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CLOSE THIS BOOKRoof Truss Guide - Design and construction of standard timber and steel trusses (BASIN - SKAT, 1999, 187 p.)
4 TIMBER TRUSSES
VIEW THE DOCUMENT4.1 Design Considerations
VIEW THE DOCUMENT4.2 Timber Quality
4.3 Fixings and Fastenings
VIEW THE DOCUMENT4.3.1 System Options
VIEW THE DOCUMENT4.3.2 Nails
VIEW THE DOCUMENT4.3.3 Bolts / Pins
VIEW THE DOCUMENT4.3.4 Nail Plates
VIEW THE DOCUMENT4.3.5 Plywood gusset plates
VIEW THE DOCUMENT4.4 Timber Seasoning and Preservative Treatment
4.5 Nail Truss
VIEW THE DOCUMENT4.5.1 General
VIEW THE DOCUMENT4.5.2 Design
VIEW THE DOCUMENT4.5.3 Details
VIEW THE DOCUMENT4.5.4 Bracing / Lateral restraining system
VIEW THE DOCUMENT4.5.5 Manufacturing
VIEW THE DOCUMENT4.5.6 Erecting
4.6 Nail Truss Samples
VIEW THE DOCUMENT4.6.1 Assumptions and Limits of Application
VIEW THE DOCUMENT4.6.2 Flat roof nail truss, 6 m span
VIEW THE DOCUMENT4.6.3 Flat roof nail truss, 8 m span
VIEW THE DOCUMENT4.6.4 Flat roof nail truss, 10 m span
VIEW THE DOCUMENT4.6.5 Single pitch roof nail truss, 6 m span
VIEW THE DOCUMENT4.6.6 Single pitch roof, 8 m span
VIEW THE DOCUMENT4.6.7 Single pitch roof nail truss, 10 m span
VIEW THE DOCUMENT4.6.8 Double pitch roof nail truss, 6 m span
VIEW THE DOCUMENT4.6.9 Double pitch roof nail truss, 8 m span
VIEW THE DOCUMENT4.6.10 Double pitch roof nail truss, 10 m span
4.7 Nail Truss Sample with Plywood Gusset Plates
VIEW THE DOCUMENT4.7.1 Assumptions and limits of application
VIEW THE DOCUMENT4.7.2 Double Pitch Roof Nail Truss with Plywood Gusset Plate, 8 m span
4.8 Bolted Timber Truss Sample
VIEW THE DOCUMENT4.8.1 Assumptions and Limits of Application
VIEW THE DOCUMENT4.8.2 Double Pitch Roof, Tight Fitting Bolt Truss, Span 8.0 m

Roof Truss Guide - Design and construction of standard timber and steel trusses (BASIN - SKAT, 1999, 187 p.)

4 TIMBER TRUSSES

4.1 Design Considerations

A number of design considerations need to be made before selecting timber as the basic material for the roof truss. Issues to be checked are the following:

Availability of suitable timber: In many countries timber has become very expensive or is no longer available at all because of widespread deforestation. Timber with good (natural) resistance against rot, insect and fungal attack should be used.

Exposure to the weather: Timber should not be used if the truss cannot be fully protected from the weather.

Roof form: This is generally a matter of roof drainage and architectural considerations. Timber trusses are highly suitable for common roof forms but may not be first choice for extravagant roofs. In this guide, only the flat, single and double pitch roof forms are dealt with.

Fire rating: Lightweight timber trusses have low fire resistance due to their slender dimensions (boards, 25 mm thick) and cannot be used for buildings requiring a high fire rating.

Jointing system: For timber trusses, the most common jointing systems are nails and bolts. Adequate drilling equipment is required.

Transport and on-site handling: Due to their light weight, timber trusses are particularly suitable for construction in remote areas and at sites without hoisting devices. Most timber trusses can be handled manually.

4.2 Timber Quality

A method for classifying sawn timber has been given in the MCR Toolkit No. 24, p. 66. Appendix 3 of the same guide allocates common timber species to the classes specified. Sample trusses of Section 4 are valid for lowest class C with admissible bending stress of between 8 to 10 N/mm2.

Unlike the properties of most other construction materials, the properties of timber can vary considerably within a single element. Timber to be used in structural elements must be properly selected. Boards and sections with natural defects must be eliminated through visual inspection. The criteria to be applied are as follows:

Knots

The size of knots must not exceed % of the height of the element.


Figure

Through-knots are knots extending through a piece of timber. Elements with through-knots must not be used for tension members of trusses (lower chords).

Sloping grain


Figure

Pockets of decay, fungal and insect attacks are not tolerated for structural timber.

4.3 Fixings and Fastenings

4.3.1 System Options

The construction of timber trusses is governed to a large extent by the availability of suitable fastening systems that permit the jointing of truss members. In many cases, the sizes of truss members are determined by the connectors used and not by the stress in the member since connectors require a certain contact area.

An appropriately selected fastener can increase the quality, durability and the economic feasibility of a timber truss. The options available are the following:

Nails: The use of nails is the preferred option for many applications as nails are available throughout the world, come in various sizes and can be applied by simple means. Slip in nailed joints can be reduced if a large number of small diameter nails are used instead of a few large ones. Pilot-drilling improves the joint as the grains are less disturbed. For nailing in hardwood elements, pilot-drilling is indispensable.

Screws: Screws have practically identical performance to nails except for withdrawal resistance where the screw is highly superior. However, as screws are much more labour-intensive and joints in roof trusses seldom require particular withdrawal resistance, the nail joint is preferred in all standard timber trusses. Where there is a danger of joints becoming loose due to repeated swelling and shrinkage of the timber (moisture and temperature fluctuations), screws should be used instead of nails.

Bolts: Bolts can transmit relatively large loads. However, slip in bolted connections is high and bolted trusses tend to have extensive sag. This problem can be overcome if shear pins with close tolerances are used. The holes to be drilled have the same diameter as the metal pin, and slip of the joint can be considerably reduced. It is clear that for connections using tight fitting pins, the requirements on workmanship are high (precise drilling). In addition, the timber must be well seasoned otherwise shrinkage will cause the timber to split between pins. To conclude, the bolted truss is to be used only after careful consideration of its pros and cons.

Split rings: The split-ring connector consists of a steel ring that is installed by cutting matching circular grooves in the faces of the lapped pieces of wood. The joint is held together by a bolt. As this type of connector requires special cutting tools and high levels of accuracy, the split ring connector is not recommended for standard roof trusses.

Nail or tooth plate connectors: A perforated or spiked metal plate is pressed into the sides of the joined timber pieces, functioning both as a gusset plate and a connecting device. A variety of multi-nail spiked connectors are now available in many countries under specific brand names. Those connectors that can be hammered in on site without the need of a hydraulic press and are very appropriate fasteners for timber roof trusses. Aesthetics of timber trusses using nail plaits is, however, somewhat impaired.

Plywood gusset plates: As an alternative to metal connectors, plywood gusset plates can be used in combination with nails.

Glued finger or laminated joints: Glued structural joints require specific know-how and equipment and are therefore not readily applicable by general carpenters and contractors.

4.3.2 Nails

Steel quality: min. tensile strength 600 N/mm2

Nails are available as galvanised or blank wire nails. They must be galvanised if timber is treated with copper-chromium-arsenic (CCA).

Designation: Nail connections are specified as follows:

No of nails; nominal diameter (dN) × length (l)

Generally, the length of the nail should not be less than twice the thickness of the timber being secured.

Nailed joints:

A nail may be used to connect one or two elements at the same time.


Figure

single shear plane of a nailed connection:


Figure

The nail must enter the second timber at least 12 times the diameter of the nail (penetration depth S1 > or = 12 dN)

double shear plane:


Figure

The nail must enter the third timber at least 8 times the diameter of the nail (penetration depth S2 > or = 8 dN)

Nail spacing and edge distance:

Nail connections often fail as a result of inadequate distance to the edge or centre to centre distance between nails. The grains of the timber act like a hammock for the nails preventing them from tearing the edge. In order to obtain the full loading capacity of a nail connection the following spacings and edge distances must be used.

· from centre to centre

- Parallel to the wood grain: min. 10 dN
- Perpendicular to the grain: min. 5 dN

· Distance to loaded edge / direction of the force

- Parallel to the wood grain: min. 15 dN
- Perpendicular to the grain: min. 6 dN

· Distance to non-loaded edge

- Parallel to the wood grain: min. 7 dN


Figure

- Perpendicular to the grain: min. 5 dN


Figure

Admissible loads of nailed connections (load perpendicular to wood grain, softwood, class C)

Diameter (mm)

2.8

3.1

3.5

4.0

4.5

5.0

5.5

6.5

7.0

Length of nail (mm)

60

75

90

100

110

130

150

180

215

70

80

120

140

160

200

230

150

215

Minimum thickness of the timber piece

(mm)

18

20

20

25

30

40

45

60

60

20

25

25

30

35

45

50

70

70

25

45

50

60

80

Admissible load per nail

· Single shear plane (kN)

0.25

0.35

0.45

0.55

0.70

0.85

1.0

1.3

1.5

· Double shear plane (kN)

0.50

0.70

0.90

1.10

1.40

1.70

2.0

2.6

2.9

The admissible loads are valid if the edge and centre to centre distances of the nails as outlined above are observed.

4.3.3 Bolts / Pins

Steel quality: commercial bolt, strength grade 4.6 (nominal tensile strength 400 N/mm2, nominal yield stress 240 N/mm2)

Bolted connections:

For timber trusses, only tight-fitting pins must be used. It is nevertheless usual practice to fit large-size washers on each side. This not only ensures that the pin cannot become loose, it also improves the rigidity of the joint.

Note, tight-fitting bolted joints must not be used if the timber is not well seasoned as shrinking timber between two tight fitting bolts in different timber elements will cause at least one of the timbers to crack.


Figure

Admissible loads of bolted joints:

Load parallel to wood grain, single shear, softwood, class C

Bolt / pin diameter
(mm)

Minimum thickness of timber piece
(mm)

Admissible load per bolt
(kN)

12

50

2.70

16

75

4.50

20

100

6.50

24

125

9.00

Load parallel to wood grain, double shear, softwood, class C

Bolt / pin diameter
(mm)

Minimum thick-ness of timber piece
(mm)

Admissible load per bolt
(kN)

outside

middle

12

50

75

5.40

16

75

100

9.00

20

75

125

13.00

24

100

150

17.80


Figure

Load perpendicular to wood grain, single shear, softwood, class C

Bolt / pin diameter
(mm)

Minimum thickness of timber piece
(mm)

Admissible load per bolt
(kN)

12

75

1.90

16

100

2.90

20

125

4.00

24

150

5.30


Figure

Load perpendicular to wood grain, double shear, softwood, class C

Bolt / pin diameter
(mm)

Minimum thickness of timber piece
(mm)

Admissible load per bolt
(kN)

outside

middle

12

75

100

3.70

16

100

125

5.80

20

100

150

8.00

24

125

200

10.60


Figure

Values for load / grain angles between 0° and 90° may be interpolated from the above tables.

4.3.4 Nail Plates

The multi-tooth connectors that are designed for on-site installation by a hammer are Tylok and Teco-Nail-on Plates. Admissible loads of these connectors may be obtained through their official outlets.

The use of nail plates eliminates the need of multiple-piece members since joints are made by gusset plates and by lapping the members. These single-piece members are thicker than those of the composite truss members. This is a genuine advantage as the availability of thin boards (25 mm) of adequate quality is still limited in many countries.


Figure

4.3.5 Plywood gusset plates

Only structural plywood must be used as gusset plates for timber trusses. The grain direction of the face ply is important for the load bearing capacity of the plywood gusset plate. The following symbol is usually used on drawings:


Figure

A nail truss with plywood gusset plates has the advantage that only single-piece members are used. These members are thicker than those of the simple nail truss and for example ceilings can better be fixed to the bottom chord.

The sample truss presented here uses a plywood sheet of 27 mm thickness.

4.4 Timber Seasoning and Preservative Treatment

The shrinking and swelling of well-seasoned timber due to climatic variations (moisture and temperature) is usually not a problem as the resulting dimensional changes are small. Special attention must, however, be paid to the greater changes in dimensions that accompany the drying of green timber. In hardwood, tangential shrinkage may amount to 15 % during drying from green to air-dry state. If a green truss member is used in conjunction with a plywood or metal gusset plate, disaster is inevitable. The green truss member will shrink but the gusset plate will not. Either the gusset plate is damaged or the timber member is split.

The timber truss samples presented in this guide are designed for seasoned timber only. The use of green timber will not give satisfactory results.

Preservative treatment of timber may be required for timber species, which are less resistant to biological hazards.

4.5 Nail Truss

4.5.1 General

The nail truss makes a highly economic roof structure. However, it has a low fire rating and it sometimes does not fulfil the client's requirements on aesthetics when the truss remains visible. Spans of nail trusses can vary from 6m up to 20m. Larger spans are possible (30 m) but only for light loads and small distances between trusses.

The nail trusses presented in this manual require relatively thin timber boards as truss members. Cutting such boards may be difficult in some areas as no adequate saws may be available. If circular saws or even chain saws are used, the loss of material may be excessive to produce such thin boards.

4.5.2 Design

Upper and lower chords are usually composed of two parallel timber pieces with diagonals in between. The nails will then be loaded for double shear. In order to avoid buckling of the top chord members, spacers or packing plates are nailed between the two members to form a composite compression chord. The packing plate is fixed with two rows of nails at 300 mm spacing.


Figure

Note: In open buildings, the lower chords may at times also become compression members due to wind suction forces (see Section 2.4). It is essential that in such cases packing elements are introduced at the lower chord, too.

4.5.3 Details

Stiffened struts: One-piece diagonals with compression forces need to be stiffened to avoid buckling. Timber boards are nailed onto the small face of the strut and fixed with nails at 300 mm spacing.


Figure

Support details: For lightweight roofing materials, the trusses need to be secured against uplift from wind suction forces.

Trusses may be fixed to the wall plate by means of a bolt and galvanised wire. More elaborate supports include steel angle bars. In some countries, special cyclone ties and straps designed as nail-on plates are available to connect trusses to timber wall plates. Needless to say that the wall plate must itself be properly secured to the solid wall (brick, concrete, etc.) below.


Figure

4.5.4 Bracing / Lateral restraining system

Lateral restraining systems are designed to maintain the stability of the truss. Nail trusses receive a bracing system on the upper chord. The traditional bracing system consists of timber planks nailed in diagonal direction to the under side of the top chord. For roofs up to 10 m long, 4 diagonals are used. For roofs up to 20 m, 6 diagonals should be applied. Diagonals consist of a timber plank 25 × 150 mm stiffened by a lath of 50 × 50 mm fixed with nails 2.7 - 70 mm at 300 mm centres. Connections with the chord member of the truss involve six nails 2.7 - 70 mm at 25 mm centres.

More recent bracing systems consist of metal strips with pre-punched holes which allow easy fixing with nails. The strips must always come in pairs (crossed) as they are only good for tensile forces. The strips are placed onto the upper side of the top chord.


Figure

4.5.5 Manufacturing

Nail trusses are fabricated with the aid of a template. The truss is drawn to scale on a flat surface. All truss members can be cut and assembled on the basis of this template without repeating the measuring work. Each panel point requires a template that permits the marking of the nails.

4.5.6 Erecting

Installation of nail trusses can be done manually. The weight of the standard nail trusses presented in Section 4.6 is below 200 kg.

In order to arrive at a regular spacing of the trusses, a lath with the exact centre-to-centre distance needs to be prepared. This lath can be fitted together with the trusses.

Laths for temporary bracing need to be provided and fixed to the truss members with nails.

It is important that all trusses are properly aligned so that the final roof line will be level and free from waves.

4.6 Nail Truss Samples

4.6.1 Assumptions and Limits of Application

The nail truss samples given herein are valid under the following assumptions:

· Softwood, category C (see FCR / MCR Toolkit Element 24)
· Moisture content of timber: seasoned to approximately air-dry conditions.
· All structural members of the truss are protected from the weather.
· Nails: min. tensile strength 600 N/mm2
· Loads are permissible loads (not ultimate capacities)
· Loads are applied through the top chord

4.6.2 Flat roof nail truss, 6 m span

Mass of truss: 70 kg (softwood)

Volume of sawn timber: 0.12 m3 / truss (net)

System

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 2.7/70 at 300 mm centres


Figure


Panel points of flat roof, 6.0 m span

4.6.3 Flat roof nail truss, 8 m span

Mass of truss: 125 kg (softwood)

Volume of sawn timber: 0.22 m3 / truss (net)

System

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 2.7/70 at 300 mm centres


Figure


Panel points of flat roof, 8.0m span

4.6.4 Flat roof nail truss, 10 m span

Mass of truss: 160 kg (softwood)

Volume of sawn timber: 0.28 m3 / truss (net)

System

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 2.7/70 at 300 mm centres


Figure


Panel points of flat roof , 10.0m span

4.6.5 Single pitch roof nail truss, 6 m span

Mass of truss: 105 kg (softwood)

Volume of sawn timber: 0.19 m3 / truss (net)

System

Roof slope

min. 22°


max. 30°

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 2.7/70 at 300 mm centres


Figure


Panel points of single pitch roof , 6.0m span

4.6.6 Single pitch roof, 8 m span

Mass of truss: 155 kg (softwood)

Volume of sawn timber: 0.27 m3 / truss (net)

System

Roof slope

min. 22°


max. 30°

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 2.7/70 at 300 mm centres


Figure


Panel points of single pitch roof , 8.0m span - 1


Panel points of single pitch roof , 8.0m span - 2

4.6.7 Single pitch roof nail truss, 10 m span

Mass of truss: 175 kg (softwood)

Volume of sawn timber: 0.30 m3 / truss (net)

System

Roof slope

min. 22°


max. 30°

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 2.7/70 at 300 mm centres


Figure


Panel points of single pitch roof , 10.0m span - 1


Panel points of single pitch roof , 10.0m span - 2

4.6.8 Double pitch roof nail truss, 6 m span

Mass of truss: 80 kg (softwood)

Volume of sawn timber: 0.14 m3 / truss (net)

System

Roof slope

min. 22°


max. 30°

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 2.7/70 at 300 mm centres


Figure


Panel points of double pitch roof , 6.0m span

4.6.9 Double pitch roof nail truss, 8 m span

Mass of truss: 120 kg (softwood)

Volume of sawn timber: 0.21 m3 / truss (net)

Roof slope

min. 22°

max. 30°

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 2.7/70 at 300 mm centres


Figure


Panel points of double pitch roof , 8.0m span

4.6.10 Double pitch roof nail truss, 10 m span

Mass of truss: 155 kg (softwood)

Volume of sawn timber: 0.27 m3 / truss (net)

Roof slope

min. 22°

max. 30°

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 2.7/70 at 300 mm centres


Figure


Panel points of double pitch roof , 10.0m span

4.7 Nail Truss Sample with Plywood Gusset Plates

4.7.1 Assumptions and limits of application

Same as for standard nail truss samples

4.7.2 Double Pitch Roof Nail Truss with Plywood Gusset Plate, 8 m span

Distance between trusses 1.5m

Total load 1.0 KN/m2

Packing and stiffener plates to be nailed with 4/100 at 300 mm centres


Figure


Panel points of double pitch roof , 8.0m span (Timber truss with plywood gusset plates, thickness 27 mm)

4.8 Bolted Timber Truss Sample

4.8.1 Assumptions and Limits of Application

The bolted truss sample given herein is valid under the following assumptions:

· Same as for nail truss sample

· Bolts: commercial bolt, strength grade 4.6 (nominal tensile strength 400 N/mm2 nominal yield stress 240 N/mm2)

4.8.2 Double Pitch Roof, Tight Fitting Bolt Truss, Span 8.0 m

Mass of truss: 155 kg (softwood)

Volume of sawn timber: 0.27 m3 / truss (net)

Roof slope

min. 22°

max. 30°

Distance between trusses 1.5m

Total load 1.0 KN/m2


Figure


Panel points of double pitch roof , 8.0m span (Timber truss with tight-fitting bolted joints)

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