2. Climatic Impacts and Loads

In structural engineering, climatic loads are extreme impacts resulting from the fluctuating state of the Earth’s climate system in general or of processes in the Earth’s atmospheric in particular. They are considered as single events such as extreme storms (e.g. hurricanes, extratropical cyclones) to ensure structural safety or as long-term processes to ensure durability. When planning development projects in urban areas not only the safe and durable design of the building or structure in question is of importance. With a trend to a high level of global urbanization, the effect of the local microclimate on human well-being has increasingly gained attention. A resourceful and climate conscious design of both, the built (structures and buildings) and urban (space between buildings) environment is indispensable to ensure sustainable urban planning [11].

Wind

Few other forces have so universally shaped the diverse terrains and waters of the earth. Few other phenomena have exerted such profound influence on the history of humankind, on the way we live and how we build and where [12]. Being one of the principal forces in structural engineering, wind loads are determined by two main factors: the local climate and the shape of the object exposed to the wind. With the latter, the architectural form language plays a significant role for the loading the structure or building has to sustain throughout its lifetime. Hence, the load-carrying system on the inside and the outside shape of the building skin are key elements of any structural optimization process.

Wind loading is a fluctuating process governed by the aerodynamic behavior of the building, structure or structural element. To identify the extreme characteristic used for structural design, a large quantity of data is usually required and obtained through simulations with experimental or numerical models. Main challenge in these simulations is the accurate reflection of the turbulent atmospheric boundary-layer (ABL) in the lower part of the Earth’s atmosphere. A mismatch in the turbulent structure and velocity distribution has a significant influence on the safety [13] or lifetime prediction of the structure designed with the data obtained through simulation. Wind tunnel testing (Figure 3a) is a proven and reliable method to investigate the complex wind loading process and allows amassing a large amount of data for reliable statistical analysis and probabilistic modelling of the structural response. On the other hand, a disadvantage of wind tunnel model is limit of shape variability. Depending on the type of model, a shape-load optimization has to done manually, i.e. through stepwise variation of the model shape (force balance models). Here, the numerical simulation provides the possibility of an automated process, linking aerodynamic optimization of the outer shape with a topology optimization of the load carrying structure. The present state-of-the-art of numerical simulation software and computational power puts some limits to a reliable and reasonably fast application. Current research effort within the group ‘Climate&Structures’ aims on numerical modelling strategies of sufficient accuracy and acceptable computation time [14],[15].

Icing

Icing of structures is part of phenomena occurring under cold climate conditions starting just under 0°C. The type of icing and magnitude of its effect on structural and aerodynamic loading is a function of temperature, wind speed and precipitation. All types have in common that the precipitating water is still liquid at the instant of impact on the object’s surface and solidifies gradually in the subsequent thermodynamic process. Air temperature, wind speed, droplet size (median volume diameter), liquid water content (LWC) in air, object’s surface and core temperature, thermal conductivity, geometric shape and surface roughness determine the ice accretion process and with that the quantity of additional mass on the object and the alteration of the object’s geometry, i.e. its aerodynamic performance. The phenomenon of atmospheric icing on structural elements is the observable manifestation of a complex thermodynamic process which is best studied by recreating the governing climatic and structural boundary conditions as true-to-life as possible. For this reason, the ‘Climate&Structures’ group uses a climatic wind tunnel (CWT) jointly developed and operated with the Department of Hydro- and Aerodynamics of FORCE Technology [16]. Amongst other, in-cloud icing on structural bridge cables [17] and wind turbine wings are investigated. The wind tunnel allows using prototype samples of cables and wing section are studies at reduced scale compared as validation data to numerical icing simulations [18] (Figure 3b,c). Apart from additional mass and increase of aerodynamic resistance amplifying static loads or reducing energy production, ice accretion also bears a risk of aerodynamic instabilities such as galloping or flutter. Both can lead to large-amplitude vibrations reducing significantly structural lifespan and/or lead to structural failure. With respect to structural ecology, controlling and reducing the driving mechanisms behind aerodynamic instabilities for extreme and changing climate conditions allows minimizing maintenance and material consumption. The CWT is part of a wind tunnel network at DTU including test facilities at the Departments of Civil and Mechanical Engineering and of Wind Energy.

Figure 3: Examples for investigations of wind actions on civil engineering structures under different climatic conditions.

Snow

With respect to structural loading snow is a considerable factor in cold climates, which in extreme cases exceeds the carrying capacity of roofs and, as in Arctic regions, can remain for months on the structure leading to densification and water penetration. Long-term penetration leads to mold growth deteriorating the integrity of the mostly wooden load carrying system and contaminating the indoor climate. The research group on ‘Climate&Structures’ looks at the relation of architectural design of Arctic residential buildings and snow accumulation on and around the buildings. The former address snow loading events whereas the latter focuses on the usability of buildings and their accessibility to formulate guidelines and recommendation for Arctic architecture and urban planning. Especially for Arctic regions, an increase of the annual mean temperature may lead to an increase of snow and wind event frequency and intensity. To investigate the mechanics of wind-driven snow accumulation the small closed-circuit boundary-layer wind tunnel at DTU Civil Engineering is fitted with a seeding mechanism of substitutional material. For validation of model-scale tests, a reference cube was installed in Nuuk (Greenland) with simultaneous monitoring of climatic conditions and snow accumulation throughout, up to now, two winter seasons.