What do small U-shaped reinforcing bars in beams aim to resist?

Small U-shaped reinforcing bars, also known as stirrups, are specifically designed to resist diagonal tension in beams. When a beam is subjected to loads, it experiences various stresses, and one of the critical factors to account for is shear. Shear forces, particularly in beams that span a distance, can result in diagonal cracks occurring from the top of the beam downwards towards the supports.

The U-shaped stirrups help to provide additional tensile strength in these diagonal shear planes, effectively confining the concrete and preventing these cracks from propagating. By doing so, they enhance the overall structural integrity of the beam, allowing it to carry loads safely without failure. The presence of these stirrups thereby improves the beam's capacity to resist shear by holding together the concrete and providing the necessary tensile response required in areas where diagonal tension develops.

In this context, other options like compression, vertical loads, and shear loads might be involved in the overall load-bearing scenario of a beam, but the primary purpose of the small U-shaped bars specifically pertains to resisting diagonal tension due to the nature of the forces and moments acting in a beam subjected to bending and shear.