Timber Frame Houses: Design Principles

Timber Frame Houses: Design Principles Dr Robert Hairstans 19 August, 2009

DESIGN PRINCIPLES: Material Properties Wood is a natural, heterogeneous, anisotropic, hygroscopic composite material. Its structural properties are highly variable as a result of a whole range of influencing factors. What has to be considered is the level of effect the influencing factors have in relation to the structural properties of the timber section being considered. If it can be considered negligible in the overall scale of investigation then it can be ignored. When designing with timber it is important to have an appreciation of what affects its strength: Density Moisture content Temperature Time Grain deviation Knots a) Cell wall organisation of a mature tracheid b) Diagrammatic representation of a wedge shaped segment cut from a five year old hardwood tree showing the principal structural features Cellular and structural features of timber 2

DESIGN PRINCIPLES: Material Properties Simple tension Cross grain tension Splintering tension b) Cross grain tension example Brash tension Compression Horizontal shear a) Failure types of clearwood in bending with span parallel to the grain c) Splintering tension example Influence of grain deviation on failure mode of small clear samples in bending 3

DESIGN PRINCIPLES: Material Properties Diagonal Compression near a knot b) Diagonal example Localised cross-grain tension b) Localised cross grain tension Influence of grain deviation & knots on failure mode of larger samples in bending 4

DESIGN PRINCIPLES: Strength Class & Grading Characteristic values for some common strength classes of solid softwood (British Standards Institution (BSI), 2003) Strength class Property Symbol Units C16 C24 C27 Characteristic bending strength, f m,k 16 24 27 Characteristic tensile strength parallel to the grain, f t,0,k 10 14 16 Characteristic tensile strength perpendicular to the grain, f t,90,k 0.5 0.5 0.6 Characteristic compressive strength along the grain, f c,0,k 17 21 22 Characteristic compressive strength f perpendicular to grain, c,90,k N/mm² 2.2 2.5 2.6 Characteristic shear strength, f v,k 1.8 2.5 2.8 Mean value of modulus of elasticity parallel to the grain, Fifth percentile value of modulus of elasticity, Mean value of modulus of elasticity perpendicular to the surface grain E 0,mean 8000 11000 11500 E 0,05 5400 7400 7700 E 90,mean 270 370 380 Mean value of shear modulus, G mean 500 690 720 Characteristic density, r a k 310 350 370 kg/m³ Mean density, r b mean 370 420 450 a Used for calculating the strength of mechanically fastened connections b Used for calculating weight 5

DESIGN PRINCIPLES: European Structural Code of Practice EN 1990 EN 1991 Structural safety, serviceability and durability Actions on structures EN 1992 EN 1993 EN 1994 EN 1995 EN 1996 EN 1999 Design and detailing EN 1997 EN 1998 Geotechnical and Seismic design 6

DESIGN PRINCIPLES: European Structural Code of Practice Ultimate limit states are those associated with the collapse or with other forms of structural failure. Ultimate limit states include: loss of equilibrium; failure through excessive deformations; transformation of the structure into a mechanism; rupture; loss of stability. Instance where ultimate limit state has been reached Serviceability limit states include: deformations which affect the appearance or the effective use of the structure; vibrations which cause discomfort to people or damage to the structure; damage (including cracking) which is likely to have an adverse effect on the durability of the structure. Instance where serviceability limit state has been breached 7

DESIGN PRINCIPLES: European Structural Code of Practice Advantages of Eurocode: Facilitate further the free trade of construction products and services within Europe Provides the designer with more scope for design input. Facilitate a wider selection of materials and components. Provides more guidance on the design of built up components facilitating the incorporation of new engineered products and allow future products to be integrated for use. Result in timber design which is economic, serviceable and ultimately safer. Disadvantages of Eurocode: More complicated design code and contains hundreds of design expressions for predicting the resistance of structural components. Factors, have the potential to affect significantly the economics of one construction material over another depending on the numerical value selected. Due to the inherent flaws in timber, partial safety factor: γ M = 1.3 Comparison between poor and high quality design expressions 8 (Byfield and Nethercot, 2001)

DESIGN PRINCIPLES: European Structural Code of Practice Consider a beam in bending (y-y axis only) 9

DESIGN PRINCIPLES: European Structural Code of Practice Consider a beam in bending (y-y axis only) s m, y, d f m, y, d where σ m,y,d = maximum design bending stress about y axis = M y,d /W y f m,y,d = k h k crit k sys k mod f m,k /γ M M y,d - W y - f m,k - k h - k crit - k sys - k mod - γ M - Bending moment Section modulus Characteristic bending strength Depth or width factor Factor used for lateral buckling System strength factor Strength modification factor for duration of load and moisture content Partial factor for material properties 10

DESIGN PRINCIPLES: Responsibility The Engineer has overall responsibility for: Strength, Stability & Structural serviceability Primary concern is load bearing elements Duty of care concerning durability Meet the requirements of the client and the relevant Building Regulations 11

DESIGN PRINCIPLES: Responsibility Building use and location Determine the imposed loads Requirements for resistance to disproportionate collapse Requirements for the corrosion protection of metal fasteners Protective treatment of timber. a) Ring beam over lintel providing redundancy to system b) Tying of system together Methods of introducing system robustness 12

DESIGN PRINCIPLES: Responsibility Design life A design life for the building should be specified. A properly designed and maintained timber building can last for centuries, but most commonly a design life of 50 years is specified. Timber frame systems can also be used for less permanent structures where a design life of 10 years may permit the use of higher strength properties. Performance indicator Initial value Serviceability level Normal maintenance SLS Repair ULS Time Evolution with time of a structure Visible damage 13

DESIGN PRINCIPLES: Responsibility Design situations The building must be designed to have adequate strength, stability and structural serviceability in the following situations: During construction (the execution phase). In designated use throughout its design life In accidental design situations Timber frame under construction 14

DESIGN PRINCIPLES: Design procedures Developer Architectural Information Timber Frame Supplier Preliminary layout of building Roof Truss System Supplier Initial Design Timber Frame Designer Initial Design Floor System Supplier Initial Design Roof Truss System Supplier Final Design Timber Frame Designer Final Design Floor System Supplier Final Design Timber Frame Designer Indemnification of Design Timber Frame Supplier Collation of design information Timber Frame Supplier Final Design Developer Certification of structural design 15

DESIGN PRINCIPLES: Design procedures Architect s Layouts DESIGN PRINCIPLES: Design procedures Select Roof System Type & Initial Make-Up Specification: Trussed Rafter Stressed Skin Panels Prefabricated timber joists Solid timber Select Floor Type & Initial Make-Up Specification: Solid Timber Joist Engineered Wood Joist Rim Beam Material Decking Make-Up Initial Make-Up of Timber Frame Walls: Wall thickness & details Timber grade & dimension Sheathing material & arrangement Fixing specification a) Fink c) Attic Truss type Building layout Initial System Dimensioning & Sizing Designation of Wall Types (Load Bearing & Non- Load-bearing). Roof & Floor Orientations & Spans Calculate Actions: Self Weight Imposed Loads (Wind, Snow, Live etc) Roof System 1. Detail Connections 2. Check Member Sizes 3. Check Bracing & Holding Down Floor System 1. Detail Connections 2. Check Member Sizes 3. Check Bracing No Are ULS and SLS criteria satisfied? Are ULS and SLS criteria satisfied? No Yes Yes a) LVL b) LSL c) PSL Wall Diaphragm Timber composites Detail Connections Does wall contain openings? Yes Check Stud & Lintel Specification No Check Stud Specification Check Racking Resistance a) Solid section b) I-Joist Floor options Does capacity exceed applied actions? Yes Check Wall Panel Overturning & Sliding No Accept Yes Building Stability Requirements Check Overturning & Sliding Specify Holding down Straps & Shear Fixings Does capacity exceed applied actions? No 16

DESIGN PRINCIPLES: Wind loading & system overturning The principles of timber platform frame design are such that it is normal to consider system stability in two parts: 1. Overall system resistance to sliding and overturning as a result of the applied wind action: Timber frame buildings are relatively lightweight, therefore it is necessary to verify their overall stability under wind loading with respect to overturning, sliding and roof uplift, both during the execution phase and after completion. During the execution phase the weight of the roof tiles should be excluded. For the majority of circumstances the self weight of the system results in a holding down moment and, as a result of friction, a resistance to sliding, both of which are greater than the applied overturning and sliding forces. A point for further consideration is the common practice of levelling due to poor foundation tolerances by inserting proprietary plastic shims, this reduces frictional resistance to sliding to an unknown level and as a result additional resistance to sliding may require to be specified. Timber frame during construction Proprietary shims reducing level of frictional resistance 17

DESIGN PRINCIPLES: Wind loading & system overturning 2. The transmission of applied shear to the foundation: Applied wind loading on a building is transferred to the foundations by diaphragm action. The side walls, considered to be simply supported at roof and foundation, transfer one half the total wind load to the roof level. The roof diaphragm, acting as a deep horizontal beam, transmits the load to the end shear walls, which in turn transfer the load to the foundation via shear connections and holding down straps. Transmission of applied shear to foundation Temporary bracing during construction for stability 18

DESIGN PRINCIPLES: Wind pressure Recommendations for low rise timber frame: Use a single reference height z e equal to the total height of the building above the ground (EC1-1-4 Figure 7.4). Base the external pressure coefficients for walls on the height of the wall to the eaves, rather than dividing the wall height into zones. For overturning, sliding, roof uplift and racking resistance calculations involving more than one value of coefficient of pressure c pe on the roof, first apply a single conservative value to the whole roof. If the structure fails, calculate the overturning moment or the sliding, uplift or racking force more accurately. To check structures during the execution phase the seasonal factor c season may be used to modify the basic wind velocity (EC1-1-4 4.2(3)). For the execution phase it is expected that a value for c season based on a 2 year erection period will be specified in the National Annex to BS EN 1991-1-6 #10.5. For small scale timber frame projects a 1 year period might be considered appropriate, for which the corresponding value of c season is 0.749. This reduces the wind pressure by a factor of 0.749² = 0.56. 19

DESIGN PRINCIPLES: Wind pressure For certain pitches of roof two sets of external pressure coefficients are given, and the critical coefficients may differ for different verifications. Illustrative values of c pe,10 for overall stability and racking resistance verifications Verification Overturning about z-z Wind coefficient zone Comments F G H I J Wind perpendicular to the ridge q = 0-0.5-0.5-0.2-0.4-0.5 Sliding +0.7 +0.7 +0.4-0.4-0.5 Roof uplift N/A N/A N/A -0.4-0.5 Calculate uplift on more severe side of ridge, resisted by half the roof weight* Racking +0.7 +0.7 +0.4-0.4-0.5 Use for horizontal racking load and for uplift which reduces vertical load on wall panels Wind parallel to the ridge q = 90 Overturning about z-z -1.1-1.4-0.8-0.5 Sliding N/A N/A N/A N/A Wind friction forces may generally be disregarded (see EC1-1-4 5.3(4)) Roof uplift -1.1-1.4 N/A N/A Assume roof trusses are separate members and check worst case Racking -1.1-1.4-0.8-0.5 Use for horizontal racking load and for uplift which reduces vertical load on wall panels * If necessary a more accurate calculation using the moments about the opposite eaves exerted by all the wind coefficient zones may be used in conjunction with the restoring moment of the whole roof. Wind zones on a 30 duopitch gable roof (EC1-1-4 7.2.5) 20

DESIGN PRINCIPLES: Masonry shielding Both testing and experience in the UK have demonstrated that within certain limits masonry walls will reduce the wind load onto the timber frame of buildings. BS 5268-6.1:1996 (British Standard Institution (BSI), 1996) makes allowance for this applying a wind load reduction factor. The IStructE Manual for the design of timber building structures to Eurocode 5 provides guidance to the application of a similar factor in Eurocode (IStructE & TRADA Technology, 2007) to reduce the applied wind action. The resulting reduced wind load F w is considered to act uniformly over the entire area of the adjacent timber frame wall. When the wind blows on or off a gable wall the total wind load on or off the adjacent timber frame wall should be calculated as: F w = k masonry F masonry + F spandrel where k masonry = wind shielding reduction factor. F masonry = total wind load on or off the masonry wall excluding the spandrel area F spandrel = wind load on or off spandrel. In other cases it should be calculated as: F w = k masonry F masonry where k masonry = wind shielding reduction factor. F masonry = total wind load on or off the masonry wall Masonry clad timber frame houses 21

DESIGN PRINCIPLES: Masonry shielding Since k masonry depends on the proportion of openings in the wall it may differ on windward and leeward faces, therefore it must be used in conjunction with the surface pressure method of EC1 (see EC1-1-4 Clause 5.3(3)). k masonry may be used only in accordance with the following conditions: only the first four storeys of masonry not exceeding 10m in total height can be considered to contribute wind shielding the external dimensions of the masonry walls are used to calculate the wind loads the masonry walls are constructed in accordance with BS EN 1996-1-1:Eurocode 6 Design of Masonry Structures (EC6-1-1) and BS EN 1996-2: Eurocode 6. Design of masonry structures (EC6-2) from a material designated in EC6-1-1. FT Wall Tie (dimensions in mm) the mortar conforms to the relevant part of BS EN 1996-1-1 with a minimum strength class of M4 the masonry walls are at least 100 mm thick and have a minimum mass of 75 kg/m² the masonry cladding is connected to the timber frame with wall ties that have sufficient strength and stiffness to transfer wind forces to the timber frame wall manufactured in accordance with BS EN 845-1 k masonry is applied to the wall as a whole up to eaves level, to the top of the fourth storey of masonry or up to 10m of masonry, whichever is less. k masonry should not be applied to the design of individual elements, for example studs. k masonry should not be used when checking the execution phase. High Movement (HM) Wall Tie (dimensions in mm) Courtesy of Cullen Building Products 22

DESIGN PRINCIPLES: Masonry shielding Values of k masonry according to IStructE & TRADA Technology Manual for the design of timber building structures to Eurocode 5 Percentage of shielded wall occupied by openings Number of storeys shielded by masonry 1 and 2 3 4 A B C A B C D E F 0 0.45 0.60 0.75 0.50 0.68 0.85 0.60 0.74 0.88 10 0.50 0.64 0.78 0.55 0.71 0.87 0.64 0.77 0.89 20 0.56 0.68 0.80 0.60.074 0.88 0.69 0.80 0.91 30 0.61 0.72 0.83 0.65 0.78 0.90 0.73 0.83 0.93 40 0.66 0.76 0.85 0.70 0.81 0.92 0.77 0.86 0.95 50 0.71 0.80 0.88 0.75 0.84 0.93 0.81 0.89 0.96 60 0.77 0.84 0.90 0.80 0.87 0.94 0.86 0.92 0.98 70 0.82 0.88 0.93 0.85 0.91 0.96 0.90 0.95 1.00 >70 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 KEY A: For masonry walls with buttresses or returns of length >= 550 mm and spaced at not more than 9 m centres B: For masonry walls with buttresses or returns of length >= 550 mm at one end only, wall length <= 4.5 m C: For masonry walls other than A and B D: For masonry walls with buttresses or returns of length >= 950 mm and spaced at not more than 9 m centres E: For masonry walls with buttresses or returns of length >= 950 mm at one end only, wall length <= 4.5 m F: For masonry walls other than D or E. NOTES (1) In calculating the percentage of wall occupied by openings, the height of the wall should be taken as the height to the eaves, the top of the fourth storey of masonry or 10m, whichever is less. (2) Values for intermediate percentages of wall occupied by openings may be obtained by linear interpolation. (3) For walls longer than 9m the tabulated values may be used provided that additional buttresses or returns are added to the masonry wall spaced at not more than 9m centres. (4) If the selected support conditions do not extend to the full shielded height of the wall in question then the number of storeys and percentage of loaded wall should be based on the height to which the selected support conditions reach. 23

DESIGN PRINCIPLES: Masonry shielding Additional ties required at door and window openings (courtesy of Cullen Building Products) Maximum net surface wind pressures for the FT range of brick / timber wall-ties (courtesy of Cullen Building Products) 24

DESIGN PRINCIPLES: Overturning The overturning calculation can be illustrated considering the previous example: A F, A G z-z = total area of roof region F, G etc = centre line of structural wall beneath roof a F,a G, etc = horizontal distances between z and centre of areas A F, A G etc. h F,h G, etc = vertical distances between z and centre of areas A F, A G etc. b, l = plan dimensions of structural walls α = roof pitch q p F building,k = peak velocity wind pressure (without masonry shielding reduction) = characteristic dead weight of building (excluding the tile weight in the execution phase) Wind zones on a 30 duopitch gable roof (EC1-1-4 7.2.5) 25

DESIGN PRINCIPLES: Overturning For wind perpendicular to the ridge the clockwise design overturning moment about z-z is M 0,d = 1.5[q p cosa(0.5a F a F + 0.5A G a G + 0.2A H a H + 0.4A I a I + 0.5A J a J ) + q p sina(-0.5a F h F - 0.5A G h G - 0.2A H h H + 0.4A I h I + 0.5A J h J ) +F w,total,k h e /2] where F w,total,k = total characteristic wind force on windward and leeward walls, after allowing for any masonry shielding in completed phase. The design restoring moment about z0-z0 is M R,0,d = γ G F building,k b/2 = 0.45F building,k b with the partial load factor for permanent load, γ G, taken as 0.9 according to BS EN1990:2002, Table A1.2(A). F building,k = characteristic dead weight of building (excluding the tile weight in the execution phase) 26

DESIGN PRINCIPLES: Overturning For wind parallel to the ridge the clockwise design overturning moment about z-z is M 90,d = 1.5[q p cosa(1.1a F a F + 1.4A G a G + 0.8A H a H + 0.5A I a IJ ) + F spandrel,total,k (h e + h r /3) + F w,total,k h e /2] where F w,total,k F spandrel,total,k h e h r = characteristic wind force on windward wall for end-of-terrace building or total wind force on windward and leeward walls for detached building, excluding the spandrel area of the gable walls. = characteristic wind force on windward spandrel for end-of-terrace building or net wind force on windward and leeward spandrels for a detached building. For hip-ended buildings or buildings with a flat roof F spandrel,total,k = 0. = height to eaves = height of roof from eaves The design restoring moment about z90-z90 is M R,90,d = γ G F building,k l/2 = 0.45F building,k l with the partial load factor for permanent load, γ G, taken as 0.9 according to BS EN1990:2002, Table A1.2(A). 27

DESIGN PRINCIPLES: Overturning If M d > M R,d A restraining or holding down method should be specified. The restraints should provide a total design restraining force along each wall of (M d -M R,d )/b or (M d -M R,d )/l. Timber frame holding down methods 28

DESIGN PRINCIPLES: Sliding The sliding calculation can be illustrated considering the previous example, where F w,total,k and F spandrel,total,k are as previously defined. For wind perpendicular to the ridge the design sliding force is F d = 1.5[q p sina(-0.7a F h F 0.7A G h G 0.4A H h H + 0.4A I h I + 0.5A J h J ) + F w,total,k ] For wind parallel to the ridge the design sliding force is F d = 1.5[F w,total,k + F spandrel,total,k ] The maximum value of F d is F d,max. The Engineer is therefore recommended to specify positive restraints around all the structural perimeter walls providing a total design restraining force of at least F d, max. If friction is utilise, a coefficient of 0.25 is recommended and a partial factor of 0.9 should be applied to the characteristic dead weight of the building; any further lateral resistance still required may then be provided by more positive restraints. Wind zones on a 30 duopitch gable roof (EC1-1-4 7.2.5) 29

DESIGN PRINCIPLES: Roof uplift It is generally regarded as good practice to attach every trussed rafter to the wall plate with truss clips, whether or not there is a possibility of roof uplift. Truss clips make a significant contribution to the strength of the horizontal diaphragm in the ceiling plane. Truss clips reduce the potential damage skew nailing can cause to connector plates, rafters or wall plates by offering a positive fixing on two planes. Truss clips 30

DESIGN PRINCIPLES: Roof uplift For wind perpendicular to the ridge the simplest approach is to calculate the uplift force on the more severely loaded side of the ridge and compare this with half the roof weight. In this case: F d = 1.5q p cosa(0.4a I + 0.5A J ) The design resisting force applied by half the roof weight is: R d = 0.5 γ G F roof,k = 0.45F roof,k with the partial load factor for permanent load, γ G, taken as 0.9 according to BS EN 1990:2002, Table A1.2(A). If necessary a more accurate calculation can be calculated using the moments exerted by all the wind coefficient zones in conjunction with the restoring moment of the whole roof. If F d > R d specify truss clips to attach the roof trusses to the head binder or top rail of the wall panels. The truss clips should provide a total design restraining force of at least (F d R d ) on each side of the roof. Wind zones on a 30 duopitch gable roof (EC1-1-4 7.2.5) 31

DESIGN PRINCIPLES: Roof uplift For wind parallel to the ridge the design uplift force should be calculated for one side of a single truss in the most severely loaded zone: F d = 1.5q p cosa(1.1a F + 1.4A G ) x 10s/2e where s = trussed rafter spacing e = the cross-wind building width or twice its height, whichever is smaller. The design resisting force applied by the roof weight on one truss is R d = 0.5sγ G F roof,k /2l = 0.225sF roof,k /l with the partial load factor for permanent load, γ G, taken as 0.9 according to BS EN1990:2002, Table A1.2(A). Each truss, at least in the most severely loaded roof zones, should be restrained by a truss clip at each eaves point with a design resistance to uplift of at least (F d R d ), determined as for wind perpendicular to the ridge. Wind zones on a 30 duopitch gable roof (EC1-1-4 7.2.5) 32

DESIGN PRINCIPLES: Racking Requirements 2. The transmission of applied shear to the foundation: Applied wind loading on a building is transferred to the foundations by diaphragm action. The side walls, considered to be simply supported at roof and foundation, transfer one half the total wind load to the roof level. The roof diaphragm, acting as a deep horizontal beam, transmits the load to the end shear walls, which in turn transfer the load to the foundation via shear connections and holding down straps. (a) Area of gable wall transferring wind load to front racking wall b) Diaphragm action of roof trusses and ceiling transferring wind on gable wall to front wall Racking load on first floor front wall from wind on gable wall 33

DESIGN PRINCIPLES: Racking Requirements Structurally graded C16 framing members, specified with no wane, cross-section 38mm x 89mm, 38mm x 140mm or 44 x 97mm (depth governed by thermal insulation requirements and method of insulation). Stud spacing 600mm (maximum); where possible spacing should match joist centres which are normally 600mm but may be 400mm or 450mm to reduce joist depth. Top and bottom rails nailed to studs with a minimum of 3.0mm galvanised smooth round steel wire nails or 3.1mm machine-driven galvanised steel nails, 75mm long, 2 no. per 89 mm stud or 3 no. per 140 mm stud. External sheathing 9.0 mm thick OSB/3; fastened to studs with 3.0mm galvanised smooth round steel wire nails or 2.8mm galvanised machine-driven steel nails; for Class 2 buildings fastened to studs with 3.35mm galvanised smooth round steel wire nails or 3.1mm galvanised machine-driven steel nails; all at least 50mm long, spaced at 150mm on perimeter, 300mm on internal studs. 12.5mm thick gypsum plasterboard suitable for 30 minutes fire resistance fastened to the internal face with 2.65mm plasterboard nails or plasterboard screws at least 40mm long, maximum fastener spacing 150mm around perimeter and on internal studs if relevant. Standard timber frame wall panel 34

DESIGN PRINCIPLES: Racking Requirements Internal walls are constructed in a similar manner to external walls except that 12.5mm plasterboard is used on both sides and the stud size may be reduced to 38mm x 63mm. If they are required to carry vertical or horizontal loads the stud depth should increase to at least 72mm, and if necessary an additional layer of structural sheathing materials may be introduced beneath the plasterboard to provide additional racking resistance. Internal & external panels Party wall 35

DESIGN PRINCIPLES: Racking Requirements Timber frame party walls consist of two separate wall panels with a gap between them. Normally they are sheathed only on the interior face of each unit with two layers of plasterboard, 19.5 mm thick and 12.5mm thick respectively, the joints being staggered. It is particularly important that the inner layer is fixed to the framing with specified fasteners at the specified spacings. In order to provide sufficient racking resistance it may be necessary to specify solid timber diagonal braces in the cavity, taking care to preserve a gap of at least 50mm. Alternatively structural sheathing on the inner side of each leaf can be specified, but this can result in drumming as it is not tied to masonry or other cladding, and it is therefore normally avoided. Typical timber frame party wall Gypsum plasterboard Designed as two individual wall units separated by a cavity, the sound performance is comparable to that of a 240 mm thick concrete wall. Each wall unit has plasterboard linings on its sides and is filled with insulation between the wall studs. Any additional bracing must be accompanied by adequate holding-down arrangements to prevent party wall panels from overturning. Standard external (EX) timber frame wall panel 36

DESIGN PRINCIPLES: Racking Design (BS5268) Current UK design method BS 5268-6.1 has been used successfully for over 20 years. It is a permissible stress design method where structures are designed so that materials are kept within their elastic limits. Racking resistance s are based on the results of tested wall assemblies and are expressed in terms of kn/m. Test panel were constructed from Hem-fir, hand driven clout nails and outdated sheathing materials. Modification factors K 101, K 102 & K 103 are applied to the basic racking resistance to account for variations in nail diameter, sheathing thickness and nail spacing. Basic racking resistances for a range of materials and combinations of materials Modification factors K 104, K 105, K 106 & K 107 are applied to the basic racking resistance to account for variations in wall dimensions, the presence of framed openings and applied vertical loading. 37

DESIGN PRINCIPLES: Unified European Code of Practice Design Method Currently Eurocode 5 contains a Method A & Method B for racking design. At present the U.K National annex to EC5 specifies the use of Method B, a conversation of BS 5268. The conversion process has been ineffectual and it is widely accepted that method B gives inaccurate results. As a result work has been on-going to create a unified Method C. Forces acting on sheathing- to- frame fasteners under idealised linear-elastic behaviour). Forces acting on sheathing to frame fasteners under idealised plastic behaviour 38

39 DESIGN PRINCIPLES: Plastic Design Method L 1 L L L L L L 5 0. 0.5 1 2 1 1 0.5 0.5.5 0 L L L F 0 F L r n t, F L r n b, M 0 1 0.5 1, L r L H F n b 2 2, 2, 1 0.5 1 1.5 0 L r L r H F n b n b 2 1.5 0 H L 0 0.5 0.5 2 H L H L H L H L H L 0.5 2 0.5 0.5 4 1 1 H L H L 1 1 2 At top rail, At bottom rail, (at bottom rail) H F

DESIGN PRINCIPLES: BSI, UKTFA & Edinburgh Napier University Collaboration Range of panels tested in accordance with BS EN 594:1995 (150/300mm nail spacing unless specified). Standard (C-1,C-2) 300mm Studs - 300mm sheet widths (C-3,C-4,C-5,C- 6) 75/150 spacing - Double end studs (C-7,C-8,C-9,C-10) 50/100 spacing - Double end studs (C-11,C-12,C-13,C-14) 50/100 spacing - double end studs - double sheathed (C-15,C-16) 1200mm Panel width (C- 17, C-18,C-19,C-20) 40

DESIGN PRINCIPLES: BSI, UKTFA & Edinburgh Napier University Collaboration Points of note from testing Racking rig set-up Hydraulic ram Vertical restraint of windward stud through hold down strap detail. Vertical restraint of windward stud through hold down strap detail. 41

Average racking strength (kn) DESIGN PRINCIPLES: BSI, UKTFA & Edinburgh Napier University Collaboration Test results (150/300mm nail spacing unless specified). 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 A Standard Average Test value Average/Design calculated value - 1.156kN fastenener capacity*1.2 mod. factor Load at 7.2mm deflection measured from test Failure Mode A or B A 300mm Sheet widths A 300mm Sheet widths - VIL A Dense Nailed 75 B Dense Nailed 75 -HD A Dense Nailed 50 B Dense Nailed 50 -HD B Double Sheathed 50 -HD A Double Sheathed 50 Failure Mode A lead stud lifting and sheathing breaking away from bottom rail A 1.2m Panel B 1.2m Panel - HD Failure Mode B Sheathing buckling out of plane Average Ultimate strength value, F max, for each panel configuration NB. VIL refers to the application of a Vertical Imposed Load, HD refers to the inclusion of Holding Down detail 42

DESIGN PRINCIPLES: BSI, UKTFA & Edinburgh Napier University Collaboration The following are critical to the racking performance of timber frame panels: Facing brick Wall Panel 1. Connection between the sheathing and timber studs. 2. Bottom runner to sole plate connection detailing. 3. Method of holding down. 4. Sole plate to substrate connection. Footer Sole Plate Floor slab Wall Footing Foundation Timber frame construction in section 43

DESIGN PRINCIPLES: Sheathing to timber connection The lateral load carrying capacity of a nailed sheathingto-timber connection can be calculated using the equations laid down in EC5 Section 8.2.2. Equations in EC5 are set up based on the minimum fastener spacing s, edge and end distances specified in EC5 Table 8.2. By adhering to these values it is ensured that failure of the connection shall occur in a predictable ductile fashion as illustrated by the range of possible failure modes specified by EC5 Clause 8.2.2 Equation 8.6 (Figure 2.18). F v.rk = min t 1 t 2 (a) (b) (c) (d) (e) (f) F F v, Rk F v, Rk v, Rk F F F fh,1, k t1 d fh,2, k t2 d fh,1, k t1 d 1 v, Rk v, Rk v, Rk f 1.05 h,1, k 2 t 2 1 t t1 d 2 fh,1, k t2 d 1.05 1 2 2 1.15 1 2M 2 1 t t 2 1 2 3 t 2 t 1 4(2 ) M y, 2(1 ) 2 f t d h,1, k 1 2 Rk 2 4(1 2) M 2 (1 ) 2 f t d y, Rk f h,1, k F d 4 ax, Rk h,1, k 2 t 1 t 2 1 F 4 y, Rk F 4 ax, Rk F 4 Where F v,rk is the characteristic load-carrying capacity per shear plane per fastener; f h,k is the characteristic embedment strength in the timber member; t i is the timber or board thickness or penetration depth, with i either 1 or 2; d is the fastener diameter; M y,rk is the characteristic fastener yield moment; β is the ratio between the embedment strength of the members; F ax,rk is the characteristic withdrawal capacity of the fastener. ax, Rk ax, Rk 44

DESIGN PRINCIPLES: Holding down detail Timber frame holding down strap Typical holding down details (courtesy of Cullen Building Products) EC5 Section 8.2.3 Steel-to-timber connections - For a thin steel plate in single shear: t 1 F v.rk = min (a) (b) t 1 F v, Rk 1.15 F 0. fh, k t1 d v, Rk 4 2M y, Rk f h,1, k F d 4 ax, Rk Where F v,rk is the characteristic load-carrying capacity per shear plane per fastener; f h,k is the characteristic embedment strength in the timber member; t 1 is the timber or board thickness or penetration depth. d is the fastener diameter; M y,rk is the characteristic fastener yield moment; F ax,rk is the characteristic withdrawal capacity of the fastener. 45

DESIGN PRINCIPLES: Holding down detail Holding Down Strap (ST-PFS/ST-PFS-M) Performance (courtesy of Cullen Building Products) NAIL SPECIFICATION 6No. 3.35 x 50mm stainless steel annular ring shank nails (ST-PFS) 4No. 3.35 x 50mm stainless steel annular ring shank nails (ST-PFS-M) 46

DESIGN PRINCIPLES: Sole plate to substrate connection details KMN 72 Shot Fired Dowel Masonry anchor Masonry anchor Express nails fasteners 47

Characteristic lateral load carrying capacity - N DESIGN PRINCIPLES: Sole plate to substrate connection details 6000 5000 4000 Caculated value based on characterisitc properties Characteristic values from test EC5 Section 8.2.3 Steel-to-timber connections - For a thick steel plate in single shear: t 1 F v, Rk fh,1, k t1 d 3000 (c) 2000 1000 F v.rk = min t 1 (d) F v, Rk f h, k t1 d 2 4M f h, k y, Rk 2 d t1 Fax, 1 4 Rk 0 t 1 F v, Rk 2.3 M y, Rk f h, k F d 4 ax, Rk (e) Fastener Type Where F v,rk is the characteristic load-carrying capacity per shear plane per fastener; f h,k is the characteristic embedment strength in the timber member; t 1 is the timber or board thickness or penetration depth; d is the fastener diameter; M y,rk is the characteristic fastener yield moment; F ax,rk is the characteristic withdrawal capacity of the fastener. 48

DESIGN PRINCIPLES: System continuity Party wall Party wall strap 49

DESIGN PRINCIPLES: System continuity Q R Σs x No. of storeys R R R R s d s d s d Characteristic load carrying capacity, s k = 1.6kN Q s d s d s d s d s d s d s d s d s d s d s d s d s d s d s d Where: R is the total racking force of each of the building units s d is the available design shear transfer from the party wall connector Characteristic load carrying capacity, s k = 3.2kN Acoustic wall strap (courtesy of Cullen BP) 50

DESIGN PRINCIPLES: Racking resistance in asymmetric buildings Where several walls parallel to the wind direction resist the wind load on a timber platform frame building it is normally assumed that they share the load in proportion to their strength. Assumption: strength of a wall is proportional to its stiffness and that the horizontal diaphragms create a stiff structure. F v, d, i F v, d R R d, i d, i where F v,d,i = design load on racking wall i F v,d = total racking load R d,i = design racking resistance of wall 51

DESIGN PRINCIPLES: Racking resistance in asymmetric buildings If the shear walls on one side of a building are significantly less strong and stiff than those on the other side then the share of the load which they carry may be greater: A L In such cases it is assumed that the building acts like a rigid box which resists both the shear force of the wind load and a torsional moment. Steel Goal Post (A) C.R G.C b W This torsional moment is equal to the wind load multiplied by the distance between the geometrical centre of the building and the building s centre of rotation (CR) measured perpendicular to the wind direction. B A aa Wind direction Steel Goal Post (B) B Plan of timber frame system 52

DESIGN PRINCIPLES: Racking resistance in asymmetric buildings For building plans on an x-y grid with an origin (0, 0) in one corner, the distance of the CR from the origin for wind perpendicular to the x-axis is calculated from the formula: x 3 x R d, i R x d, i i R1 (x 1 = 0) R 3 where R d,i = design resistance of racking wall i which is parallel to the wind direction = distance of CR from origin, measured along x-axis x i = distance of wall i from origin, measured along x-axis (0,0) x 2 x mean R 2 Therefore: R1 ( x x1 ) R2 ( x x2 ) R3 ( x3 x) x Wind direction hence x R 1 x 1 R R 1 2 R x 2 2 R R 3 3 x 3 53

DESIGN PRINCIPLES: Racking resistance in asymmetric buildings The resulting torsional moment, is resisted by all the walls, with each wall contributing to the total moment in proportion to its (stiffness) (lateral displacement) (perpendicular distance to the centre of rotation), i.e. x 3 where F v,d x mean k x z i F ( x xmean kx Rd, izi v, d ) = design racking load on building (sum of wind force on windward and leeward walls) = distance of geometrical centre of building from the origin, along x-axis =a constant calculated from the above equation =perpendicular distance of any racking wall i from CR, i.e. 2 R1 (x 1 = 0) (0,0) x 2 R 2 R 3 ( x xi ) or ( y yi ) as appropriate. x mean The additional load which each wall perpendicular to the x-axis takes to resist the torsional moment is then: x F tor,d,i = k x R d,i x i The total load carried by each wall perpendicular to the x-axis is then: F d,i = F v,d,i + F tor,d,i Wind direction And it is checked that: F d,i R d,i 54

DESIGN PRINCIPLES: Additional racking due to masonry Masonry cladding with a minimum height of 2.4m and a minimum width of 600mm attached by suitable wall ties to storey height timber frame walls can increase their racking resistance. The walls ties and their fasteners should have a design horizontal shear strength of at least 225N at deformations of 5mm or more and a characteristic horizontal shear stiffness of at least 30N/mm for deformations up to 5mm. The additional racking resistance, F v,masonry,rd, provided by the masonry subject to the conditions above, is: F v,masonry,rd = minimum of 0.25F masonry v, Rd q masonry where F v,rd = design racking resistance of attached timber frame wall in kn l masonry = length of masonry wall in m q masonry = 0.75 kn/m for 4.4 ties/m² (e.g. 600 mm horizontally, 380 mm vertically) = 0.6 kn/m for 3.7 ties/m² (e.g. 600 mm horizontally, 450 mm vertically) 55

DESIGN PRINCIPLES: Design of wall studs Wall stud design verifications: 1. Combined compression and bending stress (strength check): s f c,0,d c,0,d m,y,d m,y, d 1 2. Column stability (to prevent buckling as a column): 2 s f sc,0, d sm, y, d 1 sc,0, d sm, z, d kc, y fc,0, d f 1 m, y, d k f f c, z c,0, d m, z, d Wall studs in-situ 3. Lateral torsional stability (to prevent torsional instability as in a beam) : k s crit m, d f m, y, d 2 k s c, z c,0, d f c,0, d 1 σ c,0,d σ m,y,d σ m,z,d f c,0,d f m,y,d f m,z,d k c,y or k c,z k crit Design compressive stress along the grain Design bending stress about the principal y-axis Design bending stress about the principal z-axis Design compressive strength parallel to the grain Design bending strength about the major y-axis Design bending strength about the minor z-axis Instability factor Factor used for lateral buckling Wall studs in-situ 56

DESIGN PRINCIPLES: Design of wall studs Wall stud design information: For simplicity it is normally assumed that a stud resists the full vertical load and full net wind load i.e. sheathing is ignored. For the calculation of k crit about the stronger y-y axis a value of 0.85l may be used for the effective length, where l is the length of the stud within the frame. In the traditional UK design of buildings not exceeding four storeys it is normally assumed that wall studs are fully restrained against buckling about their weaker axis by their connection to the sheathing. Wall studs aligned with I-joists However in cases such as party wall where sheathing is limited, the load capacity is reduced, so some caution is recommended, particularly for buildings above four storeys. To support the ends of lintels single or multiple studs will be required at each end. If they are made of the same material and section as the main wall studs the total number required is at least equal to the number of wall studs removed by the opening. Beneath a window sill studs are normally provided in the position that the full height wall studs would have been. Wall studs supporting lintel over opening 57

DESIGN PRINCIPLES: Design of wall studs Notching and drilling of studs Wall studs should not be notched. Unless otherwise justified by calculation, drilling of studs should conform to the following requirements: Holes should be drilled on the centreline, avoiding knots. Hole diameters should not exceed one quarter of the stud depth. Holes should be no nearer than 150 mm and no further than a quarter of the stud length from either the top or bottom of the stud. Centre-to-centre hole spacing should be at least 4 hole diameters. Wall studs under an opening Deflection The effect of axial load on the horizontal deflection of a wall stud subject to wind loading may be generally be ignored, except in the case of slender studs subject to high wind loads, when ignoring axial load may result in excessive deflection. Bearing strength of bottom rails The bearing strength of the bottom rail should be verified. Intermediate studs should be checked rather than edge studs as they carry more load. Continuity across a goal post 58

DESIGN PRINCIPLES: Design of lintels Lintels above windows, doors and patio windows may consist of two solid timber members fastened together with nails, screws, dowels or bolts, a single LVL or hardwood member, or where necessary a bolted steel flitch beam. For lintels consisting of two or more solid timber members securely fastened together so that both members can share the load the strength properties including the bearing strength may be increased by a factor k sys of 1.1. A deflection limit of w fin 250l under dead + imposed load is recommended. Lintel over opening Screw size: 3.1mm dia. 75mm long galvanised screws at 300mm centres staggered mid distance between edge and centreline. No screw closer than 60mm to end of lintel. 59

DESIGN PRINCIPLES: Design information for Roofs Before designing a roof the Engineer should assemble the following data: site location, height, ground roughness and reference to any unusual wind conditions overall site plan indicating any adjacent buildings or features which might affect the wind loading height of building from ground level to eaves building type and whether access to the roof is required for purposes other than maintenance or repair intended use of roof space reference to any unusual environmental conditions which may affect steel or timber the type of any preservative treatment required plan and elevations of roof including overhangs and other eaves details, window lights, hatches, stairwells, chimney, and support details (nature, position and breadth) including intermediate supports (e.g. load-bearing walls) type and weight of roof tiles or covering weight of any sarking, insulation materials and plasterboard the size and position of all water tanks the weight and position of any permanent ancillary equipment to be supported on the ceiling joists preferred spacing of rafters any limitations on member size, e.g. to accommodate insulation or to match existing members, or minimum thicknesses for fixing ceiling boards or sarking rafter bracing method to be used (solid timber bracing or sarking using a specified panel product, or possibly steel ties in the case of larger roof structures) limitations on vertical deflection for rafters and ceilings joists, and on horizontal deflection at the eaves relative to the gable walls. any unusual site conditions (e.g. low loading limit) which may affect the design and assembly method Sarked attic trusses Roof layout drawing 60

DESIGN PRINCIPLES: Design information for Roofs The Engineer in turn should obtain the following output information from the roof designer: the basis of design, including any design assumptions made not covered below detailed drawings showing all trussed rafters in the roof and their positions and spacing timber strength classes or grades and species, and crosssectional dimensions the type, sizes and positions of all jointing devices with tolerances, or the number of effective teeth or nails required in each member at each joint the positions and sizes of all bearings the loadings and other conditions for which the trussed rafters have been designed the positions, fixings and sizes of any lateral supports necessary to prevent buckling of compression members such as rafters and struts the location and support method for tanks and ancillary equipment or loads, plus the capacity and magnitude of any additional loads assumed, e.g. weight of water the reactions to be accommodated at the bearings for each separate action (see Table 7.1) or load case (see Table 7.2) including asymmetrical snow loads and exceptional snow drifts where relevant maximum initial and final deflections of rafters and ceiling joists instructions concerning the fixing of any girder trusses or other special connection details Type A Roof truss details from MiTek Software (Designed by Donaldson Timber Engineering Ltd) 61

Force, F (kn) DESIGN PRINCIPLES: Roof system points of note Glued joints Split ring Punched metal plate Double sided toothedplate Dowel Bolt Bolted connection of steel truss shoe TS 100 truss shoe Slip (μmm) Nail Experimental load slip curves for joints in tension parallel to the grain (Racher, 1995) Steel truss shoe Example of truss nail plates 62

DESIGN PRINCIPLES: Roof system bracing Bracing of the system forms two basic functions: 1. Stability bracing holds the trusses firmly in place and keeps them straight so that they can resist all the loads applied (with the exception of wind). 2. Wind bracing, often required in addition to stability bracing so wind forces on the roof and walls can be withstood. Eurocode guidance for bracing in the plane of the rafters and the ceiling of trussed rafter roofs which fall within certain dimensional limits will be contained in in BS PD 6693: Complementary information for use with Eurocode 5. British Standards Insittuion. London. Outside these limits the roof designer should design the rafter bracing in accordance with EC5 9.2.5.3 and the ceiling bracing using the EC5 method described in subsection 5.5.2. 63 BS5268-3:1998Standard bracing for rafter and web members of duopitch trussed rafters

DESIGN PRINCIPLES: Roof system designed for lifting The upgraded bracing would function as bracing once the roof is in service and would improve the structural integrity of the system as it is an over-specification. In accordance with BS 5268:1998 Part 3 Annex A.1 all bracing members are of minimum width 89mm and minimum depth 22mm and the following points from the code are noted due to their level of importance: Lifting of roof Reinforced bracing 1. All bracing members are nailed to every trussed rafter they cross with two 3.35mm diameter galvanized wire nails with a minimum length equal to the bracing thickness plus 32mm. Therefore, the minimum nail length to be used is 77mm. Diagonal Bracing Element to be fixed to Gable Panel Longitudinal Bracing Element to be fixed to Gable Panel Gable Panel System Truss 2. Where bracing members are provided in two pieces, they are lap jointed over at least two trussed rafters and nailed as described above. Bracing detail Bracing element fixed to headbinder of system On-site application 64

Recommended texts: IStructE & TRADA Technology (2007) Manual for the design of timber building structures, The Institution of Structural Engineers, ISBN 978 0 901297 Porteous & Kermani (2007) Structural Timber Design to Eurocode 5, Blackwell Publishing, ISBN 978 14051 4638 8 65

Centre for Timber Engineering Edinburgh Napier University 10 Colinton Road Edinburgh EH10 5DT United Kingdom http://cte.napier.ac.uk/ r.hairstans@napier.ac.uk 66