We combine some important theoretical mathematical results with practical aspects from engineering in a nice framework ofspiders, which forms a space with global nonpositive curvature. Some concavity properties into such framework are derived. As an application, we consider a mathematical model for an engineering problem. More precisely, we are trying to model the behaviour of tension forces appearing in a metallic roof supported on seismic isolators. In fact, we are looking to find the properties of good seismic isolators in order to reduce the destructive power of an earthquake. The answer consists of a concave behaviour in terms of the displacements on the 3D axes of the Cartesian system.