Life-Savers for Buildings
Saving people’s lives from the disastrous results of major earthquakes is the most important part of California’s building codes, as indeed it should be. But what about saving the lives of buildings?
We bring this subject up because an extreme seismic event is likely to damage buildings – even those constructed in compliance with the current codes – to such a degree that repairing them would be the equivalent of re-building them. Their destruction and re-building would involve a huge expenditure of energy and carbon emissions which, in effect, would cancel whatever energy-saving measures had been used in their construction and operation.
What is being done to ameliorate this crippling situation? Examples of structural components that could lessen the damage to the building frame and the consequent huge cost of repair are being developed. One such component, the Pin-Fuse Joint, was patented in 2004 by Mark Sarkisian, structural engineer and director in the firm SOM.
The Pin-Fuse Joint operates in much the same way as some joints in the human frame; for example, the movement of the shoulder joint, as shown in the drawing below.
As shown above, the horizontal steel beams end in a circular plate that connects to the steel of the associated columns within the moment-resisting frame. The columns connect the curved steel end plates. A steel pin or hollow steel pipe in the center of the moment-frame beam provides a well-defined rotation point. Under typical conditions including wind and moderate seismic events, the joint remains fixed if the exterior forces do not overcome the friction resistance provided between the curved end plates. In an extreme event, the plate is designed to rotate around the pin joint, with the slip-critical bolts sliding in long-slotted holes in the curved end plates. With this slip, rotation is allowed, energy dissipated, and “fusing” occurs.
The rotation of the Pin-Fuse Joint during extreme seismic events, depicted above, occurs sequentially in designated locations within the frame. As the slip occurs, the building frame is softened. The dynamic characteristics of the frame are altered so that smaller forces are attracted to the frame and deformations are reduced. After the seismic event, the elastic frame finds its pre-earthquake position. The brass shim located between the curved steel plates provides the predictable coefficient of friction (0.4) required to determine the onset of slip and enables the bolts to maintain their tension with Belleview washers from the original tightening. The joints re-establish their fixity after the earthquake.
Given the threat of catastrophic earthquakes in the Bay Area and other heavily populated centers of our state one would think that this and other such eminently useful structural components would be recognized by building codes. But this has not happened.
Surprisingly, the Leadership in Energy and Environmental Design organization, the LEED, which awards building designers by giving points for energy conservation and environmental responsibility, does not recognize environmental impacts related to the construction process. Points are given for using recycled products such as rebar, but there is no overall recognition of the environmental impact of buildings at the time of their construction and throughout their existence.
Furthermore, even though need for reducing the carbon footprint of buildings is something we hear about on a regular basis, the LEED does not address how the issue figures in the overall creation of the structure. What kind of leadership is this?