Complex sentence: The building was not originally designed to include the flying buttresses around the choir and nave but after the construction began, the thinner walls (popularized in the Gothic style) grew ever higher, stress fractures began to occur as the walls pushed outward.
Split into simpler sentences: The building was not originally designed to include the flying buttresses around the choir and nave. After the construction began and the thinner walls (popularized in the Gothic style) grew ever higher, stress fractures began to occur as the walls pushed outward.