Working capability of composite wood-rubcon (rubber concrete) reinforced bridge beams under static loads

. The results of experimental studies of the performance of bent composite wood-rubcon reinforced bridge beams under static loads are presented and the bearing capacity of composite wood-rubcon reinforced bridge beams is determined. A method for calculating wood-rubcon reinforced composite bridge beams is proposed.


Introduction
Wood is one of the first effective building materials from which man built housing, bridges and other structures. The main advantages of wood include light weight, ease of transportation and handling, chemical resistance, high mechanical strength, low sound and heat conductivity and, finally, frost resistance. The effectiveness of the use of wood-rubcon composite bridge beams can be increased by reinforcing the stretched area of the beams, which increases their strength and rigidity [1][2][3][4][5][6][7][8].
At present, wood with large diameters has become scarce, from which bridge and transverse beams were made, as well as sleepers. The direction of work associated with the reinforcement of the tensile zone of wooden composite bridge beams is an urgent task, as it makes it possible to work with wood of small diameters [9][10][11][12].
With the introduction of new double-layer wood-rubcon reinforced structures, it becomes necessary to experimentally and theoretically study the operation of wood-rubcon reinforced structures under static loads .
The purpose of the work was to determine the strength and deformability of woodrubcon reinforced composite bridge beams with transverse bending, to determine theoretical and experimental deflections and to compare experimental data with the results of theoretical calculations and to analyze the causes of possible deviations.
Scientific novelty of the work: -the experimental and theoretical bearing capacity of wood-rubcon reinforced composite bridge beams has been determined; -an analysis was made of the efficiency of compounds of wood with rubcon under static loads.

Production of wood-rubcon reinforced composite bridge beams
Structurally, the wood-rubcon reinforced composite bridge beams consist of two layers, the first upper layer includes wooden beams (1) with the frame (2) (Figure 1.), then the second lower layer is made of rubcon (3) (Figure 2). Interlayer contact is reinforced with steel cage frames (Figures 3, 4). After laying the rubcon layer, the double-layer element is placed in a dry heating chamber, where the process of vulcanization occurs at a temperature 120±5°C.   In order to reduce the loads on wooden composite bridge beams during testing, the width and height of the cross section and the length of the experimental samples were halved, which did not lead to a change in the operation of composite bridge beams, since all natural defects are manifested at a width of 100 mm [13].

Experimental and theoretical studies of the operation of woodrubcon reinforced composite bridge beams under static loads
The

Determination of actual dimensions of wood-rubcon reinforced composite bridge beams
The following materials were used for the manufacture of wood-rubcon reinforced composite bridge beams:  wood -pine grade I;  rubcon;  reinforcing bar of class A-300.
Initial data:  wood moisture -20%;  cross-sectional height of a wood-rubcon reinforced composite bridge beam, h=120 mm;  width of the cross-section of a wood-rubcon reinforced composite bridge beam, b=100 mm;  height of the cross section of the 1st layer of wood, hd=90 mm;  cross-sectional height of the 2nd layer of rubcon, hk=30 mm;  beam length, l=1600 mm;  design beam span, lp=1100 mm.

Determination of the design characteristics of the material of a woodrubcon reinforced composite bridge beam
Design bending resistance of wood ( р ) equals: where -the design resistance of wood, MPa, shown in Es -design modulus of elasticity of reinforcing bar, Es=240 000 MPa.

Determination of the bearing capacity of a wood-rubcon reinforced composite bridge beam from the calculated normal bending stresses
where = 2 • 0,140; -design load. From formula (7), we assume that the voltage is equal to the rated resistance and we determine the rated load

Determination of the bearing capacity of a wood-rubcon reinforced composite bridge beam from the estimated tangential stresses in bending
where Q = 2 , then = 3 4 ≤ с From formula (9), we assume that the shear stress is equal to the design resistance when shearing and determine the design load

Determination of the bearing capacity of a wood-rubcon reinforced composite bridge beam from the stiffness condition
The standard deflection of the beam is 1/170 span, hence The calculated load is equal to the minimum calculated by the formulae (8), (10), (12). Destructive load is determined by multiplying the design load value by the material safety factor γ м = 1,15, then . = • γ м =32,50•1,15 = 37,38 kN

Drawing up a design diagram of a wood-rubcon reinforced composite bridge beam and determining the forces
Design scheme. The design scheme ( Figure 9) of a wood-rubcon reinforced composite bridge beam is taken in the form of a single-span beam on two supports.

Verification of a wood-rubcon reinforced composite bridge beam according to groups I and II of the limiting state
а) Checking beam according to the I ultimate state of strength: where R p -design bending resistance, MPa [2]; М -maximum bending moment.    To obtain the calculated characteristics of the rubcon, prism samples were made with a dimension size of 40 mm × 40 mm × 160 mm [19] ( Figure 6). We tested them for compressive strength. We plotted the graph of the relationship between stress and strain ( Figure 13).