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Timber Joint Loading Using nails screws , bolts and adhesives

For an outline review of joints used in carpentry, architecture, and joinary ref to Timber Joints

Introduction..... Notation..... nails..... Screws..... Bolts..... Adhesives.....



Introduction

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

Relevant Standards..For comprehensive list of standards Wood related Standards

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






Symbols
d = the fixing item diameter.(mm)
F = Basic shear load of component as tabled (N)
Fadm = Permissible Load (N)
K = Factor related to number of nails,or loading condition
Ns = Number of shear planes sharing load
n = number of fixings
a = distance (m)
α = angle of grain (deg /rads)
A = Area (m2)
b = breadth of beam/thickness (m)
E = Modulus of Elasticity (N/m2
Emean = mean value Modulus of Elasticity (N/m2
Emin = min value Modulus of Elasticity (N/m2
G = Modulus of Rigity (N/m2 /Pa
h = depth of section (m)
i =radius of gyration (m)
I = Second Moment of Area (m4
L =Length /span/ (m)
Le =Effective Length /Effective span (m)
m = mass (kg)
n = number
λ = slenderness ratio
Q = First moment of area(m3
ρaverage = average density (kg / m3)
M = Moment (Nm)
σm.a,ll = Applied bending stress parallel to grain (N/m2)
σm.g,ll = Grade bending stress parallel to grain (N/m2)
σm.adm,ll = Permissible bending stress parallel to grain (N/m2)
Fv = Applied shear Force (N)
τm.a,ll = Applied shear stress parallel to grain (N/m2)
τm.g,ll = Grade shear stress parallel to grain (N/m2)
τm.adm,ll = Permissible shear stress parallel to grain (N/m2)
τr.a,ll = Applied rolling shear stress parallel to grain (N/m2)
τr.adm,ll = Permissible rolling shear stress parallel to grain (N/m2)
Δm = bending deflection (m)
Δs = shear deflection (m)
Δtotal = total deflection (shear + bending) (m)
Δadm = pemissible deflection (m)
σc.a,ll = Applied compressive stress parallel to grain (N/m2)
σc.g,ll = Grade compressive stress parallel to grain (N/m2)
σc.adm,ll = Permissible compressive stress parallel to grain (N/m2)
σc.a,l- = Applied compressive stress normal to grain (N/m2)
σc.g,l- = Grade compressive stress normal to grain (N/m2)
σc.adm,l- = Permissible compressive normal parallel to grain (N/m2)
σt.a,ll = Applied tensile stress parallel to grain (N/m2)
σt.g,ll = Grade tensile stress parallel to grain (N/m2)
σt.adm,ll = Permissible tensil stress parallel to grain (N/m2)

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






Nails

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

Fadm = F . K. . n . Ns.

In the standanard K = K43.K46 K48.K49.K50.

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

Nail diaSoftwoods ***
Not predrilled
Hardwoods
pre-drilled
Standard
Thickness/
Pentration
C14C16/18/22C24C30,35,40Min. PentrationD40D50
mmF (N)F (N)F (N)F (N)mmF (N)F (N)
2,7 32 249 258 274 281 22 386 427
3 36 296 306 326 335 24 465 515
3,4 41 364 377 400 412 27 582 644
3,8 46 438 453 481 495 30 709 785
4,2 50 516 534 567 583 34 897 939
4,6 55 600 620 659 678 37 996 1103
5 60 689 712 756 778 40 1155 1279
5,5 66 806 833 885 910 44 1368 1515
6 72 930 962 1022 1051 48 1595 1767
7 84 1200 1240 1318 1355 56 2094 2319
8 96 1495 1546 1643 1689 64 2649 2933

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







Screws

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

Fadm = F . K. . n . Ns.

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

Screw shank dia Penetration Softwoods
pre-drilled
Hardwoods
pre-drilled
Headside Pointside C14 C16/18/22 C24 C27/30/35/40 D30/35/40 D40/60/70
mm mm N N N N N N
3 11 21 193 206 233 243 308 355
3,5 12 25 264 282 306 317 399 459
4 14 28 338 361 392 406 515 594
4,5 16 32 432 453 488 506 643 744
5 18 35 524 550 594 616 785 910
5,5 19 39 621 647 698 724 921 1065
6 21 42 729 760 821 851 1086 1259
7 25 49 968 1010 1092 1133 1454 1690
8 28 56 1094 1170 1309 1359 1760 2056





Bolts

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 Ns shear planes

Fadm = F . K. n . Ns.

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

Timber Grade Penetration Loading Parallel to grainLoading perpendicular to grain
mm kN kN kN kN kN kN kN kN
Min Thickness M8 M12 M16 M20 M8 M12 M16 M20
C16 47 1,22 1,80 2,30 2,73 1,13 1,56 1,91 2,19
  72 1,46 2,68 3,52 4,19 1,39 2,39 2,93 3,36
  97 1,46 3,13 4,63 5,64 1,39 2,79 3,94 4,52
C24 47 1,33 2,04 2,59 3,09 1,23 1,76 2,16 2,47
  72 1,55 2,93 3,97 4,73 1,47 2,64 3,30 3,79
  97 1,55 3,42 5,05 6,37 1,47 3,07 4,43 5,11
D40 47 1,83 3,08 3,92 4,67 1,83 3,08 3,92 4,67
  72 1,91 4,02 5,98 7,16 1,91 4,02 5,98 7,16
  97 1,91 4,21 6,93 9,32 1,91 4,21 6,93 9,32
D50 47 2,12 3,78 4,81 5,73 2,12 3,78 4,81 5,73
  72 2,12 4,66 6,92 8,78 2,12 4,66 6,92 8,78
  97 2,12 4,66 8,09 10,82 2,12 4,66 8,09 10,82

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






Adhesive joints

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.

Intended UseExposure ConditionsTypical exposureAdhesive type
ExteriorHigh HazardFull weather exposure with exposed gluelines
Oly Laminated timbers should be used for this application
Phenolic/ aminoplastic adhesives
satisfying spec'n of type I
in accordance with BS EN 301
ExteriorLow HazardProtected from weather and roofs of non important structuresPhenolic/ aminoplastic adhesives
satisfying spec'n of type I or type II
in accordance with BS EN 301
InteriorHigh HazardDamp locations, unventilated
loft spaces, chemical works Laundries
Phenolic/ aminoplastic adhesives
satisfying spec'n of type I
in accordance with BS EN 301
InteriorLow hazardHeated and ventilated areasPhenolic/ aminoplastic adhesives
satisfying spec'n of type I or II
in accordance with BS EN 301


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 Fadm.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.





Links Providing relevant information
  1. Wood Handbook -- Wood as an Engineering Material..Downloads ..Comprehensive Document (American ) Excellent
  2. Wood Guide.. Friends of the Earth Guide - Including very useful information
  3. Effect of Heat treatment on Spruce.. Informative report
  4. Canadian Wood Council ..Excellent site on Wood Engineering - my words
  5. Timber Trade Federation ..The Timber Trade Federation is the official voice of the UK timber trade.
  6. PanelGuide ..An extremely informative document on plywood.

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Last Updated 8/05/2009