The reference standard for calculating the wind load acting on a tower crane is FEM1.001, which has particularly easy settings due to its conceptual and implementation simplicity. This calculation method assumes that the wind is acting horizontally, that it can blow against the tower crane from any direction at a constant speed, and that it is generating a static force.
Dynamic pressure is thus calculated with the well-known formula:

To determine the force caused by this pressure on a structural element, the following expression is used:

The standard includes a table of the form factors for the most common profiles used in carpentry construction. In order to determine the resulting force on the whole machine, the force acting on the individual components is calculated using (2) and, finally, the total sum is calculated. The last step of the process, which is to evaluate the shielding effects that a generic element will have on the next one, is perhaps the most critical and can place the design engineer in difficulty. While the standard has many rules and parameters that can help the designer to evaluate these shielding effects, engineers must typically be guided by their intuition and experience to understand how much one element is shielded by another and how intensely they are affected by the air flow.

As mentioned above, the method proposed by the FEM1.001 standard allows engineers to easily perform a numerical evaluation of the wind force on a crane, using a completely traditional calculation. However, some critical aspects persist, such as:

• The accurate prediction of the shape coefficient of a given profile
• The evaluation of the shielding effects of one element on the next

These issues represent a real challenge for the engineer performing the calculations. The computational fluid dynamics (CFD) calculation discussed below, therefore, presents an innovative attempt to overcome these uncertainties and to assess the feasibility and accuracy of the traditional approach. The Ansys CFD analyses were performed for a Terex flat-top CTT172-8 crane (Fig. 1), according to the FEM1.001 standard mentioned in paragraph 2, and the resulting force was used as a reference for the comparison between the two methods.

Geometry and mesh

Fig. 2 - CAD models

Fig. 3 - CFD external domain, with Inlet (blue) and Outlet (red)

A CAD model of the tower crane was supplied by Terex: the geometry consisted only of the upper part of the tower crane, specifically the operator cabin, the concrete counterweight and the jib. Three different crane lengths were considered: 65, 60 and 55 meters (Fig. 2). The crane lengths were obtained by “cutting” the farthest part of the jib, while the counterweights were reduced accordingly.
To accurately calculate the fluid dynamics surrounding the crane, a suitably large external domain was considered in the CFD model. The dimensions of the box in this case were 1.0 x 0.8 x 0.2 km (Fig. 3). Different wind directions were evaluated by simply rotating the cylindrical domain of the crane relative to the wind inlet. Clearly, the major challenge of this CFD analysis concerned the over dimensions of the geometry, together with the extremely detailed CAD of the solid parts of the crane.
Small bolts, screws, and panels were common and the geometric anomalies within the CAD were too long and cumbersome for the user. Fluent Meshing’s fault-tolerant mesh workflow was crucial to successfully mesh the fluid geometry, preserving the relevant part shapes of the crane while neglecting all other details. Thanks to this powerful tool a triangular surface mesh of the tower crane was created, from which it was possible to extract the fluid domain directly.
A tetrahedral and prism approach was then used to mesh the cylindrical section of the tower crane, while the external box was meshed using a structured strategy. The resulting meshes have very large numbers of elements, due to the overall dimensions of the model and the level of mesh detail (Fig. 4).

Fig. 4 - Mesh

Fig. 5 - Velocity contour on the longitudinal section

Solution and results

The Ansys CFD model was essentially configured for an external aerodynamics problem with incompressible, turbulent, steady state conditions.
An air temperature of 20°C was selected for the working fluid, with a constant wind inlet velocity of 20 m/s. Three different wind directions were considered: headwind (front), side and tailwind (rear), which were achieved by rotating the cylindrical domain of the crane. The k-omega shear stress transport (SST) turbulence model without ground contour effects was used. Free flow (symmetry) conditions were used on the sides of the bunding box.
Results were both qualitative and quantitative, in terms of the pressure and velocity contours (Fig. 5) and overall pressure on the crane surfaces. The shielding effect of the wider surfaces on those that follow is evident in Fig. 5.
Terex was mainly interested in predicting the resulting forces along the three Cartesian directions: longitudinal (X), vertical (Y) and transverse (Z). Fig. 6 shows the cumulative plots of the X (longitudinal) force for the case with headwind. The similarity of the results for the three cases is noticeable, with a slight increase in overall force intensity from 55 to 65 meters, in line with expectations. It can be also noted that the operator’s cabin and the slewing unit are responsible for most of the resulting overall force, while the jib has a much lower effect on the force per unit length.