Qualitative Analysis of Ice Accumulation and Shedding on a Cable Stayed Bridge

The accumulation of ice on cable stayed bridges due to precipitation in winter has been a periodic problem for many cable stayed bridges in North America and Europe. The accumulation of ice and the potential danger of it shedding on the traffic below has been a concern for the public and the Departments of Transportation. This paper pertains to the research done on one such bridge in northwestern Ohio. Analysis of all the events over the past nine years has led to classification of the different methods of accumulation of ice on the stays and its subsequent shedding. This analysis was done based on the weather information obtained from various sources like local airports, local weather stations, Road Weather Information System (RWIS) stations and a local suite of sensors installed on the bridge. To help in taking traffic management actions during icing events and to have all information needed to make decisions in one place, a real time icing monitor application was developed. The monitor is populated with data collected from the above-mentioned weather sources.

Accumulation of ice on stays is a factor of various atmospheric and the structural conditions such as air temperature, stay temperature, precipitation type and quantity etc. Based on the previous significant events between 2007-15, accumulation can be categorized into five methods. The five methods of accumulation are:

1. Cold and Wet
2. Warm and Snow
3. Cold and Snow
4. Temperature or sunshine affected restructuring of ice
5. In cloud ice formation without recorded precipitation

The shedding of ice from the stays was previously thought to occur due to one of two reasons, rising temperatures or solar radiation. On thorough examination of shedding phenomena over 13 significant events, it has been observed that shedding can be categorized into 9 types which are, shedding due to:

1. Warm air temperature due to sunshine
2. Cold stays in warm ambient conditions
3. Temperature or sunshine affected restructuring of ice
4. Solar radiation
5. High speed wind gusts
6. Sublimation of ice
7. Warm stay cables
8. Wet and wind gusts
9. Warm stay cables in warm ambient conditions

In the above-mentioned accumulation and shedding types ‘Cold’ refers to temperature below 0⁰C (32⁰F) and ‘Warm’ is above 0⁰C (32⁰F). ‘Wet’ refers to precipitation of fog, rain, drizzle and other types of liquid precipitation and ‘Snow’ refers to all kinds of snow like precipitation reported in the METAR (Meteorological Terminal Aviation Routine Weather Report) format. The condition ‘Sunshine’ is derived based on time of the day and cloud cover conditions obtained from METAR reports. ‘Solar radiation’ is based on the response from a solar radiation sensor installed on the bridge.

Some of these types are subsets of others, but each type has a different meaning in the context of the events they have occurred in. The accumulation and shedding types in these categories are not mutually independent within themselves; more than one type could be contributing to accumulation or shedding at a given time.

Classification of accumulation and shedding phenomena will help in understanding the type of event occurring at the bridge. Identifying the type of accumulation and possible shedding will help operators in making appropriate control or traffic management actions. This classification was done based on the correlation of real weather data with on field reports and the type of traffic management actions taken during those events. Although this classification has been done based on real weather data, the signatures of these types of accumulation and shedding can be identified in weather forecast data to help the operator prepare for such events in advance.