# Investigation of the Pressure Coefficients Correlation Field for Low-Rise Building Roofs
## 3 Experimental Wind-Tunnel Results
### Mean Wind Direction 0\({}^{\circ}\)  
Observing the distribution of the correlation coefficient in the maps of Figure 5 (RLC), with the wind blowing from 0\({}^{\circ}\), it can be observed that the values of the pressure field at the taps close to the edges of the roof have a very small correlation with the surrounding areas. This is particularly the case for the pressure taps along the edge hit directly by the wind, and even more so in the areas near the two orthogonal edges (see Figure 5 Aa-Ae). This is due to the fact that the flow near the edges detaches from the roof and is consequently disturbed and affected by eddies which induce the fluid thread on the roof to proceed chaotically. For the same reason (chaotic detachment of vortices from the edge hit directly by the wind) an absence of symmetry was found between the level curves of the correlation function of the pressure taps Aa and Ae, which for this angle of wind (0\({}^{\circ}\)) were nominally symmetrical with respect to the longitudinal axis of the roof. As proof of the good quality of the experiment, meanwhile, the expected symmetry of the correlation functions was observed for most of the symmetrical pairs of pressure taps with respect to the longitudinal axis of the roof, e.g., pressure taps Ab and Ad, Ba and Be, Bb and Bd, Ea and Ee. They were found to be \"mirrored\" to each other, except for unavoidable small differences physically related to the statistical variations of the phenomena under consideration, as highlighted by two of the authors in a previous paper [41]. The situation at the central part of the roof (e.g., pressure taps Cb, Cc, Cd, Dc) was very different, because the flow confined by the upward curvature across the wind was for most of the roof attached along the downward curvature in the direction of the wind, with consequent high correlation (correlation index > 0.7) for portions of the roof up to about one third of its transverse and longitudinal dimensions. This circumstance can be of considerable importance in the characterization of average pressures or suctions to be considered in the design and verification of the main load-bearing elements of this type of structure, affected by the wind-induced loads on extended portions of the roof [42]. As shown recently by two of the current authors [42], the average pressure coefficients for this kind of structure generally decrease significantly when the influence area of the structural element increases, as a consequence of the decreasing correlation between local actions at points far away from each other, according to the indications reported in technical codes [39; 40] for ordinary geometries. For complex geometries such as those under examination, the variations of the correlation functions for the central and perimetral parts of the roof are larger, as already seen. This may lead to a strong time correlation of wind-induced loads for points comparatively far from each other, which may result in wind-induced loads significantly larger of those found for regions with analogous extension but where the correlation of wind flow is smaller. Comparing the data of Figure 5 (0\({}^{\circ}\) RLC) with those illustrated in Figure 6 (0\({}^{\circ}\) RLF), which refers to a roof of similar height (although not the same) but with a flatter curvature, differences can be observed. In this case, the effect described for the previous model (differences between correlation functions for the edges and the central part of the roof) was again observed. However, it was noted that for pressure taps near the edges on the flatter roof (Figure 6) the correlation with surrounding points had higher values than for the more curved roof (Figure 5), especially for the edge first hit by the flow. Furthermore, the higher correlations in the central part of the roof affected slightly larger regions than on the more curved roof.