Truss Assessment

The queen-post timber truss evaluated belongs to the roof structure of an old warehouse of Adico industry, located at Avanca (55 km South from Oporto). The exact date of the construction is not known but the industry exists since 1920 and some plans of the village from 1942 already show the warehouse. Trusses are the main elements of the roof structure, covered with ceramic tiles, 27┬░ slopes and rafters spaced 50 cm over the purlins and the ridge. The free span of the trusses is 11.8 m and the average distance between their centres is 3.5 m.

The geometry of the truss is particular: the configuration is typical of a king-post truss, but queen-posts were added connecting the joint strut/rafter to the tie-beam. This is not the traditional queen-post truss geometry, in which the king-post is substituted by a straining beam connecting horizontally (in the upper part) the two queen-posts, those located below the higher purlins, and the struts connecting the bottom part of the queen-posts to the lower purlins. Clearly, it is an example of a timber truss with an incorrect configuration for the span of the roof. The correct queen-post truss geometry should have been used or two extra posts (princess-posts) should have been

Figure 1. Truss geometry.
Figure 2. Emergence holes over the surface of sapwood.

Measurement length (mm)

Figure 3. Calculation of the cross-section reduction based in a Resistograph test profile.

Measurement length (mm)

Figure 3. Calculation of the cross-section reduction based in a Resistograph test profile.

Figure 4. Map of the decay extension in the truss.

Figure 4. Map of the decay extension in the truss.

located below the lower purlin. Point loads out of the joints, causing bending moments in the rafters, are the most common error detected in the preliminary survey performed in previous steps of the research program (Branco et al. 2006).

The truss is made of maritime pine (Pinuspinaster, Ait.). The timber members of the truss are slender, as characteristic of traditional Portuguese roofs structures, with cross-sections varying from 80 x 145 mm2 for the struts to 80 x 220 mm2 for the tie-beam. The tie-beam is suspended to the posts by iron straps nailed into the posts. Between the tie-beam and the king-post there is a gap of 5 cm while queen-posts are in contact with the tie-beam. Connections between the others timber members are made by single step joints, in some cases nailed, and the queen-posts/rafters connections have a heel strap nailed (25 mm wide and 5 mm tick), Figure 1.

Despite the apparent good condition of the timber members of the truss, the visual inspection revealed insects attack in the tie-beam, queen-posts and struts. In these timber members, emergence holes over the surface of sapwood are visible however, without signs of active infestation, Figure 2.

To evaluate the extension of decay in the timber truss, Pilodyn® and Resistograph® non-destructive tests were performed. The Pilodyn 6J was used with the aim to assess the surface hardness through the depth penetration of the pin steel (2.5 mm) measured in each test performed. Resistograph permits to plot profiles (drill resistance versus penetration depth) that can be used to determine the location and extent of voids, allowing the calculation of the residual cross section (since decayed wood presents lower penetration resistance), and variation in material density.

3 TEST SETUP, INSTRUMENTATION AND PROCEDURE

The main goal of the tests was to evaluate the overall behavior of the timber truss selected under symmetric and non-symmetric loading. Moreover, the influence of the location of point loads application, without and with eccentricity relatively to joints, was assessed.

This is the consequence of the roof geometry, which exhibits two purlins located with an eccentricity relatively to the intermediate joint of the rafter. Therefore, firstly, joints loads were applied in the joints (Fj, F2 and F3) and, in a second step, loads were applied over the purlins and ridge (Fj, F4, F5, Fg and F7).

Loading and unloading were recorded and an attempt to measure the creep of the structure under symmetric loading was made. The behavior of the truss under non-symmetric loading was evaluated, in the first scheme, only by one test and in the second with two tests (one in each pitch side). Table 1 resumes the tests performed.

Wood pallets suspended to the truss by four steel cables (^ g mm) supported the 35 kg cement bags used as loads. Each loading and unloading procedure was divided in steps of 175 kg (5 bags). A total load of 2625 kg (3 x 875 kg) and 2975 kg (5 x 595 kg) was used in the first (three point loads) and second (five point loads) schemes, respectively. The difference in the maximum load value applied between both schemes, 350 kg, is due to the difficulty to increase the number of bags over the pallets in the first case. To record the deformation of the truss during the tests, eight LVDTs (Linear Variable Differential Transformer) and six dial gauges (DG) were used. LVDTs were responsible for measuring the

Table 1. Summary of the tests performed.

Table 1. Summary of the tests performed.

six DGs.

global displacement (LVDTs 1 to 3), the behavior of the king-post/tie-beam connection (LVDT-5) and the displacement below the purlins (LVDTs 4 to 8) also used to calculate the rotational behavior of the joints rafter/tie-beam and rafter/strut. The values of the LVDTs during the tests were acquired by a Data Acquisition System, with 8 channels, using a LabVIEW program (version 8.2). Dial gauges measured the opening of the queen-post/tie-beam connections (DG 3 and 4), the horizontal displacement of the rafter in the rafter/tie-beam connections (DG 5 and 6) and two additional points to calculate the rotation of rafter/tie-beam connections (DG 1 and 2). Figure 5 shows the instrumentation used in the tests performed.

Displacement (mm)

Figure 6. Displacement recorded by LVDTs 1, 2 and 3 during 3F-C test.

Displacement (mm)

Figure 6. Displacement recorded by LVDTs 1, 2 and 3 during 3F-C test.

In every loading and unloading step the displacement values of the LVDTs were recorded; however, in the case of the dial gauges, only some steps were acquired, as results of the little variation verified.

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