Chapter 11Glued Laminated Construction

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 648

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11.1 Developments in the field of synthetic adhesive have brought gluing techniques within the range of engineering practice. Timber members of larger cross-sections and long lengths can be fabricated from small

sized planks by the process of glulam. The term glued laminated timber construction as applied to structural members refers to various laminations glued together, either in straight or curved form, having grain of all laminations essentially parallel to the length of the member.

PART 6 STRUCTURAL DESIGN — SECTION 3 TIMBER AND BAMBOO: 3A TIMBER

11.1.1 Choice of Glue

The adhesive used for glued laminated assembly are ‘gap filling’ type. A ‘filler’ in powder form is introduced in the adhesive. Structural adhesives are supplied either in powder form to which water is added or in resin form to which a hardener or catalyst is added. For choice of glues, reference may be made to accepted standard $[6-3A(11)]$ . However, it is important that only boiling water proof (BWP) grade adhesives shall be used for fabrication of glulam in tropical, high humid climates like India.

11.1.2 Manufacturing Schedule

In absence of a systematic flow-line in a factory, provisions of intermediate technology shall be created

for manufacturing structural elements. The schedule involves the following steps:

a) Drying of planks;

b) Planing;

c) End-jointing by scarfs or fingers;

d) Machining of laminations;

e) Setting up dry assembly of structural unit;

f) Application of glue;

g) Assembly and pressing the laminations;

h) Curing the glue lines, as specified; and

j) Finishing, protection and storage.

11.1.3 The information with regard to block shear test results on glued timber joints is given in Table 18.

Table 18 Block Shear Test Results on Glued Timber Joints(Clause 11.1.3)
SI No.	Timber Species	Ultimate Shear Stress(Inside Location) $N/mm^2$
SI No.	Timber Species	Solid Timber Along the Grain (Un-Jointed)(3)	With Urea Formaldehyde(4)	With Phenol Formaldehyde(5)
(1)	(2)	Solid Timber Along the Grain (Un-Jointed)(3)	With Urea Formaldehyde(4)	With Phenol Formaldehyde(5)
i)	Abies pindrow (Fir)	5.984	7.652	9.614
ii)	Acacia catechu (Khair)	15.107	7.456	—
iii)	Acacia milotica (Babul)	14.126	5.199	3.237
iv)	Albizia Odoratissima (Karasiris)	15.009	14.126	5.984
v)	Azadirachta indica (Neem)	12.557	7.259	8.240

11.2 Design of Glued Laminated Beams

11.2.1 General

Glued laminated structural members shall be fabricated only where there are adequate facilities for accurate sizing and surfacing of planks, uniform application of glue, prompt assembly, and application of adequate pressure and prescribed temperature for setting and curing of the glue. Design and fabrication shall be in accordance with established engineering principles and good practice. A glued laminated beam is a straight member made from a number of laminations assembled both ways either horizontally or vertically (see Fig. 13). While vertical laminations have limitations in restricting the cross-section of a beam by width of the plank, horizontally laminated section offers wider scope to the designer in creating even the curved members. Simple straight beams and joists are used for many structures from small domestic rafters or ridges to the light industrial structures.

FIG. 13 POSSIBLE ORIENTATION OF PLANKS IN GLULAM

11.2.2 Design

The design of glue laminated wood elements shall be in accordance with good engineering practice and shall take into consideration the species and grade of timber used, presence of defects, location of end joints in laminations, depth of beams and moisture contents expected while in service. Beams of large spans shall be designed with a suitable camber to assist in achieving the most cost effective section where deflection governs the design. The strength and stiffness of laminated beams is often governed by the quality of outer laminations. Glued laminated beams can be tapered to follow specific roof slopes across a building and/or to commensurate with the varying bending moments. Glulam allows the engineers to mix grades within a beam. Obviously, the lower tensile portion should be of high grade material. However, it is possible to incorporate a considerable volume of lower grade material near the neutral axis taking care of the horizontal shear stress. Similarly, species can also be mixed depending upon the modulus of elasticity.

Straight beams that will be simply supported should be cambered during fabrication to allow for deflection due to dead loads over a period of time. The usual camber

is $\frac{\text{Span}}{400}$ , unless otherwise specified.

NOTE — The information given in the above table is based on the block shear test results on glued timber joints conducted on the basis of BS 4169:1970 ‘Specification for manufacture of glued-laminated timber structural members’ (since revised and subsequently superseded by BS EN 392:1995, BS EN 391:1995, BS EN 386:1995), and the failures observed with regard to glue line and the wood. Ultimate shear stresses for 29 species were studied keeping design glue shear strength at least one-third of ultimate. Glue mixing, spreading, pressing and curing was done as per the instructions of adhesive manufacturers under improvised techniques locally developed. Sustainable period before delamination of glue bond (urea formaldehyde) in weathering tests was recorded for over a year. Limited number of species survived with most of them failed within a period of six months

400Span

11.2.2.1 Design outline for horizontally laminated beams

A design outline for horizontal laminated beams, specifying data required and a possible solution is given below.

Data required:

a) Span $(L)$ , loads including self weight of beam $(W)$ , species of timber, thickness of lamination proposed; and

b) Mechanical properties of timber species (permissible extreme fibre stress in bending-tension, permissible compressive stress perpendicular to grain, modulus of elasticity, permissible shear stress parallel to grain).

The permissible shear stress at the glue line is at least the same as the permissible shear stress parallel to grain of the species in design, unless otherwise tested.

Solution:

a) Let, $b =$ width of beam, and $d =$ depth of beam, for lateral stability, $b : d = 1 : 4$ (say).

b) Determine bending moment on the basis of span, loading and support conditions (say, WL/8 for beams with both ends fixed).

c) Also, Bending moment $(M) =$ Bending stress $(f_{\mathrm{b}})\times$ Section modulus $(Z)$ .

Therefore,

M = f _ {\mathrm{b}} \times \frac {b d ^ {2}}{6} = \frac {W L}{8}

\text {or} b d ^ {2} = \frac {W L}{8} \times \frac {6}{f _ {\mathrm{b}}}

d) From $bd^2$ obtained at (c) and $b:d$ assumed at (a), arrive at appropriate values of $b$ and $d$ .

e) Check for deflection of beam. Since dead load is for long term, double this for deflection check, that is,

W = (2 \times \text {Dead load}) + \text {Imposed load}

Deflection is to be limited to the specified requirement, say 0.003 of span.

Substituting values of $b$ and $d$ from (d) and $I$ ( $= bd^3 / 12$ ) in the formula for deflection, perform check against the limit for deflection.

Decide the number and thickness of laminations for the required depth of section.

PART 6 STRUCTURAL DESIGN — SECTION 3 TIMBER AND BAMBOO: 3A TIMBER

11.2.3 Material

Laminating boards shall not contain decay, knots or other strength reducing characteristics in excess of those sizes or amounts permitted by specifications. The moisture content shall approach that expected in service and shall in no case exceed 15 percent at the time of gluing. The moisture content of individual laminations in a structural member shall not differ by more than 3 percent at the time of gluing. Glue shall be of type suitable for the intended service of a structural member.

11.2.4 Fabrication/Manufacture

In order to assure a well-bonded and well-finished member of true shape and size, all equipment, end-jointing, glue spread, assembly, pressing, curing or any other operation in connection with the manufacture of glued structural members shall be in accordance with the available good practices and as per glue manufacturers' instructions as applicable.

11.2.4.1 End joints shall be scattered in adjacent laminations, which shall not be located in very highly stressed outer laminations.

11.2.5 Testing

Quality control, inspection and testing during production is essential to structural glulam. Special attention needs to be given to members exposed to weather.

For examining the quality of glue and its relative strength vis-à-vis species of timber in glued laminated construction, it is necessary to conduct block shear and other related tests in accordance with accepted standard [6-3A(11)]. Structural loading tests on prototype sizes provide information on the strength properties, stiffness or rigidity against deflection of a beam. Information

on different strength functions as obtained from such indigenous tests carried out on laminated timber beams are given in Table 19, for guidance in design.