The Cost of a Missing Wall – Discrete-Block Analysis of Lateral Restraint Loss in the Arch of Ctesiphon

The Arch of Ctesiphon is the largest unreinforced brick vault to survive from antiquity, and it now stands with one of its flanking walls gone and the wall its courses lean on cut down from the top. This thesis puts a number on what the second of those losses costs. A three dimensional discrete-block model of the vault was built on a catenary mid-surface fitted to the surveyed geometry, with the nine inclined leaves of pitched brickwork resolved as 416 separate blocks and meshed into 52,975 elements. The blocks meet only through frictional contact, so the assembly can open its joints, slide and part wherever the masonry itself can. Lateral restraint enters as a surface condition on the front face and is withdrawn progressively, the height to which it still reaches serving as the single control parameter. Five heights between 28.0m and 21.5m were analysed by explicit dynamics under self-weight. The first three remain quasi-static, with kinetic-to-internal energy ratios near 0.012 and peak displacements below 19mm. Between restraint heights of 25.0m and 23.5m the response changes character: the energy ratio rises to 0.504 and then 2.515, and peak displacement reaches 3.62m and 7.11m. The computed mechanism is a longitudinal detachment in which adjacent leaves part along the depth of the barrel, beginning at the crown towards the exposed end. Its orientation matches a discontinuity recorded on the intrados, and the documented height of the lowered wall, about 22.7m, falls below the threshold.