The collapse of WTC twin towers: General aspects and considerations on the stability under exceptional loading of columns with partial-strength connections

The collapse of WTC started a wide scientific debate in the engineering community about tall buildings' design and safety measurements. It has been proven that a main role on the sudden failure of the twin towers has been played by connections. The structural system featured partial strength continuity connections at columns distributed along the height and width of the building with different configurations, in so-called staggered patterns. This was mainly due to economical reasons, since small bending moment values in such zones do not always require full resistance, and to space considerations regarding the overall dimensions of the joints. As a result, vertical members were designed mainly against vertical loads, while under exceptional loadings they may possibly undergo lateral deflection, as dramatically occurred on the September 11th. In this paper, a brief description of the partial strength connections of WTC perimeter columns is presented. Then, the effect of partial strength column splices and of their positioning along the height of the column on the ultimate behaviour of the member is evaluated through parametric nonlinear analyses. The column is represented by two elastic elements connected by a rigid-plastic spring. P-δ effects are taken into account, while residual stress and out of camber are implemented using the concept of equivalent geometrical imperfection.