How did Michelangelo adapt Donato Bramante’s design for St. Peter’s Basilica?

When Michelangelo was appointed chief architect of St. Peter’s Basilica in 1546, he sought to restore the integrity of the original vision conceived by Donato Bramante.12

Michelangelo’s adaptations included the following:

• Reversion to the Original Plan: He discarded the modifications made by intervening architects and returned to Bramante’s initial concept of a Greek cross plan, featuring four equal arms, rather than the Latin cross design that had been introduced in the interim.12
• Unification of Space: He modified Bramante’s interior to create a more unified space. By enclosing the structure with massive semicircular wall sections, he created interior spaces that harmonized with the hemispherical volume of the dome.2
• Structural Dynamism: On the exterior, Michelangelo applied thin pilasters against the curving walls. These elements provided a sense of "pulsing dynamism" and horizontal rhythm, reflecting his background as a sculptor who utilized the interplay of stone projections and recessions.2
• Dome Support: Around the base of the dome, he designed a columned walkway that functioned similarly to Gothic flying buttresses, helping to manage the dome's heavy downward thrust while maintaining a classical aesthetic.2