The study, published in Case Studies in Construction Materials, was led by Yi Ding, with co-authors Xinru Wang, Man Xu, and Huiwen Tian. Their work addresses a deceptively simple question: how do you calculate the shear strength of a squat wall when part of it is made not of ordinary concrete, but of ultra-high performance concrete, or UHPC — a fiber-reinforced cementitious material that can reach compressive strengths above 130 megapascals, roughly double or triple that of conventional structural concrete? The answer, they found, requires rewriting one of the key equations that engineers use to size these walls, because the new material fundamentally changes how forces flow through the structure.

UHPC is not just stronger concrete in a loose sense. It is engineered through optimized particle packing — arranging fine sand, cement, and admixtures so that almost every void is filled — and through the addition of steel fibers that bridge cracks and hold the material together after it fractures. In the team’s earlier laboratory tests, shear walls in which UHPC was cast only in the boundary elements, the heavily loaded vertical zones at the edges of the wall, showed remarkable improvements. Compared with identical walls made entirely of normal-strength concrete, the UHPC-reinforced versions carried 6.79 to 20.64 percent higher peak loads, showed 2.54 to 24.11 percent greater ductility, and dissipated 44.88 to 57.65 percent more cumulative energy under simulated earthquake loading. Just as striking, the steel fibers restrained crack growth so effectively that the walls avoided the catastrophic spalling, bar buckling, and longitudinal bar fracture that rendered the conventional walls essentially unrepairable after testing.

But predicting how strong such a hybrid wall should be turned out to be a genuine theoretical challenge. The researchers first benchmarked three established calculation methods against their experimental data from thirteen full-scale wall specimens, which included rectangular and barbell-shaped cross-sections, UHPC distributed at various heights and widths, and axial load ratios of 0.25 and 0.45. The Chinese standard JGJ 3-2010 overestimated strength by an average factor of 1.51, the American standard ACI 318-19 by 1.56, and even the best-performing option — the softened strut-and-tie model developed by Hwang and colleagues — overshot by 1.38 on average. None of these methods was designed with a material as strong and as crack-resistant as UHPC in mind, and none accounted for how locally concentrating that material in the boundary elements redistributes the internal load paths.

To understand the failure mechanism directly, the team built highly refined three-dimensional finite element models in the LS-DYNA simulation environment. Concrete and UHPC were represented with the Karagozian and Case material model, a sophisticated constitutive framework that captures strength surfaces, damage evolution, and failure, with parameters specially calibrated for UHPC’s much higher strength. Steel reinforcement was modeled explicitly with beam elements, and the interface between UHPC and normal concrete was treated with shared nodes, reflecting the experimental observation that the two materials remained fully bonded without separation. After validating the modeling approach against independent low-rise wall tests by other researchers — achieving peak load prediction errors within 5 percent — the team turned the simulation loose on squat walls with an aspect ratio of 1, the regime where shear dominates.

The simulations revealed the failure sequence in vivid detail. At first cracking, tiny macroscopic fractures appeared precisely at the horizontal interface where the UHPC boundary element meets the normal-strength concrete web, marking the point where tensile strain first exceeded the material’s limit. As lateral drift increased, the outermost longitudinal bars in the boundary elements reached their yield stress — around 509 megapascals on the tension side and 501 on the compression side — while diagonal cracks spread through the web. By peak load, these diagonal cracks had carved the web into a lattice of small zones, and the concrete between them consolidated into a set of diagonal compression struts. The wall ultimately failed by crushing along its principal diagonal strut, exactly the mechanism that the strut-and-tie theory describes: steel bars acting as tension ties, concrete acting as compression struts, and the two working together in a self-equilibrating truss embedded within the wall.

That confirmation opened the door to the model’s central innovation. In the classical strut-and-tie framework, the depth of the diagonal compression strut is tied to the height of the compression zone at the base of the wall, and existing formulas for that height were derived from elastic theory for ordinary reinforced concrete columns. The researchers found that casting UHPC into the boundary elements shifts the compression zone significantly — but because squat walls are disturbed regions where the familiar assumption that plane sections remain plane breaks down, they could not simply adjust the flexural theory. Instead, they ran sixty parametric simulations, sweeping aspect ratios from 0.5 to 1.5, concrete strengths from C30 to C80, UHPC strengths from 120 to 180 megapascals, UHPC widths from 10 to 40 percent of the wall width, UHPC heights from 30 to 100 percent of the wall height, and axial load ratios from 0.05 to 0.45.

The parametric results distilled into a remarkably clean relationship. The height of the compression zone increases linearly with the normalized axial load — heavier vertical compression pushes the crushing zone deeper into the wall — but decreases with the width of the UHPC region following a power law, because the stronger material concentrates compression into a narrower band. The team encoded both effects in a single equation, bounded by sensible limits, and embedded it in a five-step design procedure that retains the familiar machinery of the softened strut-and-tie model: determine the geometry, select the dominant tie mechanism based on the strut’s inclination angle, compute the strut-and-tie coefficient that captures how distributed reinforcement enhances capacity, evaluate the softened concrete strength along the strut, and finally project the diagonal crushing capacity into the horizontal shear strength. Notably, the team deliberately kept the concrete softening coefficient conservative rather than increasing it for UHPC, even though studies of full-section UHPC walls suggest the material’s tensile toughness could justify a higher value — a choice made to avoid overestimating strength in a safety-critical application.

The validation was the most convincing part. Across sixty-eight walls — fifty-nine from the new simulations and nine from independent experimental programs by three other research groups — the proposed model achieved an average predicted-to-actual strength ratio of 1.03 with a standard deviation of 0.20. The existing methods fared considerably worse: JGJ 3-2010 averaged 1.19 with a standard deviation of 0.27, ACI 318-19 averaged 1.11 with 0.31, and the original Hwang model averaged 1.14 with 0.22. In other words, the new model is both nearly unbiased and the most consistent, which is precisely the combination a design engineer wants. The model did show characteristic blind spots: it underestimated precast walls in which UHPC also filled lap-splice zones and vertical columns, because those extra contributions lie outside its scope, and it overestimated two specimens that failed not by strut crushing but by fracture of longitudinal bars at a weak post-cast interface — a different failure mode entirely.

The authors are candid about these limits, noting that the model applies to cast-in-place walls where UHPC sits primarily in the boundary elements and works in reliable composite action with the surrounding normal concrete, and that further validation is needed for other UHPC configurations and interface conditions. Even so, the significance is hard to overstate. Squat shear walls are among the least forgiving elements in seismic design, and the ability to exploit UHPC’s extraordinary strength and crack-bridging fibers in exactly the zones where they matter most — without paying the cost of casting an entire wall in the premium material — could reshape how engineers protect tall buildings in earthquake-prone regions. A formula that turns laboratory performance into trustworthy numbers is the bridge between a promising material and a safer built environment, and this study has just laid a carefully calibrated plank across it.

Subject of Research: Shear strength prediction of squat UHPC-reinforced concrete shear walls under seismic loading

Article Title: A strut-and-tie model for predicting shear strength of squat shear walls reinforced with UHPC under seismic loading

Article References: Ding, Y., Wang, X., Xu, M., & Tian, H. (2026). A strut-and-tie model for predicting shear strength of squat shear walls reinforced with UHPC under seismic loading. Case Studies in Construction Materials, 25, Article e06575. https://doi.org/10.1016/j.cscm.2026.e06575

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06575

Keywords: UHPC, shear walls, strut-and-tie model, seismic design, finite element analysis, squat walls, concrete structures, earthquake engineering, boundary elements, shear strength, structural engineering, LS-DYNA

A study published in the Bulletin of Earthquake Engineering by Qi Cao of Dalian University of Technology and colleagues offers a way to resolve that compromise. The research team fabricated and tested seven full-scale concrete beam-column joint specimens reinforced with a hybrid combination of glass fiber reinforced polymer, or GFRP, bars and highly ductile stainless steel bars. Both materials are corrosion resistant, so the assembly can in principle survive aggressive environments, while the pairing of a brittle, linear-elastic composite with a ductile, strain-hardening metal restores the energy-dissipating behavior that pure FRP reinforcement notoriously lacks. Beam-column joints are the critical nodes where columns and beams meet in reinforced concrete frames, and their behavior under cyclic loading often determines whether a building sways and recovers during an earthquake or collapses outright.

The core problem the researchers set out to address is well known in the seismic engineering community. FRP bars are extraordinarily strong in tension, immune to electrochemical corrosion, and much lighter than steel, which makes them attractive for structures built with seawater and sea sand concrete in regions where fresh water and sound aggregates are scarce. But unlike steel, GFRP does not yield. Its stress-strain relationship is essentially a straight line until sudden, brittle rupture. Conventional seismic design of concrete frames relies on steel yielding in the beams near the joints, forming plastic hinges that absorb earthquake energy through controlled inelastic deformation. A structure reinforced entirely with FRP has no such mechanism, which is why design codes have historically restricted FRP-reinforced concrete in high seismic zones.

Hybrid reinforcement offers an elegant workaround. By placing GFRP bars and stainless steel bars together in the same cross-section, the composite bars contribute their high strength and corrosion immunity, while the stainless steel provides a genuine yield plateau and a long, ductile post-yield response. Highly ductile stainless steel alloys are particularly suitable for this role because, unlike many high-strength steels, they combine substantial elongation capacity with strength that continues to rise as strain accumulates. The result is a member that can crack, deflect, and dissipate energy like a well-detailed steel-reinforced element, while resisting the chloride attack that would cripple ordinary reinforcement within a few decades of coastal exposure.

To find out how this concept performs at the joint level, the team varied three experimental parameters across the seven specimens. The first was the type of longitudinal reinforcement used in the columns. The second was the GFRP bar replacement rate in the beams, meaning the proportion of the beam’s tension reinforcement provided by GFRP rather than stainless steel. The third was the ratio of GFRP stirrups in the joint core region, the confined zone at the intersection of beam and column where shear demands during an earthquake are most severe. Each specimen was then subjected to reversed cyclic loading, the standard laboratory proxy for the back-and-forth shaking a frame experiences during an earthquake, while researchers recorded load, displacement, and deformation patterns.

The findings carry practical weight for designers. Perhaps the most consequential observation is that the type of reinforcement placed in the columns did not change the failure mode of the joint under the conditions of this study. Instead, the GFRP replacement rate in the beams emerged as the dominant factor controlling how the joints ultimately failed. Joints with lower GFRP proportions failed in flexure within the beams, the most desirable mechanism in seismic design because beam bending is ductile and keeps damage away from the joint itself. As the GFRP share increased, failure shifted first to a combined bending-shear mechanism and, at still higher replacement rates, to brittle shear failure in the joint core, the least favorable outcome because core shear damage compromises the entire frame connection.

Ductility, the measure of how much deformation a joint can sustain beyond first yield, told a nuanced story. Hybrid reinforcement in the beams significantly improved joint ductility compared with configurations lacking that ductile steel component, confirming the central premise of the hybrid concept. But the rate of improvement depended critically on the governing failure mode. When specimens transitioned from beam bending failure to combined bending-shear failure, the growth rate of the ductility coefficient dropped, and it dropped again as specimens moved from combined failure to joint core shear failure. In other words, simply swapping more steel for GFRP does not proportionally erode performance; instead, ductility is preserved across regimes until a threshold is crossed, after which the benefit of the ductile component is increasingly throttled by brittle shear mechanisms.

The deformation results were equally revealing. Increasing the GFRP replacement rate in the beams enhanced the ultimate drift ratio overall, a metric of how far a structure can sway before losing capacity, conventionally taken as the point at which load falls to 85 percent of its peak value. And within any single failure mode, the deformation concentrated in the beam plastic hinge zone remained roughly constant. That regularity suggests that once a designer knows the failure mode, the plastic hinge behavior can be anticipated with reasonable confidence, a valuable property for performance-based seismic design, in which engineers target specific deformation and damage states rather than relying on prescriptive rules alone.

Energy dissipation, the ultimate currency of seismic resilience, followed the same logic as ductility and failure mode. The study found that the GFRP replacement rate in the beams was the main factor influencing how much energy the joints could absorb through repeated cycles. Configurations that preserved beam flexural failure dissipated the most energy, while those drifting toward core shear failure lost that capacity. This aligns with a foundational principle of earthquake engineering: the best structure is one whose beams, not its columns or joints, take the damage, because beams are comparatively easy to inspect and repair after an event, whereas joint core damage can be effectively unrepairable.

Beyond the laboratory findings, the study contributes analytical machinery for design. The researchers developed deformation and strength relationships parameterized in terms of the GFRP and stainless steel reinforcement areas, replacement rates, stirrup ratios, and plastic hinge dimensions, calibrated through regression analysis of the test data. Such tools allow practitioners to estimate yield and ultimate displacements, ductility coefficients, and drift capacities for hybrid joints without testing each configuration, which is essential if hybrid FRP-steel reinforcement is to move from promising research to routine specification. The work was supported by the National Natural Science Foundation of China and the Science and Technology Plan Project of Transportation of Henan Province.

The broader significance extends to infrastructure policy. Coastal and marine structures, from port facilities to bridges, face a well-documented corrosion crisis that costs economies billions in repairs annually, while seismic regions cannot accept the brittle behavior of pure FRP reinforcement. A reinforcement scheme that satisfies both demands simultaneously opens the door to concrete frames built with seawater and sea sand in earthquake-prone coastlines, structures that are durable over their full service life and dependable on the day the ground shakes. The seven joints tested here will not be the last word; further studies across scales, loading histories, and environmental exposure will be needed before codes fully embrace the approach. But the study demonstrates with clarity that the failure mode, governed primarily by how much ductile stainless steel remains in the beam, is the master variable for hybrid joints, and that gives designers a clear, actionable rule: preserve beam flexural yielding, and the hybrid system rewards the structure with ductility, drift capacity, and energy dissipation worthy of seismic regions.

Subject of Research: Seismic performance of corrosion-resistant concrete beam-column joints reinforced with hybrid GFRP and stainless steel bars

Article Title: Seismic performance of corrosion-resistant concrete beam-column joints reinforced with hybrid FRP and stainless steel bars

Article References: Cao, Q., Zhang, T., Luo, T., & Zeng, Z. (2026). Seismic performance of corrosion-resistant concrete beam-column joints reinforced with hybrid FRP and stainless steel bars. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02674-8

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02674-8

Keywords: GFRP bars, stainless steel reinforcement, hybrid reinforcement, beam-column joints, seismic performance, ductility, energy dissipation, plastic hinge, concrete structures, failure mode, earthquake engineering, corrosion resistance