Selecting and testing the CIB components

The basic CIB is a combination of flanges and webs. Flanges are made of timber or LVL 45-mm deep and 90 mm wide. Beams were manufactured in two different lengths, 2.3 and 4.8 meters.

3.3.2.1 Selecting the timber flanges

Radiata pine timber was used for the flanges of beams. Suppliers mechanically graded this timber as MGP 10, which means that the average modulus of elasticity (Emean) of these timbers was 10 GPa (kN/mm²). However for this project, the modulus of elasticity of each timber flange was tested again in the SCION laboratory.

3.3.2.2 Matching the timber flanges

CIBs of different profile are comparable if individual components of each beam being compared possess similar material and mechanical properties. Of the three CIB components, web, infill material and flanges, timber flanges by nature possess the highest material variability (e.g. Timber Engineering step 1, 1995). As a result it was necessary to measure timber strengths and then to spread them (wood flanges of known stiffness) evenly among different designs before fabricating the beams. There is correlation between timber strength and some characteristics of the timber that can be measured non-destructively. Those characteristics are as follows:

1. Knots

2. Annual ring width 3. Density

4. Modulus of elasticity (MoE)

5. Combination of Knots and annual ring width 6. Combination of knots and density

7. Combination of knots and MoE

A number of research studies have been conducted to determine relationships between the engineering and material properties of timber. (Hoffmeyer …(et al.) 1999, Johansson …(et al.) 1992, Lackner … (et al.) 1988.) All this research showed that the modulus of elasticity has highest correlation with bending strength, demonstrating

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the modulus of elasticity (MoE) is recognized as the best individual predictor of strength in timber. (Johansson 2003) In New Zealand, matching and sorting procedures were carried out in two steps described below.

1. Matching the timber flanges

In this stage timber flanges, base on their MoE, were evenly distributed between various groups. Each group was used for fabrication of one type of CIB profile.

The outcome was even distribution of MoE between various groups or profiles.

Detailed descriptions of the procedure are given in the next section - (3.3.2.3).

2. Sorting and matching the beams

In this step fabricated beams of each group were divided into sets of three members and sorted in such a way that each set had flanges of similar MoE values. Section 3.4 describes the method.

These procedures allow both even distribution of MoE among the different profiles and within each set of the group. As explained in chapter 4, this method is used to evaluate the effect of web openings of different diameter within the profile and also to compare them with other profiles.

3.3.2.3 Testing and grouping the timber flanges

Three hundred sections of timber were visually examined for natural defects and 267 pieces of the timber were selected for use. Those timber sections were cut into 2.75 meter lengths and were used for manufacturing 96 CIBs with 2.3 meter final length.

All the timber sections were numbered and tested using a four point bending test to evaluate their modulus of elasticity. Variation of the Young modulus was between 5 to 15 GPa. The test results were sorted by ascending modulus of elasticity and renumbered accordingly from 1 to 267: see Table A.1 in Appendix A. As a result the higher flange number (ID) reflects the higher flange MoE. Any pieces with module of elasticity lower than 7 GPa were rejected, which meant rejecting first any faulty flanges with flange ID in the range 1 to 40.

The remaining pieces were divided into nine matched groups, each group represented one profile. A group contained matched samples with MoE ranging from low to high, equally spread, as shown in Table A.2 in Appendix A. This was achieved by

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spreading the 9 flanges with lowest acceptable ID among the nine groups and then the next 9 flanges with higher ID values were assigned to groups. In this way flanges with numbers 41 to 49 were spread among the nine different profiles accordingly followed by the number 50 to 58 and this was continued until the last row starting from 257 and ending at 265. Each cell in a table contains a pair of flange IDs, which represents top and bottom flanges of the relevant profile. As is shown in Table A.2 in Appendix A, each group was also coded: see also Figure 3.6. This colour-coding system was used to identify each design during the machining process and manufacturing.

In the same fashion 56 pieces of timber were selected, including a spare, for the manufacture of 24 CIBs 5.4 meters long. These beams were also tested and sorted in a similar way to the short beams as previously described: see Tables A.3 and A.4 in Appendix A).

Figure 3.6 Colour-coded flanges after machining, ready for assembly

3.3.2.4 Selecting the LVL flanges

One hundred pieces of LVL each 2.75 meters long, 45 mm deep and 90 mm wide were selected to manufacture 48 CIBs with LVL flanges.

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Material properties of LVL are more consistent then solid timber, which reduces the variation of MoE and therefore there was no reason to test all the pieces. As a result the modulus of elasticity was determined only for 20 pieces, which were chosen randomly from the stack and results appear in Table A.5 in Appendix A.

Length of the flanges

The length of the flanges was trimmed in three stages:

1. Length during the machining

All the flanges were cut to 2.75 and 5.4 meter lengths for the machining.

2. Length during the assembling

Timber and LVL were further trimmed to 2.45 and 5 meter lengths.

3. Final stage

After the manufacturing process was completed and polyurethane injected into the frame, the CIB beams were trimmed to the designed sizes which were 2.3 meter and 4.5 meter lengths.

Flanges were not sized to their final length in the first place, because during the machining process, or during, transferring some damage might occur to the end of the beams. Similarly, after injecting the polyurethane some of the foam overflowed from the open end of the beam. This extra length provided the required margin, and eventually this is trimmed to the required design size.

Expanding the project

After designing the new profiles, designs 9 and 16, it was decided to manufacture those sections along with the previous profiles. There was also a short fall in the manufacturing plan for fabricating long CIB beams with LVL flanges so I-beams were considered. Such was the interest shown by NZ SCION research in the CIB project, that additional funding was provided in order to expand the testing program, to incorporate LVL in the long CIBs. Having these long beams created the opportunity for carrying out the full-scale comparative bending test on them along with the previous designs.

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Beams with timber flanges

• Profile 9 and 16, each one replicated three times over 2.3 m length

• Profile 9 and 16 each one replicated three times over 4.5 m length

• Profile 1 three beams over 4.5 m length

Beams with LVL flanges over 4.8 m length

• Profile 2, two beams

• Profile 4 and 11 , each one replicated twice

• Profile 8 , two beams

A total of 16 LVL flanges and 26 Timber-flanges over 4.8 meter length were selected and tested for MoE. The results of the MoE tests for six pieces of LVL flange are presented in Table A.6 in Appendix A. Those LVL pieces had been chosen randomly from 16 pieces. The results of the Young Modulus (E) for timber flanges are given in Table A.7 in Appendix A.

A similar method to that in the previous section was adopted to match the top and bottom timber flanges in such a way that each profile has a comparable modulus of elasticity: see Table A.8 in Appendix A.

3.3.2.5 Testing and preparing the webs

Three-ply structural plywood sheets grade DD, 9 mm thick, were used as a web material. Plywood sheets were manufactured to AS/NZS 2269 at the Origin Plywood Plant in New Zealand.

Testing the Plywood

Six specimens, which were randomly selected out of the 140 plywood sheets, were tested according to BS 4512 for modulus of rigidity, moisture content and density:

see Figure 3.7(a). A summary of the results is given in Table A.9(a) in Appendix A.

Modulus of elasticity (Figure 3.7(b)), bending strength with face grain parallel to the span and face grain perpendicular to the span were also determined for five samples and results are in Tables A.9(b) and A.9(c) in Appendix A.

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a)Testing modulus of rigidity b)Testing modulus of elasticity

Figure 3.7 Testing material properties of the 3ply-plywood

Preparing the webs

All the plywood sheets had an initial dimension of 1200×1200 mm. They were tongued and grooved by passing them through the double ended tenon-maker shown in Figure 3.5. The tongue and groove profile was created parallel to the face grain direction. Then Plywood webs were cut to 220, 240 and 290mm height across their face grain. The plywood webs, which were 220mm in height, were tapered on either edge by passing them through the planer-moulder to fit the groove machined in the flange.

Cutting across the face grain

The plywood used was 3-ply and 9-mm thick. Shear and bearing capacity of the ply veneer parallel to the grain is higher than perpendicular to the grain direction. By cutting across the face grain, two of the ply veneers are positioned such that the face grain is vertical toward the flange grain direction, while cutting along the face grain provides only one ply grain in desired direction.

3.3.2.6 Machining the flanges

The planer-moulder shown in Figure 3.4 was employed to machine three different flange designs, namely single grooved, double grooved and recessed as shown in Figure 3.8.

The planer-moulder used in this project could run up to six cutters simultaneously.

The machine was adjusted in a way to create the profile and to remove a millimetre from both sides of the flanges. The main reason for machining the sides of the flange

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is to produce a clean surface for laminating by removing any dust or dirt. Adhesives cannot bond well on the laminating area if the surface is covered by the dust or dirt and the outcome would be a weak glue line.

(a)Machining single groove (b)Grooving cutter for matching double groove

(c)Machining recessed profile

Figure 3.8 Machining different flange profiles

Each design required special cutter knives, shown in Figure 3.8(b), which were made in advance in NZ Waiariki Institute of Technology. The color-coded timber flanges and LVL flanges were divided into four groups and placed on the moulder in-feed table.

Four different groups were prepared for machining:

• Group 1: Profiles 6 and 13 colour coded orange

• Group 2: Profiles 3, 4, 10 and 11 colour coded red

• Group 3: Profiles 7, 8, 14 and 15 colour coded yellow, but profiles 1 and 2 also in this group were coded colourless

• Group 4: Profiles 5 and 12 colour coded blue

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After completing each group, the process had to be stopped, in order to change the cutter-head for the next group. At the same time flanges were collected from the out-feed table and stacked again in four groups ready for transferring to the assembly building. A similar approach was used to machine the additional designs.

To achieve the right dimension for the timber profile it is important to have straight wood, of sufficient size, on the in-feed to machine the profile cleanly. If not, the machined profile can show “hit and miss”. This point is especially important when using already dried and dressed timber, which can have some variation along the width; for instance such timber can be a few millimetres thinner or thicker. The planer-moulder is set to a fixed size and when thinner timber passes through the machine an asymmetrical profile is created.

In the assembly building, all the CIB flanges were cut to 2450mm in length.