In order to verify the influence of different connection types on the mechanical behavior of wood trusses, and explore the reinforcement effect of multi-timber trusses on the mechanical behavior of single wood trusses, four kinds of wood trusses were tested under static load according to GB/T 50329 (2012) (Standardization Administration of China, 2012b). Parallel trusses (PTs) were chosen to be processed, which are commonly used in the floor system and specimen number. Two singles trusses were used as the control group. The code of the single truss specimen was expressed as "S". Another one was a single wood truss (double) specimen with twice the cross-section size of the member, which was denoted by "D". Besides, two connection types were considered in the test to study the influence of different connection types on the mechanical behavior of wood trusses, which were the wooden pin used in the new-style wood truss represented as "P" and the nail which was widely used in practical engineering represented as "N". The connection positions of the nailed girder trusses and wood pin connected girder trusses were all between the tooth plate joints of upper and lower chord. Specific sample numbers and the description were shown in Table 1.
Sample number Section size, number of truss × (height×width (mm)) Schematic section Description PT-S 1×(89×38) Standard single light wood truss connected by tooth plate PT-D 1×(89×76) A single timber truss made of double width cross-section wood and connected by tooth plate PT-G-P 2×(89×38) Girder truss made of two standard single wood trusses connected by wooden pins PT-G-N 2×(89×38) Girder truss made of two standard single wood trusses connected by nails Note: the same below.
Table 1. Sectional dimensions and description of specimen
The material used in the test was Larix gemlimii, imported from Russia. The material grade was Class II, the density was 0.657 kg/m3, and the moisture content was 17.4% with the standard deviation of 0.049, based on general requirements for physical and mechanical tests of wood (GB/T 1928 (2009)) (Standardization Administration of China, 2009a).
Modulus of elasticity, compressive strength and tensile strength were tested from 20 specimens respectively, and the results are shown in Table 2.
Item Value Modulus of elasticity (MPa) 12220.9±6.21* Bending strength (MPa) 85.32±1.18* Compressive strength of parallel grain (MPa) 45.15±4.3* Tensile strength of parallel grain (MPa) 10.21±1.25* Compressive strength perpendicular to grain (MPa) 7.6±1.8* Note: * represents standard deviation.
Table 2. Material parameters of specimen
The tooth plate used in the experiment was galvanized tooth plate made in China. The performance parameters are shown in Table 3.
Item Value Thickness of tooth plate (mm) 0.90 Tooth density (a/mm2) 0.012 Plate tooth length (mm) 8.6 Elastic modulus of steel (GPa) 203 Tensile yield strength of steel (MPa) 248
Table 3. Performance parameters of metal-plate
Six single wood trusses were processed in this test. The truss was connected by a tooth plate. The tooth plate was located manually and then pressed into the platen by a press machine, from which the pressure was 13 MPa. The truss span of this test was 5200 mm, but the length of the purchased larch dimension wood was only 4000 mm long, so when the upper and lower chords of the truss were processed, the dimension material must be butted. The connection type of butt joint also adopted the tooth plate connection. The specific test piece machining size and docking mode are shown in Fig. 2, where B' and L' is the docking node.
The two girder trusses were both made up of two single wood trusses respectively as shown in Fig. 3, in which the PT-G-N was nailed and the PT-G-P was wood pin connected to the trusses. The nail with a diameter of 4 mm and length of 80 mm, while the wood pin was a beach material with a diameter of 20 mm and a length of 80 mm. The moisture content of beech pins at the test was 14.2% and the standard deviation was 0.035. The hole diameter was 19.5 mm, by screwing into the wood pin with a diameter of 20 mm to increase the tightness of the connection and thus increase the synergy among the trusses of girder trusses.
As mentioned above, the test was carried out based on the method of hierarchical loading test for trusses in the Standard for Test Methods of Timber Structures: GB/T 50329 (2012) (Standardization Administration of China, 2012b), and the loading system is shown in Fig. 4, where Pk was calculated according to the Load Code for the Design of Building Structures: GB 50009 (2012) (Standardization Administration of China, 2012a), and the result was Pk=2.0 kN. The results of internal force analysis showed that the ultimate failure of the truss was tensile failure and the allowable stress was 2.92 kN. The single timber truss was loaded according to Pk, girder trusses were loaded according to 2Pk, and the schematic diagram of loading steps was shown in Fig. 4.
The method of gravity loading which means loading sand bags of definite weight was used in this test, and a truss loading system was designed (Fig. 5). A lever principle was used to amplify the load on the joints of the trusses. It was easy to change the gravity load into the centralized load. The loading device contains three systems: the loading system, supporting system and measuring system. During the test, the absolute displacement and relative displacement of each part of the specimen were measured by the displacement gauges. At the same time, an automatic, multi-channel, scanning data logger (TDS-530), produced by Tokyo Sokki Kenkyujo Co., Ltd., was used to collect data.
2.2. Specimen processing
2.3. Loading system and device
First, the single wood truss PT-S appeared obvious instability phenomenon in the post-loading section, and the final damage was also caused by the rapid change of the geometry of the structure. The whole truss was completely out of bearing capacity, which was a kind of unstable failure (Fig. 6). Load-displacement curve showed that its displacement did not appear obvious trend of growth and the destruction was somewhat sudden. In addition, another obvious failure feature of PT-S was the failure at the butt joint, as shown in Fig. 7, which was the phenomenon of the tooth plate bulging at the top chord butt joint L' of the truss. However, there was no obvious damage feature at the butt joint B' of the lower chord. This is mainly due to the mechanical properties of the toothed plate, which was an important connecting component in wood structure and can withstand tensile and shear stresses better. However, when subjected to compressive stress, the tooth was prone to be destroyed (He and Sun, 2008). The upper chord of the parallel chord truss was compressed under the upper load. The lower chord was pulled, so the upper chord of the truss was prone to damage, and there was a bend at the butt joint, which also intensified the instability of the trusses in the plane. Therefore, when a single wood truss was used in a large-span structure, the butt joint material should be avoided as the upper chord of the truss.
The cross-section width of PT-S was only 38 mm, so it was easy to be instability. However, the section width of PT-D was increased to 76 mm, and the instability was greatly improved by using the PT-D method of the single timber truss. However, due to the low connection strength of the common tooth plate to the large section member, the tooth plate of the truss PT-D was pulled out completely at the K-joint, resulting in the complete shedding of the diagonal web member (CK), thus destroyed the whole truss. In addition, tooth pulling phenomenon occurred at other joints of the truss in varying degrees (Fig. 8), and there was no damage to the spalling CK member. Therefore, simply increase the cross-section size of the timber can not effectively enhance the load-bearing capacity of the truss, while the actual performance of the timber can not be fully used because of the damage of the tooth plate connection.
The failure of the joints would lead to the failure of all the members connected to it, and this failure did not have any precursors, showing the obvious characteristics of brittle failure. The displacement was still in linear growth, and there was no sudden increase in the load- displacement curve. The whole failure of timber structure caused by the failure of the connecting parts did not conform to the design concept of "strong node weak member" in structural design (Que et al., 2012).
Girder trusses with different connection types showed different failure modes. The PT-G-N of the nail-jointed wood truss showed obvious instability failure characteristics, as shown in Fig. 8. And the wood pin-connected girder truss PT-G-P was finally destroyed by the damage of the lower chord which was mainly due to the connection between the two trusses. In the failure stage, with the increasing of load, the nail was continuously plastically deformed, and the two single trusses would be staggered to cause the whole damage. In case of the ultimate failure of the trusses, the pin can still be kept within the elastic range of the truss when the timber frame was connected by a wood pin, there was still good synergy between the two single trusses.
The load-carrying capacity and anti-deforming ability of the new wood pin-connected wood truss PT-G-P improved remarkably, the form of failure was no longer the result of instability or the pulling out of the tooth plate, but the whole damage of the truss was caused by the failure of the members. As shown in Fig. 7a, there was a knot at the diagonal web member (AI) of truss Ⅱ in PT-G-P, and the knot had a great influence on the mechanical properties of timber (Que et al., 2016). The PT-G-P failure was accompanied by the sound of the continuous tearing of wood fiber. At first, the wood produced a tiny crack. With the increasing of the load, the crack spread out and through the whole bar, making it completely lose its bearing capacity. However, the damage of single truss did not lead to the overall damage of the girder truss, but concentrated the tension on the lower chord of truss to another truss connected with it, and finally the tooth plate of the lower chord of the other truss was completely pulled apart, and the truss was ultimately destroyed (Fig. 7b). This process lasted for more than ten seconds, and the final stage had an acceleration of the displacement in the load-displacement curve. Therefore, girder trusses have better ductility failure characteristics.
By comparing the load-displacement curves of the four trusses during the failure stage (Fig. 9), we can see that the conventional single truss PT-S and the two girder trusses exhibited ductile failure characteristics under the constant load and closed to failure, the displacement increased sharply, especially in case of two girder trusses. However, the single timber truss PT-D with increased cross-section size of the member did not appear in this feature. Before the failure, the deflection increased linearly, and the brittle failure was obvious. This phenomenon can be well explained by the corresponding failure modes of the four kinds of trusses.
After the static test, the moisture content in the damage zone of the specimens was tested according to the National Standards for Determination of the Moisture Content of Wood: GB/T 1931-2009 (2009) (Standardization Administration of China, 2009b). The moisture content of each truss member were all between 11% and 13%, which conformed that the water content of log and square timber members with small deviation would not exceed 25% according to the Code for Design of Timber Structures: GB 50005 (2003) (Standardization Administration of China, 2003). The load level at the time of failure of the specimen was the final loading level, which was taken as the ultimate bearing capacity. The ultimate load of each specimen was shown in Fig. 10, in which the bearing capacity of ordinary single wood truss could reach 3.7 kN, while the theoretical value was only 2.9 kN. The measured value was 28% higher than the theoretical value, which showed that the single wood truss had a higher safety reserve performance. In addition, the bearing capacity of the single wood truss PT-D with increased cross-section size was improved by about 10.8% compared with the single wood truss PT-S. Usually, the depth of single-side indentation is one-fourth of the thickness of the member. The cross-section size was increased while the tooth depth was not, which made the joint between the rack and plate the weakest part of the whole truss.
Compared to the two single wood trusses, the capacities of the girder trusses increased by 92% and 73%, respec-tively; wood pin connections increased by 157% and 132%, respectively. And the different connection types between the wood trusses had a great influence on the bearing capacity of the wood trusses. The bearing capa-city of PT-G-P increased by 34% compared with that of PT-G-N. This was mainly caused by the difference in the connection mechanism between the nail connection and the wood pin connection, the nail connection was a kind of fastening type connector, and it was not easy to move back and forth between them. However, when the vertical load reached a certain value, the nail deformation affected the synergy between the trusses, thus affected the vertical bearing capacity of girder trusses; In addition, the wood is a kind of viscoelastic material, which has a certain elastic recovery ability, so that the synergy between the trusses could be maintained for a long time.
According to the Standard for Methods Testing of Timber Structures: GB/T 50329 (2012) (Standardization Admini-stration of China, 2012b). Under the standard load, the wood truss must conform to the limit of deflection in the Technical Code for Light Wood Truss: JGJ/T 265 (Ministry of Housing and Urban-Rural Development of China, 2012). As shown in Table 4, the deformation value of each truss under standard load in this test was compared with the limit value in the Code. The deformation values of all kinds of trusses met the requirements of the Code by comparing under the standard load.
Position Standard limit PT-S PT-D PT-G-N PT-G-P Upper chord internode 4.73 3.42 1.69 2.52 1.63 Lower chord internode 2.37 2.55 1.78 2.15 1.76 Maximum deflection of bottom chord 28.89 13.53 10.81 8.38 8.08
Table 4. Comparison with standard limit (mm)
Table 4 is the mid-span deflection of four trusses under Pk standard load. The mid-span deflection of each truss decreased as PT-S, PT-D, PT-G-N, PT-G-P, the deflection values of which were 13.53 mm, 10.87 mm, 8.38 mm and 8.08 mm, respectively. The maximum deflection of the lower chord of PT-D reduced by 28% compared to PT-S, while the PT-G-N and PT-G-P of the two girder trusses increased by 38% and 40%. This indicated that even the results were not so clear, the overall anti-deforming ability of light wood truss could be strengthened by increasing section size under normal use. However, the anti-deforming ability of the girder trusses was significantly improved, which showed that the girder trusses can adapt to the actual application under complex stress conditions. However, there were little differences in the stiffness of wood trusses with different connections under standard loads. Therefore, in normal use of limitation, the anti-deforming ability of the girder trusses was better than that of single wood trusses, but the connection type between the trusses had little effect on it.
Figure 11 shows the load-displacement curve of the joints of the lower chords of four trusses in the T2 stage. It can be seen that the deflections of the lower chords of the four trusses tend to decrease from the middle to the two ends, and the deflection of the mid-span D node was the largest. The single truss, especially PT-S, had a big discreteness between the nodes, and the cooperation performance of the whole truss was not good. However, the performance of girder trusses was improved greatly, the nodes were arranged in a relatively tight arrangement, and the trusses showed good cooperative performance. This was mainly caused by the influence of the butt joint which affected the anti-deforming ability of the adjacent B node. In the combination of girder trusses, a single truss on one side had a butt joint and a single truss on the other side did not have a butt joint. Therefore, when the truss span is large, the combination of girder trusses can effectively improve the coordination between the nodes of the truss.
In the T2 stage of the test, the single wood truss was subjected to 24-hour standard loads, and the girder trusses were subjected to 24-hour double standard loads. The creep resistance and elastic recovery performance of the trusses could be well evaluated by such continuous-loads.
Figure 11e shows the load-displacement curves of the mid-node D of the lower chord of each truss in the T2 stage. Through the comparison of four trusses, the deflections of two wood trusses under 2Pk load were slightly lower than those of PT-S. In addition, the area covered by the load-displacement curve represented the energy dissipation performance of a member, as shown in Fig. 12b, the area covered by the load-displacement curves of two wood trusses was obviously larger than that of two single wood trusses, especially the pin-connected wood trusses, whose envelope curve was plumper and showed the best energy dissipation performance.
As shown in Fig. 12, the creep behavior of trusses under 24-hour continuous-load was also quite different. Similar to the displacement value under the standard load, the creep value of the two girder trusses was the lowest. The different was that the maximum creep value was the single truss PT-D with the increase of the section size. This was mainly due to the size effect of timber (Madsen and Buchanan, 1986; Zhou et al., 2012). The larger the wood size, the more defects it contains. Therefore, the mechanical properties will decrease, and the creep resistance will reduce as the result. Because of the cooperation of two trusses, the creep of the girder trusses was small. The PT-G-P, had the least creep, which was only 54% of the largest single truss PT-D, and the girder trusses showed better creep resistance performance.
The creep of the wood trusses with different connection types also had great difference. The creep of the nail-jointed wood truss was 5.68 mm, which was about 51% higher than that of the wood pin-jointed timber
trusses. This was mainly due to the different material pro-perties of the connectors used in the two types of multi- wood trusses. Steel is prone to irreversible plastic deformed under long-term loads, while wood is viscoela-stic material and will not undergo irreversible deforma-tion as steel under long-term loads. Therefore, the girder truss connected by wood pins can maintain good synergy under long-term load.
From Fig. 4 (load system diagram), after 24 h of continuous-loading, the load was unloaded to zero and lasted 30 min, during which the truss would recover, but some of the residual deformation was irrecoverable. The residual deformation is an important index to describe the elastic resilience of the member. The larger the residual deformation is, the weaker the elastic resilience of the member is. In Fig. 12, the residual deformation of PT-S, PT-D, PT-G-N and PT-G-P was 5.66 mm, 6.18 mm, 4.86 mm, and 4.32 mm, respectively. Residual deformation and creep were basically matched. Girder trusses, especially wood pin- connected wood trusses, showed better elastic recovery ability.
As shown in Fig. 13a, the mid-span deflection of single-truss PT-S, PT-D and PT-G-P varied in the loading process under the Pk standard load. Two single trusses were tested in three stages, while the girder truss was tested in the first two stages. In the stages of T1 and T2, the three trusses were in the elastic range, and were similar to the standard load of 2.3 kN. The anti- deforming ability of the girder truss was obviously higher than that of the other two single wood trusses. The deflection of PT-D, which had a larger size, was lower than that of PT-S, but not obvious. However, the two single wood trusses showed different situation in T3. First of all, under the load of 2Pk, the trusses still showed good elasticity, but after more than 2Pk, the deflection of the trusses increased suddenly with the load increasing, when near the destruction, the PT-D had a sudden increase with increased cross-section size of the member, while the single wood truss was near the destruction and when the load was constant, the displacement still increased a little, which indicated that the single wood truss showed a certain nonlinear near the failure. When it is used in the building structure, the ductility of the whole structure would increase, and the single wooden truss PT-D with increased cross-section size of the members shown obvious brittle failure.
Thus, the anti-deforming ability of the light wood truss can be increased to a certain extent by increasing the cross-section size of the members. However, the trusses may lose the advantage of good ductility by using the tooth plate connection.
Figure 13b shows the variation of mid-span deflection of two girder trusses under the Pk standard load. As shown in Fig. 13a, different connection types had little effect on the overall anti-deforming ability of the girder trusses. In the first two stages, the girder trusses connected by wood pined the PT-G-P should be slightly lower than the nails connected PT-G-N. However, in the T3 stage, the nails-connected girder trusses took the lead in a steep increase of the displacement, while the wood pin-connected truss still maintained a relatively flat growth. From this point of view, the new type of wood pin-connected girder trusses is superior to the traditional ones in the connection of nails.