Keywords: special truss moment frames, progressive collapse, nonlinear analysis, energy based design.

Special truss moment frames (STMF) consist of steel columns and open-web truss girders rigidly connected to form effective seismic load-resisting systems. The advantage of using the STMF systems is that the truss girders can be used over longer spans with less steel, and greater overall structural stiffness can be achieved by using deeper girders. Piping and ductwork can be placed through web openings, resulting in better utilization of storey space. Another advantage of the system is that the truss girders require relatively simple detailing for moment connections, and the structure can be repairable after being damaged by earthquakes, thus avoiding down-time. According to the American Institute of Steel Construction (AISC) seismic provisions, STMF are required to be designed to maintain elastic behaviour of the truss members, columns, and connections, except for the members of the special segment that are involved in the formation of the yield mechanism.

This study investigated the progressive collapse resisting capacity of the special truss moment frame structures. To this end analysis model structures with Vierendeel special segment were designed as per the AISC seismic provisions. The design parameters such as the length of the special segment, depth of panels, span length, and the number of stories were considered in the investigation. The progressive collapse potential of the structures was evaluated based on the arbitrary column loss scenario recommended in the GSA (General Service Administration) guidelines. A design procedure was proposed based on the energy-balance principle to prevent progressive collapse of the STMF structures, and the validity of the proposed procedure was evaluated using nonlinear static and dynamic analyses of four analysis model structures. A closed form formula was derived to obtain the required section moduli of the members in the special segment to prevent progressive collapse based on the energy balance concept. The remaining elements were resized based on the AISC seismic provisions to ensure plastic hinge formation only in the special segment.

The pushdown analysis results of the model structures show that the maximum load factor of the original structure designed as per the AISC seismic provision is less than 0.5, well below the required value of 1.0. The nonlinear time history analysis results show that the vertical displacement is unbounded when the column is suddenly removed. The model structures redesigned using the design procedure developed satisfied the acceptance criteria of the GSA guidelines to prevent progressive collapse. The nonlinear static pushover analysis of the redesigned structures showed that plastic hinges formed only in the special segment as required by the seismic provisions.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £50 +P&P)