PRP-108 (APA, 2001) in 1990s other structural panel products, such as OSB, can and have been used interchangeably in structural panel applications. 

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Desch, H. E. and J. M. Dinwoodie (1996), Timber Structure, Properties, Conservation and Use, Macmillan, London. 

The real advantage of timber structures over steel and concrete equivalents usually lies in their strength to weight ratio. Wood has a very high strength to weight ratio and therefore requires a less massive foundation for an equivalent load. Timber is also aesthetically pleasing and non-corrosive. 

EN 518. European Committee for Standardization ECS (1995) Grading of structural timber requirements for machine strength graded timber and grading machines. European Committee for Standardization ECS (1995) Design of timber structures. Eurocode 5 1-1. European Committee for Standardization... 

P. Glos and H. Horstmann, in Proc. CIB-W18A Meeting 22, Vol. I, Paper 22-7-8, International Council for Building Research Studies and Documentation, Working Commission W18A Timber Structures, Berlin, 1989. 

Resorcinol-formaldehyde (RF), and phenol-resorcinol-formaldehyde (PRF) eold-setting adhesives are used primarily in the manufacture of structural, exterior-grade glulam fingerjoints, and other exterior timber structures. They produee bonds not... 

This is a useful heuristic applicable when dealing with a timber structure or pieces of timber. Instead of using metal plates supporting applied forces, timber plates carrying these forces could be coated with ferromagnetic particles, and an electromagnetic field could be used to keep them in place. 

Externally bonded FRP plates, sheets and wraps for the strengthening of reinforced concrete, steel, aluminium and timber structural members... 

In structural design in the context of civil engineering, there are three construction materials that are dominant steel, reinforced concrete, and timber. Each has its own design specifications. Thus, steel structure design, reinforced-concrete structure design, and timber structure design are three main design disciplines. 

Other undergraduate courses in structural engineering may include design of timber structures, masonry structures, advanced structural analysis, and pre-stressed concrete structure design. 

CEN/TC 250 Structural Eurocodes , SN EN 1995-1-1 2004 Eurocode 5 Design of timber structures — Part 1-1 General — Common rules and rules for buildings, SIA Schweizerischer Ingenieur- und Architektenverein Zurich, Switzerland (2(X)5). 

CEN/TC 193 Adhesives , SN EN 302-2 2004 Adhesives for load-bearing timber structures — Test methods — Part 2 Determination of resistance to delamination, SNV Schweizerische Normen-Vereinigung, Winterthur, Switzerland (2004). 

Kommission SIA 164 "Holzbau", SIA 265 2003 Timber Structures, SIA Schweizerischer Ingenieur- und Architektenverein, Zurich, Switzerland (2003). 

This article discusses briefly the use of adhesives on the construction site in the context of structural repair and reinforcement the requirements and practical dilficulties in the work on site with regtirds to the strength and durability of the rehabilitated timber structure and the consequent need for queility control. It also highlights the characteristics and requirements that must be fidfilled by structural adhesives and reinforcing materials factors affecting performance and durability of bonded joints tmd ways to improve adhesion and durability. Finally, it points out some research needs and future developments identified by the authors. 

Timber structures, rehabilitation, on-site polymerized adhesives, limitations, requirements, performance, durability, quality control... 

Fibre reinforced polymers (FRPs) are composed of a reinforcement material (glass, aramid or carbon fibres) surrounded and retained by a (thermoplastic or thermosetting) polymer matrix (unsaturated polyester, epoxy, vinyl ester, or polyurethane). FRPs were first used in the rehahiUtation of reinforced or pre-stressed concrete, but they have also been widely used in the reinforcement of timber structures. 

Possible approaches to deal with this problem include partial reconstruction of timber structural members by replacing the degraded timber with epoxy grout cast on-site into a permanent timber formwork, or, preferably, replacing decayed material with a new timber splice in both cases, the load is transferred to the remaining sound timber by FRP or steel rods or plates (Fig. 5). 

The shear strength of a bonded joint is also a function of the time for which a given temperature is sustained or has been sustained. This is important when considering the case of an epoxy-repaired timber structure exposed to fire, as compared to situations where the bondline would be subjected to prolonged or repeated exposure to hot environments, e.g., in roof trusses in countries with hot summers [20]. 

Little information is available on epoxy-repaired timber structures exposed to fire, although there has been some investigation about the effects of fire on wood structures, in general. 

The temperature range between 80°C and 100°C that has been recommended [28] as safe surface temperatures for wood exposed for long periods is not valid for all adhesive types. Figure 12 shows the thermal behaviour of eleven typical adhesives used in the rehabilitation of timber structures. It can be seen that all adhesives exhibit a pronounced decrease in their stiffness with the increase in temperature. If one considers the temperatures these adhesives will have to withstand in service (usually up to 50°C), a very careful selection of the adhesive is necessary. Thus, the temperature of the adhesive joint in service must be below the safe working temperature precribed by the adhesive manufacturer, which should correspond to 10-20°C below the adhesive glass transition temperature (taken from the peak of the tan delta curve, which is the most prevalent criterion appearing in the literature and the most used by adhesive manufacturers) [29]. 

In summary, the rehabilitation and repairing techniques for timber structures involving structural adhesives should always take into consideration the effect of service temperature on the adhesive performance, thus care should be taken regarding the structural joint design and the adhesive selection. 

Rehabihtation systems involving structural adhesives have been used for many years in on-site repair and strengthening of timber structures. However, because they exhibit excellent initial joint strength when tested in standard climate conditions, there has not been a major concern about their service durability and reliable and reahstic accelerated ageing tests do not, therefore, currently exist [20]. 

High initial bond strengths are relatively easy to achieve with the type of adhesives commonly used in the rehabilitation and repair of modem and historic timber structures. However, maintaining good bond durability in some situations is comparatively more difficult with these adhesives. For instance, while adhesives such as phenolic, resorcinolic, and aminoplastic resins produce durable bonds in EN 1995-1-1 [4] service classes 1, 2 and 3, the typically used on-site polymerized adhesives do not form bonds of adequate durability in service class 3, when bonding some preservative treated timbers, in situations in which the adhesive is bonded to dense hardwoods, or when bonding wood to non-wood materials, such as FRP profiles, steel rods, etc. 

The mixing procedure, preparation conditions and cure schedule all have a profound effect on the mechanical properties of the adhesives and, consequently, will affect their durability [20, 73]. Because of this, expeditious on-site tests should be conducted to detect possible deficiencies. A number of test methods have been proposed by the CEN/TC193/SC1/WG11) in the pre-standard Adhesives for on-site assembling or restoration of timber structures — on-site acceptance testing . This... 

BS EN 301 2006 Adhesives, phenolic and aminoplastic, for load-bearing timber structures. Classification and performance requirements. 

BS EN 1995-1-2 2004 Eurocode 5. Design of timber structures. General. Structural fire design. 

J. Custddio, Performance and durability of composite repair and reinforcement systems for timber structures, PhD Thesis, Department of Mechanical Engineering, School of Technology, Oxford Brookes University, Oxford (2009). 

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