Tunnel-Form Buildings Could Slash Earthquake Losses to Under One Percent of Construction Costs

Every earthquake engineer knows the uncomfortable truth: a building that satisfies the seismic code is not necessarily a building that protects its owners’ finances, its occupants’ bodies, or a community’s ability to recover. Codes are written in broad, qualitative terms, anchored to specific hazard levels and return periods, and they rarely say anything explicit about what happens after the shaking stops—the repair bills, the hospital stays, the funerals, the months spent in temporary housing. A new numerical study published in the Bulletin of Earthquake Engineering argues that one particular construction system, widely used in mass housing across the Middle East and Turkey, may quietly be one of the best answers to that post-earthquake problem: the concrete tunnel-form system.

The research, carried out by Vahid Mohsenian of the University of Science and Culture in Tehran and Luigi Di-Sarno of the University of Naples Federico II, is the first life-cycle cost analysis ever performed for this structural system. Life-cycle cost analysis, or LCC, is an economic evaluation framework that adds up every cost a building will generate over its entire service life: the initial construction price, plus the expected losses from earthquake damage, injuries, fatalities, relocation, lost rental income, and downtime, all discounted to present value. Instead of asking only whether a building prevents collapse under the design earthquake, LCC asks a harder question—which design choice minimizes the total bill that nature and time will present over decades?

The tunnel-form system itself is a product of industrialized construction. Derived from load-bearing wall construction, it involves casting both the walls and the floor slabs of a building simultaneously, in a single pour, using reusable steel tunnel-shaped formwork. The technique offers shorter construction times, lower costs, better quality control, and improved worker safety, which explains its popularity in large-scale housing projects. Structurally, the result is a box-like assembly of cross walls and slabs with considerable inherent lateral strength and stiffness. Field observations after recent strong earthquakes, including the devastating 2023 Kahramanmaraş earthquake sequence in Turkey, have repeatedly described the performance of tunnel-form buildings as exemplary, yet the system still has no dedicated seismic design guideline of its own and is treated in codes as a subset of the conventional reinforced concrete load-bearing wall system.

That classification matters, because the two systems behave differently, and because the latest edition of the Iranian Seismic Design Code mandates special ductility detailing for reinforced concrete shear walls. Meeting those requirements in tunnel-form construction is practically difficult given the constraints of the casting process, and many existing tunnel-form buildings simply do not comply. This raises a pressing question: are the special ductility provisions actually necessary for this system, or are they a conservative inheritance from a different structural family? Answering that question was one of the study’s central objectives.

To do so, the researchers designed two residential tunnel-form buildings—five and ten stories tall—for a highly seismic site with a design peak ground acceleration of 0.35 g and stiff soil conditions. Crucially, the walls were designed with only intermediate ductility provisions, deliberately omitting the special ductility requirements. The coupling beams connecting the walls were modeled as hybrid elements equipped with replaceable steel seismic fuses, a configuration intended to concentrate damage in sacrificial components. The buildings were then subjected to incremental dynamic analysis using twenty far-field ground motion records selected from the PEER database, scaled in steps of 0.05 g, with peak ground acceleration as the intensity measure and both horizontal components applied simultaneously.

The structural results were striking. The average peak ground acceleration corresponding to the onset of slight damage in the structural walls came out at approximately 0.6 g—well above both the design-basis earthquake of 0.35 g and the maximum-considered earthquake of 0.52 g. In probabilistic terms, the chance of the walls even reaching the slight damage state under the design-basis earthquake was estimated at less than ten percent, and the same held true for the moderate damage state under the maximum-considered earthquake. In other words, even without special ductility detailing, the tunnel-form walls showed such high strength and stiffness that the code’s special ductility mandate appears, in the authors’ assessment, to be conservative for this system.

But strength alone does not settle the economic question, and this is where the study’s accounting framework becomes important. The researchers divided the buildings into three component categories: structural elements, acceleration-sensitive non-structural components (things like ceilings, mechanical equipment, and piping that break when shaken), and displacement-sensitive non-structural components (partitions, facades, and finishes that crack when the building sways). For each category and each of four damage states—slight, moderate, extensive, and complete—they combined fragility curves derived from the dynamic analyses with the site hazard curve to compute the probability of exceeding each state over the building’s life. Those probabilities were then multiplied by detailed unit costs drawn from Iranian regulations and surveys: replacement costs per square meter, content costs of roughly 1,600 dollars per square meter, temporary accommodation at 20 dollars per person per day, lost rental income, and legal compensation values for minor injuries, major injuries, and fatalities.

The headline economic finding is remarkably consistent. Regardless of building height, and regardless of whether the losses originated from structural or non-structural damage, the total expected earthquake-induced loss for the tunnel-form system remained below one percent of the initial construction cost. For the five-story building, the expected losses attributable to structural, acceleration-sensitive non-structural, and displacement-sensitive non-structural components amounted to just 0.59, 0.94, and 0.55 percent of construction cost respectively; for the ten-story building, the figures were 0.46, 0.51, and 0.30 percent. Perhaps the most consequential detail, however, is which components dominate the bill: acceleration-sensitive non-structural elements generated the largest share of losses in both buildings, a result consistent with a growing body of research showing that in well-designed structures it is the contents and equipment, not the frame, that drive earthquake economics.

Height mattered in a subtler way. When structural components were treated as the primary source of injury, fatality, and downtime losses, the probable loss in the shorter building was roughly 48 percent lower than in the taller one; for non-structural components the gap was about 26 percent. The explanation lies in code mechanics: for shorter buildings, minimum code requirements tend to govern the design of structural elements, inflating their overstrength and seismic capacity well beyond what taller, more demand-driven designs achieve. The practical implication is that shorter tunnel-form buildings are consistently safer and cheaper over their lifetimes—a finding the authors suggest could guide developers, regulators, and insurance companies in selecting optimal building heights for industrialized housing programs.

The study’s implications reach beyond the tunnel-form system itself. Because no current seismic design guideline incorporates life-cycle cost assessment, the authors argue that codes should evolve in one of two directions: either by embedding LCC analysis directly into the design process, or by steering designers toward structural systems that inherently minimize lifetime losses—which their results suggest tunnel-form construction does. They also recommend that the system be recognized as an independent structural category with its own seismic parameters, and they caution that their conclusions are bounded by the models and assumptions used: near-fault motions, vertical ground motion components, aftershocks, soil-structure interaction, corrosion, aging, and multi-hazard scenarios all remain open questions. Still, for a construction technique already favored for its speed and economy, the new analysis adds a powerful third argument. In the world’s most earthquake-prone regions, the humble tunnel of cast concrete may be one of the cheapest insurance policies a city can build.

Subject of Research: Life-cycle cost and seismic performance assessment of concrete tunnel-form building systems in earthquake-prone regions

Article Title: Tunnel-form system as an ideal solution for seismic regions: a numerical study based on life-cycle cost analysis

Article References: Mohsenian, V., & Di-Sarno, L. (2026). Tunnel-form system as an ideal solution for seismic regions: a numerical study based on life-cycle cost analysis. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02632-4

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02632-4

Keywords: tunnel-form system, life-cycle cost analysis, earthquake engineering, seismic fragility, incremental dynamic analysis, shear walls, non-structural components, ductility provisions, seismic design codes, downtime, reinforced concrete, seismic hazard