dr robert hairstans seminar presentation
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Timber Frame Houses: Design Principles
Dr Robert Hairstans
19 August, 2009
2
• 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
DESIGN PRINCIPLES: Material Properties
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
3
DESIGN PRINCIPLES: Material Properties
Influence of grain deviation on failure mode of small clear samples in bending
b) Cross grain tension example
a) Failure types of clearwood in bending with
span parallel to the grain c) Splintering tension example
Simple tension
Cross grain tension
Splintering tension
Brash tension
Compression
Horizontal shear
4
DESIGN PRINCIPLES: Material Properties
Influence of grain deviation & knots on failure mode of larger samples in bending
b) Localised cross grain tension
b) Diagonal example
Diagonal
Compression near a
knot
Localised cross-grain tension
5
DESIGN PRINCIPLES: Strength Class & Grading
Property Symbol UnitsStrength class
C16 C24 C27
Characteristic bending strength, fm,k
N/mm²
16 24 27
Characteristic tensile strength
parallel to the grain,ft,0,k 10 14 16
Characteristic tensile strength
perpendicular to the grain,ft,90,k 0.5 0.5 0.6
Characteristic compressive
strength along the grain,fc,0,k 17 21 22
Characteristic compressive strength
perpendicular to grain,fc,90,k 2.2 2.5 2.6
Characteristic shear strength, fv,k 1.8 2.5 2.8
Mean value of modulus of
elasticity parallel to the grain,E0,mean 8000 11000 11500
Fifth percentile value of modulus
of elasticity,E0,05 5400 7400 7700
Mean value of modulus of elasticity
perpendicular to the surface grainE90,mean 270 370 380
Mean value of shear modulus, Gmean 500 690 720
Characteristic density, rka
kg/m³
310 350 370
Mean density, rmeanb 370 420 450
a Used for calculating the strength of mechanically fastened connectionsb Used for calculating weight
Characteristic values for some common strength classes of solid softwood
(British Standards Institution (BSI), 2003)
6
DESIGN PRINCIPLES: European Structural Code of Practice
EN 1990
EN 1991
EN 1992 EN 1993 EN 1994
EN 1995 EN 1996 EN 1999
EN 1998EN 1997
Structural safety,
serviceability and
durability
Actions on structures
Design and detailing
Geotechnical and
Seismic design
7
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.
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
Instance where ultimate limit state has been reached
8
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.
γM = 1.3
Due to the inherent flaws in
timber, partial safety factor:
Comparison between poor and high quality design expressions
(Byfield and Nethercot, 2001)
9
DESIGN PRINCIPLES: European Structural Code of Practice
Consider a beam in bending (y-y axis only)
10
DESIGN PRINCIPLES: European Structural Code of Practice
Consider a beam in bending (y-y axis only)
dymdym f ,,,, s
where σm,y,d = maximum design bending stress about y axis
= My,d/Wy
fm,y,d = khkcritksyskmodfm,k/γM
My,d - Bending moment
Wy - Section modulus
fm,k - Characteristic bending strength
kh - Depth or width factor
kcrit - Factor used for lateral buckling
ksys - System strength factor
kmod - Strength modification factor for duration of load and moisture content
γM - Partial factor for material properties
11
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
12
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.
Methods of introducing system robustness
a) Ring beam over lintel providing redundancy to system b) Tying of system together
13
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.
‘Initial
value’
Performance
indicator‘Normal’ maintenance Repair
ULS
SLS
Visible damage
Serviceability
level
Time
Evolution with time of a structure
14
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
15
DESIGN PRINCIPLES: Design procedures
Developer
Architectural Information
Roof Truss System
Supplier
Initial Design
Timber Frame Supplier
Final Design
Floor System Supplier
Initial Design
Timber Frame Supplier
Collation of design
information
Developer
Certification of structural
design
Timber Frame Supplier
Preliminary layout of
building
Timber Frame Designer
Final Design
Timber Frame Designer
Indemnification of Design
Timber Frame Designer
Initial Design
Floor System Supplier
Final Design
Roof Truss System
Supplier
Final Design
16
DESIGN PRINCIPLES: Design procedures
Architect’s
Layouts
Building layout
Initial System Dimensioning & Sizing
Designation of Wall Types (Load Bearing & Non-Load-bearing).
Roof & Floor Orientations & Spans
Initial Make-Up of Timber Frame Walls:
Wall thickness & details
Timber grade & dimension
Sheathing material & arrangement
Fixing specification
Calculate Actions:
Self Weight
Imposed Loads (Wind, Snow, Live etc)
Yes
Select Floor Type & Initial Make-Up
Specification:
Solid Timber Joist
Engineered Wood Joist
Rim Beam Material
Decking Make-Up
Select Roof System Type & Initial
Make-Up Specification:
Trussed Rafter
Stressed Skin Panels
Prefabricated timber joists
Solid timber
Yes
Check Stud & Lintel
Specification
No
No No
Yes Yes
Accept
No
Check Stud
Specification
Check Racking Resistance
Building Stability
Requirements
Check Overturning
& Sliding
Specify Holding down
Straps & Shear Fixings
Yes No
Detail Connections
Does wall contain
openings?
Are ULS and SLS
criteria satisfied?
Roof System
1. Detail Connections
2. Check Member Sizes 3. Check Bracing & Holding Down
Are ULS and SLS
criteria satisfied?
Does capacity exceed
applied actions?
Does capacity exceed
applied actions?
Wall Diaphragm
Check Wall Panel
Overturning & Sliding
Floor System
1. Detail Connections
2. Check Member Sizes
3. Check Bracing
DESIGN PRINCIPLES: Design procedures
Timber composites
a) LVL b) LSL c) PSL
a) Fink c) Attic
Truss type
Floor options
a) Solid section b) I-Joist
17
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.
Proprietary shims reducing level of frictional
resistance
Timber frame during construction
18
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.
Temporary bracing during construction for stabilityTransmission of applied shear to foundation
19
DESIGN PRINCIPLES: Wind pressure
Recommendations for low rise timber frame:
•Use a single reference height ze 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 cpe 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 cseason 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 cseason
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 cseason is 0.749. This reduces the wind
pressure by a factor of 0.749² = 0.56.
20
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.
Verification
Wind coefficient zoneComments
F G H I J
Wind perpendicular to the ridge q = 0°
Overturning
about z-z’-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 -0.5Calculate uplift on more severe side of ridge,
resisted by half the roof weight*
Racking +0.7 +0.7 +0.4 -0.4 -0.5Use 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/AWind friction forces may generally be
disregarded (see EC1-1-4 5.3(4))
Roof uplift -1.1 -1.4 N/A N/AAssume roof trusses are separate members and
check worst case
Racking -1.1 -1.4 -0.8 -0.5Use 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.
Illustrative values of cpe,10 for overall stability and racking resistance verifications
Wind zones on a 30° duopitch gable roof (EC1-1-4 7.2.5)
21
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 Fw 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:
Fw = kmasonryFmasonry + Fspandrel
where kmasonry = wind shielding reduction factor.
Fmasonry = total wind load on or off the masonry
wall excluding the spandrel area
Fspandrel = wind load on or off spandrel.
In other cases it should be calculated as:
Fw = kmasonryFmasonry
where kmasonry = wind shielding reduction factor
.
Fmasonry = total wind load on or off the masonry
wallMasonry clad timber frame houses
22
DESIGN PRINCIPLES: Masonry shielding
Since kmasonry 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)).
kmasonry 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.
•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
•kmasonry 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.
•kmasonry should not be applied to the design of individual elements, for example
studs.
•kmasonry should not be used when checking the execution phase.
High Movement (HM) Wall Tie (dimensions in mm)
FT Wall Tie (dimensions in mm)
Courtesy of Cullen Building Products
23
DESIGN PRINCIPLES: Masonry shielding
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.
Values of kmasonry according to IStructE & TRADA Technology Manual for the design of timber
building structures to Eurocode 5
24
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)
25
DESIGN PRINCIPLES: Overturning
The overturning calculation can be illustrated considering the previous
example:
AF, AG = total area of roof region F, G etc
z-z’ = centre line of structural wall beneath roof
aF,aG, etc = horizontal distances between z and centre of areas AF,
AG etc.
hF,hG, etc = vertical distances between z and centre of areas AF, AG
etc.
b, ℓ = plan dimensions of structural walls
α = roof pitch
qp = peak velocity wind pressure (without masonry shielding
reduction)
Fbuilding,k = 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)
26
DESIGN PRINCIPLES: Overturning
For wind perpendicular to the ridge the clockwise design overturning moment about z-z’ is
M0,d = 1.5[qpcosa(0.5AFaF + 0.5AGaG + 0.2AHaH + 0.4AIaI + 0.5AJaJ)
+ qpsina(-0.5AFhF - 0.5AGhG - 0.2AHhH + 0.4AIhI + 0.5AJhJ)
+Fw,total,khe/2]
where Fw,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
MR,0,d = γGFbuilding,kb/2 = 0.45Fbuilding,kb
with the partial load factor for permanent load, γG, taken as 0.9 according to BS EN1990:2002, Table A1.2(A).
Fbuilding,k = characteristic dead weight of building (excluding the tile weight in the execution phase)
27
DESIGN PRINCIPLES: Overturning
For wind parallel to the ridge the clockwise design overturning moment about z-z’ is
M90,d = 1.5[qpcosa(1.1AFaF + 1.4AGaG + 0.8AHaH + 0.5AIaIJ)
+ Fspandrel,total,k(he + hr/3) + Fw,total,khe/2]
where Fw,total,k = 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.
Fspandrel,total,k = 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 Fspandrel,total,k = 0.
he = height to eaves
hr = height of roof from eaves
The design restoring moment about z90-z90’ is
MR,90,d = γGFbuilding,kℓ/2 = 0.45Fbuilding,kℓ
with the partial load factor for permanent load, γG, taken as 0.9 according to BS EN1990:2002, Table A1.2(A).
28
DESIGN PRINCIPLES: Overturning
•If Md > MR,d
•A restraining or holding down method should be specified.
•The restraints should provide a total design restraining force along each wall of (Md-MR,d)/b or (Md-MR,d)/ℓ.
Timber frame holding down methods
29
DESIGN PRINCIPLES: Sliding
The sliding calculation can be illustrated considering the previous example,
where Fw,total,k and Fspandrel,total,k are as previously defined.
For wind perpendicular to the ridge the design sliding force is
Fd = 1.5[qpsina(-0.7AFhF – 0.7AGhG – 0.4AHhH + 0.4AIhI + 0.5AJhJ) +
Fw,total,k]
For wind parallel to the ridge the design sliding force is
Fd = 1.5[Fw,total,k + Fspandrel,total,k]
The maximum value of Fd is Fd,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 Fd,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)
30
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
31
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:
Fd = 1.5qpcosa(0.4AI + 0.5AJ)
The design resisting force applied by half the roof weight is:
Rd = 0.5 γG Froof,k = 0.45Froof,k
with the partial load factor for permanent load, γG, 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 Fd > Rd 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
(Fd – Rd) on each side of the roof.Wind zones on a 30° duopitch gable roof
(EC1-1-4 7.2.5)
32
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:
Fd = 1.5qpcosa(1.1AF + 1.4AG) 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
Rd = 0.5sγGFroof,k/2ℓ = 0.225sFroof,k/ℓ
with the partial load factor for permanent load, γG, 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 (Fd – Rd), determined as for wind perpendicular to the
ridge.
Wind zones on a 30° duopitch gable roof
(EC1-1-4 7.2.5)
33
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
Racking load on first floor front wall from wind on gable wall
b) Diaphragm action of roof trusses and ceiling
transferring wind on gable wall to front wall
34
DESIGN PRINCIPLES: Racking Requirements
Standard timber frame wall panel
•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.
35
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
36
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.
•Any additional bracing must be accompanied by
adequate holding-down arrangements to prevent
party wall panels from overturning.
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.
Standard external (EX) timber frame wall panel
37
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 K101, K102 & K103 are applied to
the basic racking resistance to account for variations
in nail diameter, sheathing thickness and nail
spacing.
•Modification factors K104, K105, K106 & K107 are applied
to the basic racking resistance to account for
variations in wall dimensions, the presence of framed
openings and applied vertical loading.
Basic racking resistances for a range of materials and
combinations of materials
38
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
39
DESIGN PRINCIPLES: Plastic Design Method
L1 L
LLLLL 5.05.012
1
15.05.05.0 LLL
0F
FLr nt ,
FLr nb ,
0M
15.01 , LrLHF nb
22
,
2
, 15.0115.0 LrLrHF nbnb
215.0
H
L
05.05.0 2
H
L
H
L
H
L
H
L
H
L
5.02
5.05.0411
H
L
H
L11
2
At top rail,
At bottom rail,
(at bottom rail)
H
F
40
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)
41
DESIGN PRINCIPLES: BSI, UKTFA & Edinburgh Napier University Collaboration
•Points of note from testing
Vertical restraint of windward stud through
hold down strap detail.
Vertical restraint of windward stud through
hold down strap detail.
Racking rig set-up Hydraulic ram
42
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
Standard 300mm Sheet widths
300mm Sheet
widths -VIL
Dense Nailed 75
Dense Nailed 75
-HD
Dense Nailed 50
Dense Nailed 50
-HD
Double Sheathed 50 -HD
Double Sheathed
50
1.2m Panel
1.2m Panel -
HD
Av
era
ge r
ackin
g s
tren
gth
(kN
)
Average Test value
Average/Design calculated value -1.156kN fastenener capacity*1.2 mod. factor
Load at 7.2mm deflection measured from test
DESIGN PRINCIPLES: BSI, UKTFA & Edinburgh Napier University Collaboration
•Test results (150/300mm nail spacing unless specified).
Average Ultimate strength value, Fmax, for each panel configuration NB. “VIL” refers to the
application of a Vertical Imposed Load, “HD” refers to the inclusion of Holding Down detail
Failure Mode A – lead stud lifting and sheathing
breaking away from bottom rail
A AA
B
A
B
B
A
A
B
Failure Mode B – Sheathing
buckling out of plane
Failure Mode A or B
A
43
DESIGN PRINCIPLES: BSI, UKTFA & Edinburgh Napier University Collaboration
The following are critical to the racking
performance of timber frame panels:
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.
Floor slab
Facing
brick
Wall Panel
Sole Plate
Footer
Wall
Footing
Foundation
Floor slab
Facing
brick
Wall Panel
Sole Plate
Footer
Wall
Footing
Foundation
Timber frame construction in section
44
t1 t2
(a)
(b)
(c)
(d)
(e)
(f)
dtfF khRkv 1,1,,
dtfF khRkv 2,2,,
4112
1
,
1
2
2
1
23
2
1
2
1
221,1,
,
Rkaxkh
Rkv
F
t
t
t
t
t
t
t
tdtfF
4
)2(4)1(2
205.1
,
2
1,1,
,1,1,
,
Rkax
kh
Rkykh
Rkv
F
dtf
MdtfF
4
)21(4)1(2
2105.1
,
2
2,1,
,22,1,
,
Rkax
kh
Rkykh
Rkv
F
dtf
MdtfF
42
1
215.1
,
,1,,,
Rkax
khRkyRkv
FdfMF
Fv.Rk = min
•The lateral load carrying
capacity of a nailed sheathing-
to-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).
DESIGN PRINCIPLES: Sheathing to timber connection
Where
Fv,Rk is the characteristic load-carrying capacity per shear plane per fastener;
fh,k is the characteristic embedment strength in the timber member;
ti is the timber or board thickness or penetration depth, with i either 1 or 2;
d is the fastener diameter;
My,Rk is the characteristic fastener yield moment;
β is the ratio between the embedment strength of the members;
Fax,Rk is the characteristic withdrawal capacity of the fastener.
45
DESIGN PRINCIPLES: Holding down detail
Where
Fv,Rk is the characteristic load-carrying capacity per shear plane per fastener;
fh,k is the characteristic embedment strength in the timber member;
t1 is the timber or board thickness or penetration depth.
d is the fastener diameter;
My,Rk is the characteristic fastener yield moment;
Fax,Rk is the characteristic withdrawal capacity of the fastener.
Typical holding down details (courtesy of Cullen Building Products)
dtfF khRkv 1,, 4.0
4215.1
,
,1,,,
Rkax
khRkyRkv
FdfMF
EC5 Section 8.2.3 Steel-to-timber connections - For a thin steel plate in single shear:
Timber frame holding down strap
(a)
(b)
Fv.Rk = min
t1
t1
46
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)
Holding Down Strap (ST-PFS/ST-PFS-M) Performance (courtesy of Cullen Building Products)
DESIGN PRINCIPLES: Holding down detail
47
DESIGN PRINCIPLES: Sole plate to substrate connection details
KMN 72 Shot Fired Dowel Masonry anchor
Masonry anchor Express nails fasteners
48
(c)
(d)
(e)
dtfF khRkv 1,1,,
41
42
,
2
1,
,
1,,
Rkax
kh
Rky
khRkv
F
tdf
MdtfF
Fv.Rk = min
DESIGN PRINCIPLES: Sole plate to substrate connection details
Where
Fv,Rk is the characteristic load-carrying capacity per shear plane per fastener;
fh,k is the characteristic embedment strength in the timber member;
t1 is the timber or board thickness or penetration depth;
d is the fastener diameter;
My,Rk is the characteristic fastener yield moment;
Fax,Rk is the characteristic withdrawal capacity of the fastener.
43.2
,
,,,
Rkax
khRkyRkv
FdfMF
EC5 Section 8.2.3 Steel-to-timber connections - For a thick steel
plate in single shear:
t1
t1
t10
1000
2000
3000
4000
5000
6000
Ch
ara
cte
risti
c late
ral lo
ad
carr
yin
g c
ap
acit
y -
N
Fastener Type
Caculated value based on characterisitc properties
Characteristic values from test
49
DESIGN PRINCIPLES: System continuity
Party wall Party wall strap
50
DESIGN PRINCIPLES: System continuity
Where:
R is the total racking force of each of the
building units
sd is the available design shear transfer from
the party wall connector
Q
R
Q – R ≤ Σs x No. of storeys
sd
sd
sd
sd
sd
sd
sd
sd
sd
sd
sd
sd
sd
sd
sd
sd
sd
sd
R R R
Characteristic load carrying capacity, sk = 1.6kN
Characteristic load carrying capacity, sk = 3.2kN
Acoustic wall strap (courtesy of Cullen BP)
51
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.
id
iddv
idvR
RFF
,
,,
,,
where Fv,d,i = design load on racking wall i
Fv,d = total racking load
Rd,i = design racking resistance of wall
52
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:
G.C
C.R
A
A
B B
aSteel Goal
Post (B)
Steel Goal
Post (A)b
W
L
a
Plan of timber frame system
•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.
•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.
Wind direction
53
DESIGN PRINCIPLES: Racking resistance in asymmetric buildings
x3
R1
(x1 = 0)R3
R2
xmean
x
x2
(0,0)
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:
id
iid
R
xRx
,
,
where Rd,i = design resistance of racking wall i which is
parallel to the wind direction
= distance of CR from origin, measured along
x-axis
xi = distance of wall i from origin, measured
along x-axis
)()()( 332211 xxRxxRxxR Therefore:
hence
321
332211
RRR
xRxRxRx
Wind direction
54
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.
2
,, )( iidxmeandv zRkxxF
x3
R1
(x1 = 0)R3
R2
xmean
x
x2
(0,0)
where
Fv,d = design racking load on building (sum of wind force
on windward and leeward walls)
xmean = distance of geometrical centre of building from the
origin, along x-axis
kx =a constant calculated from the above equation
zi =perpendicular distance of any racking wall i from CR, i.e.
)( ixx )( iyy or as appropriate.
The additional load which each wall perpendicular to the x-axis takes to
resist the torsional moment is then:
Ftor,d,i = kxRd,ixi
The total load carried by each wall perpendicular to the x-axis is then:
Fd,i = Fv,d,i + Ftor,d,i
And it is checked that:
Fd,i ≤ Rd,i
Wind direction
55
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, Fv,masonry,Rd, provided by the masonry subject to the conditions above, is:
Fv,masonry,Rd = minimum of
masonrymasonry
Rdv
q
F
,25.0
where Fv,Rd = design racking resistance of attached timber frame wall in kN
ℓmasonry = length of masonry wall in m
qmasonry = 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)
56
DESIGN PRINCIPLES: Design of wall studs
Wall stud design verifications:
1. Combined compression and bending stress (strength check):
2. Column stability (to prevent buckling as a column):
3. Lateral torsional stability (to prevent torsional instability as in a beam) :
1ff d,y,m
d,y,m
2
d,0,c
d,0,c
s
s
1,,
,,
,0,,
,0,
s
s
dym
dym
dcyc
dc
ffk 1,,
,,
,0,,
,0,
s
s
dzm
dzm
dczc
dc
ffk
1,0,,
,0,
2
,,
,
s
s
dczc
dc
dymcrit
dm
fkfk
σc,0,d Design compressive stress along the grain
σm,y,d Design bending stress about the principal y-axis
σm,z,d Design bending stress about the principal z-axis
fc,0,d Design compressive strength parallel to the grain
fm,y,d Design bending strength about the major y-axis
fm,z,d Design bending strength about the minor z-axis
kc,y or kc,z Instability factor
kcrit Factor used for lateral buckling
Wall studs in-situ
Wall studs in-situ
57
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 kcrit 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.
•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 aligned with I-joists
Wall studs supporting lintel over opening
58
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.
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.
Wall studs under an opening
Continuity across a goal post
59
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 ksys of 1.1.
•A deflection limit of wfin ≤ 250ℓ under dead +
imposed load is recommended.
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.
Lintel over opening
60
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
61
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 cross-
sectional 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)
62
DESIGN PRINCIPLES: Roof system points of note
Glued joints
Split ring
Double sided toothed-plate
Dowel
Bolt
Punched metal plate
Nail
Forc
e, F
(kN
)
Slip (μmm)
TS 100 truss shoe
Steel truss shoe
Bolted connection of steel truss shoe
Example of truss nail plates
Experimental load slip curves for joints in tension parallel to the grain (Racher, 1995)
63
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 sub-
section 5.5.2.
BS5268-3:1998Standard bracing for rafter and web members
of duopitch trussed rafters
64
DESIGN PRINCIPLES: Roof system designed for lifting
Bracing element
fixed to headbinder
of system
Diagonal Bracing Element to be
fixed to Gable Panel
Longitudinal Bracing
Element to be fixed to
Gable Panel
Gable Panel
System
Truss
On-site applicationBracing detail
Lifting of roofReinforced bracing
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:
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.
2. “Where bracing members are
provided in two pieces, they are lap
jointed over at least two trussed
rafters and nailed as described
above.”
65
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
66
Centre for Timber Engineering
Edinburgh Napier University
10 Colinton Road
Edinburgh EH10 5DT
United Kingdom
http://cte.napier.ac.uk/