The thrust line caused by arch action can come dangerously close to the outer edge of a wall (a). This danger can be offset by buttressing (b), and/or by adding weight on top of the wall, for example with a pinnacle (c).
ever higher, the engineers needed to rethink their system of buttressing against the thrust. We will see shortly how they solved this problem. Another way to pull the thrust line towards the centre of the wall is to put additional weight on top. That may be done by adding pinna- cles or statues. So while such features might look purely decorative (indeed some have said frivolous) on Gothic churches, in fact they have a structural role: in a perhaps counter-intuitive way, the walls or pillars are stabilized by piling up more mass at their tops. Architectural historians have not always understood this. Paul Frankl, believing that extra weight must inevitably be destabilizing, enthusiastically repeated the sneer that Pol Abraham directed at Viollet-le-Duc’s mechanical interpretation of Gothic in the nineteenth century: ‘neither flying buttresses nor pinnacles were necessary. Many a French cathedral had none, and acquired them only when restored by Viollet-le-Duc.’ The function of a pinnacle isn’t just to recentre the thrust line, however (and in general they are not big enough to exert a strong influence of that sort). By pushing down on the stone joints, it also helps to prevent shearing of a pillar or wall at the point where the thrust of an arch threatens to displace it.