Integrate > Workshops > Engineering, Sustainability, and the Geosciences > Activity Collection > Physical model of the failure of an unreinforced structure during an earthquake

Physical model of the failure of an unreinforced structure during an earthquake

Vince Cronin, Baylor University
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This is a partially developed activity description. It is included in the collection because it contains ideas useful for teaching even though it is incomplete.

Summary

In this lab activity, students learn about how a typical unreinforced adobe or cinder-block house is built. Then, they construct an analog physical model of an adobe structure on a simple shake table, using a dry cohesive powder (e.g., flour or drywall plaster) compacted into the four walls of a model house. A relatively heavy flat roof (analogous to a mud-covered vega-beam roof) is added to complete the structure. When the model house is complete, the students sketch (or take digital photographs of) the intact structure. The spring-supported shake table is then accelerated either horizontally or vertically, to simulate an earthquake. The students sketch (or take digital photographs of) the earthquake-induced cracks or faults in the walls of the model house, and speculate about how the observed failure would affect people living inside. Students complete the exercise by reviewing photographs of actual earthquake damage, learning about efforts to construct earthquake-resistant structures that use typical indigenous building materials, and writing a lab report about the experience.

Learning Goals

One of the important lessons students should derive from this project is the importance of using building techniques developed to mitigate the risk of failure during an earthquake. Observation of the failure of the model and consideration of the human consequences of that failure motivates the need to identify ways of preventing the sort of failure that is typical of unreinforced masonry or adobe structures. The skills involved include the ability to construct the model (following directions), document its state before failure, document its state after failure, and consider possible solutions to the problem.

One aspect of sustainability is the resilience of structures in the face of hazardous conditions in the environment, such as earthquakes, floods, and wind storms (tornadoes, hurricanes). Another aspect involves the use of renewable or abundant indigenous materials in construction.

Context for Use

I will use this activity in undergraduate geoscience laboratories for our introductory courses in physical geology or natural hazards. A more involved version of this could be used in an undergraduate engineering geology course, and a less involved demonstration might be appropriate for junior high or high-school Earth science classes. I would envision the project being done by groups of 3-6 students working as a group during a laboratory session. It would be best if a You-Tube video showing the model set-up was reviewed by all students who will be involved in the project.

Description and Teaching Materials

I envision this project beginning with students reading a written introduction that indicates that throughout the lesser-developed world, houses are commonly built with some form of sun-dried mud brick (adobe), unreinforced masonry or concrete brick (cinder block) walls and a flat roof made of a layer of mud over a woven mat supported by vega beams spanning the walls. The collapse of these sorts of unreinforced buildings contributes significantly to the loss of life and property during an earthquake. A map might be used to illustrate the areas worldwide that are at the greatest risk from earthquakes, and some statistics provided to indicate casualties and economic loss from several major earthquakes.

The purpose of this lab is to investigate the collapse of unreinforced buildings with heavy flat roofs, using a physical model on a shake table, and to think about ways to mitigate this problem through the implementation of different building practices.

Students read instructions (or view a short video) describing how to build the model building, using some wooden forms and a dry cohesive powder such as flour or drywall plaster. They construct the model building on the shake table, and carefully set the pre-made flat roof on top. Wooden forms are used to support the inside and outside of the model building, as the powder is introduced in the space between the forms and compacted. The forms are removed before the roof is added.

The shake table has a plywood base with rubber feet, four compression springs at the corners connecting the plywood base to an upper plywood sheet on which the model is constructed, and the ends of the springs are contained within a hole in a block of wood attached to the plywood sheets. While the plywood base of the shake table is resting on a lab table, the top is free to move up & down and side-to-side supported by the compression springs. Wooden blocks are fit between the plywood sheet while the model is being built, so that the upper plywood surface will not move.

When the model is fully constructed, the students make a sketch (or take a digital photo) of all 4 walls of the model house. In the first experiment, intended to investigate the effect of vertical acceleration, either [1] the upper plywood surface of the shake table is carefully pressed down, compressing the springs, and suddenly released so that the plywood and the model accelerate upward, or [2] a student uses a mallet or hammer to hit the underside of the upper plywood sheet beneath the model house. The walls will likely fail. The students sketch (or take digital photographs of) the earthquake-induced cracks or faults in the walls of the model house, and speculate about how the observed failure would affect people living inside. In the second experiment, either [1] the students pull the upper plywood surface of the shake table to one side and let go, or [2] a student hits the side of the upper plywood surface with a mallet or hammer, resulting in the failure of the model walls. As in the first experiment, the students sketch (or take digital photographs of) the earthquake-induced cracks or faults in the walls of the model house, and speculate about how the observed failure would affect people living inside.

The students then review photographs of buildings damaged or destroyed in actual earthquakes, and review documents describing how earthquake-resistant buildings can be constructed using common indigenous materials. Finally, students create a report describing their experiments, including the before-and-after sketches/photographs, and suggest ways that such disasters can be avoided.

The materials used in this lab are inexpensive and readily available. Some cleverness is needed to construct the wooden forms used to build the model. (Details to follow.)

Teaching Notes and Tips

This is a discovery-type activity, in which students construct and destroy a model that share some mechanical similarity with unreinforced buildings that are commonly found in lesser-developed countries. It is important to connect the lab activity with (1) the geography of earthquakes, (2) common building practices and materials, (3) the economic reasons for the common building practices, (4) the disastrous consequences of continuing to construct unreinforced buildings that are not resistant to earthquakes, and (5) the nature of simple solutions to these problems, using better construction methods and virtually the same inexpensive materials.

Assessment

To be completed later.

References and Resources

https://bearspace.baylor.edu/Vince_Cronin/www/EQ_Resistant_Buildings/index.html

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