Final answer:
An arch is a form that resists compressive stress by eliminating tensile stress. Concrete is good against compression but poor against shear. Liquids and gases have shear moduli near zero.
Step-by-step explanation:
An arch is a pure compression form that can resist compressive stress. It spans a large area by resolving forces into compressive stresses that eliminate tensile stresses. To prevent the arch from collapsing, the outward thrust needs to be restrained with internal ties or external bracing.
Objects like long shelves loaded with heavy books can experience both compressive stress and tensile stress simultaneously. The top surface of the shelf is in compressive stress, while the bottom surface is in tensile stress.
The shear modulus of concrete used in buildings is very small, making it poor against shear. Modern structures use steel and steel-reinforced concrete to overcome this limitation. Liquids and gases have shear moduli near zero because they flow in response to shearing forces.