Don’t Get Carried Away by Wind Loads: Understanding and Managing Wind Forces in Port and Terminal Design
- CP&A Staff
- 1 day ago
- 6 min read
Introduction
Containerization has been a pivotal factor in enabling globalization and remains integral to the global supply chain. However, it can introduce complexity into the procurement process due to variations in regional design practices. Across the globe, environmental hazards such as wind loads, are still defined using region-specific methodologies. Even within a single region, cranes and wharves may be engineered according to separate design standards.
Design codes differ in how they measure wind speed. Without careful coordination, this can result in inaccurate load estimations. For instance, the National Building Code of Canada (NBC) defines wind speed based on a one-hour average, whereas the American Society of Civil Engineers’ ASCE 7-10 standard uses a three-second average. These differing interpretations can cause confusion, miscommunications, and mistakes. This disparity can create confusion when determining loading requirements between the different engineering teams. It is not uncommon for crane manufacturers to supply estimated wheel loads, but those values may have been calculated using an entirely separate design methodology. Crane and dock designs often adhere to differing standards. Which code were the wheel loads based on? Each design code applies its own safety factors—some increase the expected demand to build in 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, ensuring that the appropriate specifications and assumptions are clearly communicated and properly aligned.
Evolution of Wind Speed Measurement
Accurate wind speed measurement has evolved significantly. Early instruments, such as Dr. Hooke’s pendulum pressure plate in 1667 and Robinson’s 1846 cup anemometer, laid the groundwork for future developments but suffered from limitations including poor responsiveness and inaccuracy in turbulent or wind conditions involving vertical components. The introduction of ultrasonic anemometers in the 1950s marked a significant improvement, using 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 the wind speed over a period of time. Modern standards vary in their approach to wind speed averaging. This can have surprising effects on the wind load.
· 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 very little consistency between different design codes. If we consider a 10-minute average, some measurements will be above the mean and some will be below. There will be a 3-second subset of the data that has a larger average than the 10-minute average. The Durst curve provides a means to convert between these metrics, ensuring consistent application of wind data across different standards. It is very important to compare wind speeds using the same averaging time. For example, a 50mph wind using a 10-minute average wind speed is equivalent to a 72 mph wind speed using a 3-second average.
While design codes have the same goal of ensuring adequate strength of a structure, they may go about achieving that goal differently. Some design codes may use a longer averaging time and apply a gust factor to account for microbursts. This is part of the reason why so many engineers consider wind load to be the most confusing design load. It is very easy to accidentally compare apples and oranges.
Before 1995, ASCE 7 measured wind speed using the concept of the fastest mile. The fastest mile measures how long it takes a theoretical particle to travel a mile under the measured wind conditions. Instead of a set length of time, the fastest mile uses a set distance to measure the wind speed. ASCE 7 switch to a 3-second average for several reasons. Most importantly the US National Weather Service stopped collecting fastest mile data. Another consideration was the introduction of handheld anemometers. On-site reporters would measure wind speeds much higher than what was being reported due to several factors such as the wind speed averaging time. Too often this often lead to public confusion and was another motivating factor to update the wind speed definition.
Determining Design Wind Speed
Design wind speed is determined based on location and the risk category of the structure. Higher-risk categories, which reflect greater potential impact on public safety, use longer mean recurrence intervals (MRI). For example, ASCE classifies a hospital as a higher risk category IV while a minor storage facility may be risk category I. The risk category determines the mean recurrence interval which points the designer to a wind map. The wind map is used to determine the design wind speed.
Piers and wharves may be designed as a risk category II structure, which would be a 700 MRI. If the cranes are designed to FEM 1.001, they may be designed for a 50-year MRI. To help illustrate the difference, let’s consider the host building of the Port & Terminal Technology conference, InterContinental at Doral Miami. For this building 50-year MRI 3 second gust wind speed is 127mph and a 700-year MRI is 166mph. This is a rather large discrepancy between design wind speeds for the same building, so it is vitally important to correctly classify the structure.
Coordination Challenges Between Cranes and Wharves
Designing port infrastructure often involves collaboration between multiple engineering teams, each using different design codes and assumptions. It is essential to clearly define wind speed criteria, load combinations, and boundary conditions when exchanging design data. Detailed load cases and reaction forces should be requested explicitly to avoid ambiguity. Crane specifications may use different wind speed definitions and load combinations than the piers and wharves they operate on. This can be further extended to the design of the wharves and the soils engineering. Typically, wharves are designed using Load and Resistance Factor Design, while it is more common for geotechnical engineers to use Allowable Strength Design.
It is very unlikely that all the international governing bodies responsible for structural design codes will come together to create a universal wind load standard. These codes are deeply rooted in regional practices, local climate data, historical performance, and even legal frameworks. Wind hazard maps would need to be revised for a consistent wind speed averaging time. This means crane manufacturers, port engineers, and dock designers need to be proactive in identifying which code assumptions are being used and how they affect the final design. Clear communication between the different design teams is of the upmost importance.
Wind Turbulence and Non-Uniform Loads
Wind is a complex, turbulent phenomenon characterized by irregular fluctuations in speed and direction across both time and spatial dimensions. In engineering applications, this turbulence leads to a highly variable pressure distribution on different parts of a structure. Designers often approximate these effects using equivalent static pressures that vary with height, simplifying calculations. However, this simplification may result in overly conservative designs, especially when codes overestimate wind loads. Overly stiff or heavy crane structures, while seemingly safer, can place excessive demands on supporting wharf structures, necessitating a more balanced approach.
A more accurate approach is important since the crane manufacturer has to provide extra structural strength which costs the crane owner more money. Too often the manufacturer is forced into extra design effort and more expensive construction details to avoid exceeding specified wheel loads with overly conservative wind loads.
Owners should care too, because they have to provide wharves, tie down anchors and stow pin sockets. Year after year owners also pay a small, but measurable penalty for energy costs to operate heavier structures.
Boundary Layer Wind Tunnel Testing
Dissatisfied with the usual wind design criteria, Casper, Phillips & Associates has, from time to time, aided in performing 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. This type of testing is quite distinct from aeronautical wind tunnel testing which, as the name implies, is used for airplane testing. While aeronautical tunnels have a uniform speed air flow over the entire length, height and width of the tunnel, boundary layer has features which deliberately create turbulent air flow from the surrounding terrain to simulate actual wind speeds as experienced by the subject structure. Even the effects of nearby buildings and other side conditions can be introduced if desired.
BLWT test data can be used to develop wind loads on a container crane and validated through Finite Element Analysis (FEA). Testing should be done to the adopted design code of the jurisdiction having authority. For example in the United States, Chapter 31 of ASCE 7-22 is likely to apply. When using test data, the user must ensure both base shear and overturning moments match measured data. Matching only base shear forces can lead to underestimations of uplift, potentially compromising structural safety.
Conclusion
The increasing complexity of port equipment and the variability of wind design criteria requires clear communication among engineering teams and stakeholders. As codes evolve and cranes grow in scale, it is vital to ensure that design assumptions remain valid and are understood across all design teams. Tools such as wind tunnel testing, FEA, and real-time monitoring, combined with a clear and shared understanding of design standards, are necessary to achieve resilient, safe, and efficient port infrastructure.
