There is a lack of significant research and seismic
performance studies on the resiliency and inherent strength redundancy of older
buildings.In specific, the capacity of existing buildings to resist ground motion associated with earthquakes has not been fully developed or thoroughly researched. Based on damage from earthquakes, especially the 2010 Canterbury and 2011 Christchurch earthquakes in New Zealand, with additional seismic activity lasting nearly one year, the general thought is that older existing buildings perform poorly in response to ground motion. When analyzed further, the damage from the Christchurch earthquake was predominantly due to acceleration in a vertical direction, literally tossing buildings in to the air. The peak vertical acceleration during the Christchurch earthquake exceeded the design criteria for today’s modern buildings. Not lessening the severity of the event, nor proposing for less stringent seismic codes, the Christchurch earthquake would flatten most modern cities regardless of building age. Adequate resistance to vertical movement cannot be achieved with current engineering techniques and therefore research and performance studies regarding the resiliency of existing structures must concentrate on horizontal ground motion.
These studies combined with documented field assessments and
field evidence of older structures surviving earthquakes and repeated ground
motion disturbances over several hundred years are available in numerous
communities and offer case study structures for further research. The numbers
of university engineering departments with “shake tables” (e.g. Portland State
University) create opportunities for joint partnership with private sector
consultants, public agencies, and professional organizations to assess and
analyze the unique aspects of archaic building materials and methodologies for
seismic response. The collaboration between university and private cooperation
for seismic research has the potential to develop a
wealth of practical and applicable information. The current collaborative
efforts involving energy consumption offer the model from which to base seismic
research.
A development of systematic research, publication, and
dissemination of the inherent strength of existing structures to resist
horizontal ground motion would benefit all communities across the globe.
I agree that more analysis needs to be done on the seismic capacity of existing but a few caveats should be noted. While vertical motion does sometimes happen in earthquakes, it is far from the norm. As you mention, the consequences can be severe. Another example was the collapse of the Bay Bridge during the Loma Prieta earthquake that failed from vertical shear.
ReplyDeleteHowever, while you mention historic buildings, which covers a variety of structural types and whose lateral performance will vay, your examples show masonry buildings. These have rightly been a concern, given the ample evidence from many earthquakes over the years that they are prone to collapse. Individual building can fare well, but as a class, they don't do well.
The current code is very unforgiving of existing masonry buildings and your call to understand how they actually perform is good. Loads path often do exist for these buildings, even if they are not recognized by code. Improving a performance based methodology would allow engineers to take advantage when these loads paths can be identified.
Dynamic analysis of buildings is already helping with reducing the impact lateral loading solutions on historic buildings. The use of hydraulic dampening systems used in the Meier & Frank renovation is a case in point. It eliminated the need for tying the terra cotta facing to the structure and for intrusive shear walls.