Climatic actions according to Eurocode
Climatic loads are a specific type of imposed loads to which almost every building object is exposed. Their nature and value is closely related to the type, geometry and location of the object. When preparing the design, the designer is obliged to include these loads in his calculations.
It has already happened in the past that incorrect consideration of this influence has led to disasters or failures. This aspect is often simplified or omitted due to a certain laboriousness of the determination of loads (especially wind loads) and their transfer to the calculation model, which will be the main subject of this article.
The current basis for the determination of climatic loads are Eurocode standards EN 1991-1-3 for snow loads and EN 1991-1-4 for wind loads.
In a similar way, these standards first determine the effect of the building location on the size of the characteristic load and divide the country into snow and wind load zones, respectively. The next step is to determine the nature of the load resulting from the geometry of the building itself – for wind it will be the external pressure zones and their distribution, while for snow it will be the roof shape factor. The whole is thus a basis and a relatively clear instruction for the determination of the ultimate snow and wind loads.
Automatic generation of loads in an FEA model
If the designer would like to determine these loads manually and apply them to the object in the calculation program, he has to reckon with a very labour-consuming task, mainly due to a multitude of coefficients leading to the final value and it is, so to say, complicated for even the simplest object. For example – we have to consider several wind directions, determine the range of external pressure zones, take into account the internal pressure, the value of pressure in individual zones, and to top it all off we have to take into account a number of dozens of variables and values (from dimensions, to location, to factors related to exposure, direction, terrain, etc.). The worst thing is that the whole thing is then drastically sensitive to change – a small change in the geometry of the building leads to a change in the external pressure zones.
Unfortunately, in most calculation programs we are forced to determine these actions manually and apply them in the form of loads to the FEM model of the structure, which often also requires us to prepare the model itself. Advance Design software has an automatic climatic load generator based on Eurocode, working on the principle of geometry discretisation to the appropriate standard schemes. The user does not have to impose any parameters connected with building geometry.
Above is a general diagram of how a climate load generator works (using wind as an example). Step 1 is practically just the preparation of the FE model for any subsequent analysis. However, it is important that the whole object is clad with cladding, i.e. panels, which do not have any mechanical properties but are only supposed to distribute the surface load on the structural elements. Their geometry is presented in step 2 – on this basis the program recognises the shape of the object and applies appropriate load schemes. Step 3 is the determination of the external pressure zones and the load values which are distributed from the cladding to the members and shells in step 4. In step 5 the final result, the wind load on the structure, is presented.
All these operations take place automatically and one could say that they are by default invisible for the user – the designer only prepares the geometry and as a result he gets the structure loaded by climatic actions. Importantly, any change to the design (geometry, assumptions etc.) allows the loads to be automatically updated to the current state of the model.
Guide to Advance Design generator
The 2020 version of the programme introduced a number of tools allowing to easily generate all cladding in an extremely short time e.g. by selecting linear/surface elements, by drawing or copying.
The cladding determines in which direction (x/y/xy/other defined by angle) it will distribute the load applied to its surface. It also determines certain parameters related to climatic loads.
The loads are determined from the parameters specified in the load cases.
The operation itself is trivial – the parameters we need to establish are those that are not possible to establish from the model but result from the project assumptions (altitude, thermal coefficient, terrain category, etc.).
After these two operations (cladding and load cases), the program is ready to generate loads. It will create exactly as many load cases as necessary from the point of view of uniform/nonuniform snow or different pressure values in the individual zones.
CNC2M – additional provisions for wind loads
Very importantly, the program implements the provisions of the CNC2M document. This document is a kind of annex to the French standars but it is universal in its nature regardless of the country. It defines rules for determining zones and pressure values for buildings much more complicated than those included in the general provisions of Eurocode, e.g. L-shaped or C-shaped buildings, awning canopies, additional provisions for wind shelters. In Poland we are not obliged to use such provisions, but it is a much more reliable approach than using a simple cuboid “cube” model for the whole building.