Engineering with a mission: 100 years

The long view

The long view
Structures solutions: From left, Tonya Nilsson, Jake Echeverria '11, and Berkeley-based engineer Henri Mannik. Photo by Charles Barry
by Justin Gerdes |
Build it safer and stronger—sustainably.

Mark Aschheim and Tonya Nilsson believe in complementary goals when it comes to construction: Make a building structurally sound so that it can withstand an earthquake that may never come. And build it with materials that won’t deplete natural resources or harm the environment.

Aschheim chairs SCU’s Department of Civil Engineering, where Nilsson is a lecturer. With assistance from undergraduate and graduate students, they’re testing the use of innovative building materials and systems that tread lightly on the earth, while ensuring that buildings and their inhabitants can survive tremors.

 

Mighty bamboo

Aschheim pioneered the use of bamboo in structural I-beams in SCU’s 2007 Solar Decathlon house. He and Nilsson are quick to extol the many sustainability and structural advantages of bamboo compared to conventional alternatives. “It grows rapidly and it sequesters carbon very rapidly. This carbon stays sequestered, assuming we don’t burn the building down. And the bamboo does not decay at the end of the useful life of the building.”

Nilsson notes that bamboo can first be harvested after only four to six years; after that, you can re-harvest every three to four.

“It’s a grass: You chop it down and it grows like your lawn. The root system stays intact,” Aschheim adds. “It sprouts new shoots and continues growing; whereas with trees it’s 30 to 50-plus years before you can harvest, and when you cut it down, it’s done.”

Bamboo is also strong for its weight—three to four times stronger than softwoods typically used in construction. One advantage is shape: Bamboo is hollow, “inherently optimal from a structural point of view,” Aschheim says. “Anything you can do with softwood you can do with bamboo. That includes load-bearing walls, shear walls, floor joists, and plywood on top of floors.”

But there’s a catch: In order for bamboo to become a mainstream building material, building codes must be updated and code enforcement officials persuaded to permit the use of the emerging technology. Aschheim and his collaborators have developed “acceptance criteria” for bamboo I-joists so that building code officials recognize its use; now the engineers are working on criteria for other bamboo components. This process will require time and patience—starting small, getting projects approved at the local level on a case-by-case basis. “At some point, demand will come, and if building officials see it more and more, then it does push code development,” says Nilsson.

Aschheim and Nilsson hope that work under way in the School of Engineering’s new Structures Lab can hasten the deployment of bamboo and straw bale, another less common but environmentally sensible building material.

 

The greatest promise—and need

The earthquake that struck Haiti in January 2010 took a heavy toll in human life for several reasons; one factor was poorly constructed buildings in densely populated Port-au-Prince. A safer but cost-effective approach to construction could save lives in the future. Aschheim, Nilsson, and their students have tested pieces of the building systems they would like to deploy in Haiti.

“The idea is to tread very lightly on the environment, using local materials, preferably recycled materials, as much as possible,” says Aschheim.

The new building system was developed in conjunction with the Ecological Building Network. It begins with a 3-foot-high wall of concrete block made using a low-cement mix and recycled concrete rubble. A bamboo framing system is anchored atop the wall. To attach the bamboo, a length of rebar is set into the concrete block wall, with one part sticking up out of the wall. The bamboo frame is set over the rebar, taking advantage of the bamboo’s hollow shape, and held in place with mortar. This connects the elements. Care must be taken, however, that the bamboo column does not touch the concrete block, because moisture could infiltrate the column. To create the necessary distance, the column is raised an inch or so off the block, which exposes a small section of rebar. The bar is flexible and ductile; during an earthquake, that protects the frame from damage. However, the rebar, if left exposed to the elements, will eventually rust. An easy solution to this problem is to protect the bar with a bottle top from a plastic Coke bottle. The bottom of the plastic bottle top is set into mortar along the top of the concrete block wall, and the top of the bottle top is inserted into the bamboo column, to keep hold in place the mortar that bonds the rebar to the bamboo column.

 

Damage assessment

Epicenter: The 2010 Haiti earthquake. U.S. Geological Society

Aschheim witnessed firsthand the life-or-death importance of sound earthquake engineering on a tour of Japan after the magnitude 9.0 Tohoku earthquake, in March 2011. Aschheim joined a National Science Foundation-funded reconnaissance team for five days in the field, inspecting the shaking damage resulting from the quake and aftershocks.

The experience prompted Aschheim to ask if engineers need to re-evaluate the conventional wisdom that prioritizes the avoidance of structural damage in buildings. “Earthquakes occur so rarely that it’s very expensive to design buildings not to be damaged at all, because the forces are quite high. The paradigm has been to design buildings that are ductile, because ductile systems can be designed at lower cost to withstand earthquake demands. Even while this approach provides for life safety, the problem is that this ensures structural damage will occur even in moderate earthquakes,” he says. “At some point you have to ask: ‘Is this serving society very well?’” After all, it does a community little good if a hospital survives a quake but is so badly damaged that it cannot treat injuries in the aftermath.

“We know how to approach this problem,” Aschheim says, “and considering different structural systems and possibly making buildings that are stronger can reduce the damage—and the larger social and economic consequences.” Building retrofits that increased stiffness, such as installing concrete shear walls or steel braced frames, were effective. Other technologies, such as buckling-restrained braced frames and base isolation, which couples rubber pads with steel plates, also have a place.

But that, too, would need to be reflected in building codes.

Summer 2014

Table of contents

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