ANALYSIS OF BENDING STRENGTH IN STEEL REINFORCED TIMBER BEAM

ABSTRACT

The aim of this project was to; obtain the mechanical properties of cypress wood and mild steel plate and to demonstrate experimentally the contribution of steel plate on timber reinforcement configurations (unreinforced timber beam, single steel plate reinforcement and dual steel plate reinforcement).

Four cypress timber beam specimens were selected for mechanical bending test. Each of these specimens was mounted onto a Hounsfield Tensometer and a central load was applied at a rate of 3.3mm/min while keeping the record of load-deflection curve on the W402 graph paper. The test machine itself deflected therefore this was included in the curve and this was subtracted for the purpose of calculating E. This was done by inserting a 25mm square steel rod in place of timber and loading up to maximum load, this gave a steep straight line (machine characteristic line), the deflection of which was subtracted from the record of each timber test. The curves were all plotted on the same graph paper and the value of E was obtained.

Mild steel plate in figure 12 was subjected to a tensile load in a Galdabini extensometer until the material failed under ductile mode of failure. As the specimens were loaded, a graph showing the relationship between the applied stress and the strain was automatically generated and displayed on a computer screen as shown in figure 21. The corresponding stress, strain, maximum load and E were also displayed on the screen.

Composite beams were formed by drilling holes on both timber beams and steel plates at points equidistant from each other as shown in figure 7 and figure 8. To test for bending, these beams (three specimens for each configuration) were each loaded at the center gradually in a bending machine as the loads and corresponding deflections were recorded until the timber beam ruptured. These data was then used to plot load-deflection curves and failure loads for each configuration were recorded and their averages were used to calculate the moment of resistance offered by the composite beam to counter the applied load.

From analysis, a single plate reinforced timber beam recorded a 44% increase in peak load while a dual plate reinforced one recorded a 33% increase in peak load. The same percentages were also recorded in the moment of resistances offered by the composite beams to counter the applied load.

It was also noted that for maximum strength to be achieved, the reinforcing elements had to be on both tension and compression faces and for maximum ductility to be achieved, these elements had to be on tension faces since timber is weak in tension.