How Building Height Rewrites Earthquake Risk in Pilotis Towers

For decades, earthquake engineers have argued about a deceptively simple question: do the brick walls that fill the frames of reinforced concrete buildings help or hurt when the ground shakes? A new computational study by S. Shivaramakrishnan and L. Pelecanos at the University of Bath, published in the Bulletin of Earthquake Engineering, argues that the question itself is poorly posed. The answer, their work suggests, depends less on the walls than on how tall the building is, how its stiffness evolves as damage accumulates, and how the frequencies of the earthquake interact with the changing dynamics of the structure. The research offers a pointed challenge to simplified design assumptions embedded in codes such as Eurocode 8.

The team focused on a configuration familiar to anyone who has walked through Mediterranean or Asian cities: the pilotis building, in which the ground storey is left open, its columns rising without the confining embrace of masonry infill walls. Buildings with this open ground floor have collapsed repeatedly in past earthquakes, including the Yushu earthquake in China, because the abrupt stiffness discontinuity between the open base and the stiff upper storeys concentrates deformation there. Yet pilotis buildings continue to be constructed because open ground floors are architecturally desirable, leaving engineers to assess the risk with tools of uncertain reliability.

To probe that risk systematically, the researchers built finite element models of reinforced concrete frames at three heights—three, six and twelve storeys—in three configurations each: bare frames, fully infilled frames, and frames with ground-floor pilotis. The plan geometry was held constant across every model, with four bays of 4.0 metres and five bays of 3.0 metres, and uniform 3.0-metre storey heights, so that only the infill arrangement and the building height varied. Reinforcement detailing followed Eurocode 2 and Eurocode 8 practice for medium-ductility frames, with longitudinal reinforcement ratios of roughly 1.7 percent in beams and 2.7 percent in columns, and confined concrete cores represented through the Mander constitutive model.

The modelling strategy balanced fidelity against computational practicality. Reinforced concrete beams and columns were represented with fibre-based force-formulated frame elements, which subdivide each cross-section into fibres whose nonlinear uniaxial stress-strain behaviour is integrated along the member, capturing the spread of inelasticity in both space and time. Masonry infills were represented through a macro-model based on Crisafulli’s double-strut formulation, in which each panel is replaced by six components: two parallel axial struts in each diagonal direction and shear springs that capture bed-joint sliding. This equivalent-strut approach accounts for four failure mechanisms—diagonal tension, sliding shear, corner compression and compression at the panel centre—without the prohibitive cost of detailed micro-modelling, making it well suited to large parametric studies.

Nonlinear time-history analyses were performed in SeismoStruct using the HHT-alpha integration algorithm with a fixed timestep of 0.05 seconds, while eigenvalue analyses employing the Lanczos algorithm established the modal properties of each frame. Three recorded earthquake motions—Landers 1992, Northridge 1994 and Kobe 1995—were deliberately selected because their response spectra peaked near the natural periods of the structures, creating challenging resonant conditions. Critically, the records were applied without scaling or spectrum matching, so the results reflect genuine recorded events, though the authors caution that the limited number of motions means the findings should be read as indicative of behavioural mechanisms rather than statistically robust design predictions.

The modal analyses confirmed that masonry infill fundamentally reshapes a building’s dynamics. Infilled frames were consistently stiffer than bare frames, with shorter fundamental periods, while pilotis frames fell between the two, slightly softer than fully infilled counterparts because of the missing ground-storey walls. As height increased, fundamental periods lengthened and the first three modes separated further apart, consistent with classical structural dynamics. Notably, the difference between fully infilled and pilotis models was small, suggesting that total infill quantity, rather than its vertical distribution, governs overall stiffness—a first hint that the notorious soft storey might not be the dominant factor engineers often assume.

The nonlinear time-history results deepened that surprise. Acceleration profiles along the height of the buildings were strikingly non-monotonic: after an initial rise, accelerations decreased and rose again in the six- and twelve-storey frames, a pattern the authors attribute to higher-mode participation shifting as stiffness degrades non-uniformly with accumulating damage. Masonry infill reduced acceleration amplification by a factor of roughly 1.5 across the model families, and the peak amplification period scaled almost linearly with building height—doubling as storey count doubled—indicating that the fundamental mode dominates response even deep into the nonlinear regime. Perhaps most counterintuitively, the twelve-storey bare frame sometimes showed lower maximum displacements than its infilled counterparts, hinting that added mass and nonlinear interaction can offset the benefits of added stiffness.

The height-dependent transition in pilotis behaviour emerged most clearly in the drift and force demands. In the three- and six-storey frames, the expected concentration of inter-storey drift at the open ground floor failed to materialise; the pilotis models behaved much like their fully infilled twins. Only in the twelve-storey building did deformation concentrate at the base, and even there, the fully infilled frame experienced comparable ground-floor drift, suggesting that infill mass rather than soft-storey flexibility alone drove the effect. Column shear forces told a parallel story: in shorter buildings, pilotis columns carried less shear than bare-frame columns, but in the twelve-storey pilotis frame, ground-floor column shear exceeded that of the fully infilled counterpart by about 50 kilonewtons, evidence that the detrimental influence of the open storey strengthens as buildings grow taller.

Comparisons across the three earthquake records revealed why the picture is so record-dependent. Under Landers, bare frames showed far higher peak spectral accelerations than pilotis frames, and the pilotis configuration occasionally proved beneficial, reducing amplification relative to the bare frame. Under Kobe and Northridge, differences between configurations shrank, and in one case the infilled frame’s peak acceleration nearly matched the bare frame’s, shifted to a longer period of about 1.0 second. The researchers traced this variability to nonlinear period elongation: as damage softens a structure, its elongated periods migrate across the frequency landscape of the input motion, sometimes sliding toward spectral peaks and sometimes away from them. Whether pilotis proves harmful therefore depends on where the resonant peak of the shaking happens to fall relative to the structure’s evolving dynamics.

The study’s most consequential finding concerns design codes. Eurocode 8 permits structures with significant infill asymmetry to be analysed as bare frames, on the assumption that this is conservative. The Bath team’s results show that such simplification can be conservative for some response quantities and configurations, but not universally so: in several cases, ignoring the infill underestimated demands or overstated benefits in ways that depended on the interplay of period elongation, spectral content and building height. The authors acknowledge limitations—a small set of unscaled records, macro-modelled infills that cannot capture local cracking, rigid diaphragm assumptions, neglected torsion and soil-structure interaction—but argue that the message stands. Assessing irregular reinforced concrete buildings accurately demands nonlinear dynamic analysis that honours infill-frame interaction, because static or linear thinking simply cannot predict where a soft storey will turn from a nuisance into a catastrophe.

Subject of Research: Nonlinear seismic response of masonry-infilled reinforced concrete frame buildings with pilotis configurations across different building heights

Article Title: Height-dependent nonlinear seismic response of masonry-infilled RC frame buildings with pilotis configurations

Article References: Height-dependent nonlinear seismic response of masonry-infilled RC frame buildings with pilotis configurations. (n.d.). https://doi.org/10.1007/s10518-026-02687-3

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02687-3

Keywords: earthquake engineering, reinforced concrete, masonry infill, pilotis, soft storey, nonlinear analysis, time-history analysis, inter-storey drift, period elongation, Eurocode 8, finite element analysis, building height