CONNECTIONS IN TIMBER STRUCTURES

1. Introduction

The competitiveness of a timber structure, relative to other building materials, may be determined by the
efficiency of the connections. In most cases, the fastening of timber to timber requires little skill or
knowledge of design. Consider the widespread use of nails in domestic situations where the handyman
routinely uses nails and bolts to construct all manner of timber structures.

In heavy construction, joints may require ingenuity and the use of specialized connectors, such as nail
plates, bolts, shear plates, split rings, coach screws or glued-in threaded rods. The application of these
requires some knowledge of design and construction skills.

The shrinkage and swelling characteristics of timber in response to drying and wetting, the possibility of
fungal decay in the presence of moisture and the need to protect metallic fasteners from fire or corrosion,
call for special construction detailing.

2. Factors affecting the detailing of connections.

Changes in moisture.

Changes in the moisture content of the timber will cause the timber to swell and shrink. The dimensional
changes in the direction parallel to the grain can be ignored in most cases. The dimensional change in the
perpendicular-to-grain direction can be large, especially if the moisture content variation is large. This
must be borne in mind when a horizontal timber member is connected to a vertical timber or steel member.
If the connectors prevent shrinkage, splitting of the timber may occur. This type of splitting often occurs
when treated timber, which generally still has a high moisture content, is bolted to uprights. Figure 1 shows
how the timber may split when movement is prevented. The splitting of the timber at the support may
reflect negatively on the shear strength of the member.

Figure 1: Splitting of timber as a result of differential shrinkage.

In the case of the connection in Figure 1, it would have been preferable to install only one larger
connector. Perpendicular-to-grain tensile strength
The drinking straw analogy for timber works well when one is designing connections. Remember that the
adhesive sticking the straws together is weak. Any connection, which tends to cleave the wood, will of
necessity be weak. Figure 2 illustrates the loads that can cause cleavage as a result of tensile loads
perpendicular to the grain. If this type of connection cannot be avoided, it is always good policy to move
the bolt down as far as possible.

Figure 2: Loading of members in tension. Cracking may occur when the end distance is insufficient.

Cleavage often occurs in trusses where one of the chords, i.e., top compression member or bottom tie, has
to transfer the loads between the web members and the web members are some distance apart. Figure 3
shows such a cleaving action.

Figure 3: Cleaving of the member as a result of tension perpendicular to the grain.

Shear strength
The horizontal shear strength of timber is low, typically one-tenth of the bending strength. This can cause
problems when there is an eccentricity between the loaded point and the support. This is aggravated when
the loaded point has damaged the supporting member, by, for instance, a bolt hole. The effective shear
transfer area is greatly reduced at the bolt hole. Figure 4 shows an eccentrically loaded support for a truss.
Note that the bolt hole is in an area of large shear as well as bending stress. The high stresses at these
supports must be borne in mind when designing the truss.
 Figure 4: Shear force and bending in member as a result of an eccentric connection and support.

3. Selection of Fasteners.

Mechanical fasteners

A mechanical fastener is any device, metallic, plastic or timber, which transfers load from one piece of
timber to another piece of timber. The most common types of fasteners are metallic and include:

• Nails
• Dowels
• Screws
• Bolts
• Coach-screws
• Toothed ring connectors
• Split rings
• Nail plates
• Proprietary or patented fasteners.

Most fasteners transfer forces through bearing on the timber and shear in the connector. Screws may under
certain circumstances be used in withdrawal, although end grain withdrawal is not recommended.

Fastener strength

The strength of the various fasteners, together with end and edge distances is given in SABS 0163. SABS
0163 does not give strength values for proprietary or patented fasteners. Strength values for these must be
obtained from the manufacturer’s literature.
Structural efficiency

Structural efficiency can be described as the load that can be transferred divided by the area required by the
connectors. It can be shown that nails or dowels into pre-drilled holes through steel plates are the most
efficient connectors. These are followed by bolts, toothed-ring connectors and split rings. Nails in pre-
drilled holes are in the region of twice as efficient as any of the other connectors. In terms of cost
efficiency, bolts may be cheaper than nails. The choice of the connector will depend on the available space
for the connection and the esthetics.

End-grain connectors, where the load transfer is through direct tension, are the most efficient connectors.
The shorter the load path can be made, the more efficient the connector becomes. Glued-in threaded rods
may be used to obtain very efficient connections.

Figure 5: Bolted connection showing the convoluted load path for the transfer of the forces.

Figure 6: Shortened load path when glued-in rods are used.

4. Designing for durability

Irrespective of the fastener type, a joint should be designed and constructed for durability. The durability
of timber structures is influenced by several factors.
Fungal decay
Decay or fungal attack is the result of the action of fungi, which break down the chemical structure
of timber if suitable conditions prevail. A combination of the following circumstances creates such
suitable conditions:
  The moisture content must be higher than 20%. It is unusual for this to happen except where timber
is exposed to rain, timber is in direct contact with trapped water or placed directly in the ground.
Fungal attacks may also occur in the following: inadequately ventilated swimming pool structures,
bathrooms, laundries, under-floor areas, saunas, cooling towers as well as bridge and pier
structures close to the water.
 Oxygen must be present even in small quantities. Timber will not decay if permanently immersed in
water.
 The temperature must be in the range of 5ºC to 40ºC. Above and below this temperature
range, decay virtually ceases. The optimal temperature range for fungal growth is 25ºC to
35ºC.
 The timber on which the fungal lives must be either naturally or chemically unprotected.

End-grain is especially susceptible to the ingress of moisture and this is where decay usually begins.

Other Timber Hazards

Timber that is exposed to high levels of ultraviolet radiation, rain and extremes of temperature can suffer
from splits, cracks and discolouration. It can also be subject to insect attacks and marine borers. These
factors do not affect the calculations of the design but must be borne in mind when the connection is being
detailed.

Joint Detailing Principles

To achieve good joint design and structural detailing, the following general principles should be observed:

 Avoid connections that can trap moisture. Ensure proper drainage and ventilation, especially the end
grain.
 Avoid exposing unprotected timber to the weather. If capping is used, ensure that all the moisture
can escape and that the capping is properly ventilated. Capping that leaks and is not ventilated will
hasten the onset of and promote fungal decay.
 Avoid placing especially the end-grain of timber in direct contact with concrete. Concrete is
hygroscopic and will increase the moisture content on the interface between the concrete and the
timber. If possible leave an air gap between the timber and the concrete. If the timber cannot be
supported away from the concrete, insert a steel plate between the timber and the concrete. The steel
plate will act as a moisture barrier.

 If moisture can enter bolt holes, treat the timber in the hole with a preservative that does not leach
out. If leaching is a problem, the bolts can be covered with grease or a silicon sealant.
  Use chemically treated timber where moisture ingress could be a problem. Remember that CCA
treatment stops fungal decay but not swelling and shrinkage due to moisture ingress. Always treat
the timber with an additional water-repellant.
 Corrosion-resistant fasteners should be used in salt-water or seaside environments. Corrosion
resistance in ascending order: steel, aluminium, stainless steel, copper and copper alloys.
 Where possible, transfer forces through the direct bearing, thereby shortening the load path.
In coastal areas, large-diameter bolts may be used, where a certain percentage of the area is sacrificial and
the bolt maintains enough strength after corrosion has taken place. Bituminous or epoxy coating can
improve the performance of bolts. Hot-dipped or electro-plated zinc-coated bolts may be used in structures
where a high chemical hazard exists.

6. Some Practical Joint Details

Figure 7: A method of avoiding splitting as a result of large tension perpendicular to the grain.

Figure 8: Air gap to prevent ingress of moisture into the end-grain of the column.
 Figure 9: Transfer of loading through bearing pads.

7. Fire Resistance

Large cross-sectional timber members are fairly fire resistant, but exposed metal connectors are not as
they lose strength fairly rapidly at elevated temperatures. They also conduct heat into the interior of the
timber, where the timber then chars. SABS 0163 provides a basis for assessing the fire resistance of the
timber section. It does not provide any guidance on the strength of metal connectors at elevated
temperatures

Where a fire rating is required, the metal connectors may be protected by an intumescent paint or by
embedding the connector in the timber. Non-conducting fibre bolts or dowels may be considered.

Figure 10: Protection of metal connection to achieve the required fire rating.
Design Check List

The following may be used as a checklist when considering the design of joints:
o Is the connection detail simple?
o Have I avoided shrinkage restraint of wet timber?
o Have I selected the connector type according to:
Structural Requirement?
o Cost efficiency?
o Practical application?
o Is corrosion protection required?
o Are fastener spacing and end distances maintained?
o Will service moisture content exceed 20% and if so has adequate provision been made for
fungal protection?
o Have moisture traps been avoided and has adequate space been allowed for ventilation?
o Has the end grain been protected against moisture ingress?
o Is it necessary to apply preservative treatment to the timber and/or the bolt holes of the
connection?
o Must fire protection be applied to the connectors?

7. Design of Connections

Nails dowels and bolts

The failure mode of a connector determines the strength of the joint. By changing the failure mode of the
metal connector, the size and strength of the connection can be vastly improved. Figure 11 illustrates the
possible failure modes of dowel-type connectors.
 Figure 11: Possible failure mechanisms for nails, dowels and bolts.

The most likely failure mechanism for a nailed connection is a failure (f) while for a large diameter bolt or
dowel, (c). The strength values, for nails and bolts, given in SABS 0163 are based on the failure
mechanisms (f) and (c) respectively. The failure mechanisms with the greatest strengths are (a) or (b). If
one can force this type of failure by ensuring the crushing of the timber, then the smallest, strongest
connection will result. This type of failure mechanism can be induced if a stiff connector is used together
with metal side plates, a fastener in double shear. The possible failure mechanisms for fasteners in double
shear are shown in Figure 12.

Figure 12: Failure on connections and connectors in double shear.

Failure type (b) would be the mechanism that is induced if the connector is very stiff, i.e., large-diameter
dowels.

Some propriety connectors use the metal to its full capacity. The strength of the connectors is based on
test results and may be substantially higher than when the strength values in SABS 0163 are used.

For detailed calculations of connection strength, the SALMA Timber manual should be consulted.
A few typical connections will be given in this chapter, to illustrate good detailing practice.

9. Column Base Details

The following details illustrate how the timber is kept away from possible moisture ingress into the end
grain.

Figure 13: Column base with two angle brackets, force transfer through bearing on the pad.

Figure 14: Column base with cruciform bracket slotted into the timber. Force transfer through direct
bearing on a metal plate.
 Figure 15: Column base with anchor bolts glued into timber column. Force transfer through direct
bearing on a metal plate.

10. Arch Bases

Arches can be either tied or the base can transfer the horizontal thrust into a concrete base that is
designed to resist the horizontal forces. The following sketches show the two types of bases.

Figure 16: Arch or portal frame base plate with the tie rod. Tie rods can be covered by an elevated
timber floor.

Figure 17: Pinned base for an arched structure.

Beam-to-column connections

Beam-to-column connections are generally not moment-resisting. A few details are given in the
following sketches.

Figure 18: Beam-to-column detail where the beams are pitched.

Figure 19: Beam to steel column detail.

Figure 20: Beam-to-column connection where the column is continuous past the beam level.
 Figure 21: Beam-to-column connection, where the connectors have been recessed and are hidden.

11. Beam-to-wall or concrete connections.

The following details show how timber beams can be connected to brickwork and concrete walls and
beams.

Figure 22: Beam to concrete connection. A channel bracket may be used to improve lateral torsional
restraint at the support.

Figure 23: Connection for sloped beam.

 Figure 24: Beam built into a brick wall with wedged blocking to improve torsional restraint.

12 Beam-to-beam connections

Beam-to-beam connections can be either exposed or hidden. It is important to avoid loading perpendicular
to the grain if at all possible. The beam that is doing the loading should load the bracket in bearing if
possible and the loaded beam should have the connectors as high as possible.

Figure 25: Force transfer in direct bearing and through coach screws or bolts. Note that rotation of the
loaded beam is prevented.

Figure 26: Harness overloaded beam, all loads transferred by direct bearing.
 Figure 27: Timber beam to concrete beam or wall. It is preferable to transfer the load indirect bearing.

a. Beam or column splices.

The following sketches may be used as guidelines for good splicing practice.

Figure 28: Splicing of the member that must transfer bending moment and shear.

Figure 29: Splicing of beams when only shear force must be transferred between the right-hand side and
the left.
 Figure 30: Splicing of members with glued-in threaded rods and end plates. Note that this method
should not be used where large moisture content variation is expected.

b. Large-span truss connections.

The following are a few illustrations of large-span truss connections that have been used very
successfully in other countries.

Figure 31: Metal plates slotted into the timber members. Connection completed by using metal
dowels in tight-fitting holes, or bolt

Figure 32: External plate connection for members with limited width.
Figure 33: Spaced chords where, where the resultant forces lie in the same plane. Large-diameter bolts
or shear plates and split rings may be used.

c. Apex joints for portal frames.

Apex joints in portal frames may be pinned or may have to transfer limited moments. In all cases, the
apex joint must be able to transfer shear forces. The following sketches show a few possible details.

Figure 34: Hinged apex connection for a portal frame.

Figure 35: Pinned connection, using a shear plate or split ring to transfer the shear.
Figure 36: Moment connection at apex affected by nailing oval nails (glulam rivets) through round
holes. The distortion of the nails ensures that the nails cannot be easily extracted.

Figure 37: Moment connection using threaded rods and steel end plates. Rods can be threaded into
the steel plates and then bent, or simply welded onto the end plates. The connection can be
assembled on-site.
Figure 38: Compression ring for the apex of the dome-type structure. Note that a certain amount of
moment transfer is possible.

Figure 39: Eaves joint for the portal frame. Steel bracket makes it possible to assemble the joint on-site.
Threaded rods must be glued under factory conditions.
 Figure 40: Eaves joint using plywood on either side and nailing.

Figure 41: Eaves joint manufactured by using large finger joints.