Retrofitting an antenna tower foundation under limited site constraints
\[ \textsf{\textit{\footnotesize{Excavation adjacent to foundation block}}}\]
About the project
The project involved the retrofitting of an approximately 39-metre-high lattice steel antenna tower forming part of a telecommunications base station in Gizdavac, Split–Dalmatia County. During excavation works, it was discovered that the existing concrete foundation was significantly shallower than expected, and the condition and bond of the original anchors could not be reliably verified. This discovery invalidated the initial design approach, which relied on extending the foundation and activating the existing anchors in tension.
\[ \textsf{\textit{\footnotesize{Global FEM model and Connection of tower reinforcing structure to the new foundation}}}\]
To retain the tower and avoid extensive replacement works, the structural concept was revised. The existing foundation was treated as a compression-only support in the global model, while all tensile reactions were redirected into a newly constructed foundation segment via an above-foundation steel adapter. This approach minimized intervention within the constrained site boundaries and allowed the tower to remain operational throughout most of the retrofit process.
\[ \textsf{\textit{\footnotesize{New foundation}}}\]
Engineering challenges
The primary challenge was the unreliable tensile capacity of the existing foundation and anchors, which could not be confirmed through investigation. In addition, the available footprint for any new foundation work was severely limited by property boundaries, existing installations, and protective corridors. The asymmetrical distribution of antennas and equipment along the tower height generated significant eccentricities and torsional effects, increasing tensile reactions at individual supports.
\[ \textsf{\textit{\footnotesize{IDEA StatiCa Connection model and analysis results of a pipe connection to HEB550 with ribs}}}\]
At a local level, the above-foundation steel adapter, formed from an HEB550 section, required careful verification of wall and flange stability. The anchoring system had to be designed within a narrow concrete block, making edge distances, group effects, and reinforcement interaction critical. All of this had to be executed while maintaining structural stability during drilling, anchor installation, and assembly, without interrupting telecommunications service.
Solution and results
The revised structural concept separated uncertainty from controllable elements. In the global FEM model, the old foundation was defined as compression-only, eliminating dependence on its tensile resistance. Tensile forces were transferred through an HEB550 steel adapter with locally optimized stiffeners into a new concrete foundation zone, anchored using chemical anchors.
\[ \textsf{\textit{\footnotesize{IDEA StatiCa Connection model and analysis results of anchoring the HEB element}}}\]
IDEA StatiCa Connection was used to model and verify the above-foundation substructure. The software enabled detailed assessment of bolted joints, prying effects, combined shear and tension in bolts, and local stability of the HEB550 section. Iterative adjustment of stiffener geometry and placement reduced peak stresses and ensured a rational force flow, resulting in an execution-friendly detail.
IDEA StatiCa Detail 3D was applied to the design of chemical anchor groups in the new foundation. All relevant failure mechanisms were checked in accordance with EN 1992-4, with full consideration of group effects and cracked concrete. A decisive advantage was the explicit inclusion of reinforcement contribution within the anchorage zone, which made it possible to achieve sufficient resistance despite narrow foundation dimensions and limited embedment depths.
\[ \textsf{\textit{\footnotesize{IDEA StatiCa Detail model of the anchoring including reinforcement}}}\]
The final solution satisfied all ultimate and serviceability limit state requirements. Tower displacements and base rotations remained within acceptable limits, torsional effects were effectively managed, and execution was completed within the constrained site without major temporary works. Compared to alternatives such as full foundation replacement or micropiles, the adopted concept reduced construction risk, duration, and cost while maintaining transparency and traceability in design.
\[ \textsf{\textit{\footnotesize{Summary of results in IDEA StatiCa Detail}}}\]
Conclusion
By redefining the support concept and transferring tensile forces to a newly designed above-foundation substructure, the retrofitting of the Gizdavac antenna tower was successfully completed without reliance on the uncertain capacity of the existing foundation. The combined use of the Connection and Detail applications enabled precise verification of joints, stiffeners, and chemical anchor groups, including reinforcement interaction. The result is a verifiable and constructible solution tailored to strict spatial constraints and continuous operation requirements, demonstrating how targeted digital analysis can significantly reduce structural risk in retrofitting projects.
About DI PLAN d.o.o.
DI PLAN d.o.o. is a Croatian structural engineering company specializing in the analysis and design of steel and concrete structures, with a strong focus on retrofitting, upgrading, and complex load-bearing systems. The company combines advanced numerical modelling with practical construction insight, delivering solutions that are technically sound, transparent, and adapted to real-world site conditions. Through the consistent use of modern tools such as IDEA StatiCa, DI PLAN d.o.o. supports clients in achieving safe, efficient, and economically rational structures across infrastructure and telecommunications projects.
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