Using CFD Analysis to determine loads on an existing telecommunications tower and shorten the investment process by 6 months

Introduction

Designing tall structures in the telecommunications industry has relied on conservative empirical-analytical procedures contained in wind standards for years. Although the PN-EN 1991-1-4 (Eurocode 1) and PN-EN 1993-3-1 (Eurocode 3) standards guarantee safety when designing telecommunications towers, their excessive pessimism sometimes becomes an investment barrier.

The traditional calculation method sums the forces acting on individual elements, completely ignoring key aerodynamic phenomena, such as antenna shielding (the aerodynamic shadow effect) or orientation relative to the wind direction. Consequently, investors often face the necessity of costly and time-consuming reinforcement of steel structures that, in reality, possess significant load-bearing reserves. The answer to such situations is wind engineering based on Computational Fluid Dynamics (CFD).

The Task: A telecommunications tower with exceeded bracing connection capacity

As a design office, we were tasked with evaluating the possibility of mounting planned base station telecommunications infrastructure for a mobile operator on an existing Protel TPM I steel tower with a total height of 50 m located in Sosnowiec.

This structure is a spatial truncated pyramid with an equilateral triangle cross-section and constant taper (base axial dimensions are 4.40 m, and the top is 1.40 m). The tower shaft consists of 5 assembly segments, each 10.0 m high. The tower was made of equal-leg angles (legs) and cold-formed channels (bracings).

The planned investment assumed mounting additional antennas on one of the existing horizontal service galleries at the top.

Our technical opinion, containing static-strength calculations, was unfavorable for our Client. The calculations showed an exceedance of the allowable load-bearing capacity in bracing connections in the lower part of the tower (in segments 0-20 m above ground level). The utilization of critical bracing-to-leg connection capacities reached 112% and 103% for the lowest assembly segments of the tower.

Conclusions drawn from calculations performed using the traditional method meant the necessity to physically reinforce the tower structure prior to mounting the additional base station. For the operator, such a scenario meant a significant increase in project costs and, worse, extending the investment process by several months due to the need to obtain required administrative permits.

The Solution: A digital CFD wind tunnel to determine loads

Classic calculations determine the wind pressure force on elements according to the simplified formula ∑F = q x A x c_f, treating each piece of equipment as independent objects with individual aerodynamic drag coefficients. This method is fast and safe, but it always overestimates the loads on structures.

Thanks to CFD, we could account for physical phenomena completely omitted by the classic calculation method:

1. The shielding phenomenon (aerodynamic shadow): Leeward antennas are located in an aerodynamic wake (an area of drastically reduced air velocity generated by windward antennas). This causes the wind pressure forces on these shielded elements to be much lower.
2. Real wind orientation: Classic calculation methods assume full, frontal exposure of all elements to the wind direction. The CFD analysis takes into account whether the antenna is oriented frontally, sideways, or at an angle to the wind, yielding lower wind pressure force values.
3. Structure-equipment interaction: Classic calculations treat the tower structure and tower equipment as separate, independent elements. In reality, there is interaction between these components, and the CFD method analyzes the tower-equipment system as a whole, thereby fully accounting for the orientation and mutual shielding of all elements.

The first step of the CFD analysis is to create a precise 3D model of the tower in question, containing the structure's geometry and the full telecommunications equipment arranged according to the planned configuration provided by our Client. The next step was to create dedicated calculating mesh and ran the analysis.

The Results: Overestimations of the classic cethod?

Comparing CFD simulations with traditional standard calculations brought significantly different results. As a result of the conducted CFD analyses, forces acting on the tower were obtained that were up to approx. 30% smaller than those calculated classically.

The differences for selected tower segments present themselves as follows:

• Segment S-1 (top, loaded with antennas): The force dropped from the standard 11.44 kN to 8.20 kN (a reduction of 28.3%). The shielding effect is clearly visible in the wide aerodynamic wake generated by the first sector antennas on the windward side
• Segment S-2 (level of planned antenna assembly): The force dropped from the standard 9.94 kN to 5.65 kN (a reduction of as much as 43%). Here, the shielding effect was further amplified by the favorable orientation of the antennas on the leeward side.

When the wind pressure loads determined via CFD were applied to the tower's computational model, the previously obtained capacity exceedances disappeared:

• The capacity utilization level of the critical bracing-to-leg connection in the bottom section (segment SM5) dropped from an unacceptable 112% to a safe 81%.
• In the adjacent segment SM4, connection utilization dropped from 103% to 76%.

The Effect: Immediate investment unblocking

Thanks to engineering insight, an innovative approach, and the application of advanced CFD fluid mechanics, we proved that the tower in Sosnowiec has sufficient load-bearing reserves and is fully safe. Physical reinforcement of the lower segment structures proved unnecessary.

For our Client, this design decision brought direct, tangible business benefits:

• Shortening the investment process by 6 months – the need to design reinforcing bracings, undergo a time-consuming administrative building permit process, and perform rope access assembly works was eliminated.

   

• Reducing the investment process cost by tens of thousands of PLN – the funds the investor would have had to spend on the investment process and construction works remained within the organization.

Summary

The case of the 50 m PROTEL tower in Sosnowiec is a proof of how a traditional design approach, based solely on simplified standard assumptions, can unnecessarily stifle investment processes and generate redundant costs.

At CarbonPro, we believe that the role of a modern civil engineer is to look for ways that allow the Investor to execute the investment process faster and cheaper while maintaining the proper level of operational safety. By combining over a decade of experience in high-rise construction with advanced Computational Fluid Dynamics (CFD) numerical modeling and Finite Element Method (FEM) calculations, we optimize structures, extend the lifespan of existing masts and towers, and enable the realization of projects that others deem impossible. We prove that structural safety and optimization can go hand in hand - the key is utilizing modern tools that have already been used in other industries for decades.