Structural Timber
Table of Contents
Why design using structural timber
Structural timber is becoming a more common material in the design and construction of new buildings. This is primarily due to technological advances and the ease of producing high strength timber that can be used for columns, beams in high rise buildings. An example is a 14-stroey timber tower called “The Tree” was built in Norway using cross-laminated timber for floors and walls.
The differences between using timber and traditional materials are listed below:
Timber:
- Medium material cost
- Low maintenance
- Quick, clean construction
- Low self-weight
- Variable properties
- Good fire resistance
- Very low \(C0_2\) emission during processing
- Very good recyclability
Steel:
- Expensive material cost (depends on the price of steel)
- High maintenance in exterior use
- Quick, clean construction
- Low self-weight
- High, reliable strength
Concrete:
- Cheap
- low maintenance
- slow, messy construction
- High self-weight (need bigger foundations)
- Variable properties
Types of engineered Timber
- Glulam
- OSB (Oriented Strand Board
- LVL, (Laminated Vaneer Lamber)
- Cross laminated Timber (CLT)
- Wood polymer composites
Advantages of the using engineer wood are below:
- More homogenous with less defects
- Designable to meet specific applications
- Versatile and available in different geometry and grade
- Maixmise the natural strength and stiffness characteristics of wood
- Easy to work with using orginary tools and basic skills
- More efficient use of wood
Disadvantages of the using engineer wood are below:
- Requires more primary energy to produce
- The adhesives used may be toxic
- Specialist contractors may be needed
These are some of the many reasons why engineers are starting to use structural timber in the construction, which is a much more sustainable source when compared to steel and concrete.
Glulam
Glulam is made up of laminated timber, which is glued (bonded) together to form a singe structural element. The grain of timber in each layer is parallel to all other layers. Glulam members can be used for structural glulam beam and column members. It is commonly used as roof beams as large spanning structures in sports halls, swimming pools and also used as bridge beams as well as trusses.
Cross laminted timber (CLT)
Cross laminated timber consists of solid sawn timber, which are glued (bonded) together into a single structural member. The grain in each layer is perpendicular to those on either side. Common uses are one-way spanning floor slabs, wall panels and roof panels. This is a lightweight solution, when compared with a concrete floor as this could save cost and time through using smaller foundations.
However, floor vibration could become an issue (gyms, dance floors) as well as acoustic issues.
Laminated Vaneered lumber (LVL)
LVL consist of thin sheets (veneers) of wood, laid out in the same orientation, coated with adhesive and hot pressed. Required member sizes can then be cut from the sheet. It is commonly used as structural beams and formwork. As they are made up of thin veneers, which reduces the effect of defects but the member sizes are generally limited to around 75mm (although larger members can be achieved by mechanically fixing/gluing panels together).
Fire Resistance and Durability of Timber
Decay
Timber decays normally occurs when the moisture content is above 20% for most of the year. This applies to internal timber members and exposed timber member if it is properly drained and ventilated. However, engineers should design them so they do not get wet or there is adequate drainage.
Insect Attack
Insect attack is an issue to be looked at in the design of timber but the UK does not have serious issues with insect attack and the main problems come from fungal attack. In the UK, insect attack mainly occurs from furniture beetles (woodworms), death watch beetles and the house longhorn beetle.
Fungal Attack
Structural timber can become susceptible to fungal attack when conditions are right for the fungi to grow when the moisture content increases to a minimum of 20%.
The two types of fungal attack are dry rot (most serious) and wet rot. Dry rot is caused by the ‘Serpula lacrymans’ which can spread and damage much timber throughout a structure. White cotton wool like strands form on the surface of the timber. Sometimes, yellow patches can be seen. This fungus weakens the timber, causing it to be dry and brittle. A serious issue is that it can spread between newly built timber and so the true of extent of structural damage may be larger than imagined.
Wet rot is caused by ‘Coniophora puteana’. This is a common cause of timber decay and is usually confined to areas where there is dampness or leaks. This occurs when moisture levels are between 30-50%. This is identified by dark brown staining on the timber as well as splitting along the grain of timber.
Fire Resistance
Structural timber has good resistance against fire and despite it being a combustible material, as it has natural insulating properties. When timber is exposed to fire, timber starts to break down into charcoal, carbon monoxide and methane. The charcoal layer forms around the timber and acts as insulation which protects the inner core of the timber which maintains the strength and stability of timber members.
The ISTRUCTE guide for the design of timber members has predicted rates for charring of timber, which is tabulated below:
| Material | Charring rate per exposed face (mm/min) |
|---|---|
| Softwood timber | 0.8 |
| Softwood glulam, and LVL to BS EN 14374 | 0.7 |
| Hardwood timber and hardwood glulam with a characteristic density of \(\geq 450\) \(kg/m^3\) | 0.55 |
Design Manual and Codes
Timber Design Codes and Capacities
Structural timber such as glulam can be used for beams and column, and just as we would check the bending and shear capacities for steel and concrete members, we do the same for timber as well as other checks depending on how we are using the structural element.
For example, if we were looking at a structural timber beam under a Uniformly distributed load pinned at each support, we would analyse the beam an element where the bottom of the beam would be in tension and top part in compression (i.e, typical bending moment for a simply supported beam), we would then look at the bending capacity of the timber beam and makes sure that it passes (i.e, bending capacity is greater than the design moment). This would be done for other similar checks.
Below are the definitions of symbols used frequently in Euroocode 5:
- \(\chi\) = characteristic value of strength property
- \(\phi\) = multiple of relevant member and system modification factors
- \(\alpha_{M}\) = material safety factor
- \(f_{m,y,k}\) = characteristic bending strength about y-y axis
- \(k_{mod}\) = is a factor to allow for service class (moisture condition) and load duration
- \(k_{h}\) = relates to depth of section
- \(k_{m,\alpha}\) = relates to bending for tapered beams
- \(k_{sys}\) = relates to load sharing systems
Load duration classes
Duration classes are is linked to the time a structural element is under loads and this affects the timber capacities in subsequent equations. This is an important factor which needs to be considered when designing structural timber members.
| Load-duration class | Order of accumulated duration of characteristic load |
|---|---|
| Permanent | more than 10 years |
| Long-term | 6 months – 10 years |
| Medium-term | 1 week – 6 months |
| Short-term | less than one week |
| Instantaneous |
Examples of Load duration
| Load-duration class | Examples of loadings |
|---|---|
| Permanent | Self-weight |
| Long-term | Storage |
| Medium-term | imposed floor load, snow |
| Short-term | Snow, wind |
| Instantaneous | Wind, accidental load |
Material Partial factors of Saftey
This table can be found in BS EN 1995-1-1 (Table 2.3)
| Solid Timber | 1.3 |
| Glued Laminated timber | 1.25 |
| LVL,plywood and OSB | 1.2 |
| Particleboards | 1.3 |
| Connections | 1.3 |
| Punched metal plate fasteners | 1.25 |
| Accidental combinations | 1.0 |
Service Classes
Service Class 1: \(20^{\circ}\) and \(65%\) relative humidity in air, timber average moisture content \(\leq12%\)
Service Class 2: \(20^{\circ}\) and relative humidity> 85% for only a few weeks/year, timber average moisture content \(leq 20%\)
Service Class 3: timber average moisture content > Service Class 2
Values for \(k_{mod}\)
| Material | Standard | Service Class | Load duration class | ||||
|---|---|---|---|---|---|---|---|
| Permanent action | Long term action | Medium term action | Short term action | Instantaneous action | |||
| Solid Timber, Plywood Parts 1,2 and 3 | EN 14081-1 | 1,2 | 0.6 | 0.7 | 0.8 | 0.9 | 1.10 |
| Glulam | BS EN 14080 | 1,2 | 0.6 | 0.7 | 0.8 | 0.9 | 1.10 |
| Solid timber, glulam, plywood part 3 and LVL | 3 | 0.5 | 0.55 | 0.65 | 0.7 | 0.9 | |
| OSB | BS EN 300 | ||||||
| OSB/2 | 1 | 0.30 | 0.45 | 0.65 | 0.85 | 1.1 | |
| OSB/3, OSB/4 | 1 | 0.40 | 0.50 | 0.70 | 0.90 | 1.10 | |
| OSB/3, OSB/4 | 2 | 0.30 | 0.40 | 0.55 | 0.70 | 0.90 | |
Depth factors \(k_h\)
Bending and tension strength for soild timber, \(k_h = min. (\frac{150}{h})^{0.2} or 1.3\) for \(h < 150 mm\)
h = depth in bending or maximum cross section
Bending and tension strength for glulam timber, \(k_h = min. (\frac{600}{h})^{0.1} or 1.1\)
h = depth in bending or maximum cross section
Bending for LVL, \(k_h = min. (\frac{300}{h})^{s} or 1.2\)
h = depth in bending in mm and s = size effect parameter declared by manufacturer
| Example of Use | Limit State | Instantaneous/Final | Recommended limits for beams spanning between 2 supports |
|---|---|---|---|
| Cracking of plasterboard, glass,ceramics etc. in roofs, ceilings or floorss | Irreversible | Final | \(w_{fin}\leq\frac{L}{250}\) where \(w_{fin} = deflection due to permanent + imposed loads + creep. |
| Apperance of roofs and ceilings with no attatched brittle finishes | Reversible | Final | \(w_{fin}\leq\frac{L}{150}\), where \(w_{fin}\) = deflection due to permanent + imposed loads + creep |
| In fire, at end of required period of fire resistance, where protecttion depends on attatched plasterboard, unless proved by test | Accidental | Instantaneous | \(w_{inst}\leq\frac{L}{150}\) where \(w_{inst}\) = instantaneous deflection due to permanent + imposed loads |
Load-sharing system factor, \(k_{sys}\)
When several equally spaced similar members, components, or assemblies are laterally connected by a continuous load distribution system, the members strength properties may be multiplied by a system strength factor \(k_{sys}\).
Provided the continous load-distribution system is capable of transferring the loads from one member to the neighbouring members, \(k_{sys} = 1.1\)
Shear modification factor, \(k_{CR}\)
A reduction in shear capacity of timber is taken to allow for drying splits and glue-line failure. This can be found in the National Annex (Table NA.4) to BS EN 1995-1-1.
| Material | Modification factor \(K_{CR}\) |
|---|---|
| Solid Timber | 0.67 |
| Glulam | 0.67 |
| LVL | 1.0 |
| Wood based panels | 1.0 |
Compression strength perpendicular to the grain for solid timber and glulam, \(k_{c,90}\)
\(k_{c,90}\) is usually taken as 1.0 but can be increased for the following:
For members on discrete continuous supports which undertakes distributed loads/ or concentrated loads further away from supports than \(l_1 = 2h\),
where \(l\) is the contract length and \(h\) is the depth of the member.
\(k_c,90\) = 1.5 for solid softwwood Timber
\(k_c,90\) = 1.75 for glued laminated softwood timber provided that \(l \leq 400\)mm
This can be found in BS EN 1995-1-1, section 6.1.5.
Bi-axial Bending Check
There are two expressions below that need to be to be satisfied.
\(\frac{\sigma_{m,y,d}}{f_{m,y,d}}+k_m\frac{\sigma_{m,z,d}}{f_{m,z,d}} \leq 1.0\) (6.11)
\(k_m\frac{\sigma_{m,y,d}}{f_{m,y,d}}+\frac{\sigma_{m,z,d}}{f_{m,z,d}} \leq 1.0\) (6.12)
\(\sigma_{m,y,d}\) and \(\sigma_{m,z,d}\) are the design bending stresses about the principle axes as shown in Figure 6.1;
\(f_{m,y,d}\) and \(f_{m,z,d}\) are the corresponding design bending strengths.
- For rectangular sections: \(k_m = 0.7\)
- For other cross-sections: \(k_m = 1.0\)
- For other wood-based structural products, for all cross-sections: \(k_m = 1.0\)
This can be found in BS EN 1995-1-1, section 6.1.6.
Modification factor for notched beams
At support locations, notches can be made into the timber, which will affect the stress concentrations in those areas.
\(k_v\) is the reduction factor and can be taken as 1.0.
This can be found in BS EN 1995-1-1, section 6.5.2
Elastic analysis of a beam - bending analysis - lateral stability
- Find \(L_{eff}\)
- Find \(\sigma_{m,eff}\)
- Find \(\lambda_{rel,m}\)
- Find \(k_{crit}\)
- Check \(\sigma_{m,d} \leq k_{crit} \times f_{md}\)
\(l_eff\) can be found in table 6.1 from BS EN 1995-1-1 which has been tabulated below:
| Beam Type | Loading type | \(l_{ef}/l^a\) |
|---|---|---|
| Simply supported | Constant moment | 1.0 |
| Uniformly distributed load | 0.9 | |
| Concentrated force at the middle of the span | 0.8 | |
| Cantilever | Uniformly distributed load | 0.5 |
| Concentrated force at the free end | 0.8 |
\(^a\)The ratio between the effetice length \(l_{ef}\) and the span \(l\) is valid for a beam with torsionally restrained supports and loaded at the centre of gravity. If the load is applied at the compression edge of the beam, \(l_{ef}\) should be increased by 2h and may be decreased by 0.5h for a load at the tension edge of the beam.
For exmaple, in a 2m simply supported beam with a height of 300mm with a uniformly distributed load would have an effective length of 0.9 x 2.0 + 2 x 0.3 = 2.4m
\(\sigma_{m,crit} = \frac{0.78b^2}{hl_{ef}}E_{0,05}\)
\(\lambda_{rel,m} = \sqrt{\frac{f_{m,k}}{\sigma_{m,crit}}}\)
\(k_{crit}\) can be determined from expression 6.34
| \(k_{crit}\) | |
| For \(\lambda_{rel,m} \leq 0.75\) | 1 |
| For \(0.75 < \lambda_{rel,m} \leq 1.4\) | \(1.56-0.75\lambda_{rel,m}\) |
| For \(1.4 < \lambda_{rel,m}\) | \(\frac{1}{\lambda^2_{rel,m}}\) |
Deflection Design
When calculating the deflection of a structural timber member, it undergoes instantaneous deflection due to permanent and variable loads but also due to creep. The deflection due to creep is calculated using the deformation factor \(k_{def}\) which is taken from BS EN 1995-1-1, table 3.2.
| Material | Standard | Service Class | ||
|---|---|---|---|---|
| 1 | 2 | 3 | ||
| Solid timber | EN 14081-1 | 0.6 | 0.8 | 2.0 |
| Glued Laminated timber | EN 14081 | 0.6 | 0.8 | 2.0 |
| LVL | EN 14374, EN 14279 | 0.6 | 0.8 | 2.0 |
| Plywood | EN 636 | 0.8 | 1.0 | 2.5 |
| OSB/2 | EN 300 | 2.25 | ||
| OSB/3, OSB/4 | EN 300 | 1.50 | 2.25 | |
Deformation is defined in BS EN 1995-1-1, clause 2.2.3, equation 2.2
Total deformation = instantaneous deflection + creep deflection
Where \(U_{fin} = U_{inst} + U_{creep}\) which is shown in the codes as \(u_{fin} = u_{fin,G} + u_{fin,Q1} + \Sigma u_{fin,Q1}\)
\(u_{fin,G} = u_{inst,G}(1+k_{def})\) for permanent action, G
\(u_{fin,Q,1} = u_{inst,Q,1}(1+\psi_{2,1} k_{def})\) for the leading variable action, Q1
\(u_{fin,Q,i} = u_{inst,Q,i}(\psi_{0,i}+\psi_{2,i} k_{def})\) for accompanying variable action, Qi (i>1)
\(\psi_0 and \psi_2 can be found in table NA.A1.1\)
| Action | \(\psi_0\) | \(\psi_1\) | \(\psi_2\) |
|---|---|---|---|
| Category A: domestic, residental areas | 0.7 | 0.5 | 0.3 |
| Category B: Office areas | 0.7 | 0.5 | 0.3 |
| Category C: Congregation areas | 0.7 | 0.7 | 0.6 |
| Category D: shopping areas | 0.7 | 0.7 | 0.6 |
| Category E: storage areas | 1.0 | 0.9 | 0.8 |
| Category F: traffic areas, vehicle weight \(\leq 30 kN\) | 0.7 | 0.7 | 0.6 |
| Category H: traffic areas \(30 kN < vehicle weight \leq 160 kN\) | 0.7 | 0.5 | 0.3 |
| Snow loads on buildings (see EN 1991-3) | |||
| for sites located at altitude H>1000 m a.s.l. | 0.7 | 0.5 | 0.2 |
| for sites located at altitude \(H\leq1000\) m a.s.l. | 0.5 | 0.2 | |
| Wind loads on buildings (See BS EN 1991-1-4) | 0.5 | 0.2 | |
| Temperature (non-fire) in buildings (see EN 1991-1-5) | 0.6 | 0.5 |
Structural Timber Span Tables
Max spans for domestic floor joists
| Cross section (mm x mm) | Imposed load 1.5 \(kN/m^2\) – Dead load 0.5 \(kN/m^2\) | ||||||
|---|---|---|---|---|---|---|---|
| Spaced at 400mm | Spaced at 450mm | Spaced at 600mm | |||||
| C16 | C24 | C16 | C24 | C16 | C24 | ||
| 47×95 | 1.77 | 2.05 | 1.68 | 1.94 | 1.47 | 1.70 | |
| 47×120 | 2.40 | 2.67 | 2.28 | 2.57 | 2.01 | 2.32 | |
| 47×145 | 2.89 | 3.22 | 2.78 | 3.10 | 2.52 | 2.81 | |
| 47×170 | 3.38 | 3.77 | 3.25 | 3.62 | 2.95 | 3.29 | |
Preliminary Sizes for Structural Timber Roofs
| Structural form, Description and materials | Pitch | Span range (m) | Approximate proportions |
|---|---|---|---|
| Truss rafter, solid timber and punched metal plate fasteners | 15-40 | 3-15 | \(h\approx \frac{L}{75} to \approx \frac{L}{100}\) |
| Bolted timber truss with purlins and intermediate common rafters and ceiling ties | 15-45 | 5-20 | \(h\approx \frac{L}{60} to \approx \frac{L}{80}\) |
| Trussed girder, Warren type etc. Solid Timber, glulam, LVL | Less than 3 | 30-80 | \(h\approx \frac{L}{10} to \approx \frac{L}{15}\) |
Characteristic properties of Structural Timber
| Stength properties \(N/mm^2\) | Class | C16 | C20 | C24 | C27 | D40 | D50 |
|---|---|---|---|---|---|---|---|
| Bending | \(f_{mk}\) | 16 | 20 | 24 | 27 | 40 | 50 |
| Tension Parallel | \(ft_{0k}\) | 8.5 | 11.5 | 14.5 | 16.5 | 24 | 30 |
| Tension Perpendicular | \(ft90_{mk}\) | 0.4 | 0.4 | 0.4 | 0.4 | 0.6 | 0.6 |
| Compression Parallel | \(f_{c0k}\) | 17 | 19 | 21 | 22 | 27 | 30 |
| Shear | \(f_{vk}\) | 3.2 | 3.6 | 4 | 4 | 4.2 | 4.5 |
| Stiffness Properties \(kN/mm^2\) | |||||||
| Mean modulus of elasticity parallel bending | \(E_{m,0,mean}\) | 8 | 9.5 | 11 | 11.5 | 13.0 | 14.0 |
| 5 percentile modulus of elasticity parallel bending | \(E_{m,0,k}\) | 5.4 | 6.4 | 7.4 | 7.7 | 10.9 | 11.8 |
| Mean modulus of eleasticity perpendicular | \(E_{m,90,mean}\) | 0.27 | 0.32 | 0.37 | 0.38 | 0.87 | 0.93 |
| Mean Shear modulus | \(G_{mean}\) | 0.5 | 0.59 | 0.69 | 0.72 | 0.81 | 0.88 |
| Density \(kg/m^3\) | |||||||
| 5 percentile density | \(\rho_{k}\) | 310 | 330 | 350 | 360 | 550 | 620 |
| Mean density | \(\rho_{mean}\) | 370 | 400 | 420 | 430 | 660 | 740 |
Span Tables
| Example of Use | Limit State | Instantaneous/Final | Recommended limits for beams spanning between 2 supports |
|---|---|---|---|
| Cracking of plasterboard, glass,ceramics etc. in roofs, ceilings or floorss | Irreversible | Final | \(w_{fin}\leq\frac{L}{250}\) where \(w_{fin} = deflection due to permanent + imposed loads + creep. |
| Apperance of roofs and ceilings with no attatched brittle finishes | Reversible | Final | \(w_{fin}\leq\frac{L}{150}\), where \(w_{fin}\) = deflection due to permanent + imposed loads + creep |
| In fire, at end of required period of fire resistance, where protecttion depends on attatched plasterboard, unless proved by test | Accidental | Instantaneous | \(w_{inst}\leq\frac{L}{150}\) where \(w_{inst}\) = instantaneous deflection due to permanent + imposed loads |