Domed Perfection:Domed Perfection:The Pantheon,The Pantheon,

Santa Maria del Fiore,Santa Maria del Fiore,

St. Peter’s BasilicaSt. Peter’s Basilica

Roman grandeur

Pantheon is a Greek work meaning to honor all Pantheon is a Greek work meaning to honor all Gods.Gods.

History Agrippa had the original Pantheon erected in 27 Agrippa had the original Pantheon erected in 27

B.C. after he crushed Anthony and Cleopatra at B.C. after he crushed Anthony and Cleopatra at Actium. Actium.

Agrippa dedicated to the Romans’ septet of Agrippa dedicated to the Romans’ septet of planetary gods out of gratitude of his conquest. planetary gods out of gratitude of his conquest.

After two fires damaged the original structure, After two fires damaged the original structure, Hadrian rebuilt it from AD 117 to 125Hadrian rebuilt it from AD 117 to 125

Interesting fact: The front of the Pantheon says Interesting fact: The front of the Pantheon says “M. AGRIPPA L. F. COS. TERTIUM FECIT” “M. AGRIPPA L. F. COS. TERTIUM FECIT” which translated means “Marcus Agrippa, son of which translated means “Marcus Agrippa, son of Lucius, in his third consulate, made it.” even Lucius, in his third consulate, made it.” even though Hadrian actually rebuilt it.though Hadrian actually rebuilt it.

Orientation

Main entrance of Pantheon faces north, and Main entrance of Pantheon faces north, and the whole building is oriented on the north-the whole building is oriented on the north-south axis of the building.south axis of the building.

Foundation

Built on marshy, unstable earth which gave Built on marshy, unstable earth which gave serious supporting problem to builders. serious supporting problem to builders. Ring foundation rested on a bed of bluish Ring foundation rested on a bed of bluish colored river clay. In final construction colored river clay. In final construction phase, the foundation cracked at the two phase, the foundation cracked at the two ends of North-South axis.ends of North-South axis.

Configuration

Walls Walls are structurally a Walls are structurally a

series of concrete piers series of concrete piers separated by 8 large separated by 8 large niches.niches.

Thick walls act like Thick walls act like buttress to support thrust buttress to support thrust from dome.from dome.

Within these niches lie Within these niches lie great kings of Italy, great kings of Italy, important popes, and at important popes, and at one time Raphael.one time Raphael.

Dome Structure

Rotunda: inner diameter of 142.4 feet (43.4 m), Rotunda: inner diameter of 142.4 feet (43.4 m), which is about half the length of our football field.which is about half the length of our football field.

The dome could hold a 143ft. diameter perfect The dome could hold a 143ft. diameter perfect hemisphere. However, from the top and sides it hemisphere. However, from the top and sides it looks much flatter than a hemisphere. Why?looks much flatter than a hemisphere. Why?

At the bottom, the walls were ~20 feet wide, while At the bottom, the walls were ~20 feet wide, while at the top they were only ~5 feet wide. This was at the top they were only ~5 feet wide. This was one of the ways it resisted the outward thrust one of the ways it resisted the outward thrust forces at the bottom of the dome.forces at the bottom of the dome.

Construction

Old Mycenaean tombs in Greece were made by Old Mycenaean tombs in Greece were made by corbeling stone slabs over one another. corbeling stone slabs over one another.

It is likely that Romans used this principle in It is likely that Romans used this principle in placing one-step ring on another in building the placing one-step ring on another in building the rings on the dome.rings on the dome.

Compression ring at center of dome is effective in Compression ring at center of dome is effective in properly distributing compression forces at this properly distributing compression forces at this point.point.

Materials All concrete, no steel reinforcement!All concrete, no steel reinforcement! How did the concrete hold that long? Special How did the concrete hold that long? Special

concrete: volcanic ash (concrete: volcanic ash (pozzolana)pozzolana), lime, water., lime, water. Difference is: Difference is:

Common plaster – made from wet lime and plain Common plaster – made from wet lime and plain sand, crystalline atomic structure doesn’t allow sand, crystalline atomic structure doesn’t allow calcium hydroxide from lime to enter and react.calcium hydroxide from lime to enter and react.

Pozzolana: amorphous silica atomic structure Pozzolana: amorphous silica atomic structure with many holes, allowing calcium hydroxide to with many holes, allowing calcium hydroxide to enter and make a concrete gel that expands, enter and make a concrete gel that expands, bonding pieces of rock together.bonding pieces of rock together.

Building process

To build load-bearing wall:To build load-bearing wall: Create form through wood, stacked rock, or brick.Create form through wood, stacked rock, or brick. Layer of aggregate rock (hand-sized rock or Layer of aggregate rock (hand-sized rock or

broken brick), then tightly tamped pozzolan broken brick), then tightly tamped pozzolan mortar into aggregate with special tool (beetle) to mortar into aggregate with special tool (beetle) to get all the air out.get all the air out.

They didn’t mix or “pour” concrete because the They didn’t mix or “pour” concrete because the concrete was not liquid – low moisture content.concrete was not liquid – low moisture content.

Low water content and close compaction were part Low water content and close compaction were part of the reason Roman oncrete lasted so long.of the reason Roman oncrete lasted so long.

Weight-loss regimen

Coffered interior reduces weight and Coffered interior reduces weight and compression forces on bottom.compression forces on bottom.

Also, the Romans used different types of Also, the Romans used different types of rocks for top and bottom of dome. Lighter rocks for top and bottom of dome. Lighter rocks on top and heavier on bottom.rocks on top and heavier on bottom.

Fractures

Cracking in walls and domeCracking in walls and dome Fractures “reaching from base of rotunda to Fractures “reaching from base of rotunda to

summit of dome,” probably brought about by summit of dome,” probably brought about by differential settlement from uneven loading of differential settlement from uneven loading of wall, esp. near entrance of rotunda in the principle wall, esp. near entrance of rotunda in the principle niche.niche.

Length of cracks (from base of dome upwards to Length of cracks (from base of dome upwards to about 54 deg) corresponds well with natural range about 54 deg) corresponds well with natural range of hoop tension in dome.of hoop tension in dome.