Embedded steel tubes have been investigated to assess their ability to strengthen reinforced concrete (RC) beams with longitudinal voids designed to accommodate building services. The embedded tubes were seen to improve load-carrying capacity, stiffness, ductility, and energy absorption, helping to restore the structural performance of voided beams.
Study: Structural behavior of reinforced concrete beams with longitudinal voids incorporating embedded steel tubes. Image Credit: zhu difeng/Shutterstock.com
The study highlights a practical reinforcement strategy that enables electrical, plumbing, and ventilation services to be embedded, while restoring or improving flexural performance. These findings were published in Scientific Reports.
Strengthening Reinforced Concrete Beams for Modern Building Design
RC beams not only provide the strength needed to support modern buildings; they must also accommodate electrical wiring, plumbing, ventilation ducts, and communication systems.
Routing these services beneath the beams often requires suspended ceilings, increasing floor-to-floor height, material use, and construction costs. Longitudinal openings offer a practical alternative by allowing utilities to pass directly through the beam while making more efficient use of building space.
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However, introducing these openings weakens the structural member. Removing concrete from the compression zone reduces the effective cross-sectional area and disrupts the internal stress distribution. As a result, cracks develop more easily, stiffness decreases, and the beam carries lower loads before failure.
In response to these challenges, previous studies have explored strengthening techniques such as fiber-reinforced polymers, engineered cementitious composites, and externally bonded steel plates.
In the present study, researchers took a different approach, investigating whether embedded steel tubes could restore the structural performance of voided RC beams.
They examined the effects of tube geometry, orientation, reinforcement ratio, and void configuration through laboratory testing and validated numerical simulations. Their findings demonstrate that embedded steel tubes effectively reinforce voided beams while preserving the space required for essential building services.
Evaluating Embedded Steel Tubes as Internal Reinforcement
The researchers fabricated 11 reinforced concrete beam specimens to evaluate the effectiveness of embedded steel tubes. The specimens included one solid control beam, one with an unreinforced longitudinal void, and nine reinforced with different steel tube configurations.
The voids simulated openings commonly used to accommodate electrical conduits, plumbing, ventilation ducts, and communication systems.
Each beam measured 1200 mm in length and incorporated identical concrete properties and reinforcement layouts to ensure a fair comparison. After curing, the specimens underwent four-point bending tests to examine their flexural behavior.
Throughout the tests, the team monitored load capacity, midspan deflection, crack dev
elopment, stiffness, energy absorption, and failure patterns. These measurements provided a comprehensive assessment of how each reinforcement configuration influenced structural performance.
The experimental program was complemented by a three-dimensional nonlinear finite element model developed in ABAQUS. The model employed the concrete damaged plasticity approach to simulate cracking and crushing, while embedded reinforcement elements captured the interaction between the steel tubes and surrounding concrete.
Combining experiments with numerical simulations provided deeper insight into the structural behavior of reinforced voided beams.
Embedded Steel Tubes Significantly Improve Structural Performance
The experimental results demonstrated the structural consequences of introducing an unreinforced longitudinal void. Compared with the solid control beam, the voided beam exhibited a 27.5% reduction in ultimate load capacity, while stiffness and energy absorption both declined by approximately 41%.
The reduced concrete area in the compression zone also accelerated crack initiation and limited the beam's deformation capacity. These findings show that longitudinal service openings like those examined in the compression zone may require added reinforcement to maintain structural performance.
Embedding steel tubes within the void substantially improved beam performance. A single rectangular steel tube increased the ultimate load capacity by 57.1% compared with the unreinforced beam and also enhanced stiffness, ductility, and energy absorption.
Increasing the reinforcement ratio produced even greater improvements. The best-performing rectangular configuration increased the ultimate load capacity by 150% and doubled the beam's deformation capacity.
Horizontally oriented rectangular tubes showed better results compared to vertical configurations because their larger moment of inertia resisted bending more efficiently.
Circular steel tubes delivered the highest overall performance, increasing the ultimate load capacity by as much as 180% compared with the unreinforced beam. Their smooth geometry reduced stress concentrations around the void and promoted more uniform stress transfer throughout the beam.
It should be noted, however, that each configuration was only tested once, so repeatability could not be statistically assessed. However, as noted, the validated finite element model supported the findings.
A Promising Solution for Service-Integrated Concrete Structures
This study demonstrates that embedded steel tubes provide an effective way to strengthen reinforced concrete beams containing longitudinal service openings. Experimental testing and validated numerical simulations consistently confirmed the effectiveness of this approach under flexural loading.
The study also provides practical guidance for structural design. Circular steel tubes offered the best overall performance by promoting more uniform stress distribution around the voids, while horizontally oriented rectangular tubes also delivered substantial improvements.
The validated finite element model could support and optimize reinforcement designs before construction, though more validation is required before it could reduce the need for extensive physical testing.
Future research should evaluate larger structural members under realistic service conditions to support practical implementation. Additional instrumentation could also provide a better understanding of strain development, crack growth, and the interaction between concrete and embedded steel tubes.
These studies may shape future design recommendations and facilitate wider adoption in the construction industry.
Embedded steel tubes could allow building services to pass directly through reinforced concrete beams, reducing the need for suspended ceilings without significantly affecting structural performance. As modern buildings incorporate increasingly complex mechanical and electrical systems, this approach could prove to be a practical pathway toward more efficient and sustainable structural design.
Journal Reference
Fayed, S., Ghalla, M., et al. (2026). Structural behavior of reinforced concrete beams with longitudinal voids incorporating embedded steel tubes. Scientific Reports. 16(1). https://www.nature.com/articles/s41598-026-59579-4.
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