Dynamic Sensitivity of Self Support Towers
General
Latticed Self Supported towers are generally not dynamic sensitive, as they have relatively low dynamic masses. And as they are very light structures due to the latticed construction.
However, there are cases where dynamic effects can be significant due to the height of the tower. Or mass and load distribution with a higher concentration towards the top of the tower.
This article describes different approaches to evaluate the dynamic sensitivity of the towers. The approach in various design standards throughout the word is described.
Tower analysis software such as DE Tower can identify dynamic effects sensitivity for the structures that it analyzes.
Identifying the dynamic sensitivity is important as there could be significant dynamic amplification effects and, if not identified, can cause partial or full tower collapses due to gust buffeting effects.
European Standards Self Support Towers
We looked at the older BS 8100 standards part 1 and part 2, “Latticed towers and masts, Part 1: Code of practice for loading” . And Part 2: Guide to the background and use of Part 1. Despite the prevalence of the Eurocodes throughout Europe. This standard is still widely used in the UK and the Republic of Ireland.
The current standards in Europe are EN 1991-1-4 (Eurocode 1: Actions on structures – Part 1-4: General actions – Wind actions) and EN 1993-3-1 (Eurocode 3, Design of steel structures, Part 3-1, Towers, masts and Chimneys, Towers and Masts). Each European country has its national application documents.
The general method of carrying out a structural analysis of self-supported towers is to use an equivalent static analysis. The equivalent static method is a procedure where some allowance of dynamic effects is made through either gust factors (BS 8100 standards) or cscd structural factors (Eurocodes, EN 1991-1-4, and EN 1993-3-1).
The gust factors calculated in BS 8100 do not require a modal analysis, while cscd structural factors require the natural frequency of the self-supported towers.
The resonance factor R used for calculating the cscd structural factor uses the total logarithmic decrement of damping. The total logarithmic decrement of damping is made of the summation of the structural logarithmic decrement of damping and the aerodynamic logarithmic decrement of damping.
BS 8100 & EN 1993-3-1 Static Criterion
Both BS 8100 and the EN 1993-3-1 have an equivalent static criterion. The mass resistance parameter exercises a dominant role. Flare towers have commonly unfavourable ratios.
When tower widths are severely constrained for the layout of compact broadcast antennas configurations, maximum leg loads may occur within the top 20% of the height of the tower.
Dynamic augmentation happens in the higher panels of the tower, particularly when supporting a large concentration of ancillaries at the top of the tower.
Besides, the slenderness ratio of the towers (height/width ratio) is an indicator of dynamic sensitivity. DE Tower software evaluates the equivalent static criterion for each wind direction. It is important to calculate for each wind direction as the wind resistance is different for different wind directions.
The spectral method consists of evaluating the mean and fluctuating part of o tower response. Tower responses can be member loads, foundation reactions, tower displacements. The maximum response due to gusty wind, Pmax, is taken as:
Also, Read About Wind Turbine Tower
Please note that the mean load effect considers second-order effects. The modal analysis was carried out for the pole structure that has deflections due to the mean wind loads. BS PD 6688-1-4:2015, “Background information to the National Annex to BS EN 1991‑1‑4 and additional guidance”, uses the principles of spectral method procedure as an alternative of evaluating cscd structural factor.
The gust factor GEN is defined as follows:
GEN = (gf )(2JaJpIv,0 )
Ja and Jp are aerodynamic admittances, defined here for a particular height above ground level and tower of total height (H).
The above referenced published document substitutes the codification in the EN 1991-3-1:2006.
Although the methodology used comes from the spectral method procedure, by calculating admittances, it can be considered still as an equivalent static method as it considers only the first mode of vibration.
Australian Standards Self-Supported Towers
The Australian Standards (AS/NZS 1170.2:2011, Structural design actions, Part 2: Wind actions) calculates wind loads based on peak gust wind data with return periods around 500 years.
The standard uses a Cdyn, dynamic response factor based on the natural frequency of the structure. The Cdyn dynamic response factor is equal to 1 if natural frequency is greater than 1.
The formulation of the dynamic response factor is similar to the Eurocode (EN 1991-1-4) factor as it considers the following parameters:
- Natural frequency
- Wind turbulence intensity
- Turbulence intensity length
- Background factor
- Peak response factor for resonant response
- Size Reduction Factor
- Structural Damping
- Aerodynamic Damping
US Standards
TIA-EIA-222-H-2017 , “Structural Standard for Antenna Supporting Structures, Antennas and Small Wind Turbine Support Structures,” is the governing standard for the Design and analysis of self-support towers. Basic wind speeds are defined as 3-secong gust wind speeds.
However, ASCE 7-16, “Minimum Design Loads and Associated Criteria for Buildings. And Other Structures” has a formulation for the gust factor Gf for flexible structures. That is dependent on the natural frequency of the tower.
The gust factor is provided for flexible buildings and structures that do not meet the requirements of rigid structures that the fundamental natural frequency is greater than or equal to 1 Hz. This factor also accounts for the building size and gust size in the same manner as the alternate calculated factor for rigid buildings. Still, it also accounts for dynamic amplification caused by the design wind speed, the fundamental natural frequency of vibration, and the damping ratio.
The gust-effect factors account for loading effects in the along-wind direction caused by wind turbulence–structure interaction. They do not include allowances for across-wind loading effects, vortex shedding, instability caused by galloping or flutter. Or amplification of aerodynamic torsion caused by building vibration is the pure tensional mode.
