Experimental Studies on Joints of Wooden Elements with Proposed “CM Insert”

1. Introduction

Paper [1] proposes a method for restoring the bearing capacity of damaged wooden beams in support zones based on their modification with a polymer composition. Numerical calculations were performed for beams 6 m long and with a cross-section 100×200. The parameters of the modified beam and the reference “healthy” structure were compared. The shear stresses in the reinforced beam exceeded the parameters of the “healthy” beam by 15–17%. It was found that the strength of the damaged beam modified on the supports increased by 16–18% compared to the damaged beam. Based on the obtained results, the boundary conditions for applying the modification of damaged wooden beams in support zones to restore their bearing capacity were determined. If the loss of bearing capacity is more than 35%, this method is not recommended due to the advisability of replacing such structures.

Paper [2] presents the first results of an experiment with wooden elements reinforced in the tension zone with strips of natural fibers of flax, hemp, bamboo and basalt. The use of these types of composite materials made it possible to obtain wooden beams with strength and rigidity characteristics superior to unreinforced wood, which confirms one of the fundamental aspects on which the idea of reinforcement is based, i.e., obtaining a final product that becomes stronger as a result of a significant reduction in the influence of the physiological defects of wood on structural characteristics.

The aim of study [3] was to investigate the flexural properties of timber beams strengthened with carbon fiber-reinforced polymer (CFRP) plates. Five timber beams made of yellow meranti wood were tested. One of the beams was used as a control (non-reinforced), while the other four beams were reinforced before the four-point failure test. The results showed that the reinforced beams performed better than the control beam. The ultimate and service loads of the reinforced beams increased by 31.8–44.5% and 27.1–80%, respectively, when the CFRP area was 0.15–0.42%. Reinforcing the timber beams with CFRP increased their stiffness. The stiffness of the beams was increased by 32.6–87.6%. Tensile cracking and crushing occurred simultaneously (balanced reinforcement) when the CFRP was around 0.16%. As a result of this study, flexural strength and stiffness modification factors were proposed for timber beams strengthened using carbon fiber-reinforced plastic plates.

The work in [4] showed how fiber-reinforced polymer (FRP) composites can improve the structural performance of glued laminated beams, especially in terms of their flexural and shear strength. In this study, larch beams were reinforced with carbon fiber-reinforced polymer fabric in one, two and three layers. The effect of the number of storeys on the flexural properties of the reinforcing beams was investigated experimentally and numerically. As a result of the study, the best flexural properties were achieved using three-layer reinforcement. It was noted that one- and two-layer reinforcement were also significantly more effective than the reference beam. Numerical analysis showed close values to the experimental test results. As a result of comparing the results obtained using the numerical model with the experimental data, it was concluded that the use of fiberglass fabric managed to significantly improve the performance of larch wood. The model is a useful tool for studying the effect of reinforcement ratio and will be used to optimize the design of the larch beam.

In the manufacture of wooden rods, several elements must be combined to obtain the required cross-section height. The most common glued structures are formed from relatively thin boards using adhesive joints. An increased cross-section height can be obtained by combining two relatively thick solid wood beams. The easiest way to combine the beams into a single structure is to use an adhesive joint. In addition to glue, a layer of composite material can be included in the joint, which is placed between the adhesive layers. The composite material in this compound performs the role of reinforcement [5,6,7,8].

The presence of a composite material in the adhesive joint smoothes the peaks of tangential stresses, increases the plasticity of the joint and, as a result, should lead to an increase in the strength of the joint and the bearing capacity of the element as a whole. Experimental studies on the impact of a beam joint with the use of CM inserts on the cut were carried out [9,10,11,12,13].

When using CM inserts in a two-layer structure, the joint of the beams experiences maximum shear forces unevenly distributed along the length of the element. The nature of the distribution depends on the operating load, the characteristics of the inclusion of wood and composite material in the work when using an adhesive joint, and the properties of wood, composite and glue [14,15]. The features of the use of composite materials in building structures, the use of adhesive joints, and the interaction of dissimilar materials in structures are considered in a number of works.

Fiberglass and carbon fiber composite materials have been used in construction [16,17]. Carbon fiber composite materials are used to strengthen reinforced concrete, steel and wooden structures [18].

The use of carbon fiber to strengthen building structures leads to different results [14,15]. For reinforced concrete, there is an increase in crack resistance, stiffness and load-bearing capacity. For steel beams, there is a noticeable increase in load-bearing capacity with a slight increase in stiffness. A number of regulatory documents on strengthening have been developed [19].

The study of building structures reinforced with composite materials requires experimental studies that allow us to obtain objective data. Experimental studies of wooden structures are prepared in different articles [20,21,22].

To obtain objective data on the operation of the adhesive joint with a composite material liner, experimental studies were carried out to assess the bearing capacity and deformability of the proposed types of compounds on samples and structures of composite cross-sections of full-scale dimensions [23,24,25,26]. Tests and evaluations of the bearing capacity of joints were carried out according to the established method [27,28,29,30,31]. Wooden structures of composite sections were based on the studied compounds—according to the methods in [32,33]. An analysis of sources used to test the bending of wooden elements was completed [34,35,36,37].

The following tasks were performed on the samples in the course of investigating the functioning of the “CM insert” compounds:

–: Strength and deformation characteristics were determined for the “CM insert” joints;
–: Based on statistical analysis of experimental data, the load-bearing capacity and deformability of the joints were evaluated for two types of composite material.

The novelty of the study lies in the possibility of bringing the work of a traditionally flexible joint closer to rigid work and using this type of joint for gluing beams.

2. Materials and Methods

To assess the load-bearing capacity and deformability of the “CM insert” joint, tests of 2 types of samples (B-1 and B-2), of the joint of 15 samples of each type, were carried out. The two types of samples differ both in the composition of the adhesive layer and the reinforcing composite material. The proposed “CM insert” connection is shown in Figure 1.

The “CM insert” is the connection of timber elements by means of a composite seal formed during the manufacturing process on the contact surfaces of the connected elements (Figure 1). The composite insert is located in the bonding seams along the contact surfaces. This solution prevents mutual shifting of the connected elements, replacing the traditional mechanical connection.

The evaluation of the load-bearing capacity and deformability of the “CM insert” joint was carried out on the basis of the test results of two series of joint specimens, each consisting of 15 specimens. The “CM insert” connection of B-1-type samples is made using a composite material based on KDA-M epoxy resin with Etal-45 hardener and a reinforcing component in the form of three layers of fiberglass T13. The “CM insert” joint in the B-2 series of samples is made of a composite based on PolyTay 303TAE polyester resin with Butanox M50 hardener and a reinforcing component in the form of three layers of T13 fiberglass. The type of B-1 samples in the test process is shown in Figure 2.

The selection of wood samples is carried out according to the general criteria of requirements for structural wood in terms of the quantity and total volume of structural features—defects, cross-grain, knots.

The characteristics of the resin during curing include a bending strength of 111 MPa, and the modulus of elasticity during bending is 4000 MPa. As is known, epoxy resins are widely used to strengthen wooden structures, including in particularly stressed areas. For the sample parameters, such as symmetrical double-cut, the joint operation of the elements in its composition is ensured by an insert based on a composite material.

The samples were fixed in a testing machine and loaded in stages until destruction. The tests were carried out on 10-ton press testing machines WDW-100E, Instron 1000 HDX (Inston Corp., Grove City, PA, USA), at standard temperature and humidity of wooden samples.

The evaluation of the load-bearing capacity and deformability of the “CM insert” joints according to the test results was carried out according to the method in [32,33]. For each tested specimen of the B-1 and B-2 series, the destructive load Nmax, [kN]; full DF deformations [mm] and full ∆DF differential deformations [mm] at each loading stage were determined. The load N_I-II, [kN], corresponding to the upper limit of the elastic range of the specimens, and the calculated bearing capacity of the joint, Nd, were determined. Knowing the total deformations DFi [mm] of the specimens at each (i-th) loading stage before loading Ni, the average intensity of deformation growth

D_F,I-II/N_I-II [mm/kN] within the elastic work of the couplings was determined. The required reliability coefficient γV for the value of the destructive force Nmax and the calculated bearing capacity NF = Rexp of the “CM insert” compound for each sample were calculated according to Formula (1) on the basis of two criteria—the destructive force Nmax and the load N_I-II (see Formulas (2) and (3)). Of the two values obtained, the lowest is taken as the calculated resistance of the specimen:

NF = min Rexp.v,Rexp._I-II

(1)

The average cleavage stresses were determined for each specimen by the Nmax, N_I-II, and NF forces:
where N is the force Nmax, N_I–II or Nmax; F is the chipping area, determined taking into account the number of chipping planes (n = 2). Taking into account the size of the samples, the cleavage area F = (15.0 × 9.5) * 2 = 285 cm².

A statistical analysis of the strength and deformation characteristics of the “CM gluing” compounds of the B-1 and B-2 series, made on the basis of the epoxy and polyester matrix, respectively, and fiberglass T-13 was carried out. The normative and calculated cleavage resistance was determined, as well as the average for the site of the “CM insert” B-1 and B-2 joints based on an epoxy and polyester matrix:

$- normative resistance {R c l}^{n} = {R c l}^{t} (1 - η_{n} v)$

(3)

$- calculated resistance R c l = {R c l}^{t} (1 - η_{c a l} v)$

(4)

where ${R c l}^{t}$ is the temporary chipping resistance (average distribution value), MPa; $η_{n}$ = 1.65 is the quantile in the assumed statistical distribution function with a security of 0.95; $η_{c a l}$ = 2.33 is the quantile in the assumed statistical distribution function with a security of 0.99; and ν is the coefficient of variation according to the test results.

3. Results

The experimental data obtained during the tests are systematized, tabulated and presented in the form of graphs for both types of samples.

3.1. Test Results of B-1-Type Samples

The test results of the B-1-type samples of the “CM- insert” connection are presented in Table 1 and in the graphs in Figure 3.

Generalizing from the test results of the B-1 series specimens, the stress–strain state of the “CM insert” joints was based on an epoxy matrix; the average destructive load for the joint specimens was Nmax = 131.5 kN, which corresponds to the average stress at the cleavage site τcl.aver.Nmax = 4.61 MPa. The upper limit of the elastic work area of the joint was N_I-II = 78.2 kN, which corresponds to the average stress at the cleavage site τcl.aver._I-II = 2.74 MPa. The average values of total deformations of the “CM insert” joint of the B-1 series were at a load of N_I-II corresponding to the upper limit of the elastic work of the joint D_{F, I-II} = 0.113 mm; the average value of the intensity of deformation growth within the elastic work of the “CM insert” joint D_{F, I-II}/N_I-II = 0.00148 mm.

The results of the statistical processing of the strength and deformation characteristics of the B-1 series “CM insert” compound based on an epoxy matrix are presented in Table 2.

From Table 2, we see that the experimental data are stable; the variability of the strength parameters of the “CM insert” compound does not exceed the corresponding indicators for wood [33]. The coefficient of variation V for forces Nmax, N_I-II, and for stresses τaver is 18.1–18.4% (for pine wood when chipping along fibers V = 20%). The accuracy index for the strength characteristics of the “CM insert” compound based on an epoxy matrix is p = 4.68–4.92% < 5%.

The normative cleavage resistance of the “CM insert” joint Rcln for the loading mode “A”, corresponding to a linearly increasing load under standard machine tests, calculated from the values of the average stresses on the cleavage site τdestr = Nmax/Fcl with a confidence probability (security) at a minimum Rd =0.95, for the B-1 series was

${R c l}^{n} = {R c l}^{t} (1 - η_{n} v) = 4.61 * (1 - 1.65 * 0.184) = 3.21 MPa$

which is less than the normative resistance of pure wood to chipping along the fibers Rn.p = 4.5 MPa [33].

The calculated cleavage resistance of the “CM insert” joint Rcl for loading mode “A” (Table 3 [33]), corresponding to a linearly increasing load in standard machine tests, calculated from the values of the average stresses at the cleavage site, τdestr = Nmax/Fcl, with a confidence probability (security) of at least Rd = 0.99, for the B-1 series was

$R c l = {R c l}^{t} (1 - η_{c a l} * v) = 4.61 * (1 - 2.33 * 0.184) = 2.63 MPa .$

The mean value of the calculated cleavage resistance of the “CM insert” joint, calculated according to the method in [32,33], which should correspond to loading modes B and G (Table 4 [33]), with a coefficient of variation V = 0.191, was as follows

Rcl = Σ(NF, min/F)/n = 1.58 MPa

where n = 15 is the number of B-1 series samples.

The maximum probable deformation of the “CM insert” joint based on an epoxy matrix at the load level N_I-II was

Dmax0.95 = D_I-II, aver (1 + ηn ν) = 0.00148 ∗ (1 +1.65 ∗ 0.176) = 0.0019 mm.

3.2. Test Results of B-2-Type Samples

The test results of the B-1-type samples of the “CM insert “ connection are presented in Table 3 and in the graphs in Figure 4.

Generalizing from the test results of the B-2 series specimens, the stress–strain state of the “CM insert” compounds was based on a polyester matrix on average; the destructive load for the compound specimens was Nmax = 40.15 kN, which corresponds to the average stress at the cleavage site τcl.aver.Nmax = 1.41 MPa. The upper limit of the elastic working range of the joint was N_I-II = 27.8 kN, corresponding to the average stress at the cleavage site τcl.aver._I-II = 0.975 MPa. The average values of the total deformations of the “CM insert” joint of the B-2 series at a load of N_I-II, corresponding to the upper limit of the elastic work of the joint, were D_{F, I-II} = 0.059 mm; the average value of the intensity of the deformation growth within the elastic work of the “CM insert” joint was D_{F, I-II}/N_I-II = 0.0028 mm.

The results of statistical processing of the strength and deformation characteristics of the “CM insert” compound of the B-2 series based on a polyester matrix are presented in Table 4.

From Table 4, we see that the coefficient of variation V for forces Nmax, N_I-II, and for stresses taver is 20.2–26.7% (for pine wood when chipping along fibers V = 20%); the accuracy index for the strength characteristics of the “CM insert” compound based on a polyester matrix was p = 5.2–6.88% > 5%. This indicates that the variability of the obtained strength characteristics of the “CM insert” compound based on a polymer matrix exceeds the corresponding indicators for wood [33] by 1.04–1.33 times.

The normative cleavage resistance of the “CM insert” joint Rcln for the loading mode “A”, corresponding to a linearly increasing load during standard machine tests, calculated from the values of the average stresses on the cleavage site τdestr = Nmax/Fcl with a confidence probability (security) at a minimum Rd =0.95, for the B-2 series was

${R c l}^{n} = {R c l}^{t} (1 - η_{n} v) = 1.41 * (1 - 1.65 * 0.259) = 0.806 MPa$

which is less than the normative resistance of pure wood to chipping along the fibers Rn.p = 4.5 MPa [33].

The calculated cleavage resistance of the “CM insert” joint Rcl for the loading mode “A” (Table 4, [33]), corresponding to a linearly increasing load under standard machine tests, calculated from the values of the average stresses on the cleavage site τdestr = Nmax/F with a confidence probability (security) at a minimum Rd = 0.99, for the B-1 series, was

$R c l = {R c l}^{t} (1 - η_{c a l} * v) = 1.41 * (1 - 2.33 * 0.259) = 0.558 MPa .$

The average value of the calculated cleavage resistance of the “CM insert” joint, calculated according to the method of [28,29], which should correspond to the loading mode B and Г (Table 4 [33]) with a coefficient of variation V = 0.267, was

Rcl = Σ(NF, min/F)/n = 0.465 MPa

where n = 15 is the number of samples in the B-21 series.

The maximum probable deformation of the “CM insert” joint based on a polyester matrix at the load level N_I-II will be

Dmax0.95 = D_I-II, aver (1 + ηn ν) = 0.00219 ∗ (1 + 1.65 ∗ 0.178) = 0.00284 mm.

4. Discussion

The connection in question is a type of traditionally flexible connection; however, the materials used to ensure joint operation under load allow this connection to be transferred to the rigid category, where the shear deformation of the connected elements is negligible, compared to the linear dimensions of the samples.

A comparative assessment of the bearing capacity and deformability of the “CM insert” compounds made on the basis of an epoxy matrix (B-1 series) and a polyester matrix (B-2 series) was performed in relation to the compounds of the B-2 series. We compared the “CM insert” compound on a polyester matrix with the “CM insert” compound on an epoxy matrix and found the following:

–: The destructive load Nmax and temporary chipping resistance Rclt is 3.27 times greater;
–: The load of N_I-II, corresponding to the upper boundary of the elastic region of the “CM insert” joint, is 2.8 times greater;
–: The load NF corresponding to the calculated bearing capacity of the “CM insert” joint and the calculated cleavage resistance, determined by the method in [32,33], is 3.4 times greater;
–: The deformability of the “CM insert” D_F and _I-II/N_I-II joint at the level of the upper boundary of the elastic work area is 1.48 times less;
–: The normative cleavage resistance Rcln of the joint “CM insert” is 4 times greater;
–: The calculated cleavage resistance Rcl of the joint “CM insert” is 4.7 times greater.

The obtained stress values numerically correspond to the calculated studies obtained as a result of modeling the connection of wooden structures in the scientific article [1].

5. Recommendations for the Calculation of Joints of Wooden Elements with a Composite Material Based on Fiberglass

For each type of joint, strength characteristics were determined as a function of the stress state and the type of composite material used in the joint, as well as deformation characteristics, which determine the deformability within the calculated load-bearing capacity and the elastic operation of the joints.

The main normalized strength characteristics of the “CM insert” joint are the calculated joint resistances determined using the method and taking into account the statistical variability of the strength indicators with a confidence level of at least 0.95 for standard resistances and 0.99865 for calculated resistances based on the test results of the joint samples. The maximum probable deformability of the “CM insert” joint was also determined from the test results with a confidence level of 0.95.

Strength and deformation characteristics corresponding to the calculated bearing capacity of the joints have been established for the “CM insert” joints:

The temporary chipping resistance:
–
For a compound on an epoxy matrix, Rclt = 4.61 MPa;
–
For a compound on a polyester matrix, Rclt = 1.41 MPa.
The average value of the calculated cleavage resistance (according to the method [28,29]):
–
For a compound on an epoxy matrix, Rcl = 1.58 MPa with a coefficient of variation V = 0.191;
–
For a compound on a polyester matrix, Rcl = 0.465 MPa with a coefficient of variation V = 0.267.
The normative chipping resistance with a confidence probability (security) at a minimum of Rd = 0.95 for the loading mode “A” corresponding to a linearly increasing load during standard machine tests:
–
For connection on an epoxy matrix, Rcln = 3.21 MPa;
–
For a compound on a polyester matrix, Rcln = 0.806 MPa.
The calculated chipping resistance with a confidence probability (security) at a minimum of Rd = 0.99865 for the loading mode “A” corresponding to a linearly increasing load during standard machine tests:
–
For connection on an epoxy matrix, Rcl = 2.63 MPa
–
For connection on a polyester matrix, Rcl = 0.558 MPa;
The deformability of joints within the calculated bearing capacity:
–
For joints type 1 and type 3, D_F/Nd = 0.0021 mm/kN;
–
For connection type 2, D_F/Nd = 0.0031 mm/kN.

The calculated bearing capacity of the “CM insert” joint should be determined by chipping the composite material along the joint seam.

[Tcl_CM] = Rcl_CM ∗ L_CM ∗ binsert,

where Rcl_CM = Rcl/(γm ∗ γmn) is the calculated resistance to cleavage of the “CM insert” joint; Rcl_CM—see paragraph 8.64; γ_m = 1.1 is the coefficient of reliability of the composite material in the “CM insert” joint; γ_mn is the coefficient of reliability according to the method of manufacture of the joint, L_CM = L_O − 2 ∗ 50 mm is the estimated length of the “CM insert”; and binsert is the width of the strip of the composite liner in the joint.

The length of the “CM insert” joint L_O when developing a new wooden element of composite section should be at least one-quarter of the span of the element being created. The working length of the “CM insert” joint L_O when restoring an existing wooden element should be allocated according to the size of the restored section on the wooden structure but not less than one-half of the length of the restored section.

We calculated the chipping resistance of the composite material and the results are as follows:

–: For “CM insert” joints based on epoxy matrix and fiberglass, T13 Rcl = 1.58 MPa;
–: For “CM insert” compounds based on polyester matrix and fiberglass, T13 Rcl = 0.465 MPa;

6. Conclusions

In total, 15 samples were tested for each type of compound.

2.

For “CM insert” compounds using composite materials based on an epoxy matrix (B-1) and a polyester matrix (B-2), the following results were obtained:

The temporary cleavage resistance is Rclt = 4.61 MPa and 1.41 MPa;
The average value of the calculated chipping resistance is Rcl = 1.58 MPa and 0.465 MPa with coefficients of variation V = 0.191 and 0.267;
The normative cleavage resistance with a confidence probability (security) at a minimum Rd = 0.95 for the loading mode “A” corresponding to a linearly increasing load under standard machine tests is Rcln = 3.21 MPa and 0.806 MPa;
The calculated chipping resistance with a confidence probability (security) at a minimum of Rd = 0.99 for the loading mode “A” corresponding to a linearly increasing load at standard machine tests is Rcl = 2.63 MPa and 0.558 MPa;
The maximum probable deformation at the load level corresponding to the upper boundary of the N_I-II elastic region is Dmax 0.95 = 0.0019 mm and 0.0028 mm.

3.

A comparative assessment of the bearing capacity and deformability of the “CM insert” compounds revealed that the strength characteristics of the epoxy matrix-based compound are 2.8–4.7 times greater than those of the polyester matrix-based compounds, while the deformability is 1.48 times less.

4.

For use in load-bearing wooden structures, “CM insert” compounds based on a KDA-M epoxy matrix with Etal-45 hardener and T-13 fiberglass are recommended.

5.

As a result, the proposed connection made it possible to eliminate mutual shear deformations from the connection operation and proved the rigidity of the connection and the viability of the composite material used.

Author Contributions

Conceptualization, A.T.; methodology, A.T.; software, L.N. and K.A.; validation, L.N. and K.A.; formal analysis, A.T.; investigation, A.T., L.N. and K.A.; resources, A.T. and L.N.; data curation K.A.; writing—original draft preparation, L.N. and K.A.; writing—review and editing, A.T.; visualization, L.N.; supervision, A.T.; project administration, A.T.; funding acquisition, A.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by a grant issued by the National Research Moscow State University of Civil Engineering as a result of a competition among the lecturers for implementation of fundamental and applied scientific research (R&D) by the teams of NRU MGSU researchers (Order 527/130-22/06/2022-project No. 30). The research was funded by the National Research Moscow State University of Civil Engineering (grant for fundamental and applied scientific research, project No. 30-234/130).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1.
Connection of wooden elements with a composite material based on an epoxy matrix and fiberglass with a liner installed between adjacent planes of the elements to be connected—“CM insert”: 1—wooden elements to be connected; 2—welding seam; 3—composite material, 4—adhesive layer.

Figure 2.
B-1-type sample view during testing.

Figure 3.
Graphs of the “deformation load” dependence of B-1-type samples based on an epoxy matrix.

Figure 4.
Graphs of deformations of full DF [mm] samples of the “CM insert” B-2 series compound based on a polyester matrix.

Table 1.
The bearing capacity and deformability of the samples of the “CM insert” joint. B-1 series.

№ sam.	Τdestr = = Nmax/F, MPa	EBC of the Connection, Rexp		Сhipping Resistance Rcl = Rexp/F		Joint Deformations
№ sam.	Τdestr = = Nmax/F, MPa	Nmax kN	N_I-II kN	Nmax MPa	N_I-II MPa	D_{F, I-II} mm	D_{F, I-II}/N_I-II mm/kN
1	2	3	4	5	6	7	8
В-1-1	3.16	30.2	50.8	1.06	1.78	0.157	0.00238
В-1-2	5.16	51.4	71.5	1.80	2.51	0.125	0.00134
В-1-3	4.53	44.0	64.6	1.54	2.27	0.139	0.00165
В-1-4	4.21	40.8	46.2	1.43	1.62	0.091	0.00152
В-1-5	3.88	37.6	46.2	1.32	1.62	0.099	0.00165
В-1-6	4.40	42.8	50.8	1.50	1.78	0.089	0.00134
В-1-7	4.95	48.2	60.0	1.69	2.11	0.110	0.00140
В-1-8	5.47	53.4	73.8	1.87	2.59	0.087	0.00091
В-1-9	4.84	47.1	55.4	1.65	1.94	0.100	0.00139
В-1-10	4.42	42.9	60.0	1.51	2.11	0.117	0.00149
В-1-11	4.00	38.8	50.8	1.36	1.78	0.112	0.00170
В-1-12	5.79	56.7	73.8	1.99	2.59	0.116	0.00120
В-1-13	5.86	57.4	73.8	2.01	2.59	0.127	0.00132
В-1-14	3.16	30.3	50.8	1.06	1.78	0.103	0.00156
В-1-15	5.37	52.4	73.8	1.84	2.59	0.133	0.00139
Aver.	Rclt = 4.61	44.94	60.15	1.58	2.11	0.113	0.00148

Table 2.
Statistical processing of test results of B-1-type samples of the “CM insert“ connection.

Type of Load and Deformation	Meas. Units	M	S	V %	m	p %
Nmax	kN	131.48	24.21	18.4	6.25	4.75
Rclt = τaver = Nmax/F	MPa	4.61	0.85	18.4	0.22	4.75
N_I-II	kN	78.2	14.18	18.1	3.66	4.68
(D_{F, I-II}/N_I-II) *	mm/kN	0.00148	0.00032	21.4	0.00008	5.53
Rcl = N_F, min/F	MPa	1.58	0.30	0.191	0.08	4.92

Table 3.
Bearing capacity and deformability of B-2-type samples.

№ sam.	Τdestr * = = Nmax/F, MPa	EBC *** of the Connection, Rexp		Сhipping Resistance Rcl = Rexp/F		Joint Deformations
№ sam.	Τdestr * = = Nmax/F, MPa	Nmax kN	N_I-II kN	Nmax MPa	N_I-II MPa	D_{F, I-II} mm	D_F/N_I-II mm/kN
1	2	3	4	5	6	7	8
В-2-1	1.15	10.75	18.46	0.38	0.65	0.070	0.0038
В-2-2	1.13	10.52	16.15	0.37	0.57	0.065	0.0040
В-2-3	1.37	12.84	18.46	0.45	0.65	0.060	0.0032
В-2-4	1.28	12.04	18.46	0.42	0.65	0.047	0.0025
В-2-5	1.58	14.89	27.69	0.52	0.97	0.053	0.0019
В-2-6	1.86	17.59	27.69	0.62	0.97	0.072	0.0026
В-2-7	1.16	10.83	20.77	0.38	0.73	0.082	0.0039
В-2-8	1.47	13.87	20.77	0.49	0.73	0.062	0.0030
В-2-9	1.26	11.82	20.77	0.41	0.73	0.057	0.0027
В-2-10	1.79	16.91	23.08	0.59	0.81	0.064	0.0028
В-2-11	1.16	10.82	18.46	0.38	0.65	0.062	0.0033
В-2-12	2.32	22.04	27.69	0.77	0.97	0.049	0.0018
В-2-13	1.58	14.86	27.69	0.52	0.97	0.056	0.0020
В-2-14	1.08	10.05	16.15	0.35	0.57	0.045	0.0028
В-2-15	0.95	8.80	18.46	0.31	0.65	0.046	0.0025
Aver.	Rclt ** = 1.409	13.243	21.385	0.465	0.750	0.059	0.0028

Table 4.
Statistical processing of test results of samples of “CM insert” compound. B-2 series.

Type of Load and Deformation	Meas. Units	M	S	V %	m	p %
Nmax	kN	40.147	10.406	0.259	2.687	6.7
Rclt = τaver = Nmax/F	MPa	1.409	0.365	0.259	0.094	6.692
N _I-II	kN	27.80	5.609	0.202	1.448	5.209
(D_{F, I-II}/N_I-II) *	mm/kN	0.0219	0.00054	0.2445	0.00014	6.314
Rcl = NF, min/F	MPa	0.465	0.124	0.267	0.032	6.882

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Experimental Studies on Joints of Wooden Elements with Proposed “CM Insert”

1. Introduction

2. Materials and Methods

3. Results

3.1. Test Results of B-1-Type Samples

3.2. Test Results of B-2-Type Samples

4. Discussion

5. Recommendations for the Calculation of Joints of Wooden Elements with a Composite Material Based on Fiberglass

6. Conclusions

Author Contributions

Funding

Data Availability Statement

Conflicts of Interest

References

Greenberg

1. Introduction

2. Materials and Methods

3. Results

3.1. Test Results of B-1-Type Samples

3.2. Test Results of B-2-Type Samples

4. Discussion

5. Recommendations for the Calculation of Joints of Wooden Elements with a Composite Material Based on Fiberglass

6. Conclusions

Author Contributions

Funding

Data Availability Statement

Conflicts of Interest

References

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