Bamboo Reinforced Cob: Seismic Stability
A quick seismic history: "Understanding the seismic performance of structures in terms of engineering science is of recent vintage. Only in the twentieth century did information begin to emerge on how structures respond in earthquakes. Historical building practices developed with the accumulation of experience gained through trial and error. The first measurements of ground motions in damaging earthquakes were not taken until 1933, and it was not until the 1970s that the first recordings were made of a building as it responded to an earthquake that caused damage to that structure.”(GSAP 2000)
This is what the cob builder trying to get permitted is up against: “Steel and reinforced concrete are ductile materials that have linear elastic properties and good post-elastic strength characteristics. After yielding, these materials maintain most of their strength while undergoing substantial plastic deformations. They can be analyzed with reasonable accuracy using analytical or computational
methods. In contrast, the behavior of brittle, unreinforced materials—such as stone, brick, or adobe—is extremely difficult to predict after cracks are initiated, even with today’s advanced computational capabilities. Even if results could be generated with these technologies, they would not be accurate.”(GSAP 2000)
methods. In contrast, the behavior of brittle, unreinforced materials—such as stone, brick, or adobe—is extremely difficult to predict after cracks are initiated, even with today’s advanced computational capabilities. Even if results could be generated with these technologies, they would not be accurate.”(GSAP 2000)
GSAP's strap method is a retrofit to an adobe structure that uses steel to reduce out-of-plane collapse. "The retrofit systems tested in GSAP involved horizontal and vertical straps, ties, vertical center-core rods, and improvements in the anchoring of the roof to the walls. Each method proved to be successful in reducing the tendency of the model buildings to collapse.”(GSAP 2000)
To me, this shows that reinforcement is key to seismic resistance. However, I am more interested in cob structures and building with local resources. I was able to get in contact with Peter Hickson, a carpenter and building contractor with 37 year of experience. Originally I thought it was odd that he included demolition as part of his experience, but then I remembered our topic and immediately realized the significance.
In 2006 he designed and built a low cost bamboo reinforced cob house in the Philippines. The goal was to build a functional house that was both earthquake safe and affordable to the locals.
They chose a monolithic reinforced concrete slab as their foundation, then built the first floor and attic roof using coco lumber.
wall about 1 foot thick
- bamboo ladder mesh of split bamboo wired together for every 15” rise in the structure.
- Monolithic earth walls are stronger than earth brick walls in earthquakes
- Composite of bamboo and cob works like reinforced concrete (RC) with earth in compression and bamboo in tension and tying across weak points around openings.
- Vertical bamboo connected to a structural diaphragm inhibits movement therefore displacement of center of walls subjected to horizontal out of plane loads.
- Weight of the cob walling resists uplift on the building in strong winds.
- RC strength but affordable and more sustainable.
In 2012 Peter teamed up with a few University of Technology, Sydney (UTS) students as his cob structure was challenged by an engineer. “Two final year engineering students Luke Punzet and Jean-Michel Albert-Thernet worked with me as part of their final year Capstone project. We built “U” shaped test panels reinforced and unreinforced cob as a comparative study with work previously undertaken by Dominic Dowling as part of his PhD research into what he called quake safe adobe. The cob and reinforced cob walls proved much stronger than any of the mudbrick panels tested by Dom. The “U” shaped panels were so strong they were not challenged by the testing.” (Hickson 2015)
Next, they build a scaled model of the house he built in the Philippines. The model was complete with window openings, an upper floor and roof loads to represent a more accurate test of a bamboo reinforced cob building as an entire system.
After the build was completed to their satisfaction, they put it to the shake test and chose the 2001 El Salvador earthquake, measuring 7.8, as their guide. The team subjected the model house to a 100% shake, 125% (which makes that a 8.6 magnitude quake) and then two more 100% to ensure no further damage and to simulate a series of aftershocks. The structure was slightly damaged as some cracking was noticed near the top of a wall, but overall it was deemed safe enough to enter and that it might not even need repair. A Prof Samili (Engineer?) claimed that this reinforced cob system can be safely used anywhere that seismic activity is a concern.
Resources:
Peter Hickson’s reinforced cob:
No comments:
Post a Comment