Containerization and Wind Speed: Challenges in the Global Supply Chain
- CP&A Staff
- May 28
- 5 min read
Updated: Oct 2
Understanding the Complexity of Containerization
Containerization has been a pivotal factor in enabling globalization. It continues to play an integral role in the global supply chain. However, it also introduces complexity into the procurement process due to regional design practices. Across the globe, environmental hazards such as wind loads are defined using region-specific methodologies.
Even within a single region, cranes and wharves may be engineered according to separate design standards. Design codes vary significantly in their measurement of wind speed. For example, the National Building Code of Canada (NBC) defines wind speed based on a one-hour average. In contrast, the American Society of Civil Engineers’ (ASCE) ASCE 7-10 standard uses a three-second average.
Without careful coordination, this disparity can lead to inaccurate load estimations. Crane manufacturers often supply estimated wheel loads based on their design methodologies. However, these values may not align with the requirements of different engineering teams. Each design code applies its own safety factors. Some codes increase the expected demand to introduce conservatism, while others reduce the permissible stress in structural elements.
In such a multifaceted design environment, an experienced subject matter expert is invaluable. They can bridge the gap between design teams. This ensures that appropriate specifications and assumptions are clearly communicated and properly aligned.
The Evolution of Wind Speed Measurement
Accurate wind speed measurement has evolved significantly over the years. Early instruments, such as Dr. Hooke’s pendulum pressure plate in 1667 and Robinson’s 1846 cup anemometer, laid the groundwork for future developments. However, these instruments suffered from limitations, such as poor responsiveness and inaccuracies in turbulent wind conditions.
The introduction of ultrasonic anemometers in the 1950s marked a major improvement. They utilize time-of-flight measurements to detect wind speed with greater precision. Due to the turbulent nature of wind loads, it is common practice to average wind speed over a specific period.
Modern standards vary in their approach to wind speed averaging. This variance can lead to surprising effects on the resultant wind load. Consider the following standards for measuring wind speed:
ASCE 7: 3-second gust
NBC: 1-hour average
FEM 1.001: 10-minute average
Saffir-Simpson Hurricane Wind Scale: 1-minute average
Unfortunately, there is little consistency among different design codes. If we consider a 10-minute average, some measurements will be higher than the mean, and some will be lower. The Durst curve is helpful in converting between these metrics, ensuring consistent application of wind data across different standards.
For instance, a 50 mph wind using a 10-minute average is equivalent to a 72 mph wind using a 3-second average.
While design codes aim to ensure adequate structural strength, their methodologies can differ widely. Some codes may apply a gust factor to account for microbursts while using longer averaging times. This discrepancy contributes to why many engineers consider wind load to be the most confusing design load. It is straightforward to accidentally compare apples to oranges.
Before 1995, ASCE 7 measured wind speed using the concept of the “fastest mile.” This method measured the time taken for a theoretical particle to travel a mile under the current wind conditions. In 1995, ASCE 7 switched to a 3-second average for several reasons. Firstly, the US National Weather Service stopped collecting fastest mile data. Additionally, the introduction of handheld anemometers led to reporters measuring wind speeds much higher than previously recorded, causing confusion.
Determining Design Wind Speed
Design wind speed is determined based on the location and the risk category of the structure. Higher-risk categories have a greater potential impact on public safety and utilize longer mean recurrence intervals (MRI). For instance, ASCE classifies a hospital as a higher risk category IV, while a minor storage facility falls under risk category I.
The risk category influences the mean recurrence interval, which directs the designer to a wind map. This map helps determine the design wind speed relevant to their project. Piers and wharves are typically designed as risk category II structures, with a 700 MRI. If cranes are designed according to FEM 1.001, they may be scaled for a 50-year MRI.
To illustrate, consider the host building for the Port & Terminal Technology conference, the InterContinental at Doral Miami. For this building, the 50-year MRI three-second gust wind speed is 127 mph, while the 700-year MRI wind speed is 166 mph. This significant discrepancy in design wind speeds highlights the critical need for correct structure classification.
Coordination Challenges Between Cranes and Wharves
Designing port infrastructure requires collaboration between various engineering teams. Each team may use different design codes and assumptions, making clear definition of wind speed criteria, load combinations, and boundary conditions essential when exchanging design data. To avoid ambiguity, detailed load cases and reaction forces should be explicitly requested.
Crane specifications might utilize different wind speed definitions and load combinations than the piers and wharves they operate on. This extends to the design of wharves and soil engineering. Generally, wharves are designed using Load and Resistance Factor Design, while geotechnical engineers often employ Allowable Strength Design.
It is unlikely that international governing bodies will unify their structural design codes into a universal standard for wind loads. These codes are deeply rooted in regional practices, local climate data, historical performance, and legal frameworks. Updating wind hazard maps for a consistent averaging time would be necessary for standardization.
Consequently, crane manufacturers, port engineers, and dock designers must proactively identify which code assumptions are being used and how they affect the final design. Clear communication between different design teams is of utmost importance.
Understanding Wind Turbulence and Non-Uniform Loads
Wind is a complex, turbulent phenomenon characterized by irregular fluctuations in speed and direction. This turbulence results in highly variable pressure distributions across a structure. Designers often approximate these effects using equivalent static pressures that vary with height. However, this simplification can lead to overly conservative designs, especially when codes overestimate wind loads.
Overly stiff or heavy crane structures might appear safer but can exert excessive demands on supporting wharf structures. This necessitates a more balanced approach. Accurate design is crucial because crane manufacturers need to provide additional structural strength, resulting in higher costs for the crane owner.
Additionally, crane owners must consider the costs associated with providing wharves, tie-down anchors, and stow pin sockets. Each year, owners incur small but measurable penalties related to energy costs associated with operating heavier structures.
Boundary Layer Wind Tunnel Testing
Dissatisfied with standard wind design criteria, Casper, Phillips & Associates has occasionally assisted in boundary layer wind tunnel (BLWT) testing. BLWT testing provides a more accurate representation of wind effects on structures by simulating real-world turbulent wind conditions.
Unlike aeronautical wind tunnel testing, which maintains uniform speed airflow, boundary layer tests intentionally create turbulent airflow. This simulates actual wind speeds experienced by the structure and can include the effects of nearby buildings and other side conditions.
BLWT test data can develop wind loads on a container crane and can be validated through Finite Element Analysis (FEA). Testing should comply with the adopted design code of the local jurisdiction. For instance, in the United States, Chapter 31 of ASCE 7-22 is likely to apply. When using test data, both base shear and overturning moments must match measured data. If only base shear forces are matched, it may lead to underestimated uplift, potentially compromising structural safety.
Conclusion
The growing complexity of port equipment and the variability of wind design criteria underscore the necessity for clear communication among engineering teams and stakeholders. As codes evolve and cranes increase in size, maintaining valid and understood design assumptions across all teams becomes essential. Utilizing tools such as wind tunnel testing, FEA, and real-time monitoring, along with a shared understanding of design standards, is crucial to achieving resilient, safe, and efficient port infrastructure.
By ensuring a proactive approach to design coordination and communication, industry stakeholders can navigate the complexities of containerization and wind loads effectively.
