Aspec Engineering student engineers Drishti Patel and Jarred Richards discuss the impact of wind loads on key industrial structures, including conveyor galleries.
When it comes to industrial structures, understanding how wind loads interact and impact them can be the difference between successful operation and failure.
With a thorough understanding of the factors that influence wind loads, engineers can develop efficient designs that can withstand these impacts. Factoring in wind load assumptions when designing key infrastructure, like conveyor galleries, is crucial.
Wind loading is a critical consideration in structural design. It directly influences elements of safety, functionality and durability of buildings an infrastructure. While it can be challenging to assess the impact on structures like conveyor galleries, due to their elongated design, susceptibility to outside influence, the impact of dynamic effects, the ability to make accurate load calculations is vital.
While there are many facets which should be considered when determining the validation of wind loads, specifically in relation to conveyor galleries, there are four areas of critical consideration:
- Ensuring design wind speeds are determined in accordance with the relevant code, including AS/NZS 1170.2:2021
- Aerodynamic drag and pressure coefficients and the calculation of wind loading on the structure.
- Validating the results via wind tunnel testing
- Using alternative techniques for validation, including computational fluid dynamics (CFD)
AS/NZS 1170.2:2021 is considered the primary design standard for the determination of wind loads on structures. The determination of wind speed and design wind pressures for the structure encompasses several factors.
The final design wind pressure on a structural element is determined via the following formula:
Understanding AS/NZS 1170.2:2021: Wind Speed
There are multiple key factors involved in the determination of the relevant design speed including: the regional wind speed, wind direction multiplier, local terrain and height, local shielding, climate change allowances and local topography.
The terrain/height multiplier is calculated based non the height of the structure and terrain category from AS1170.2. It can have a significant impact on the design wind speed with values ranging from 0.75 to 1.39.
It should be noted that the climate change factor replaced the uncertainty factors when the standards were revised in 2021. The topographical factor is based on the surrounding topography such as hill shapes. The shielding factor considers shield from adjacent structures.
The wind direction multiplier is based on statistical probability of peak winds to occur in each cardinal direction. This allows for engineers to optimise designs based on a structure’s orientations.
The wind directional factors are separated into four groups based on the governing weather patterns in each region. The regions are as follows:
- Region A0: Dominated by non-synoptic winds (e.g. thunderstorms)
- Region A1 and A4: Dominated by extra-tropical synoptic winds (e.g. large-scale pressure systems)
- Regions A2, A3, A5 and B1: Influenced by a mix of tropical synoptic and non-synoptic winds (Note: B1 includes tropical cyclones)
- Regions B2, C and D: Dominated by tropical cyclones
Understanding AS/NZS 1170.2:2021: Aerodynamic Parameters
For complex structures like conveyor galleries, it is challenging to accurately determine aerodynamic parameters such as the aerodynamic shape factor or drag coefficient. In Appendix C of AS 1170.2:2021, methods are provided to determine aerodynamic shape factors for exposed structural members, frames and lattice towers by utilising the drag coefficient of the structure or member.
The drag coefficient is a dimensionless quantity that is used to quantify the resistance of an object in a fluid. The drag coefficient is combined with additional factors such as the aspect ratio correction factors and the shielding factor for multiple frames to determine the aerodynamic shape factor of simple shapes, individual members and a series of multiple open frame structures such as truss style conveyor galleries.
While a ‘member-by-member’ method can be effectively used to determine the wind force on a truss structure in accordance with AS1170.2:2021, applying it to conveyor galleries is more complex.
These structures often support additional equipment (e.g. walkways, conveyor idlers, conveyor belt, services, secondary structures etc.) which are common in bulk material handling assets. These attachments significantly increase the complexity of accurately determining the aerodynamic properties of the structure.
Due to the complex geometry of the equipment exposed to wind, AS1170.2:2021 can be substituted for experimental and computational methods to assist engineers in determining the aerodynamic properties of structures.
Wind tunnel testing
As a case study, wind loads on a single open conveyor gallery were assessed using the analytical methods outlined in Appendix C of AS 1170.2:2021. Wind loads were calculated perpendicular to the gallery span and the results were converted into an equivalent drag coefficient based on the total bluff area of the gallery (total length x total height), allowing for direct comparison with values obtained from small-scale wind tunnel tests.
The analytical assessment yielded an equivalent drag coefficient in the range of 1.6 to 1.7 for a gallery with a solidity ratio of around 0.2. The drag coefficient for the conveying equipment and fixtures was determined using engineering judgment and assumed drag values provided in AS 1170.2:2021.
Wind tunnel testing has shown a range of values for different conveyor gallery geometries / constructions and can be invaluable in optimising wind loads, compared with analytical methods.
Open truss galleries, which consist of exposed structural members tend to have lower drag coefficient when compared to enclosed galleries, especially if the structure is open on all sides and has low solidity.
Typical drag coefficients values for conveyor galleries with low solidity (0.2 and under) range from 0.8 to 1.4, as measured through wind tunnel testing. These experimentally determined values are generally lower than those obtained using analytical methods for galleries with similar designs and solidity ratios.
Enclosed galleries, which are clad in metal sheeting or other materials, present a more uniform wind profile. These structures typically have drag coefficients between 1.3 and 2.0 as measured by testing, depending on cross-sectional shape, aspect ratio and wind direction.
Alternative methods
Computational Fluid Dynamics (CFD) is the computational equivalent of wind tunnel testing. Its key advantages include the ability to be conducted in-house, with greater efficiency and at a lower cost compared to traditional wind tunnel testing. However, it is not recognised by AS 1170.2 as a prescribed method to determine aerodynamic parameters without additional wind tunnel testing. CFD is also limited to assessing models in steady state without dynamic effects considered in the simulation.
In conclusion, there is no universal method in determining wind loads on complex structures and careful consideration must be given each time wind loads are assessed. Wind tunnel testing remains the most accurate and reliable method to determine wind loads, particularly for conveyor galleries, and can lead to more efficient designs by reducing conservatism in wind load estimates.
In the absence in wind tunnel testing, engineers should apply and interpret relevant standards such as AS1170.2:2021 – noting they are generally designed to provide conservative results to ensure structural safety.