Strength Test On Structural Timber - Lab Results And Analysis

Arrangement – Simply supported beam with single point load at mid-span Beam 1: Timber Grade ___________________ Dimensions: b: __________ mm d: __________ mm Span: ______________ mm Failure load: __________ Beam 2: Timber Grade ___________________ Dimensions: b: __________ mm d: __________ mm Span: ______________ mm Failure load: __________ Average failure load: _______________ Theoretical bending stress failure obtained from ???? = ???? ???? ???? The theoretical bending stress must be compared against the characteristic strength listed in the corresponding table in AS 1720.1-2010 (Timber Structures). [The relevant tables are: Tables H2.1 – Table H3.1] Notes: ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………

Aims/Objectives

Since ancient times timber has been considered an essential material for construction of buildings and other structures. The structural stability of this timbers was determined by use of simple calculations and assumptions which were validated by the several practical which were carried out (Andrews, 2011). The strength capability of timber is very complicated. The strength capability of wood is a matter of several parameters which include the density, duration of the applied load, size of the members, moisture content, availability of various strength-reducing characteristics such as wane, the slope of the grains, knots.

Wood is well known to have several unique properties. The mechanical and physical properties of wood have a significant number of values that are usually determined by the extended factors. The timber properties vary from one species of timber to another, and also they vary within the same species depending on the environmental conditions under which the wood was growing. However, due to the same cell structure and the way the cells are organised in the wood, the strength of timber products mainly depends on the direction from which the load is being applied from.

Currently, it is much easier to determine the strength of the structural timber by carrying out the test on its properties and the recording the observations which can be used to analyse the strength of the structural timber by the use of the developed scientific formulae.

The primary goal of this lab session was to carry out a test on the strength of the structural timber. The students conducted a lab session where they gathered data which was to enable the students to;

To determine the density of structural timber. This was achieved by determining the volume and the mass of the test piece of the sample.To find out the bending strength and apparent modulus of elasticity of the structural timber.To establish the apparent modulus of elasticity in bending structural timber.To determine the standard test conditions which are within the set standard for structural timber in Australia.To determine the bending strength of the structural timber which was measured.To determine the beam shear strength of the tested structural timber and to determine the factors which affect the shear strength.

Apparatus

The following materials and equipment were used during the lab session to enable the students to gather the most accurate data to use in the analysis of the strength of the structural timber. All the equipment in the lab were used under the guide of the lecturer.Two long timber beams of equal width, length and height. The timber beams had to be of the same grade .flexural testing machine, computer data Acquisition machine which was to be used in the recording of point load and load cells which were to be used in the application of the load in the middle of the timber beams.

Procedure/Methodology

There were standard test conditions which were to be observed during the experiment. That was aimed at ensuring that the different samples which were to be tested were under the same conditions.The rate of deformation or rate of loading was set to determine the failure typically within two to five minutes.Both the moisture content and the temperature of the timber beams were to be determined and recorded at the time of testing.After the lab session, the students were required to return the materials which they were assigned with to their respective safe place of storage.

Procedure/methodology

The following procedures and instructions were followed during the lab session to ensure the safety of students and at the same time to ensure that student participated fully in the study to gather very accurate data. All the students who took part in the Lab session were required to follow them strictly.

The students gathered in the classroom to receive the instructions from the lecture on how to carry out the tests.All the students who were present were required to sign-in the attendance sheet to enable the lecturer to know the exact number of students who took part in the lab session.The students were allowed to proceed to the laboratory. In the laboratory, each team of the students was allowed to carry out tests on two pieces of wood which each of the team was provided with.It was mandatory for all the students to follow the Health and Occupational instructions in the laboratory to avoid the risks of accidents occurring.

All the team members gathered together to test the strength of the two pieces of wood which the lecturer assigned to them.Each student was required to record the span between the support points, the size of the elements tested and they were also required to test the failure load for each element tested in the datasheet.The students then were permitted to take photos of the equipment and tests to illustrate in their reports. The students were only allowed to share any information concerning the report apart from the test results which were recorded on the data sheet.

The students were then required to submit their data sheets which were filled to the lecturer to sign. The signed data sheet was to be included in the appendix of the report to show evidence of attendance.After the testing, all the students were required to return to their classroom to receive further instructions regarding the report from their lecturer.Again all the students who took part in the lab session were required to sign out the attendance sheet.

Assumptions

Students were required to write their reports based on the data which they collected from the laboratory. The report was to be an individual effort, and they were the team members were allowed to share the results from the lab.

Assumptions   

The temperature of the timber beams which were to be tasted was above 15oc The moisture content by AS/NZS 1080.1 of the test pieces.it was within the specified range for the reference population. 

Results

It was clarified by the lecturer that the students who don't sign –in and sign-out the attendance sheets, were assumed to be absent and they were not entitled to receive the marks for the lab report. For the students who signed only one attendance roll was deem to have The timber beams were analysed by carrying out a laboratory test which was carried out at Melbourne Polytechnic. During the Lab process, very high levels of accuracy were observed to obtain data which had minimal errors. The data which was collected is as shown below.

Channel number: 2

    END TEST STOPResultsMaximum load: 7.901 KNMaximum strength: 47.836 MPaA sample of data which was used for plotting the graph.  

Rate	0.100 kN/sec
Start load:	5.000 KN
Stop load	30 %
Width	45.000 mm
Height	70.000 mm
10.000 mm	Upper span
Lower span:	900.000 mm
Area	165.169 mm²
Time (sec)	load (KN)
0.05	-0.029
0.1	-0.055
0.15	-0.087
0.2	-0.084
0.25	-0.072
62.5	4.743
62.55	4.743
62.6	4.743
62.65	4.743
62.7	4.743
62.75	4.743
62.8	4.743
62.85	4.743
62.9	4.74
62.95	4.74

Discussions

After the data was recorded from the lab session they were used to analyse the strength of the structural timber by analysing the following;

Density The density of the timber was determined as follows.The size of the piece was a full rectangular cross-section ( d*B) of the sample piece and had a length of  L which was not to be less than d. The cross-sectional measurements were to be measured to an accuracy of +/- 0.1 mm and the length L was to be measured to an accuracy of 0.1% .The mass of the test piece was to be measured to an accuracy of +/- 0.005kg.

The density was determined as follows. 

= (m* 109)/ (l*b*d)

= (509.85 *109)/ (45*70*47)

=337.72 kg/m3

Bending strength and the apparent modulus of elasticity.

The bending strength and the apparent modulus of elasticity in the bending E of the test piece were calculated from a simply supported four-point beam test configuration. The overall length of the test piece was 20d, and the length of the test span was 18d between the reactions on which the test pieces were supported .the total F was to be applied equally at the third point of the test span L (Jones, 2011).

The test piece, when tested as a directly supported beam, was to be adequately restrained to prevent buckling during loading. For the purpose of preventing lateral buckling which was including the lateral or torsional restraint (Kasal, 2013). The lateral restraint was not to inhibit deformation from the directional load.Also, the bearing plates should be sized in such a way to reduce the crushing indentation at the bearing points.

Results

Apparent modulus of elasticity in bending.It was to be established from measuring the vertical displacement e of the point B which is on the centreline at the mid-span which is relative to the points A and C. Which were on the centreline at the end of the support (Richards, 2015).

Bending strength.For the case of the bending strength of the test piece, it was determined as follows.First the for the bending modes of failure that occurred within the zone of the constant bending moment of a test piece, the bending strength was calculated as follows;

F = (def_ault L)/bd²

= (7.9*18)/ (45²*70)

=0.045

Compared with the characteristic strength which is listed in the corresponding tables in the AS1720.1-2010 the bending stress of the timber pieces was much lower, thus showing that the strength of the tested pieces was good.

For the case where the bending mode of failure is within the other segments of the span, the bending strength was calculated as follows.

f =[3f(L-2L)]/(2bd2)

f= [3*7.9(18-12)]/(2*45*70)

3.76 * 10^-03

Beam shear strength.

The value of the beam shear strength was calculated by using three bending points.A load F was applied at a uniform rate of the loading to the test piece until the failure occurred. The mode of failure was attributed to the beam shear which was evidenced by the splitting of the grain of the test piece (Köhler, 2016). And then it was calculated as follows.

F=0.75F/bd

(0.75*7.9)/ (18*6)

0.0548

Where the mode of failure was attributed to a mode other than the shear. For example in the bending, the mode of failure was recorded, and the shear strength of the piece determine

Conclusion

In conclusion, The standardised procedures for testing the strength of the structural wood was started in the earlier 1900s.Currently, there are a complete and well-developed procedures for investigating the strength of wood depending on their usage (Lacasse, 2014). The physical properties of timber which are obtained in the laboratories test should mathematically be adjusted by the use of models which are in place.

The adjustments take into account the differences which come up from the laboratory specimens and the wood building elements which are used in building (Lyons, 2011). The timber bending strength and density have a close relationship with other properties which usually have a significant impact on the successful timer applications. For structural stability, the elements of timer should be supplied in strength class that can determine the allowable working stress (Thelandersson, 2010). 

References 

Andrews, H. J. (2011). An Introduction to Timber Engineering: Pergamon Series of Monographs on Furniture and Timber. Paris: Elsevier.

Arya, C. (2013). Design of Structural Elements: Concrete, Steelwork, Masonry and Timber Designs to British Standards and Eurocodes, Second Edition. Texas: CRC Press.

Bianco, A. (2017). On Site Diagnostics for Architectural Conservation and Restoration. Berlin: Anchor Academic Publishing.

Dickson, M. (2016). Sustainable Timber Design. Chicago: Routledge.

Dinwoodie, D. (2013). Timber; Its Nature and Behaviour, Second Edition. Chicago: CRC Press.

Domone, P. (2013). Construction Materials: Their Nature and Behaviour, Fourth Edition. Chicago: CRC Press.

Hassan, R. (2015). Proceedings of the International Civil and Infrastructure Engineering Conference 2013. Berlin: Springer Science & Business Media.

Hatt, W. K. (2014). Progress Report on the Strength of Structural Timber, Paris: U.S. Government Printing Office.

Heim, A. L. (2014). Tests of Structural Timbers. Chicago: U.S. Department of Agriculture, Forest Service.

Jones, P. (2011). Timber Structures, Test Methods, Racking Strength and Stiffness of Timber Frame Wall Panels. London: British Standards Institution.

Kasal, B. (2013). In Situ Assessment of Structural Timber. Texas: Springer Science & Business Media.

Köhler, J. (2016). Reliability of Timber Structures. London: vdf Hochschulverlag AG,

Lacasse, M. A. (2014). The durability of Building Materials and Components 8: Service life and strength of materials and components. Texas: NRC Research Press.

Lyons, A. (2011). Materials for Architects and Builders. London: Routledge.

Richards, G. (2015). Building and Construction Materials. Manchester: McGraw Hill Education (India) Pvt Ltd.

Thelandersson, S. (2010). Timber Engineering. London: John Wiley & Sons.