Comparative Assessment of the Strengths of Solid and Glued Laminated Timber

Comparative Assessment of the Strengths of Solid and Glued Laminated Timber

CHAPTER ONE

Aim and Objectives

Aim

The aim of this study is to assess the strength properties of glue laminated Ire (Funtumia, Africana) Oriro(Antiaristoxicaria) and Awun (Alstoniacongensis) species using polyvinyl acetate (PVA) glue with a view to presenting the capability of glulam elements made from the selected timber species.

Objectives

The specific objectives are;

To determine the glueability of Ire (Funtumia, Africana),Oriro (Antiaris Toxicaria) and Awun (Alstonia Congensis).
To evaluate the effects of varied temperature on strength performance of polyvinyl acetate glue and the mechanical properties of the glue laminated compression
To determine the mechanical properties such as static bending strength, compression parallel and perpendicular to grain of solid wood and glulam beams specimen of the species studied.
To compare the mechanical properties of solid and glued laminated timber

CHAPTER TWO

Literature review

Structural Glue Laminated Timber

According to the glued laminated timber association (GLTA) of the United Kingdom, glulam beams account for 50% of total glued laminated timber sales for structural application in the United Kingdom making it one of the fastest growing structural materials today; its consumption is reported to have doubled between 1985 and 1995 in Britain alone. Glued laminated timber has gained wide application and demand because of the standard sizes and availability of glued laminated structural elements such as beams and columns. This trend has made architects and specifies to give preference to glulam over other materials for a wide range of structural application in building.

Structural glued laminated timber is obtained by gluing up suitably selected and prepared pieces of stress graded wood in straight or curved form with the grain of all pieces running parallel to the longitudinal axis of the member (APA, 1996; Moody and Hernandez 1997). This technology allows the flexibility in the formation of structural elements with a wide variety of sizes, profiles, and lengths having superior strength, serviceability, and appearance to sawn timber. Structural elements such as beams, columns, floor joist and purlins can be produced as glulam elements. Glued laminated timber beams are manufactured with the strongest laminations on the bottom and top of the beam, where greatest tension and compression stresses occur so as to optimize the use of the material (Wan, 2011).

Depending on the intended use, glued laminated timber members can be manufactured using single or multiple grade of timber, similarly any specie can be used for glulam timber, provided its mechanical and physical properties are compatible and the specie glueable (APA, 1996; Zhiyong & Robert, 2007).

CHAPTER THREE

Experimental Procedure

Preamble

This research set out to determine and compare the strength properties and failure mode of solid wood and glued laminated elements of three timber species in Nigeria, namely; Ire (Funtumia Africana), Oriro (Antiaristoxicaria) and Awun (Alstoniacongensis). The glueability of the said species was investigated using Polyvinyl Acetate glue. Effects of varied temperature on strength properties and failure characteristics of both solid wood and glue laminated element were investigated. Details of materials and methods are as follows.

CHAPTER FOUR

PRESENTATIONOF RESULT ANALYSIS AND DISCUSSION

Preamble

Three strength properties were investigated for the timber species as summarized on tables 4.1 to table 4.9. Solid and glue laminated timber elements were produced from Ire (Funtumia Africana),Awun (Alstonia congensis) and Oriro (Antiaris toxicaria). The structural elements included solid and glue laminated beams and short columns. Results were analyzed, examined and discussed.

CHAPTER FIVE

Summary, Conclusion and Recommendation

Summary of Research Findings

Laboratory experiments were conducted to determine the physical and mechanical properties of three Nigerian timber species, namely: Ire (Funtumia african), Awun (Alstonia congensis) and Oriro (Antiaris toxicaria,) in accordance with ASTM D193, EN 13183-1 (2002), EN 408 (2003) and EN 384(2004). The mechanical properties were determined using three points loading for bending strength. The characteristic values of the material property were calculated and tabulated on table 4.7: Characteristic material properties. The strength properties were adjusted to 18% moisture content relevant to the equilibrium moisture content for structural use of timber in Nigeria as shown on table 4.8: Adjusted characteristic bending strength to 150mm depth. The Nigerian standard code of practice (NCP 2:1975) has sevenstrength classes ranging from N1- N7 with the highest strength in N1 and the lowest being N7. From the results of laboratory test the following were found:

From the results of Characteristic properties it was shown that solid wood hadhigher strength properties than glue laminated timber with Ire offering the highest bending strength of 62.15 N/mm² to 43.3 N/mm² followed by Awun with 9N/mm²to 30.6N/mm² and Oriro offering 15.92N/mm²to 22.44N/mm² for solid and glulam beams respectively. This fact is recognized in literature by Zhiyong and Ross, (2006) Thelandersson and Larsen, (2003).
The glulam elements developed significant clear solid wood strength of 55%95% and 143% in edgewise bendingin Ire, Awun and Oriro respectively which could be improved upon with more efficient pressing
Failure generally occurred in wood fiber and not along the glue line for theglulam elements as shown in Plates V, VII, IX and XII.
The results of bending strength and compression parallel and perpendicular tograin in the both solid and glulam elements were of structural significance as contained in table 4.7:Characteristic material properties.
For Compressive strength parallel to grain, at varied temperature, the effects of heat on glulam elements showed a progressive loss of strength with increasing temperature and thus strength and temperature were negatively correlated as shown in Fig.5: Effects of temperature variation on compressive

Conclusion

Giving the above, the following conclusions were reached:

The said species are glueable using polyvinyl acetate glue
Polyvinylacetate glued joints performed reasonably well within temperature variation from 0⁰C to 100⁰
Thestrength of glulam was not always higher than solid wood however the strength of glulam elements was of structural significance with Ire offering the highest bending strength of 62.15 N/mm² , 43.3 N/mm² followed by Awun with 9N/mm² and 30.6N/mm² and Oriro offering 15.92N/mm² , 22.44N/mm² for solid and glulam beams respectively.
Solid and glulam beams were more flexible in flat wise bending with higher MOEFor classification according to density and strength class of NCP 2 (1973) the species all fall to group N_7.
Timber classified into the least strength class of N₇ could be used for structural purpose in tandem with those from higher strength classes as wood composite.

Recommendation

Based on the strength developed, the glulam elements of the said species are useful as structural elements using polyvinyl acetate
Giving the extent of strength developed the species show useful potential for structural application as glulam elements with higher strength properties than solid wood over greater depths and span
The use of polyvinyl acetate glued elements should be restricted to temperatures at and close to room temperature.
Ire (Funtumia Africana) and Oriro (Antiaris Toxicaria) are timbers harvested andsold in the local timber market without information on their properties. These timbers fall in the same class as Awun (Alstonia Congensis) and should be included in reviewed editions of NCP 2.
According to NCP 2(1973) all Nigerian timber species are hardwood i.e. harvested from broad leaf trees regarded as deciduous trees on the bases of which the timber species especially Awun and Oriro are hard wood while Ire is a

Recommendation for Further Studies

Further studies could assess;

The glueability of other timber species as contained in NCP2 with the view to establish their glueability.
Suitability of other types of synthetic glues on local timber species for structural The mechanical properties of structural sized beams and columns of the species used in the study
The relationship between the quantity of glue and the strength of the glue line as well as the effect of species on the

REFERENCES

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