The notes below are basic guidance notes showing the spacings and permissible loads for nails,woodscrews,bolts and adhesives whan fastening timber beams together.     The information is generally based on data included in BS 5268-2:2002

The tables below show only a small part of the comprehensive data provided in the identified standard.    The information is intended to indicate the level of permissible stresses which may be considered for type of connections shown. For detailed design of timber structures it is important that the relevant standards and codes are used.

Notes relating the strength grade to the wood species are found on webpage Timber Design

BS 5268 -2 ;2002 Structural use of timber � Part 2: Code of practice for permissible stress design, materials and workmanship

BS EN 1912:2004: Structural timber Strength classes- Assignment of visual grades and species

The notes below are basic guidance notes showing the spacings for nails,woodscrewsa and bolts whan fastening timber beams together.

The values identified in the table below relate to nails made from wire driven at right angles to the grain.    For the loads given here for nails to be valid, the steel wire from which the nails are produced should have a minimum ultimate tensile strength of 600 N/mm²     In hardwood the holes normally need to be pre-drilled with diameter not bigger than 0,8 d.

Figure showing minimum Nail spacing

Note: The spacings between nails as shown can be reduced for all softwoods, except Douglas fir, by 0,8.    The edge spacings shown must be at least 5d.

The permissible loads for nail joints in service class 1 and 2 joints are derived from the equation

F_adm = F . K. . n . N_s.

In the standanard K = K₄₃.K₄₆ K₄₈.K₄₉.K_50.

K₄₃ = 0.7 for nails driven into the end grain otherwise = 1,00.
K₄₆ = 1,25 for nails driven fastening a predrilled steel component to a timber member the hole in the steel member should not be greater than d.
K₄₈ = 1,00 for long term loading for timber to timber joints, 1,12 for medium term loads, 1,25 for short/very short term loading
          {1.40 for particleboard-to-timber joints: medium-term loads & 2,1 for particleboard-to-timber / OSB-to-timber" joints: short/ very short term loading}
K₄₉ = 1,00 in service classes 1 & 2 and 0,70 for service class 3
K₅₀ = 1,00 when n _l =< 10 and = 0,9 when n _l > 10......n _l = number of nails in any line of nails parallel to applied load

For softwoods where the thicknesses of members or nail penetrations are less than the standard values given in the table below, the basic load should be multiplied by the smaller of the two ratios: (see tables below)

a) actual to standard thickness of headside member;(see tables below) or
b) actual penetration to standard pointside thickness.(see tables below)

If either ratio is less than 0,66 then the joint is assumed to have no load carrying capacity

Table of basic shear loads (F) for nails in timber to timber joints

*** For predrilled holes in softwood the values in table may be multiplied by 1,15 .

Screws should be turned, not hammered, into pre-drilled holes.    The tops of countersunk screws should be no more than 1 mm below the surface of the timber.

The values provided for the screws are for screws in accordance with BS 1210 in predrilled holes. The holes should be drilled with the screw shank dia. for the part of the hole which relates to the screw shank, reducing to a pilot hole dia = d/2 for the threaded portion of the screw.

The effective cross-section of timber taking a load be determined by deducting the net projected area of the pre-drilled holes from the gross area of the cross-section being considered.     When assessing the effective cross-section of multiple screw joints, all screws that lie within a distance of five screw diameters measured parallel to the grain from a given cross-section should be considered as occurring at that cross-section.

The permissible loads for nail joints in service for class 1 and 2 joints are derived from the equation

F_adm = F . K. . n . N_s.

In the standard K = K₄₃.K₄₆ K₅₂.K₅₃.K₅₄

K₄₃ = 1,0 for screws normal to grain direction/ = 0.7 for screws screwed into the end grain.
K₄₆ = 1,0 for timber ot timber joints/ 1,25 for screws fastening a predrilled steel component to a timber member the hole in the steel member should not be greater than d.
K₅₂ = 1,00 for long term loading for timber to timber joints:1,12 for medium term loading:1,25 for short term loading
K₅₃ = 1,00 inservice classes 1 & 2 and 0,70 for service class 3
K₅₄ = 1,00 when n _l =< 10 and = 0,9 when n _l > 10......n _l = number of nails in any line of nails parallel to applied load

For the basic loads the table to apply, the headside member thickness and the penetration of the screw in the pointside should be not less than the values given.
Where the thickness of the headside member is less than the value given in the table, the tabulated basic load should be multiplied by the ratio of the actual to the standard headside thickness, provided that the pointside screw penetration is at least twice the actual headside thickness.    The minimum headside member thickness should be not less than twice the shank diameter.

Figure showing minimum screw spacing

Table of basic shear loads (F) for screws in timber to timber joints

The values for bolted joints relate to BS EN ISO 898-1 having a minimum tensile strength of 400 N/mm² with washers which conform to BS 4320 .    Bolt holes should not be drilled more than 2mm larger than the nominal bolt diameter.    Washers should have a diameter or width of three times the bolt diameter with a thickness of 0,25 times the bolt diameter and should be fitted under the head and nut of each bolt unless a steel plate is used to provide an equivalent bearing area.  At least one complete thread should protude above the tightened nut.

The effective cross-section of the timber taking loads should be determined by deducting the net projected area of the bolt holes from the gross area of the timber cross-section.    All bolts that lie within a distance of two bolt diameters, measured parallel to the grain, from a given cross-section should be considered as occurring at that cross-section.

The permissible load for a bolted joint for N_s shear planes

F_adm = F . K. n . N_s.

In the standard K =K₄₆. K₅₆.K₅₇

K₄₆ = 1,00 for normal timber to timber joints/ =1,25 Where a steel component is bolted to a timber member loaded parallel to the grain.
K₅₆ = 1,00 in service classes 1 & 2 / = 0,70 for service class 3 timber used in that service
K₅₇ = 1 - 3( n _l - 1)/100 when n _l =< 10 and = 0,9 when n _l > 10....n _l = number of in-line bolts in any line parallel to applied loads.

Figure showing minimum Bolt/Dowel spacing

Table showing single shear loads for one grade 4,6 bolt in a two member timber connection
Important Note: The table below shows only a selected number of bolt diameters subject to long term loading

The above show load which act perpendicular to the axis of the bolt, and parallel or perpendicular to the grain of the timber

The adhesive used should be appropriate to the environment in which the joint will be used. the table below identifies some adhesives which are acceptable.

If the timber is correctly specified and the manufacturing process is to an acceptable standard then the adhesive is always stronger than the timber used and the strength of the joint is based on the weaker of the timber joint components being glued.     The load duration factors and the load sharing factors apply in determining the acceptable strength of the joint.

It is important to note that adhesive joints are primarily shear loaded joints.    Tensile components of stress perpendicular to the plane of the glueline are not acceptable.

The relevant section in BS 5278 part 2 is limited to the type of structural joints as shown below

When considering adhesive joints in timber the risk resulting from shrinkage, distortion and stress concentration at the joint should be considered.

The glued faces are normally held together until the glue sets and surface force of about 0,7 N/mm is recommended for softwoods     If mechanical fasteners are used to hold the glued faces together these should not be considered to add strength to the bonded joint.    When nails are used to hold the glued faces together the permissible shear stength used in the calculations should be multiplied by a nail/glue modification factor K₇₀ = 0,9

A typical glued timber joint withstanding a force F is shown below.

.

When the components of the joint are loaded parallel to the grain, the permissible shear stress fot the bonded lap joints should be taken as the lesser of the permissible shear stresses parallel to the grain.    When one of the faces is loaded at an angle to the relevant grain α, the permissible shear stress τ _adm,a for the glueline should be calculated using the following equation.

When one of the faces is loaded across the grain the the permissible stress is 1/3 of the permissible grade shear stress parallel to the grain.    In this circumstance the shear is referred to as rolling shear because there is a tendency for the fibres to roll like a rolling cylinders at alower stress level

For the loaded lap joint system as shown above the permissible force F_adm.a =

..

( ref to Page Modifying factors )

This equation is based on forces parallel to the grain and the surfaces being clamped to achieve adhesive bonding. If nails are used to hold surfaces together to achieve good adhesive bonding then the equation below should be used.