This study examines the structural behaviour of latticed LVL web beams. I-joists of 241 mm and 305 mm depth were subjected to bending and shear tests to evaluate their structural properties. Two different adhesive types were used in the manufacture of the webs, namely, Urea Formaldehyde (UF) and Phenol Resorcinol Formaldehyde (PRF). The specimens were designated as 241-1-B, 305-2-S etc. The first three digits denote the depth of the joist. The second digit denotes the specimen number and the final letter denotes the test configuration (B = bending test set-up, S = Shear test set-up).

Fig. (1). Latticed LVL-webbed I-joist.

For manufacturing of the lattice components, 1.8 to 2.0 mm veneer sheets were bonded in a specially designed mould and subjected to heat and pressure using a radio frequency press. Radio frequency curing is especially suitable to curved elements with uniform heating of the veneer sheets and rapid curing of the adhesive. Curing time of four minutes was required for the UF adhesive and 5 minutes was required for the PRF adhesive. The press pressure applied was approximately 1000 kN/m² and was suitable for for the duration of the curing. Upon removal from the press, the cured LVL curved lamina is cut to width. The width of the LVL lattice components was 30 mm for both joist heights, but the thickness was 31.5 mm for the 241 mm deep joist and 36 mm for the 305 mm deep joist. Each end was then cut to make finger jointed, like male and female type joint, as seen in Fig. (2).

A total of 30 joists were manufactured. The 241 mm and 305 mm deep joists had flange sections of 60 x 50 mm² and 65 x 55 mm², respectively. Grade C24 Norway spruce timber was utilised for the flanges of the joists with the exception of two joists which utilised Irish-grown Sitka spruce timber. The cross-section of the 241 mm and 305 mm deep joists can be seen in Figs. (3) along with the manufactured joist. A groove routed the entire length of the top and bottom flanges to house the LVL webs. Groove depths were 15 mm for the 241 mm deep joists and 17 mm for the 305 mm deep joists. The LVL webs were finger jointed prior to being adhered within the routed grooves. As previously mentioned, two different adhesive types were used to manufacture the LVL webs and to be consistent, the same adhesive was used to bond the flanges to the latticed LVL web. The joists were assembled, glued, clamped under pressure and allowed to cure. The ends of each joist contain a solid end block. The solid end blocks provide some flexibility for on-site trimming to a shorter span if desired, without compromising the structural integrity of the joist. All joists were conditioned at a relative humidity of 65% and a temperature of 20°C prior to experimental testing.

Bending and shear tests were carried out for each joist depth. Joists subjected to bending tests were tested in four-point bending over a span of 18 times the joist depth and joists subjected to shear tests were tested in four-point bending over a span of 10 times the joist depth in accordance with the standard, EOTA TR002 [18]. The bending and shear tests set-ups for the 241 mm and 305 mm deep joists can be seen in Figs. (4 and 5), respectively. The locations of the load points were adjusted to coincide with the top arch of the LVL web in each case. Lateral restraints lined with frictionless Polytetrafluoroethylene (PTFE) plates were provided to avoid failure through torsional buckling of the member. The vertical deflection at the mid-span of each joist was measured using a Linear Variable Displacement Transformer (LVDT) positioned on the top face of the joist. After a preload, each joist was initially loaded to 40% of the maximum failure load (F_max) to determine the modulus of elasticity. Each member was then loaded to failure at a rate 0.05 mm/s to ensure failure occurred within 600 ± 300s in accordance with EOTA Standard TR002 [18] allowing the load-displacement behaviour and failure mode to be recorded.

Characteristic values (5^th percentile) have been calculated by examining the distribution of experimental test results in accordance with EN 14358 [19]. The characteristic value is calculated using Eq. (1) where is the mean value and is the standard deviation corrected by the factor, k_s. The k_s factor is an adjustment factor dependent upon the sample size. This factor provides an additional degree of safety when determining characteristic values by penalising smaller sample groups. Values for k_s and the associated sample size are given in EN 14358 [19].

(1)

Fig. (2). Latticed LVL web element with finger jointed detail.

Fig. (3). I-joist cross-section and component manufacture.

Fig. (4). Lattice LVL web joist geometry and bending test set-up.

Fig. (5). Lattice LVL web joist geometry and shear test set-up.

The load-deflection response for all 241 mm deep joists is presented in Fig. (6). The majority of the joists displayed linear elastic behaviour until failure, however, three joists experienced residual capacity after an initial failure followed by an ultimate failure of the joist. The bending stiffness, first failure load, peak failure load, peak deflection, failure mode and the bending moment are presented in Table 1. As all joists failed either in the web or the web-flange interface, the shear force is also presented in Table 1. Three failure modes were observed during testing, namely, tension splitting of perpendicular to the grain in the LVL web (T), pop-out of the 1^st LVL web from the top flange (P), and separation of the solid end block from the bottom flange (S). For all tests performed, the bending moment and shear force at failure were calculated using the first failure load. The first failure load was defined by a loss of load greater than 10% of the final maximum load. Three joists experienced residual capacity after an initial failure, namely 241-1-B, 241-3-B and 241-5-B as seen in Fig. (6).

The failure modes associated with these joists was predominantly due to tension failure perpendicular to the grain in joist 241-1-B and 241-3-B. Joist 241-5-B failed through pop-out of the 1^st LVL web from the top flange followed by tension failure perpendicular to the grain causing ultimate failure of the joist.

The mean flexural stiffness was found to be 0.3486 kN/ mm. This value is comparable to the theoretical flexural stiffness value of 0.4932 kN/mm for a solid OSB-webbed joist with no openings and having the same depth and flange dimensions calculated using a linear elastic model.

The linear elastic model calculated the neutral axis and stiffness of the composite I-joist section using a series of transformed section analyses. The location of the neutral axis (y_NA) is calculated using Eq. (2).

(2)

where A_i is the transformed cross-sectional area of the i^th component, and y̅_i is the distance from the top surface to the centroid of the i^th component. The second moment of area (I) is then calculated using the parallel axis theorem shown in Eq. (3).

(3)

where I_i is the second moment of area of each component of the composite beam and y_i is the distance from the neutral axis of the transformed section to the centroid of the i^th component. The composite stiffness of the joist is then calculated by multiplying the second moment of area (I) by the modulus of elasticity of the top flange to which the other components were transformed. For the purpose of this model, the elastic modulus of the solid timber flanges and OSB web was assumed to be 12000 N/mm² and 5000 N/mm², respectively. It is worth noting that the interaction between the web and flange is considered to be fully composite and no partial interaction or slip in the joint was considered. This was assumed as no relative slip occurred between the web and flanges prior to reaching the point of failure. For beams that failed at this interface, failure was sudden and complete. Models examining partial interaction may be seen in Stojic and Cvetkovic [20] and Faella et al. [21]. The mean peak load and characteristic peak load were found to be 7.24 kN and 5.19 kN, respectively for the 241 mm deep joists. This corresponds to a mean bending moment and shear force of 4.21 kNm and 3.57 kN, respectively. The characteristic bending moment and shear force calculated in accordance with EN 14358 [19] were 2.84 kNm and 2.41 kN, respectively.

Fig. (6). Load-displacement curves for 241 mm deep joists in bending.

Examples of each failure mode can be seen in Fig. (7). Fig. (7a) shows tension perpendicular to the grain failure in the LVL web. This failure mode was observed in many beams manufactured with UF adhesive and was observed to be the cause of initial failures in joist 241-1-B and 241-3-B. Fig. (7b) and Fig. ( 7c) demonstrates pop-out failure of the LVL web from the top flange and the separation of the solid end block from the bottom flange, respectively. The latter failure modes commonly occurred one after the other with pop-out of the flange occurring first followed by separation of the solid end block. The bending test results have shown that the structural response of webs manufactured using UF and PRF adhesive was different with UF webs tending to fail in tension perpendicular to the grain. While LVL is not generally subject to tensile stresses perpendicular to the grain, utilising LVL in a horizontal lay-up of curved latticed elements makes it susceptible to this failure mode. The PRF web members were not as susceptible to failure perpendicular to the grain. The PRF joists generally failed due to a pop-out of the first LVL flange from the top flange followed by a separation of the end-block from the lower flange at one of the support points. The PRF joists performed better than the UF joists. The mean failure load, ultimate bending moment and shear force was greater for the PRF joists. One of the problems identified with the earlier tests was related to the height of the end-block which appeared to cause premature failure in one of the PRF joists. This was monitored closely during the subsequent manufacturing phase. For the 241mm PRF joists, the characteristic peak load, bending moment and shear force is 5.14 kN, 3.04 kNm and 2.57 kN, respectively. This is compared to a characteristic peak load, bending moment and shear force of 4.71 kN, 2.40 kNm and 2.03 kN for the 241 mm UF joists, respectively.

Due to the superior performance of the PRF adhesive, all of the 305 mm deep joists were manufactured using this product. The bending tests results are tabulated in Table 2 and presented graphically in Fig. (8). All joists failed through pop-out of the first LVL web from the top flange and then separation of the solid end block from the bottom flange, with the exception of joist 305-1-B, which failed through tension perpendicular to the grain in the LVL web. The mean peak failure load was 9.34 kN and the corresponding bending moment and shear force are 8.47 kNm and 4.67 kN, respectively. The characteristic peak load calculated on the basis of the eight bending test results is 7.33 kN. This corresponds to a characteristic bending moment and shear force of 6.66 kNm and 3.67 kN, respectively.

To assess the performance of the latticed LVL webbed I-joists, the experimental characteristic bending moment capacity is compared to that of commercially available I-joists in Table 3. Commercial I-joists with overall depths of 241 mm and 305 mm, having solid timber flanges and solid OSB webs, are considered [22]. For both joist depths, the flange section is 63.5 mm wide and 38.1 mm deep. The bending moment capacities of the commercial joists are reported in terms of the Allowable State Design (ASD) moment [22]. The short-term bending moment capacity of these joists may be determined by multiplying the ASD moment by a factor of 1.5. The short-term moment is considered to be more appropriate for comparison with the experimental test results. For the 241 mm deep commercial joist, the ASD and short-term design moment are 4.60 kNm and 6.90 kNm, respectively. The characteristic bending moment determined from experimental tests is 61.7% of the ASD moment and 41.4% of the short-term design moment of the commercial I-joist. The characteristic bending moment capacity of the 305 mm deep latticed joists compared better with commercial I-joists than was the case with the 241 mm joist achieving 112% and 74% of the ASD and short-term moments, respectively.

Fig. (7). Failure Modes: a) tension perpendicular to the grain, b) pop-out of the LVL web from flange, c) separation of solid end block.

Fig. (8). Load-displacement curves for 305 mm deep joists in bending.

Irrespective of the large openings in the latticed LVL web, the deeper joists had high bending moment capacities relative to the solid OSB webbed I-joists. It is considered that the relatively poorer performance of the 241 mm series joists was due to the different adhesives used in their manufacture. As demonstrated during experimental tests, joists made with UF adhesive were susceptible to tension failure perpendicular to the grain, which has contributed to reduced capacity. To reduce premature failure of the web, it is recommended to use PRF adhesive in future tests. The deeper 305 mm joists, manufactured with PRF adhesive, performed significantly better than the 241 mm deep joists relative to their solid web counterpart.

The joist stiffnesses are also compared in Table 4. For a joist depth of 241 mm, the stiffness of the experimental I-joist achieved 77.4% of the commercial I-joists. When comparing the 305 mm I-joists, the experimental I-joist demonstrated a greater stiffness than the commercial I-joist notwithstanding the large openings in the web of the experimental joists. It is likely that this is mainly due to the larger flange area of the experimental I-joists.

The results of the shear tests performed on 241 mm and 305 mm joists are summarised in Tables 5 and 6, respectively. The load-displacement curves for the 241 mm and 305 mm deep joists can be seen in Figs. (9 and 10), respectively. In all tests, the response was linear elastic up to a first failure load. The load continued to increase after this point with large increases in deflection prior to the ultimate failure of the test specimen. All joists experienced web failure with a pop-out failure of the first latticed LVL web from the flange followed by separation of the solid end block. The shear capacity of each joist was taken as the shear force at the first failure load. The mean first failure load was found to be 14.90 kN and 17.28 kN, for the 241 mm and 305 mm joists, respectively. This corresponds to a mean shear load of 7.45 kN and 8.64 kN, for the 241 mm and 305 mm joists, respectively.

When examining the permissible shear load values of commercial I-joists [22], values of 7.80 kN and 10.02 kN are reported for the 241 mm and 305 mm deep joists, respectively. Although the shear load was based on the first failure load and not the peak failure load, the mean experimental values compare very well to the values reported for commercial I-joists. The characteristic test value of 7.35 kN for the 241 mm joist is 94.2% of the shear capacity reported for the commercial joist. The larger 305 mm joists achieve 54.3% of the shear capacity reported for the commercial joists. This is particularly interesting given the size of the openings in the web of the latticed LVL web joists. It is noted that the sample size is small, and the reported characteristic values should be assessed carefully. More tests are required to confirm these findings generally. The manufacturer [23] also provides equations to account for openings in the web. They state that the shear capacity of a joist with a circular opening (V_circle) is calculated using Eq. (4)

Fig. (9). Load-displacement curve for 241 mm deep joists in shear.

Fig. (10). Load-displacement curve for 305 mm deep joists in shear.

(4)

where V_k is the shear capacity of the joist with no openings, D is the diameter and H is the joist height. For a joist with a square opening, the shear capacity is calculated using Eq (5)

(5)

where D is the hole depth and W is the hole width.

Utilising Eq. (4) and (5) to account for a circular hole of diameter 50 mm and a square hole 60 mm wide and 50 mm deep, the shear capacity of the 305 mm deep joist is reduced to 6.28 kN for the circular opening and 5.73 kN for the square opening. Compared to the experimental results, the latticed LVL web joist achieved similar shear loads with much larger openings in the web. The shear capacity demonstrated experimentally on the 305 mm latticed LVL web joists achieved 72.9% and 79.9% of the commercially available joist with one circular and square opening, respectively.