In northern Ethiopia there is a ruined wall a little under two metres thick, built from broken phonolite stone set in solid clay mortar and threaded through with layers of wooden beams. It has been standing, in one state or another, for roughly 2,800 years. Earlier this year two engineers took one corner of that wall and one doorway from it, rebuilt both inside a computer as finite element models, loaded them with the worst material properties the stone and clay could plausibly have had, applied the safety factors a modern structural engineer is obliged to apply, and asked a simple question. How much more building would the wall still carry?
The answer came back at roughly sixteen floors. The reconstruction archaeologists had already drawn for the same building has eight.
That is a doubling, and it is worth being exact about what kind of doubling it is. It is a modelling output, not an observation. Nothing excavated at the site says a sixteen storey building ever stood there. What the study establishes is a ceiling on capacity, which is a different thing from a measurement of height, and the gap between those two ideas is where almost every write up of this work has gone astray. Within days of the paper reaching the general press, headlines were calling the place one of the world’s earliest skyscrapers. The two outside specialists who were asked about it both pushed back on precisely that word.
The building is called Grat Be’al Gibri, and it sits at Yeha, a small place in the north of the country that very few readers outside Ethiopia have heard of and that a specialist in the early first millennium BC would name without hesitation. The complex is enormous for its date, roughly sixty metres on a side. It stands on a stone podium about six metres high whose walls are around two point two metres thick. The ground floor walls above that are about one point nine metres thick. At the entrance there were square stone pillars something like ten metres tall. Radiocarbon dating puts the construction at around 800 BC. At some point in antiquity the building burned, and what survives is the lower part of a structure whose upper part is gone entirely.
A few hundred metres away stands the other reason anyone knows the name Yeha. The Great Temple there is generally described as the oldest standing structure in Ethiopia, dated to roughly 700 BC by comparison with buildings in southern Arabia and supported by inscriptions found locally. The archaeologist David Phillipson credited its survival to two things, the care its builders took to set it level on uneven bedrock, and its rededication as a Christian church, possibly as early as the sixth century AD. A building that keeps being used is a building that keeps being maintained. The palace next door had no such second life, which is why one of them is a roof over your head and the other is a footprint.
Yeha has been dug at, abandoned and dug at again for most of a century. A British traveller poked around the site in the 1890s. The Ethiopian Institute of Archaeology began excavating in 1952. That work was interrupted under the Derg, and a French team resumed in 1993. Rock cut shaft tombs were found in the early 1960s, with some speculation that one held a royal burial. More recently the German Archaeological Institute has worked there, and Mike Schnelle of the institute’s Orient Department, the second author on the new study, is a long standing Yeha specialist. His co-author, Martin Drieschner, is not an archaeologist at all. He works at the chair of structural analysis and dynamics at the Brandenburg University of Technology in Cottbus, which is to say he is a civil engineer who normally models things that have not been built yet.
That pairing is the whole reason this study exists, and it is more interesting than the number it produced.
Here is the problem the two of them had to solve. If you want to know whether a wall can carry a load, you need to know what the wall is made of, in numbers. Compressive strength, tensile strength, stiffness. For a concrete frame going up next year, you look those up, because the concrete has been specified and tested. For a wall built around 800 BC out of locally quarried rubble, locally dug clay and hardwood beams, you do not know them. You can take samples, and archaeologists have, but the material varies from stone to stone and the clay has had nearly three millennia to change. Any single number you plug in is a guess dressed up as data.
So rather than guess, they modelled the ignorance itself. The study runs what is called a nested fuzzy interval stochastic analysis, which in plain terms means the material properties are not entered as single values but as ranges, with a structure that distinguishes between genuine random scatter and the plain fact that nobody knows the right figure. The model is then run across those ranges rather than at one point. You come out not with one answer but with a spread, and crucially you come out knowing which of your uncertainties actually mattered.
That last part produced the finding most likely to survive the next decade, and it has nothing to do with floor counts. The variation in the wood barely moved the result. Change the stiffness or strength of the beams across their plausible range and the wall’s capacity hardly notices. What dominated was the masonry, and the specific failure mode that decided everything was tensile failure in the clay mortared rubble. The weak point of a timber reinforced wall, in other words, is not the timber.
Which is a slightly unexpected thing to learn about a construction system whose entire distinguishing feature is the timber. Horizontal wooden beams were laid into the rubble in alternating directions, longitudinal then transverse, course after course, with clay layers between. The wall section the engineers modelled was three point three six metres high and contained fourteen separate layers of beams. Two wood species have been identified at the site, African olive and Cordia africana, both hardwoods, and both understood to have been chosen in part for their resistance to insects. Timber lacing of this general kind shows up independently in a number of building traditions, often in places that shake, because wood gives a rigid masonry wall somewhere to put movement. The Yeha walls use horizontal beams only, which distinguishes them from the South Arabian practice of using both horizontal and vertical members.
That distinction matters more than it looks, because it bears on an argument about where the whole technique came from.
The conventional telling has long leaned on southern Arabia as the source. The Great Temple at Yeha is Sabaean in style. The inscriptions point across the Red Sea. The site belongs to the world historians call Ethio Sabaean, where South Arabian and African building traditions were in contact, and for a long time the default assumption in that contact was a one way transfer. Kwasi Konadu, formerly an endowed chair in Africana and Latin American studies at Colgate University, made the point bluntly when asked about the new paper. The result pushes against the older habit of treating Arabia as the origin of these systems in the northern Horn of Africa. What the engineering shows, he said in effect, is that African builders were calculating height, material and load with real competence roughly three thousand years ago. Davide Tanasi, a classical archaeologist at the University of South Florida, made the complementary point. Archaeologists already regarded this construction system as sophisticated. What was missing was a quantitative assessment of it, and equally sophisticated traditions existed in the Ethio Sabaean world where the two building cultures met.
So why was anyone counting floors in the first place? Because the question has a long paper trail, and it is not an engineer’s question originally. It is a textual one.
A tenth century Yemeni historian, al Hamdani, described a palace at Sanaa called Ghumdan and gave it a height of twenty storeys. Rock paintings and wall paintings in southern Arabia show palace like buildings of at least nine storeys. None of that is evidence in the archaeological sense, but it is persistent, and it has shaped how anyone reconstructing a monumental building in this region thinks about the vertical. At Yeha itself there is one piece of actual structural evidence for height. The footprint of the pillared entrance porch on the southeast side corresponds exactly to a central projecting bay of the building, which implies additional floors above the porch, and on that basis the complex has been argued to have stood at least three storeys. From there the published reconstruction takes a careful step further, five full floors with three smaller recessed levels above them, eight in total, and it is honest about leaning on the painted depictions to get there.
Eight, in other words, was never a measurement either. It was a reasoned drawing, built out of a bit of structural logic and a lot of comparative imagery. What the new study does is take that drawing to a structural engineer and ask whether the surviving wall could even have borne it.
It could, comfortably. Under favourable assumptions about the materials the modelled corner section carried the equivalent of something like forty eight additional floors and the doorway section around fifty six, numbers so far beyond any plausible building that they function mainly as a demonstration that the wall is nowhere near its limit. Push the materials to the worst end of their plausible range, combine that with heavy loading, then apply standard safety factors, and both sections still came out at around sixteen floors. The figures that reached the general press varied, with some coverage reporting at least nineteen rather than sixteen, which is a reasonable sign that readers should treat any single number here as indicative rather than final. The robust version of the finding is the comparison, not the digit. The wall system could carry roughly double what the reconstructed building would have put on it.
The authors are also clear about something that pushes the reserve higher still. Only two sections were modelled, both from the ground floor, where the walls are at their thickest and carry the most. A real building of this type would have thinned its walls as it rose, shedding weight with every level, so the structure as a whole had more margin than two ground floor samples can show.
None of which makes it a skyscraper, and the word is doing real damage. A wall carrying nineteen floors is not a structure that stood nineteen floors high. Konadu’s formulation is the one to keep. What the analysis actually establishes is that the building system was more capable than the surviving ground floor would lead you to assume, which is a finding about engineering knowledge rather than about silhouette. Tanasi put the arithmetic plainly. The floor counts indicate how much additional load the wall could theoretically take before failing. That is a stress calculation, and reading a building height out of it is a category error.
Drieschner, for his part, seems most pleased about the method rather than the result. What he found exciting, he said, was that tools from modern civil engineering, finite element modelling and the handling of fuzzy material parameters, transfer directly onto a 2,800 year old structure. That is the quietly useful part. There are ruins all over the world whose material properties nobody can pin down, which has historically meant that structural questions about them got answered with judgement and analogy. A method that takes the uncertainty as an input rather than a problem to be papered over can be pointed at any of them.
There is a limit to all of this that is worth stating plainly, because a computer model of a ruin is not an experiment on a building. What gets modelled is an idealisation, a clean geometry with defined material zones, and the real wall was never clean. The clay mortar being sampled today has had almost three thousand years of wetting, drying and settling, so even the ranges fed into the model are ranges for an aged material rather than a fresh one. The fire that destroyed the upper structure will have done things to the stone that no material range captures. A model like this is best read as a boundary, telling you what is impossible and what is comfortably possible, and leaving a wide middle where it cannot help.
And there is one piece of evidence the model does not need, which is that the thing is still there. A wall built of rubble and clay, in a building that burned, has held its line through roughly 2,800 years of weather in the open air. That is not a controlled test, and nobody would write it up as one, but it is the kind of fact that should make an engineer suspect the margins were generous before any software is opened. The eight storey reconstruction was conservative in the ordinary professional sense of the word, which is to say its authors chose the smallest building the evidence would support rather than the largest, because that is what you do when the evidence is a painted wall in another country. What the new work adds is the other end of the range, and the finding is that the two ends are further apart than anyone had cause to assume.
The building at Yeha burned a very long time ago and nobody knows how tall it was. That has not changed. What changed is that the eight storeys on the reconstruction drawing are no longer the ceiling of what the evidence permits, and the reason they were ever the ceiling turns out to have been the limits of the drawing rather than the limits of the wall. Someone built that thing out of rubble, clay and olive wood, in a place where the whole technique was supposedly borrowed, and built it with so much capacity to spare that two engineers with a modern model had to push the materials to their worst imaginable state before the numbers came anywhere near the building above.