What import shape gave the Roman bridges and apueducts their strength?

The Roman bridges and aqueducts were primarily strengthened by the use of the arch shape. This architectural design allowed for the even distribution of weight and the ability to span large distances without the need for excessive material. The semicircular arch not only provided structural integrity but also enabled the construction of higher and more durable structures, making it a hallmark of Roman engineering. Additionally, the use of concrete further enhanced the resilience and longevity of these constructions.