The Pantheon's original portico truss roof

[Don P]

 We usually think the 1800's industrial age is when metal trusses appeared. 

The Romans were the first to make widespread use of timber and timber reinforced with metal, trusses. Greeks mostly used prop and lintel, post and beam technology for their roofs. Lots of posts in the way in a building of any size. Imagine trying to make an auditorium in a sea of posts, there is a post in half the audience's view. The roman basilica using the truss was a hit. When Christians copied that design they held on to it  :D. In the portico below the suspended ceiling would have been level on each side aisle and barrel vaulted in the center.

The entry portico of the Pantheon, the round drum shaped building with its impressive coffered lightweight concrete dome in Rome, was built around 125AD. It replaced the previous 2 temples that had burned, fire was on their minds, no wood in the building, this may be the first, or it is possible it was one of several at that time. The others were at least bronze sheathed and ceilinged. It is hard to suss it out exactly in the writings of the time, this roof however survived. At least into relatively modern times. The portico roof was spanned by 3 truss assemblies made of bronze plates riveted together. Each truss contained 60 rivets which were about 20" long. The only thing that remains of that roof is one rivet in a museum in Germany. Metal was precious. The roof survived until 1625, so right at 1500 years old, when Pope Urban VIII melted down 152 tons of bronze trusses and purlins to make canons to protect his dear rear.



 
Happily, Francesco Borromini an interesting young Italian architect was there in 1625 when the ancient roof was being dismantled. That is one page of several. A number of sketches by various people exist, this thing had existed for a millennia and a half, in good condition. This is another very good sketch of the truss, hopefully it'll hold enough zoom so you can see how they riveted the plates together. C channel purlins, they had it going on.



 

The main members are plates cast about 1" thick x 2' deep. Depending on the stack of plates at any given joint the rivet would be passing through that and a paired sister truss, notice it is "doubled", making the stack with a central void about 18" thick. There are 3 truss assemblies spaced across the ~50' deep x ~100' wide portico.


In one paper a grad student had studied the rivet, found the bronze was 90% copper/10%tin and did an engineering analysis of the truss design that had stood for 1500 years. It was a low stress assembly. They were very conservative and building for the long haul. Alas any jackass can kick a barn down.

The canons were to protect Castel Sant Angelo... Hadrian's tomb, which the popes by that time had dumped Hadrian and his buddies ashes in the Tiber and taken to using the mausoleum as a fortress refuge when unpleasantness was going on in Rome. The bronze ceiling of the portico and the bronze roof tiles that were originally covering the dome and portico of the Pantheon had been pilfered by the 7th century. That 152 tons of bronze the pope stole was just the roof structure itself. Originally it was metal skinned above and below in bronze.

At any rate this was a fairly substantial span using riveted metal trusses about 1600 years before we normally think of that technology being understood and used. They obviously understood what they were about 

Reading while hangin with a sick bud.

[barbender]

What a shame that a building of that antiquity was scrapped!

[thecfarm]

I betcha the ones tearing it down said the same thing.

[Don P]

The people were not happy, I came across this last night and a pic of the remaining rivet.  
THE PANTHEON EXPLAINED - PAGE 2 (atouchofrome.com)


edit to add;

The citizens of Rome were outraged that their beloved Pantheon, such an important and well-preserved part of their long history, was subjected to such injury. In response, a very clever satire targetting Pope Urban VIII, whose family name was Barberini, began circulating among the people:

"Quod non fecerunt Barberi, fecerunt Barberini"
"What the barbarians did not do, the Barberini did."

[Don P]

Wandering down that rabbit hole  :D
This is a paper on the oldest well recorded Roman wooden truss. San Paolo fuori le Mura it burned in the 1820's but was well documented and restored, it has a clear span of about 80';
STR19014FU1.pdf (witpress.com)

Look at the double layered lower end of the top chord, the heeljoint and bolster secured by an iron clamp, the forelock bolt holding the 2 plies together further up. The "kingpost" for lack of a better term is hanging from the "straining beam" and then supporting the bottom chord.



 

Choisy observed in 1873 that "the modern trusses of Italy... resemble those of the Christian basilicas, and those, constructed in a time where architecture had no other basis than memories, more or less altered from Roman practice, which are evidently nothing but copies of the originals that are lost to us" [1]. This study concurs with Choisy's observation that Imperial Roman buildings destroyed centuries ago are echoed in the form of Early Christian basilicas. These Early Christian basilicas consistently used the simple double truss, which was later used by Roman builders in the Medieval era.

[ppine]

The Pantheon is the grandest building in the world.  It proudly stands in Roma.  I was able to visit there once and went to the Pantheon every day.  It is possible to have a quiet conversation between two people at opposite sides of the doma.