Assessing the condition of existing steel structures built decades ago is a recurring task in design practice, particularly when the original design documentation is incomplete or the structure’s condition has changed over time. One such assessment involved the structural verification of several high-voltage transmission line support types using Consteel. The original design documentation of the supports presented here dates back to around 1968, while the assessment itself was carried out in accordance with currently applicable standards.

The two support types presented gave different results: the PIC 400 passed all checks, while the PAS 400101 required strengthening – this latter case is presented in detail below.

STANDARDS AND METHODOLOGY

The structural verification was performed on a spatial (3D) finite element model, with loads determined based on the actual spans, in accordance with SR EN 50341-1:2013 [1] and SR EN 50341-2-24:2019 [2], the standards governing overhead lines exceeding AC 1 kV. The reliability level applicable to the supports is Level 1 (50-year return period), with a partial safety factor for the maximum wind load of γW = 1.0.

The assessment covered the following load combinations as specified by the relevant standard: N1, N2a, N2b, N3, N4a, N4b, as well as the security load cases A1 and A2. The most critical combinations are generally the combined action of wind and ice loads perpendicular to the conductor (N2a, N2b), and the simultaneous action of nominal wind and extreme icing under conductor breakage (A1), particularly for suspension and tension supports located at angle points. Every support examined was checked for both strength and global stability, using the governing combinations for the given support type. Utilization ratios were determined in accordance with the standard, with stability checks carried out using the General Method[3], which automatically evaluates the structure’s resistance to loss of stability without requiring manual input of buckling lengths – a significant simplification compared to traditional analytical methods. (For a simple example illustrating the difference between the two methods, see this link.)

INCORPORATING THE ACTUAL GEOMETRY INTO THE MODEL

A key element of the assessment is that the calculation was based not on the geometry of the original design documentation, but on the structure’s actual, present-day condition. The field survey was carried out using laser scanning technology, enabling point-cloud-based analysis; from this, the support types were identified (distances between legs and conductors, dimensions of the main structural members). Building the calculation model in Consteel is quick and straightforward, and if the structure has also been modeled in Tekla, the model can be easily transferred into Consteel, avoiding the need to build the geometry twice.

Determining the vertical deviation (tilt) of the structural members was carried out as a separate step: the calculation model and the point cloud were compared in Tekla Structures, and the resulting deviations were incorporated into the final Consteel model as global imperfections. Since the standard does not specify deflection or tilt limits, the verticality of each support was assessed individually, using the same General Method that automatically evaluates loss of stability without requiring input of buckling lengths.

The survey also included verification of the wall thickness of the structural members: although the corrosion protection coating showed significant degradation, no actual reduction in cross-section was detected.

EXAMINED TOWER TYPES

Based on the visual survey, three of the most common tower types with the largest spans/loads were selected for detailed assessment. Two of them are presented below; a third type, designated SAC 400107, was also verified, but its results are not detailed within the scope of this article.

The PAS 400101 tower – requiring strengthening

The PAS 400101 is one of the most common suspension tower types in the network; its main load-bearing frame consists of L70×7 leg members and the transverse bracing elements. The conductors are attached to the cross arm, at the insulator connection points, this zone was therefore given particular attention during the assessment.

Load combinations and evaluation

Under the N1 load combination, all structural members remained within the permissible limit, with the highest utilization (96.9%) occurring at the main legs. Under the N2a combination, a utilization of 108.8% was already obtained at the upper part of the cross arm, at the insulator connection point, while the main structural members remained below 80%, so the cross arm required local strengthening.

The most critical condition was represented by the A1 combination (conductor breakage): breakage of the upper right-hand conductor was modeled, and the highest utilization reached 168.4%, at the lower foot of the cross arm. The overload at this point resulted from the section being subjected to a force perpendicular to its principal axis of inertia, which led to loss of stability. This was not the only point exceeding 100%: values above 100% were also obtained at five further points within the cross arm zone, indicating that the problem does not stem from a single local weakness, but from an inadequate member layout of the cross arm as a whole. Meanwhile, the main legs operated with a utilization below 60% throughout.

VERTICALITY CHECK

Comparison of the point cloud with the ideal geometry showed a horizontal deviation of about 50 mm, which was incorporated into the model as an imperfection; upon re-analysis, the highest utilization reached 96.9%, confirming that the structure met the stability requirements.

STRENGTHENING PROPOSAL

This solution ensures adequate load-bearing capacity of the cross arm zone and eliminates the identified overloads.

The structurally safe PIC 400 tower

The structural configuration of the PIC 400, through its main legs and diagonal bracing, provides increased stiffness for both transverse and longitudinal loads. Under the N2a combination, the highest utilization was 91.7%, near the tower tip; under the A1 combination (conductor breakage), it was 85.2% at the lower foot of the transverse member. Comparison of the point cloud with the ideal geometry showed a horizontal deviation of about 140 mm at the tower tip; once this was incorporated into the model as an imperfection, utilization in the affected zone rose slightly, to 80.5–82.5%. The magnitude of the deformation is significant, but based on the stability assessment the structure can be operated safely, and the deviation does not result in a critical condition.

SUMMARY

This assessment clearly demonstrates how an existing steel structure, designed decades earlier, can be verified against current standard requirements, taking into account its actual geometric condition as surveyed by laser scanning. The PIC 400 passed all examined load combinations; for the PAS 400101, however, a clearly identifiable structural issue, inadequate member layout of the cross arm zone, was uncovered, for which a targeted strengthening proposal was also developed.

REFERENCIÁK

[1] SR EN 50341-1:2013 – Overhead electrical lines exceeding AC 1 kV – Part 1: General requirements – Common specifications

[2] SR EN 50341-2-24:2019 – Overhead electrical lines exceeding AC 1 kV – Part 2-24: National Normative Aspects (NNA) for Romania

[3] EN 1993-1-1:2006 – Eurocode 3: Design of steel structures – Part 1-1: General rules and rules for buildings 6.3.4