Why did British and French forces shell Tianjin in 1860?

In 1860, British and French forces shelled Tianjin as part of the resumed hostilities of the Second Opium War.21

The conflict had been reignited in 1859 when the Chinese refused to allow British and French diplomats to pass by the Dagu forts at the mouth of the Hai River to reach Beijing for the ratification of the Treaties of Tianjin. After the Chinese government failed to ratify these agreements, which had been signed in 1858, the allied forces launched a military campaign in August 1860 to enforce the terms. They destroyed the Dagu batteries and proceeded upriver to Tianjin, eventually capturing Beijing and forcing the Chinese to sign the Beijing Convention.2341