AU2021101455A4

AU2021101455A4 - Method for adjusting the bearing capacity of spf parallel chord wooden trusses by web member angles

Info

Publication number: AU2021101455A4
Application number: AU2021101455A
Authority: AU
Inventors: Guanben DU; Yanjun Liang; Mingli Qiang; Yan Xia; Zhe YUAN; Xiaojian Zhou
Original assignee: Southwest Forestry University
Current assignee: Southwest Forestry University
Priority date: 2020-03-20
Filing date: 2021-03-22
Publication date: 2021-05-13
Anticipated expiration: 2029-03-22
Also published as: CN111400803A

Abstract

This invention discloses a method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles, which includes: S1, applying the Smsolver mechanics solver model to analyze and calculate a variation range of web member angles of a parallel chord wooden truss, and obtaining an optimal web member angle, and upper and lower critical angles; S2, through static loading experiments on the parallel chord wooden truss, obtaining an ultimate bearing capacity of the parallel chord wooden truss, an ultimate deflection, axial strain, main failure modes, and deformation patterns of each main node of the parallel wooden truss. In this invention, by adjusting the angle of the parallel chord wooden truss web members, the redundant load of the parallel chord wooden truss is controlled to ensure the bearing capacity while saving materials, facilitating the construction, which provides evidential support for the application of parallel chord wooden trusses in wood structure buildings. 1/2 Si, applying the Smsolver mechanics solver model to analyze and calculate a variation range of web member angles of a parallel chord wooden truss, and obtaining an optimal web member angle, and upper and lower critical angles S2, through static loading experiments on the parallel chord wooden truss, obtaining an ultimate bearing capacity of the parallel chord wooden truss, axial strain, main failure modes, and deformation patterns of each main node of the parallel wooden truss S3, observing damage characterizations, analyzing the failure modes, verifying the theoretical analysis in S 1, and obtaining an optimal web member angle and component failure modes of an SPF parallel chord wooden truss Fig. 1 Fig. 2 2/2 A c - K M N 0 - F ML Fig. 3 Anti-arm beam Hydraulic device M- c C -3 Force sensor Parallel Chord nnn nn nn nn uuSupport Gantry Fig. 4 T1 T2 Pu 5 P I5 5 5 2P/3 5 5 5 5 5 5 i5 P/6 5 5 0 30 T(min) Fig. 5

Description

1/2

Si, applying the Smsolver mechanics solver model to analyze and calculate a variation range of web member angles of a parallel chord wooden truss, and obtaining an optimal web member angle, and upper and lower critical angles

S2, through static loading experiments on the parallel chord wooden truss, obtaining an ultimate bearing capacity of the parallel chord wooden truss, axial strain, main failure modes, and deformation patterns of each main node of the parallel wooden truss

S3, observing damage characterizations, analyzing the failure modes, verifying the theoretical analysis in S 1, and obtaining an optimal web member angle and component failure modes of an SPF parallel chord wooden truss

Fig. 1

Fig. 2

2/2

A c - K M N0

- F ML

Fig. 3

Anti-arm beam

Hydraulic device M- c C -3 Force sensor

Parallel Chord

nnn nn nn nn uuSupport

Gantry

Fig. 4

T1 T2

Pu

5 P I5 5 5 2P/3 55 5 5 5 5 i5 P/6 5 5 0 30 T(min)

Fig. 5

METHOD FOR ADJUSTING THE BEARING CAPACITY OF SPF PARALLEL CHORD WOODEN TRUSSES BY WEB MEMBER ANGLES

FIELD OF THE INVENTION This invention generally relates to wood structure design, in particular, to a method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles.

BACKGROUND OF THE INVENTION Parallel chord wooden trusses are a main load-bearing component of modern light-weight wooden structure buildings. They are made of dimensional lumber as the base material and mostly assembled by tooth plate connectors. They have a lightweight, high-strength, economical and environmentally friendly structure. Their main components include upper and lower chords, web members and connectors, etc. Existing specifications do not clarify the relationship between the angle of web members and the bearing capacity of the truss. When processing parallel chord wooden truss, the determination of the positions of the web members under empirical estimation has led to the bearing capacity of parallel chord wooden truss far exceeding the requirements of building load codes, wasting materials and causing inconvenience to pipeline construction. The main references are as follows:

[1] Zhou Haibin, Ren Haiqing, Lv Jianxiong, etc. The length-size effect of the flexural strength of Chinese fir gauge lumber for timber structure [J]. Journal of Building Materials, 2009, 12(4): 501-504

[2] Xu Ming, Gong Yingchun, Li Xiazhen. Research status of structural specification lumber in my country and preliminary exploration of product certification [J]. World Forestry Research, 2018, 31(2): 72-76.

[3]RAINER. Performance of wood-frame building construction in earthquakes[J] .Australasian Journal of Construction Economics & Building, 2010, 10(3): 80-81.

[4]SARAHA, STEVENSON, AYMAN, et al. A practical modelling technique to assess the performance of wood-frame roofs under extreme wind loads[J]. Engineering Structures, 2019, 191(15): 640-648.

[5]P. Munaf6, F. Stazi, C. Tassi, F. Davl. Experimentation on historic timber trusses to identify repair techniques compliant with the original structural-constructive conception[J].Construction and Building Materials, 2015, 87(3):54-66.

[6]GRZEGORZ P, ANDRZEJ N, ANNA L, et al. Wood as a building material in the light of environmental assessment of full life cycle of four buildings [J] .Construction and Building Materials, 2014, 52(3):428-436.

[7]Rakesh Gupta, Milan Vatovec, Thomas H .Miller .metal plate connected wood joints: a literature review[J] .Research Contribution, 1996(13) .

[8]Rakesh Gupta .metal-plate connected tension joint under different loading conditions[Z] Department of Forest Products Oregon State University Corvallis, OR 97331. 1993.

[9] Wang Zhiqiang, Wang Zhuanzhuan, Cui Fancheng, et al. Analysis of bearing capacity of fast-growing fir parallel chord wooden truss [J]. Construction Technology, 2017, 48(4): 424-426.

[10] "Wood Structure Design Standard" (GB50005-2017), [S].

[11] "Wood Structure Test Method Standard" (GB/T50329-2012), [S].

[12]BIP 2198-2012, ((Concise Eurocode for design of timber structures: BS EN 1995: Eurocodes 5)) [S]

. Parallel chord wooden trusses are the main load-bearing component of modern light-weight wooden structure buildings. They have been widely concerned around the world for their advantages of light weight, high strength, and durability. At present, experts have conducted thorough research on parallel chord wooden trusses in terms of joint bearing capacity and member bearing capacity. Gupta conducted pure tension and tension-bending experiments on the toothed plate connection nodes, which showed that the tensile bearing capacity of the toothed plate connection nodes decreased with the increase of the bending moment; Wang Zhiqiang conducted a static loading test on fast-growing Chinese fir parallel chord wooden truss, which indicates that the premature failure of the parallel chord wooden truss structure is caused by the tooth plate at the node being pulled out of the wood. In contrast, there is little research on web member angles, such as the influence of web member angles on the bearing capacity of trusses.

SUMMARY OF THE INVENTION In view of the shortcomings of the prior art, the purpose of the present invention is to solve the problem of obtaining the optimal angle and the upper and lower critical angle of the truss through theoretical analysis and calculation in the early stage. The above-mentioned technical purpose is achieved through the following technical solution: a method for web members to control the bearing capacity of SPF parallel chord wooden truss. In the above-mentioned method of controlling SPF parallel chord wooden truss bearing capacity with web members angles, theoretical analysis and calculation is conducted in the early stage, and then a truss optimal angle and upper and lower limit critical angles are obtained. A method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles, includes: S1. Applying the Smsolver mechanics solver model to analyze and calculate a variation range of the parallel chord wooden truss web members angle, and obtaining the optimal web members angle and the critical angle of the upper and lower limits; S2. Through static loading experiments on parallel chord wooden truss, obtaining the ultimate bearing capacity of parallel chord wooden truss, ultimate deflection of each main node, axial strain, main failure modes, and deformation patterns; S3. Observing the damage characterization and analyzing the damage mode. The objective is to verify the correctness of the theoretical analysis in S1, and obtain the SPF parallel chord wooden truss optimal web member angle and component failure modes. Preferably, according to the member bearing capacity and stability check calculation method specified in the "Wood Structure Design Standard" (GB50005-2017), when the S1 web members angle is less than 34, the truss bearing capacity check value is greater than 1, which does not meet the requirements; when the web members angle is 34-60, the truss bearing capacity check calculation value is less than 1, which meets the bearing capacity check calculation requirements; therefore the lower limit critical value of the parallel chord wooden truss web members with a span of 2m is 34. When the web members angle is 34-60,the stability check value of the truss is less than 10, that is, the stability check calculation requirement is met within this angle range, and the stability check value of the truss at an angle of 47 is the smallest; and when the web member angle is 47, the truss meets the requirements of bearing capacity, indicating that the optimal value of the parallel chord wooden truss web members angle is 47; The stability check value of the truss with a web member angle of 600 is greater than 10, indicating that the member has been unstable under the design load, and the upper critical value of the web member angle is 60°. The conditions of S2 require experimental materials, test piece preparation, measuring point layout, experimental equipment and devices, and a loading system. The experimental material is based on SPF, with a water content of 13.2% and a density of 0.43 g-cm 3 .

The test piece is prepared with SPF as the base material, the size of the members was determined according to the truss structure. Dimensional lumber is sawn into corresponding sizes, and tooth plates are pressed into each node of the truss in sections by a flat pressing method. The test piece is cured for a week in an environment of 20°C and 60% relative humidity to balance the stress distribution of the test piece.

Displacement dial gauges are set at each of the two ends of the support (D-2, in the lower chord node corresponding to the loading point and in the vertical direction of the middle of the span-@; 2 pieces of 20mm strain gauges are placed at the middle of the lower chord, 2 pieces on each side of the mid-span, 2 pieces each on the left and right sides of the loading point, 2 pieces each on the oblique web members at the supports at both ends. The equipment used in the experiment includes a DH5908 wireless dynamic strain acquisition system, a dial indicator, and a DY2200-K1T2 force sensor. With reference to the "Wood Structure Test Method Standard" (GB/T50329-2012), the test adopts a graded loading system and preloads before the formal loading. According to the "Building Structure Load Code" (GB50009-2012), the calculated design load P for the parallel chord wooden truss with a span of 2 m is 4.58 kN; (1) Preloading T1 The pre-loading takes p/6 as one grade, and includes: loading one grade every 5 minutes; holding the load for 5 minutes after loading to the fourth grade; then unloading one grade every minutes according to a two-grade unloading method until empty; and keeping an empty load for 30 minutes; (2) Formal loading T2 In the formal loading, p/6 is one grade, and the loading time interval of each grade is 5 minutes. The formal loading includes: loading to the design load in 6 grades, holding the load for minutes, then observing damages and changes of the instrument value, continuing to use p/6 as one grade with the loading time interval of each grade being 5 minutes; The formal loading also includes observing the conditions of the members and nodes during the loading process, loading until the truss is damaged, and measuring and reading the data immediately after each grade of loading is completed. The S3 experimental results and analysis of the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles: Average ultimate truss load values of the web member angles 34, 470 and 600 are 21.79 kN, 24.93 kN and 35.09 kN respectively, which are 4.75, 5.44 and 7.66 times the design load respectively, and the variation coefficients are 3.99%, 2.89% and 3.54% respectively; The experiment carried out a static load test on three parallel chord wooden trusses. The failure of the truss caused by the pull-out of the node tooth plates was the main failure form, and the failure position mainly appeared at the ends on both sides and at the two loading points; In the experiment, it was found that trusses with the web members angles of 34 and 60 have different degrees of dislocation when the trusses are broken, which is caused by the roll force that gradually reduces the truss stability during the loading process. Parallel chord wooden trusses with the web member angle of 47 remained in the original position and did not dislocate when they were destroyed, indicating that parallel chord wooden trusses with the web member angle of 47 have better stability, which is consistent with the theoretical analysis. The above-mentioned the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles is based on theoretical calculation for truss structure design, and is composed of upper and lower chords and web members connected by tooth plates. The chords and web members are based on SPF, with a cross-sectional size of 38x89 mm, a moisture content of 13.2%, and an air-dry density of 0.43g*cm. The tooth plates are made of Q235 Carbon structural steel, with a yield strength of 277 MPa and a tensile strength of 363 MPa.

The dimensions of the tooth plates at the nodes are all 75 mm x 133 mm, except for the tooth plates at the two ends of the support which are 57 mm x 75 mm. In the above-mentioned the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles, the design load P of a parallel chord wooden truss with a span of 2 m is 4.58 kN. In the above-mentioned the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles, the preloading has 0.763 kN as one grade, and one grade is loaded every 5 minutes. In the above-mentioned the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles, if the tooth plate is pulled out for 8 mm or the load drops to 80% of the peak load when the wooden material splits at any node, it is considered that the truss is broken. The beneficial effects of this technology: 1. Through theoretical analysis and calculation, the optimal web member angle of parallel chord wooden trusses is obtained, so that the parallel chord wooden trusses have better stability under the premise of meeting the bearing capacity; 2. By adjusting the angle of the parallel chord wooden truss web members, the redundant load of the parallel chord wooden truss is controlled to ensure the bearing capacity while saving materials, facilitating the construction, which provides evidential support for the application of parallel chord wooden trusses in wood structure buildings; 3. This technology provides theory support to SPF parallel chord wooden truss strength and quality control, to make up for the shortcomings of existing technology and specifications. On the basis of solving the lack of stability and bearing capacity of parallel chord wooden trusses, the following shortcomings of the prior art are overcome: 1 Due to the unique properties of wood, parallel chord wooden trusses have large redundant loads. Wood is a non-homogeneous natural material and in the case of a large span, the uneven distribution of natural defects causes the stress in the parallel chord wooden truss to be unevenly distributed, which in turn generates redundant loads. 2 The installation problems of decoration engineering equipment during construction, such as the arrangement of pipes and wires. 3 There are few technical specifications and research results on web member angles. Existing researches on parallel chord wooden trusses mainly focus on theoretical analysis and calculation methods, node connection, and bearing capacity performance, which strengthens the basic theoretical research of parallel chord wooden trusses, but weakens the quality control research in practical application, and results the incompleteness of the theoretical research of light wooden truss design.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall flowchart of the present invention FIG. 2 is a schematic diagram of the parallel chord wooden truss specimen of the present invention; FIG. 3 is a layout diagram of measuring points of the present invention; FIG. 4 is a schematic diagram of the experimental loading device of the present invention; FIG. 5 is a schematic diagram of the parallel chord wooden truss loading system of the present invention.

DETAILED DESCRIPTION The embodiments of the present invention will be described in detail with reference to the drawings of the present invention. Referring to Figures 1 to 5, a method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles includes: S1, applying the Smsolver mechanics solver model to analyze and calculate the changes in the angle of parallel chord wooden truss web members Range, obtaining the optimal web members angle and the critical angle of the upper and lower limits; S2. Through static loading experiments on parallel chord wooden truss, obtaining the ultimate bearing capacity of parallel chord wooden truss, ultimate deflection of each main node, axial strain, main failure modes, and deformation patterns; S3. Observing the damage characterization and analyzing the damage mode. The objective is to verify the correctness of the theoretical analysis in S1, and obtain the SPF parallel chord wooden truss optimal web member angle and component failure modes. According to the member bearing capacity and stability check calculation method specified in the "Wood Structure Design Standard" (GB50005-2017), when the S1 web members angle is less than 34, the truss bearing capacity check value is greater than 1, which does not meet the requirements; when the web members angle is 34-60°, the truss bearing capacity check calculation value is less than 1, which meets the bearing capacity check calculation requirements; therefore the lower limit critical value of the parallel chord wooden truss web members with a span of 2m is 34. When the web members angle is 34-60°, the stability check value of the truss is less than 10, that is, the stability check calculation requirement is met within this angle range, and the stability check value of the truss at an angle of 47 is the smallest; and when the web member angle is 47, the truss meets the requirements of bearing capacity, indicating that the optimal value of the parallel chord wooden truss web members angle is 47; The stability check value of the truss with a web member angle greater than 60 is greater than 10, indicating that the member has been unstable under the design load, and the upper critical value of the web member angle is 60°. The conditions of S2 require experimental materials, test piece preparation, measuring point layout, experimental equipment and devices, and a loading system. The experimental material is based on SPF, with a water content of 13.2% and a density of 0.43 g-cm 3 .

According to "Wood Structure Design Standard" (GB 50005-2017), the size of the member is determined according to the truss structure. Dimensional lumber is sawn into corresponding sizes, and tooth plates are pressed into each node of the truss in sections by a flat pressing method. The test piece is cured for a week in an environment of 20°Cand 60% relative humidity to balance the stress distribution of the test piece. Displacement dial gauges are set at each of the two ends of the support (D-2, in the lower chord node corresponding to the loading point and in the vertical direction of the middle of the span-@; 2 pieces of 20mm strain gauges are placed at the middle of the lower chord, 2 pieces on each side of the mid-span, 2 pieces each on the left and right sides of the loading point, 2 pieces each on the oblique web members at the supports at both ends. The equipment used in the test includes a DH5908 wireless dynamic strain acquisition system, a dial indicator, and a DY2200-K1T2 force sensor. With reference to the "Wood Structure Test Method Standard" (GB/T50329-2012), the test adopts a graded loading system and preloads before the formal loading. According to the "Building Structure Load Code" (GB50009-2012), the calculated design load P for the parallel chord wooden truss with a span of 2 m is 4.58 kN; (1) Preloading T1 The pre-loading takes p/6 as one grade, and includes: loading one grade every 5 minutes; holding the load for 5 minutes after loading to the fourth grade; then unloading one grade every minutes according to a two-grade unloading method until empty; and keeping an empty load for 30 minutes; (2) Formal loading T2 In the formal loading, p/6 is one grade, and the loading time interval of each grade is 5 minutes. The formal loading includes: loading to the design load in 6 grades, holding the load for minutes, then observing damages and changes of the instrument value, continuing to use p/6 as one grade with the loading time interval of each grade being 5 minutes; The formal loading also includes observing the conditions of the members and nodes during the loading process, loading until the truss is damaged, and measuring and reading the data immediately after each grade of loading is completed. The S3 experimental results and analysis of the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles: Average ultimate truss load values of the web member angles 34, 470 and 600 are 21.79 kN, 24.93 kN and 35.09 kN respectively, which are 4.75, 5.44 and 7.66 times the design load respectively, and the variation coefficients are 3.99%, 2.89% and 3.54% respectively; The experiment carried out a static load test on three parallel chord wooden trusses. The failure of the truss caused by the pull-out of the node tooth plates was the main failure form, and the failure position mainly appeared at the ends on both sides and at the two loading points; In the experiment, it was found that trusses with the web members angles of 34 and 60 have different degrees of dislocation when the trusses are broken, which is caused by the roll force that gradually reduces the truss stability during the loading process. Parallel chord wooden trusses with the web member angle of 47 remained in the original position and did not dislocate when they were destroyed, indicating that parallel chord wooden trusses with the web member angle of 47 have better stability, which is consistent with the theoretical analysis. The above-mentioned the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles is based on theoretical calculation for truss structure design, and is composed of upper and lower chords and web members connected by tooth plates. The chords and web members are based on SPF, with a cross-sectional size of 38x89 mm, a moisture content of 13.2%, and an air-dry density of 0.43g~cm-3. The tooth plates are made of Q235 Carbon structural steel, with a yield strength of 277 MPa and a tensile strength of 363 MPa. The dimensions of the tooth plates at the nodes are all 75 mm x 133 mm, except for the tooth plates at the two ends of the support which are 57 mm x 75 mm. In the above-mentioned the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles, the design load P of a parallel chord wooden truss with a span of 2 m is 4.58 kN. In the above-mentioned the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles, the preloading has 0.763 kN as one grade, and one grade is loaded every 5 minutes. In the above-mentioned the method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles, if the tooth plate is pulled out for 8 mm or the load drops to 80% of the peak load when the wooden material splits at any node, it is considered that the truss is broken. A method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles includes: S1. Applying the Smsolver mechanics solver model to analyze and calculate a variation range of the parallel chord wooden trusses web members angle, and obtaining the optimal web members angle and the critical angle of the upper and lower limits; S2. Through static loading experiments on parallel chord wooden truss, obtaining the ultimate bearing capacity of parallel chord wooden truss, ultimate deflection of each main node, axial strain, main failure modes, and deformation patterns; S3. Observing the damage characterization and analyzing the damage mode. The objective is to verify the correctness of the theoretical analysis in S1, and obtain the SPF parallel chord wooden truss optimal web member angle and component failure modes. Theoretical analysis: The truss is applied to the load-bearing structure of the floor beam of indoor bathrooms, and its component design load is determined according to the "Building Structure Load Code" (GB50009-2012), including a dead load and a live load. The dead load includes: a ceiling (wood keel + a layer of 9 mm thick gypsum board) load with a standard value of 0.12 kN/m 2, a truss self-weight with a standard value of 0.084 kN/m 2, a 18mm thick OSB (Oriented Strand Board) load with a standard value of 0.09kN/m 2, a 5cm thick concrete cushion load with a standard value of 1.25kN/m 2, a 10mm thick floor tile load with a standard value of 0.25kN/m2; the live load includes: the standard value of uniform live load of bathroom and toilet, which is 2.5kN/m2; and the design load value P of 4.578 kN under the standard combination. According to the member bearing capacity and stability check calculation method specified in the "Wood Structure Design Standard" (GB50005-2017), when the S1 web members angle is less than 34, the truss bearing capacity check value is greater than 1, which does not meet the requirements; when the web members angle is 34-60°, the truss bearing capacity check calculation value is less than 1, which meets the bearing capacity check calculation requirements; therefore the lower limit critical value of the parallel chord wooden truss web members with a span of 2m is 34. When the web members angle is 34-60°, the stability check value of the truss is less than 10, that is, the stability check calculation requirement is met within this angle range, and the stability check value of the truss at an angle of 47° is the smallest; and when the web member angle is 47, the truss meets the requirements of bearing capacity, indicating that the optimal value of the parallel chord wooden truss web members angle is 47°; The stability check value of the truss with a web member angle of 60° is greater than 10, indicating that the member has been unstable under the design load, and the upper critical value of the web member angle is 60°. The Step S2 requires: experimental materials, test piece preparation, measuring point layout, experimental equipment and devices, and a loading system. 1 The material The test piece is based on SPF (spruce-pine-fir), and Ilc grade dimensional lumber is selected according to the requirements of "Wood Structure Design Standard" (GB50005-2017) and "Wood Structure Engineering Construction Quality Acceptance Specification" (GB 50206-2012). After testing, the moisture content of the dimensional lumber is 13.2%, the density is 0.43g~cm-3, and the main physical and mechanical properties are shown in Table 1. Table 1 main physical and mechanical properties of SPF dimensional lumber Tre Along- Elastic Along-Grai Along-gr Along-Grain e grain Modulus(GPa) n Compression ain Tensile Shear Resistance species Bending Resistance(N/ Resistance (N/m 2

) and Resistance( m2) (N/m 2

) strength N/m 2 )

grades R A Ra A Ra A R A Ran vg. ange vg. nge vg. nge vg. ange vg. ge SPF 4 1 9. 3 27 9 8 7 6.7 7.3 1.69- 0.06 24-12. 2.37 .07-37. 6.81 8.17- .47 4-8.21 2 96.12 44 64 114.7 5

The tooth plates are made of Q235 carbon steel, with a thickness of about 1mm, a tooth length of about 10mm, a tooth width of about 3mm, a yield strength of 277MPa, a tensile strength of 363MPa, and an elongation rate is 34%. The strain gauge adopts domestic welding-free strain gauge, the model is 120-20AA, the length is 20mm, and the resistance value is 1200. 2 The test piece preparation According to "Wood Structure Design Standards" (GB 50005-2017), the size of the members was determined according to the truss structure. Dimensional lumber is sawn into corresponding sizes, and tooth plates are pressed into each node of the truss in sections by a flat pressing method. The test piece is cured for a week in an environment of 20C and 60% relative humidity to balance the stress distribution of the test piece. The test piece is shown in Fig. 2. 3 The measuring points layout In this embodiment, Displacement dial gauges are set at each of the two ends of the support CD-2, in the lower chord node corresponding to the loading point and in the vertical direction of the middle of the span-@; 2 pieces of 20mm strain gauges are placed at the middle of the lower chord, 2 pieces on each side of the mid-span, 2 pieces each on the left and right sides of the loading point, 2 pieces each on the oblique web members at the supports at both ends. The measuring points layout is shown in Fig. 3. 4 The experimental equipment and devices The equipment used in the test includes a DH5908 wireless dynamic strain acquisition system, a dial indicator, and a DY2200-K1T2 force sensor. 6 The loading system With reference to the "Wood Structure Test Method Standard" (GB/T50329-2012), the test adopts a graded loading system and preloads before the formal loading. According to the

"Building Structure Load Code" (GB50009-2012), the calculated design load P for the parallel chord wooden truss with a span of 2 m is 4.58 kN; (1) Preloading T1 The pre-loading takes p/6 as one grade, and includes: loading one grade every 5 minutes; holding the load for 5 minutes after loading to the fourth grade; then unloading one grade every minutes according to a two-grade unloading method until empty; and keeping an empty load for 30 minutes; (2) Formal Loading T2 In the formal loading, p/6 is one grade, and the loading time interval of each grade is 5 minutes. The formal loading includes: loading to the design load in 6 grades, holding the load for minutes, then observing damages and changes of the instrument value, continuing to use p/6 as one grade with the loading time interval of each grade being 5 minutes; The formal loading also includes observing the conditions of the members and nodes during the loading process, loading until the truss is damaged, and measuring and reading the data immediately after each grade of loading is completed. The loading system is shown in Fig. 5. The S3 experimental results and analysis: Bearing capacity. Average ultimate truss load values of the web member angles 34, 470 and 600 are 21.79 kN, 24.93 kN and 35.09 kN respectively, which are 4.75, 5.44 and 7.66 times the design load respectively, and the variation coefficients are 3.99%, 2.89% and 3.54% respectively; Failure modes. The experiment carried out a static load test on three parallel chord wooden trusses. The failure of the truss caused by the pull-out of the node tooth plates was the main failure form, and the failure position mainly appeared at the ends on both sides and at the two loading points; Stable state. In the experiment, it was found that trusses with the web members angles of 340 and 600have different degrees of dislocation when the trusses are broken, which is caused by the roll force that gradually reduces the truss stability during the loading process. Parallel chord wooden trusses with the web member angle of 47 remained in the original position and did not dislocate when they were destroyed, indicating that parallel chord wooden trusses with the web member angle of 47 have better stability, which is consistent with the theoretical analysis. 1. Through theoretical analysis and calculation, the optimal web member angle of parallel chord wooden trusses is obtained, so that the parallel chord wooden trusses have better stability under the premise of meeting the bearing capacity; 2. By adjusting the angle of the parallel chord wooden truss web members, the redundant load of the parallel chord wooden truss is controlled to ensure the bearing capacity while saving materials, facilitating the construction, which provides evidential support for the application of parallel chord wooden trusses in wood structure buildings; 3. This technology provides theory support to SPF parallel chord wooden truss strength and quality control, to make up for the shortcomings of existing technology and specifications. On the basis of solving the lack of stability and bearing capacity of parallel chord wooden trusses, the following shortcomings of the prior art are overcome: 1) Due to the unique properties of wood, parallel chord wooden trusses have large redundant loads. Wood is a non-homogeneous natural material and in the case of a large span, the uneven distribution of natural defects causes the stress in the parallel chord wooden trusses to be unevenly distributed, which in turn generates redundant loads. 2) The installation problems of decoration engineering equipment during construction, such as the arrangement of pipes and wires. 3) There are few technical specifications and research results on web member angles. Existing researches on parallel chord wooden trusses mainly focus on theoretical analysis and calculation methods, node connection, and bearing capacity performance, which strengthens the basic theoretical research of parallel chord wooden trusses, but weakens the quality control research in practical application, and results the incompleteness of the theoretical research of light wooden truss design. In summary, the failure of the truss caused by the pull-out of node tooth plates is the main failure mode of parallel chord wooden trusses. The location of the failure is at the end node and the two loading points; a truss of with the web member angle of 47 has a higher stability while satisfying the bearing capacity requirements, and the web member angle of 47° is the optimal member angle for parallel chord wooden trusses with a span of 2 m, which is consistent with the theoretical analysis results. While particular elements, embodiments, and applications of the present invention have been shown and described, it is understood that the invention is not limited thereto because modifications may be made by those skilled in the art, particularly in light of the foregoing teaching. It is therefore contemplated by the appended claims to cover such modifications and incorporate those features which come within the spirit and scope of the invention.

Claims

Claims What is Claimed is: 1. A method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles, comprising: S1, applying the Smsolver mechanics solver model to analyze and calculate a variation range of web member angles of a parallel chord wooden truss, and obtaining an optimal web member angle, and upper and lower critical angles; S2, through static loading experiments on the parallel chord wooden truss, obtaining an ultimate bearing capacity of the parallel chord wooden truss, an ultimate deflection, axial strain, main failure modes, and deformation patterns of each main node of the parallel wooden truss; and S3, observing damage characterizations, analyzing the failure modes, verifying the theoretical analysis in S1, and obtaining an optimal web member angle and component failure modes of an SPF parallel chord wooden truss.
2. The method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles according to claim 1, wherein according to the member bearing capacity and stability check calculation method specified in the "Wood Structure Design Standard" (GB50005-2017), the S1 web member angle is less than 34°, and a calculated truss bearing capacity value is greater than 1, which does not meet the calculated bearing capacity requirements in the "Wood Structure Design Standard", and this means that the lower limit of the web member angles of a parallel chord wooden truss with a span of 2m can be obtained is 34, wherein when the web member angle is 34°-60°, a calculated truss stability value is less than , that is, this range meets the calculated stability capacity requirements in the "Wood Structure Design Standard", wherein a calculated stability value of a truss with a web member angle of 47 is the smallest and the web member angle 47 also meets the calculated bearing capacity requirements, indicating that the optimal value of the web member angle of the parallel chord wooden truss is 470, wherein the calculated stability value of the truss with a web member angle greater than 60 is greater than 10, indicating that the web member is unstable under the designed load, and therefore the upper critical value of the web member angle is 60°.
3. The method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles according to claim 2, wherein the Step S2 requires experimental materials, specimen preparation, measurement point layout, experimental equipment and devices, and a loading system.
4. The method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles according to claim 3, wherein the experimental materials are based on SPF 3 (spruce-pine-fir), with a moisture content of 13.2% and a density of 0.43g*cm .
5. The method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles according to claim 3, wherein the specimen preparation includes: using SPF as the base material; determine sizes of the web members according to the structure of the truss; sawing dimensional lumber into pieces with predetermined dimensions; pressing a tooth plate into each node of the truss by a flat pressing method; and curing the specimen for a week in an environment with a temperature of 20°C and a relative humidity of 60% to balance the stress distribution of the specimen.
6. The method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles according to claim 3, wherein the measurement point layout is as follows: displacement dial gauges set at each of two supports of the truss CD-@, and at the lower chord node corresponding to the loading point and in the vertical direction of the middle of the span -@;and 2 pieces of 20 mm strain gauges are placed on each side of the middle of the lower chord and the middle of the span, 2 pieces of 20 mm strain gauges are placed on each left side and right side of the loading points, and 2 pieces of 20 mm strain gauges are placed on oblique web members on the supports at both ends.
7. The method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles according to claim 3, wherein the experimental equipment and devices include: a DH5908 wireless dynamic strain acquisition system; a dial indicator; and a DY2200-K1T2force sensor.
8. The method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles according to claim 3, wherein the loading system refers to the "Wood Structure Test Method Standard" (GB/T50329-2012), adopts a graded loading system, and includes a preloading before a formal loading, wherein the design load P of the parallel chord wooden truss with a span of 2 m is 4.58kN, according to the "Building Structure Load Code" (GB50009-2012), wherein during the preloading T1, every one sixth of the design load P is one grade, one grade is loaded every five minutes, after loading the fourth grade the load is held for 5 minutes, then a two-level unloading method is used to unload one grade every 5 minutes until there is no more load, and then there is no load for 30 minutes, wherein during the formal loading T2, every one sixth of the design load P is one grade, one grade is loaded every five minutes, the truss is loaded to the design load in six loads, then the load is held for 5 minutes, and damage and instrument value changes are then observed.
9. The method for adjusting the bearing capacity of SPF parallel chord wooden trusses by web member angles according to claim 1, wherein at step S3 the average limit loads of the trusses with web member angles of 34, 470, and 600 are 21.79 kN, 24.93 kN, and 35.09 kN respectively, which are 4.75, 5.44, and 7.66 times of the design load respectively, and the coefficient of variation is 3.99%, 2.89%, and 3.54% respectively, wherein static load tests are conducted on parallel chord wooden trusses with the three different web member angles, which find that failures are mainly truss failures caused by the pull-out of joint tooth plates, and failures mainly appear at both ends and the two loading points, wherein different degrees of dislocation occur when the trusses with the web member angles of 34 and 600 break, which is caused by a roll force that gradually decreases the stability of the trusses during the loading process, wherein in experiments parallel chord wooden trusses with the web member angle of 47 remain in their original positions and do not move relatively to each other when they are destroyed, indicating that parallel chord wooden trusses with a web member angle of 47 have better stability, which is consistent with the theoretical analysis.