Timber connections Centre for Offsite Construction ...

1 Andrew LivingstoneCentre for Offsite Construction + Innovative Content1)Introduction2)Nails, Screws, Bolts & Dowels 3)Glued joints4) Timber connectors5)Connection plates6)Specification of connections7)Eurocode Design of Dowel Type Fastener 8)Other design considerations 9)Connection calculations examples 10)Comparing C16+ vs C16 Timber connections11)Summary1) IntroductionIt is commonly stated that a structure is a constructed assembly of joints separated by members (McLain ,1998) and in Timber engineering the joint is generally the critical factor in the design of the structure.

2 The strengthof the connectorsin the joint will normally dictate the strength of the structure; their stiffness will greatly influence its overall behaviour and member sizes will generally be determined by the numbers and physical characteristics of the connector rather than by the strength requirements of the member are crucial points in many Timber structures because they can determine the overall strength and lengthof structural Timber is generally shorterthan the required spans and as a result splicing or composite structures ( trusses)

3 Must be between members are most often transferredthrough lap joints, either by adhesives (glues) or by laterally loaded dowel-type fasteners (nails, bolts, screws, dowels or nail plates). rightSibelius Hall, Lahti, Finland Bottom Scottish Parliament, Edinburgh, ScotlandTop leftPost & beam Increasing spans through connectionsExamples of different truss systems where connections have been used to combine Timber elements of different lengths to achieve longer Flitch beam Increasing spans through connectionsBolted Flitch beam Increasing spans through Connection typesTraditional Timber joint (a) Scarf joint(b) Horizontal finger joint(c)

4 Vertical finger jointGlued jointsTRADA Wood information sheet Connection typesDowel type connectors(a) Dowels(b) Connection types(b) Hexagonal head bolt(c) A modern self-drilling wood Connection types2) Nails, Screws, Bolts & , ,forexample, NailsNails may be driven by hand or by pneumatically operated portable machines. When nails are to be driven into dense timbers there is a danger that excessive splitting will occur. Methods of avoiding splitting are blunting the pointed end of the nail so that it cuts through the Timber fibres rather than separating them or to pre-drill a hole in the Timber less than 80% of the nail diameter.

5 Pre-drilling is not normally carried out on timbers with a lower characteristic density of 500 of pre-drilling: The lateral load carrying capacityof the nail is increased. The spacingbetween the nails and the distances between the nails and the end and edge of the Timber may be reduced thus producing more compact joints. Less slipoccurs in the : Labour intensive and as a result expensive. Reduces the cross sectional area of the ScrewsWood screws are especially suitable for steel-to- Timber and panel to Timber joints, but they can also be used for Timber -to- Timber joints.

6 Such screwed joints are normally designed as single shear are inserted by turning and this can be done either by hand or by power actuated tool depending on the situation. The main advantage a screw has over a nail is its additional withdrawal capacity. DowelsDowels are circular rodsof Timber , steel, or carbon-reinforced plastics which have a minimum diameter of are driven into identically or marginally undersized holes. These holes must either be drilled through all members in one operation or made using CNC with dowels are used in Timber Construction to transmit high forces.

7 Dowels are an economic type of joint which is easy to (M12 M14withacoarsehead) , ) Carriage boltb) Hexagonal head boltc) Square head bolta, b & c TRADA Wood information Sheet 52 d Canadian wood council ( )d) Lag screw3) Glued jointsKey advantages: of glued joints Structural glued joints are generally stiffer, require less Timber and have a betterappearance than mechanically fastened connections . They are resistant to corrosive atmospheres Joints made with thermosetting resins are saferin firethan mechanically fastened connections .

8 Key disadvantages are: stringent quality control is required unsuitable in conditions of fluctuating moisture contentif dissimilar materials are involved or if there is a change in the angle of grain at their interfaces. unsuitable if there is a significant component of load perpendicular to the plane of adhesion. AdhesiveApplicationSetting process and cure timeAdvantages / DisadvantagesThermo-PlasticPolyvinyl Acetate, CatalyzedPolyvinyl Acetate (PVA) interiorbut some special formulations are waterproofnon-reactive, 40 minutes at roomtemperatureeasy to work withHot MeltsInterior, high speed production linesnon-reactive, sets by coolinggrips on contact when hotThermo and Room Temperature SetResorcinol formaldehyde (RF)

9 Fully exterior, laminating, finger jointing, wood jointingreactive, sets in 2 minutes with heat and 6 hours at room temperaturewaterproof, high cost, marine-plywoodPhenol-resorcinol formaldehyde (PRF)waterproofPhenol formaldehyde (PF)fully exterior, plywood, some particalboardThermo-SetMelamine formaldehyde (MF) semi-exterior and Interior, plywood, particleboard, formwork panels. (not often used alone in the UK)reactive, sets with heat in 2 minutes and 30 minutes to 12 hours at room temperaturemoisture resistant, low costMelamine urea formaldehyde (MUF)semi-exterior and Interior, laminating, plywood, particleboard, finger jointingUrea formaldehyde (UF)interior, plywood, particleboard, wood jointing, bent laminations10 to 12 hours to cure.

10 There are liquid catalysts that will allow the resin to cure in 20 minuteseasy to work, withsomewat gap filling, moisture resistant, foundry sand moldsIsocyanates and Polyurethanes(Most Polyurethane are thermo-set but thermoplastic are available)isocyanates fully exterior, polyurethane semi-exterior and moist interior where temperature does not exceed 50 , laminatingreactive, one component sets with heat in 2 minutes, from to 2 to 60 minutes at room temperature for two-part resinsability to set in high moisture conditions, suitable for multiple martials, 100% solid, good gap filling properties, low glue spread rate, expensiveCatalystEpoxy resinssemi-exterior and Interiorreactive.