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Glued Laminated Timber



Introduction

Glued laminated timber is often called Glulam and is manufactured as timber laminations bonded together using adhesive.   Glulam can be made to almost any size and used for a wide variety of structural features including arches, portals, roof, lintel and floor beams, for columns, rafters and 'A' frames etc etc   Glulam structural members can make use of smaller and less desirable dimensions of timber, yet are engineered to be stronger than similarly sized members made of solid wood.    They also suffer less than solid timbers from defects and movement due to moisture changes such as checking, warping and twisting.    The laminations used are also generally of a more uniform quality compared to solid timber sections.   

Compared to most structural materials, Glulam has a relatively high strength per unit weight.    It can be used to produce a lighter superstructure, compared to steel or concrete, with a consequent economy in foundation construction.    Glulam will not corrode. It also has a high resistance to chemical attack and aggressive and polluted environments.   

Glulam is manufactured in accordance with BS En 14080.    The timber used is graded in accordance with BS EN 14081.    The various parameters required to design using this material are covered in BS 5268-2 2002 section 3


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

BS EN 14080:2005 Timber structures. Glued laminated timber. Requirements
BS EN 14081-1:2005 Timber structures. Strength graded structural timber with rectangular cross section. General requirements
BS 5268 -2 ;2002 Structural use of timber Part 2: Code of practice for permissible stress design, materials and workmanship



Horizontally glued laminates

This type of laminate is designed to resist loads normal to the plane of the laminates.    The grades stress for horizontally glued timber laminations is obtained as the product of the relevent stresses found in tables and the relevant modification factors found in the table. Modification factors for horizontally glued single grade laminates

It should be noted that the modification factors K7, and K14 apply to the size of the glulam section and not to the timber lamination size.



Horizontal Timber Glued Laminate - all one grade

The above rule cover only applies when laminations of one strength grade only used.    It is acceptable to use laminations of different grades when the grades are up to two grades apart and when a maximum of 25% of the depth at the top and bottom of the section are of the superior grade.    When timbers of different grades are use the calculated grades stresses relevant to bending tension and compression, parallel to the grain, should be multiplied by 0,95.    The calculated grade stress for shear parallel to the grain should be multiplied by 0,8



Horizontal Timber Glued Laminate - combined grades

Summerising the derivation of the relevant permissible stresses for the glued laminate sections..

Bending parallel to grain
For single grade glulam .. σm.adm,ll = σm,g,ll. K2.K3.K6.K7.K8.K15

For combined grade glulam .... σm.adm,ll = σm,g,ll. K2.K3.K6.K7.K8.K150,95



Tension parallel to grain
For single grade glulam .. σt.adm,ll = σt,g,ll. K2.K3.K8.K14.K16

For combined grade glulam .... σt.adm,ll = σt.,g,ll. K2.K3.K8.K.K160,95



Compression parallel to grain
For single grade glulam .. σc.adm,ll = σc,g,ll. K2.K3.K8.K12.K17

For combined grade glulam .... σc.adm,ll = σc,g,ll. K2.K3.K8.K12.K170,95



Compression normal to grain
For single and combined grade glulam .. σc.adm,l- = σc,g,l-. K2.K3.K4.K8.K18



Shear parallel to grain
For single grade glulam .. τt.adm,ll = τt,g,ll. K2.K3.K5K8.K19

For combined grade glulam .... τt.adm,ll = τt,g,ll. K2.K3.K5K8.K19.0,80



Modulus of Elasticity
For single and combined grade glulam ..EGlulam = Emean. K20




Vertically glued laminates

This type of laminate is designed to resist loads parallel to the plane of the laminates.    The grades stress for horizontally glued timber laminations is obtained as the product of the relevent stresses found in tables and the relevant modification factors found in the table. Modification factors for vertically glued single grade laminates

It should be noted that the modification factors K7, and K14 apply to the size of the glulam section and not to the timber lamination size.

Summerising the derivation of the relevant permissible stresses for the glued laminate sections..

Bending parallel to grain
σm.adm,ll = σm,g,ll. K2.K3.K6.K7.K8.K27



Tension parallel to grain
σt.adm,ll = σt,g,ll. K2.K3.K8.K14.K27



Shear parallel to grain
τt.adm,ll = τt,g,ll. K2.K3.K5K8.K27



Compression parallel to grain
σc.adm,ll = σc,g,ll. K2.K3.K8.K12.K28



Compression normal to grain
σc.adm,l- = σc,g,l-. K2.K3.K4.K8.K29



Modulus of Elasticity
.EGlulam = Emean




Glued end joints relevant to glued laminated timber

According to BS 5268-2, finger joints, used to produce longer gluded laminated beam lengths from shorter lengths, should have characteristic bending strengths of not less than the characteristic bending strength of the strength class for the lamination when tested in accordance with BS EN 385.

Note : As identified on webpage Timber design the characteristic bending strength is the ultimate strength of the timber prior to inclusion of the various safety margins and is identified as the number in the grading system e.g a c14 strength graded timber has a characteristic bending strenth of 14 N/mm2

Links Providing relevant Links
  1. moelven..Supplier website with lots of useful information
  2. Glulsmbeams co.uk/Specificationsb....Supplier website with lots of useful information
  3. Wood Handbook -- Wood as an Engineering Material..Downloads ..Comprehensive Document (American ) Excellent

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Last Updated 8/05/2009