TABLE OF CONTENTS Frequently in civil engineering construction beams are fabricated from comparatively inexpensive materials of low strength which are reinforced by small amounts of high- strength material, such as steel. In this way a timber beam of rectangular section may have steel plates bolted to its sides or to its top and bottom surfaces. Again, concrete beams are reinforced in their weak tension zones and also, if necessary, in their compression zones, by steel-reinforcing bars. Other instances arise where steel beams support concrete floor slabs in which the strength of the concrete may be allowed for in the design of the beams. The design of reinforced concrete beams, and concrete and steel beams is covered by Codes of Practice and relies, as in the case of steel beams, on ultimate load analysis. The design of steel-reinforced timber beams is not covered by a code, and we shall therefore limit the analysis of this type of beam to an elastic approach.
The timber joist of breadth b and depth d shown in Fig. 12.1 is reinforced by two steel plates bolted to its sides, each plate being of thickness t and depth d. Let us suppose that the beam is bent to a radius R at this section by a positive bending moment, M. Clearly, since the steel plates are firmly attached to the sides of the timber joist, both are bent to the same radius, R. Then, from Eq. (9.7), the...
