Tāq Kasrā, one of the last surviving buildings of the ancient Sasanian Empire’s capital city, has the largest unreinforced brick arch in the world.
I’m not a student of architecture, but it seems that prior to modern times the two most important elements for architects to master were pillars and arches. Certainly, the arch (and its architectural siblings the vault and the dome) were responsible for glorious Byzantine architecture like the Hagia Sofia and the dizzying heights of Gothic cathedrals. But one of the most impressive archways constructed in ancient times has to be the archway of Tāq Kasrā in modern Iraq.
This arch is huge: 37 metres high and 26 metres wide. When built it was part of a large complex, thought to be a palace of the Sasanian Empire (the Persian / Iranian Empire that ruled large parts of the Middle East for more than four centuries). Now the rest of the palace – and indeed the rest of the capital city around it – are gone, but the arch remains. No-one has conclusively dated its construction, but estimates fit between the middle of the 3rd and the middle of the 6th century CE.
Tāq Kasrā is an iwan, a very distinctive part of Persian architecture. It’s a large vault, enclosed on three sides but open to the world on the remaining side. This structure was later adopted by Islamic architecture; the front of the Taj Mahal, for example, bears a huge iwan as its entrance. This arch was built of brick and quick-drying cement, and for a long time it was a record-breaker – variously described as the largest vault, the largest arch, the largest man-made arch, the largest brick arch, and the largest unreinforced brick arch in the world. I don’t know if it can truly lay claim to all of those titles, but it is still an unprecedented feat of engineering.
Up until the end of the 19th century CE the archway was flanked by two large facades, but one of them collapsed due to flooding in 1888. There have been some attempts to restore and reconstruct the lost sections, but it was damaged again last year and its long term survival remains in question.
[Thanks to Siobhan L. for suggesting this topic.]