Applying Post-Tensioned Connections to Prevent Progressive Collapse of Buildings

In progressive collapse, a local failure in a building leads to the failure of the adjoining members or even the entire structural collapse. When a single column (or a wall) is removed, beams and slabs act together to develop catenary action. This prevents collapse progression by redistributing gravity loads through reliable paths. If a structure has good alternative loading paths, the initial failure will not expand to other parts of the structure and thus the damage will be restricted. In this research, the influence of applying post-tensioned connections, which can increase catenary action, to prevent progressive collapse is investigated. A post-tensioned (PT) steel moment resisting frame is a self-centering earthquake resistant steel frame having post-tensioned strands, compressing the beam flanges against the column flanges at the connections. The post-tensioned strands contribute to the moment capacity of the connections and provide an elastic restoring force to return the frame to its pre-earthquake position. The most important advantage of the PT connection is that the beams and columns remain elastic, while inelastic deformations (and damage) in the connections provide considerable energy dissipation. The damage in the connection occurs mostly in the angles which can be easily replaced after the earthquake. In this regard, two types of six-story, four-bay steel frames are designed; one having post tensioned and the other having regular rigid connections. The frames are modeled in OpenSees program and their progressive collpse possibilities are studied. To study the influence of the design parameter (αa) on the progressive collapse, five different details are considered for the PT connections. The resultant frames have different αas (αa =0.95, 1.1, 1.25, 1.4 and 1.55), where αa is the ratio of connection moment, corresponding to the angle yielding, to the design moment of the beam at the column face.
To study the possibility of progressive collapse in the building, the gravity loads are firstly applied to the structure, then the column of the first floor is suddenly removed as the scenaio of the progressive collapse, based on the code, GSA. Time histories for deflections of the critical point (top of the removed column) of these frames are compared with the results of the frame having rigid connections. It has been shown that increasing αa in the frames with PT connections raises the resistance against progressive collapse and the one with αa >1.55 is even better than rigid connection frame in preventing progressive collapse. A sensitivity analysis is also carried out to study influences of the PT connection details on the progressive collapse. Different PT connections with different details can have the same αa; incresing the size of the conenction angle sections or raising the strands force can both increase αa. A six-story, four-bay steel frame is designed, having post-tensioned connections, the frame is considered as Frame A. Two other frames are also designed with greater αa; Frame B and Frame C have the same αa in their PT connections: Frame B has the same connection angle sections of Frame A, but higher T0. In Frame C, the angles’ size is just increased. For each of these cases (Frame A, Frame B and Frame C) the gravity loads are firstly applied to the structure, and then progressive collapse analysis is performed dynamically by sudden removal of the middle column in the first floor. Deflections of the critical points (top of the removed column) for these frames are compared. It has been shown that if the middle column at the first floor of Frame B is suddenly removed, the node at the top of the removed column vibrates vertically with the maximum and residual displacement of 10.37 cm and 4.85 cm, respectively. The maximum vertical displacement and residual displacements in the Frame C under the same scenario are 11.21 cm and 8.56 cm, respectively. Comparing the results of these two frames shows that increasing the strands posttensioning force is more effective against the progressive collapse. In summary, the obtained results of this paper show that applying PT connections with αa >1.55 is more efficient against progressive collapse than moment resisting frames with regular rigid connections; the greater αa supplies higher resistance against such collapse. This can be applied both by raising the connection strands’ force or increasing the connection angle section size; however, the former is more efficient in preventing progressive collapse.