Environment-friendly demountable precast concrete frame building system for minimization of seismic downtime

Reinforced concrete (RC) frame buildings and precast concrete buildings with traditional connections between precast members turn into a monolithic form. As a result, such buildings can be terminated only by demolition, which renders their structural components unusable. This is against the philosophy of sustainable building design. After an earthquake, conventional concrete buildings, which are damaged but are structurally repairable, incur significant seismic losses mainly because of the downtime in addition to the repair cost required to restore their functionality. For these reasons, researchers are exploring alternative concrete frame building systems that are sustainable, adaptable, and repairable in a quick time without causing a lengthy downtime. In this paper, schematic development of a seismically resilient demountable precast concrete frame building system, in which the structural components are connected using strong “dry” steel connections, is presented. In the proposed building system, any damaged structural elements can be replaced with new ones; thereby rendering it an unambiguously and quickly repairable low-loss and low-downtime system, despite not being a completely damage avoidance system

Are stronger, stiffer buildings indeed costlier? Case study of RC frame buildings

Reinforced concrete (RC) buildings designed as per current seismic codes/standards are expected to satisfy “life safety” in a design level earthquake, but modern buildings have suffered significant damage in recent earthquakes, which were either irreparable or required lengthy and costly repair. Seismic codes/standards allow buildings to be designed for different lateral strength as long as the associated ductility and drift demands are catered for. The design seismic force is primarily controlled by a force reduction factor; a higher reduction factor leads to weaker/softer building but requires stringent confinement detailing to achieve a higher ductility. Buildings designed for higher reduction factor are more likely to suffer structural damage in moderate earthquakes. It is intuitively believed that stronger buildings, which respond elastically in a design level earthquake, are prohibitively costlier. However, cost of building skeleton, which governs its strength, is only a minor contributor to the total project cost. This paper conducts cost analysis on RC frame buildings designed with different reduction factors to investigate their effect on the construction cost. Based on the cost breakdown, it is found that the structural material cost of low-medium rise RC frame buildings is normally 25-30% of the total project cost, which varies within ±10% for the full range of force reduction factors allowed by modern building standards. The additional initial cost required to design RC frame buildings (with ductile detailing) that respond elastically in design-level shakings is insignificant when compared against the cumulative reduction in damage repair costs and financial losses due to business interruption over their lifetime. Hence, to achieve low-damage design through traditional measures, engineers should be encouraged to design stronger and stiffer buildings so that they remain in “operational” state after a DBE

Feasibility of pinned-base connections for demountable precast frame building systems

In recent Canterbury earthquakes, structures have performed well in terms of life safety but the estimated total cost of the rebuild was as high as $40 billion. The major contributors to this cost are repair/demolition/rebuild cost, the resulting downtime and business interruption. For this reason, the authors are exploring alternate building systems that can minimize the downtime and business interruption due to building damage in an earthquake; thereby greatly reducing the financial implications of seismic events. In this paper, a sustainable and demountable precast reinforced concrete (RC) frame system in which the precast members are connected via steel tubes/plates or steel angles/plates and high strength friction grip (HSFG) bolts is introduced. In the proposed system, damaged structural elements in seismic frames can be easily replaced with new ones; thereby making it an easily and quickly repairable and a low-loss system. The column to foundation connection in the proposed system can be designed either as fixed or pinned depending on the requirement of strength and stiffness. In a fixed base frame system, ground storey columns will also be damaged along with beams in seismic events, which are to be replaced after seismic events; whereas in a pin base frame only beams (which are easy to replace) will be damaged. Low to medium rise (3-6 storey) precast RC frame buildings with fixed and pin bases are analyzed in this paper; and their lateral capacity, lateral stiffness and natural period are scrutinized to better understand the pros and cons of the demountable precast frame system with fixed and pin base connections