Project # 6:Buildings that Resist Earthquakes Better

Faculty Sponsor:

Dr. Anant R. Kukreti

Associate Dean for Engineering Education Research

Professor of Civil and Environmental Engineering

Department of Civil and Environmental Engineering

University of Cincinnati

Office:  701F Engineering Research Center

E-Mail:  Anant.kukreti@uc.edu

Phone:  (513)-556-4105

 

And

 

Dr. Kelly Cohen

Associate Professor

Department of Aerospace Engineering & Engineering Mechanics

University of Cincinnati

Cincinnati , Ohio

Office: 732 Rhodes Hall

E-mail: Kelly.Cohen@uc.edu

Phone: (513) 556-3523

Project Summary

On 1/26/01 an earthquake of magnitude 7.9 on the Richter scale, centered in India left tens of thousands of people killed or injured whereas on 1/28/01 the Nisqually Earthquake near Olympia, Washington had a comparable magnitude of 6.8 but resulted in only one death, and a few hundred minor injuries. The difference in outcomes from these earthquakes is attributed to the use of aseismic design techniques, the main strategy in which is to employ various energy-absorbing and base isolation devices. In this project the teachers will explore such technology, and learn how it can be geared toward educating and exciting physics and math students.

To understand the behavior of structural systems with aseismic devices, one needs to perform physical testing of complete structures subjected to earthquake motions.  Because of the size limitations and expense of seismic simulators (shake tables) available and the need to test complete building structures, small-scale models are often the only choice for testing.  It is generally agreed that testing of larger scale models (1/3 size) gives better information about localized behavior such as member cracking, however much of the same global behavior can be obtained at a much lower expense by testing much smaller models (1/12 to 1/24 size).  The expense reduction due to the use of small-scale models is due to lower specimen production and instrumentation costs and drastically lower simulator (shake table) costs.

The objective of this project is to test, evaluate, and compare results from small-scale (1/24) models of steel frame building structures fitted with various types of base isolators and damping devices and subjected to base motions.  Base isolation systems are a means of allowing the ground to move horizontally underneath a building due to an earthquake while the building remains relatively steady.  To accomplish this, base isolation systems must have very low horizontal stiffness between the building being isolated and the supporting ground.  Also, the isolation system must have a relatively large vertical stiffness to support the weight of the building and prevent the building from overturning.  These systems are gaining popularity as more economical and effective isolators have been developed.  It is desirable to create accurate small-scale models of these new systems so that the overall performance of scaled structures supported by them can be studied on seismic simulators. 

There are other acceptable seismic isolation devices that can be used, besides rubber bearings.  The fluid viscous damper consists of a stainless steel piston with a bronze orifice head and is filled with silicone oil.  The piston head utilizes specially shaped passages which change the flow of the damper fluid.  This change in flow alters the resistance of the damper.  This damper behaves as a shock absorber.  The movement within the damper fluid absorbs kinetic energy and makes the building undergo less horizontal movement, and therefore less damage during the ground motion.  Fluid viscous and friction dampers are “passive control” devices which utilize the response of the structure to develop a control force and hence require no external power supply for operation.  Other devices include tuned mass dampers and tuned liquid dampers.

Since the RET participants may not have a background in Structural Dynamics, an understanding of basic vibration principles and use of electronic sensors (displacement and velocity transducers and accelerometers) and computerized data acquisition systems (DAS) to measure dynamic response parameters of interest will be taught in a special lab developed for testing small-scale structural models.  The project will begin with learning of basic vibration principles through experiments on 1/24-scale one-story small-scale building models to explore their use to: experimentally determine their frequencies, mode shapes and damping characteristics.  The building model structures used will consist of four spring steel columns for each floor, which have fixed connections at the base, and a large steel block mass for each floor.  The first set of experiments will deal with free vibration motion characteristics of one-story models mounted on the laboratory table, deformed, and released to freely vibrate, as shown in Figure 1.  The displacement, velocity, and acceleration response will be recorded.  For all these experiments, prefabricated parts of the model structures and other needed devices will be utilized.  The teachers will be required to assemble the components, install the necessary instrumentation (displacement, velocity, and acceleration transducers), and acquire and process the data using the computer software LabVIEW.  These models will not be tested to failure and will be reusable.

           

                              (a)                                                                          (b)

Figure 1.  (a) Small Building Frame Model Details and (b) Free Vibration Test Set-Up

In the next set of experiments the one-story model will be mounted with and without base isolators on a small unidirectional shake table that has a 6.25” stroke, 30 lb force, 50 lb maximum payload capacity, and a 9” x 12” horizontal table matched with the shaker, as shown in Figure 2.  For these tests, using the knowledge gained from the first series of free vibration tests, teachers will design the models with a known fundamental natural frequency.  Each test will be repeated for three different base (i.e., shake table) motion frequencies--lower, about equal and greater than the model fundamental frequency. 

Cylindrical rubber mounts are commonly used as supports in mechanical machinery, and their use as base isolators for aseismic design will be first investigated.  The teachers will conduct two series of tests.  In the first series, the response of the one-story bare frame model will be measured.  In the second series the cylindrical rubber mounts will be installed under each column of the model frame so that the model is de-coupled from the shake table over which it is mounted, as shown in Figure 3.  Rubber mounts of various diameters and heights will be tried.  The test will be repeated for each type of cylindrical rubber mount and the response for each will be recorded and compared to that obtained for the bare model.  Using these test results, teachers will investigate and quantify the effect of the different sizes of base isolators in decreasing the amplitude of the displacements and accelerations.

     

Figure 3.  Test Frame Model with Base Isolators Mounted on the Shake Table

Finally, in this series the teachers will repeat the frame model tests fitted with a viscous damper, as shown in Figure 4.  They will repeat tests with viscous dampers filled with three liquids of different viscosities, viz., water, canola oil, and glycerine.

    

Figure 4.  Test Model Frame with Viscous Damper

After having tested the passive viscous damper device, the teachers will then be introduced to active methods of seismic mitigation, and they will test the performance of an active mass damper on a test frame, shown in Figure 5 on the right.  For the test frame the teachers will compare the time it will take for a given excitation to be dampened out, with and without the active mass damper.

Possible Ideas for Classroom Implementation 

To convey their research experiences, the teachers may consider the following classroom implementation plan:

·         ENGAGE:  Students will spend approximately 1-2 days on an engagement activity that will introduce them to earthquakes and their affects on buildings.  A video called “Earth Science: Earthquakes” will be watched and a guided reading called “How Earthquakes Affect Buildings” will be completed. 

·         EXPLORE:  Students will spend 3-5 days on exploratory activities that revolve around seismic waves and simple harmonic motion.  In several of these activities, students are asked to make connections between the motions of the lab set-up (pendulums, spring-mass, etc.) and the motion of a building during an earthquake. 

·         EXPLAIN/ELABORATE:  Students will spend 2 days learning about the connections between simple harmonics and buildings through a lecture using PowerPoint.  Students will also be introduced to the engineering design process.  These PowerPoint’s are in the process of being created.  They will be accessible on the RET site.  During this project, students will spend one day on the Golf Ball Drop challenge, an introductory activity for engineering design.

·         EVALUATE:  Students will spend approximately 5-7 days on the Engineering Design project.  Students will work on the design, testing, analysis and presentation of a earthquake retrofit system for a single story K’Nex structure, using the test platform or shake table built from Pascar and Lego Mindstorms NXT components (see Figure 6).

     

                              (a)                                                                          (b)

Figure 6.  (a) Classroom Demonstration Frame Mounted on the Test Platform (b) Close-up of the Prime Moving Mechanism of the Model Shake Table

 


2009 RET Flyer