Which characteristic is true of an unreinforced concrete beam with primary failure in flexure?

The statement that an unreinforced concrete beam with primary failure in flexure fails by yielding of steel first is not accurate in this context. In an unreinforced concrete beam, the primary load-carrying mechanism is the concrete itself, which is strong in compression but weak in tension. Therefore, when such a beam reaches its failure point, it typically does so due to the cracking and eventual crushing of the concrete rather than yielding of steel reinforcement, which is not present in unreinforced designs.

Concrete beams are capable of withstanding compressive stresses, but the tensile stresses result in the formation of cracks. This leads to the primary mode of failure being characterized by the crushing of the concrete once the tensile stresses exceed the tensile strength of the material, which is considerably lower than its compressive strength.

Furthermore, unreinforced concrete beams are generally discouraged in construction due to their inability to adequately support tensile loads, making them potentially unsafe in structural applications. Building codes typically require reinforcement in concrete members to improve their performance under loads and to provide adequate safety margins. This is especially true for elements subject to bending or flexural forces, where steel reinforcement is critical to extending the structural capacity and delaying failure.