Domed Perfection: The Pantheon, Santa Maria del Fiore, St. Peter’s Basilica
Roman grandeur • Pantheon is a Greek work meaning to honor all Gods.
History • Agrippa had the original Pantheon erected in 27 B.C. after he crushed Anthony and Cleopatra at Actium. • Agrippa dedicated to the Romans’ septet of planetary gods out of gratitude of his conquest. • After two fires damaged the original structure, Hadrian rebuilt it from AD 117 to 125 • Interesting fact: The front of the Pantheon says “M. AGRIPPA L. F. COS. TERTIUM FECIT” which translated means “Marcus Agrippa, son of Lucius, in his third consulate, made it.” even though Hadrian actually rebuilt it.
Orientation • Main entrance of Pantheon faces north, and the whole building is oriented on the north-south axis of the building.
Foundation • Built on marshy, unstable earth which gave serious supporting problem to builders. Ring foundation rested on a bed of bluish colored river clay. In final construction phase, the foundation cracked at the two ends of North-South axis.
Walls • Walls are structurally a series of concrete piers separated by 8 large niches. • Thick walls act like buttress to support thrust from dome. • Within these niches lie great kings of Italy, important popes, and at one time Raphael.
Dome Structure • Rotunda: inner diameter of 142.4 feet (43.4 m), which is about half the length of our football field. • The dome could hold a 143ft. diameter perfect hemisphere. However, from the top and sides it looks much flatter than a hemisphere. Why? • At the bottom, the walls were ~20 feet wide, while at the top they were only ~5 feet wide. This was one of the ways it resisted the outward thrust forces at the bottom of the dome.
Construction • Old Mycenaean tombs in Greece were made by corbeling stone slabs over one another. • It is likely that Romans used this principle in placing one-step ring on another in building the rings on the dome. • Compression ring at center of dome is effective in properly distributing compression forces at this point.
Materials • All concrete, no steel reinforcement! • How did the concrete hold that long? Special concrete: volcanic ash (pozzolana), lime, water. • Difference is: • Common plaster – made from wet lime and plain sand, crystalline atomic structure doesn’t allow calcium hydroxide from lime to enter and react. • Pozzolana: amorphous silica atomic structure with many holes, allowing calcium hydroxide to enter and make a concrete gel that expands, bonding pieces of rock together.
Building process • To build load-bearing wall: • Create form through wood, stacked rock, or brick. • Layer of aggregate rock (hand-sized rock or broken brick), then tightly tamped pozzolan mortar into aggregate with special tool (beetle) to get all the air out. • They didn’t mix or “pour” concrete because the concrete was not liquid – low moisture content. • Low water content and close compaction were part of the reason Roman oncrete lasted so long.
Weight-loss regimen • Coffered interior reduces weight and compression forces on bottom. • Also, the Romans used different types of rocks for top and bottom of dome. Lighter rocks on top and heavier on bottom.
Fractures • Cracking in walls and dome • Fractures “reaching from base of rotunda to summit of dome,” probably brought about by differential settlement from uneven loading of wall, esp. near entrance of rotunda in the principle niche. • Length of cracks (from base of dome upwards to about 54 deg) corresponds well with natural range of hoop tension in dome.