Retrofit of Structures to Meet Progressive Collapse Guidelines

A novel approach for the implementation of the progressive collapse guidelines of UFC 4-023-03 in an existing building is presented. The existing building is a 5 stories concrete flat plate structure with perimeter beams. It was determined that the beams and columns would be overstressed as a moment frame due to the light reinforcement of the beams and lack of continuous top reinforcement. The proposed solution is to hang the columns from a new steel transfer girder, located on top of the roof, which is designed to span over the lost column. Since concrete has limited tensile strength, new high strength Dywidag steel rods are used to carry the load from the floors up to the transfer girder. The progressive collapse event was modeled as a time history in SAP2000. It was found that a strain compatibility analysis was necessary in order to obtain an adequate estimate of the force distribution between the concrete columns, concrete column reinforcement and the new steel rods. For this particular building, it was found that most of the load was carried by the column and its reinforcement up to the roof level. As a result a very robust connection was required at the upper levels to transfer the load from the damaged column to the rods and up to the steel transfer girder. The analyses presented herein take into account a lower yielding stress in the rebar due to insufficient development length and considers various concrete tensile capacities. Finally, the design of the steel transfer girder has a considerable impact in the calculation of the peak load experienced by the system. If the girder is allowed to develop plastic deformation during the collapse event - but remains elastic in the steady state condition - the peak response forces at the column, steel rods and connections are considerably smaller from the ones obtained when the girder remains elastic.