Paper ID #26587

Board 17: Design and Small-Scale Testing of 3D Printed Seismic Isolators

Dr. Jenna Wong P.E., San Francisco State University

Dr. Wong is a structural engineer broadly focused on seismic design of critical facilities. Her doctor-ate research at UC Berkeley investigated the applicability of seismic isolation and supplemental viscousdamping to nuclear power plants with focus on seismic resilience and safety. The work identified isolationparameters for the optimization of design to produce high performance levels of both structural responseand secondary systems. After receiving her PhD, Dr. Wong began a post-doctoral fellowship at LawrenceNational Laboratory focusing on developing a modern computational framework for the nonlinear seismicanalysis of Department of Energy nuclear facilities and systems. This work seeks to expand the under-standing of soil structure interaction for these structures and the means of modeling this behavior boththeoretically and experimentally. In addition to her research experience, Dr. Wong also has worked forthe public and private engineering sectors in the areas of water infrastructure, transportation, data systems,and project management. She joined San Francisco State University in 2014 as lecturer and is currentlyan assistant professor of Civil Engineering in the School of Engineering. Her research interests focus onthe application of seismic technology for critical facilities and engineering education. She is a member ofASCE, EERI, SEAONC, CAIES, and SWE.

Ms. Lakshmipriya Lakshmipathy, Indian Institute of Technology, MadrasMr. Panfilo Jesus ArmasMr. Andres Ernesto ParedesChris ParkJorge Antonio Campos

c©American Society for Engineering Education, 2019

Design and Small-Scale Testing of 3D Printed Seismic Isolators Abstract 3D printing is a versatile technology with applications spanning from toy production to biomedical devices. With the ability to bring small-scaled prototypes into the classroom, 3D printing offers educators an excellent opportunity to enhance the learning experience. However, in looking at its applications in engineering, civil/structural engineering still falls behind in taking full advantage of this technology which is not only accessible but also relatively inexpensive. This paper will focus on the design and testing process of 3D printed seismic isolators and the observations and lessons learned as part of a senior design project. Civil engineering students at San Francisco State University explore the use and effectiveness of seismic isolation by designing, printing, and testing an isolated system using a small scale shake table testing. The structural system is a balsa wood tower using 3D printed friction pendulum seismic isolators at the base. Through design work and dynamic testing, the students see first-hand how isolation works and identify means of sizing the isolators for optimum structural performance. Utilizing these prototype isolators not only reinforces seismic isolation theory but also allows for scaled testing otherwise inaccessible to most students due to the cost of real seismic isolators. This project establishes a foundation of work needed to develop 3D printing applications for civil/structural engineering topics for both classroom and outreach purposes. 1. Introduction Marvel keychain. Crochet hook. Incubator parts. These are just a few designs recently accessed on Thingiverse, a database for 3D printing designs. 3D printing or rapid prototyping has surged in popularity over the past decade with increased accessibility to printers at malls, libraries and even homes. This technology allows users to create computer aided designs (CAD) and then print a three-dimensional reproduction within the matter of minutes to hours depending on the design complexity at a minimal cost. This process engages users in a way that teaches key skills such as critical thinking, problem solving, and logic analysis in a creative setting very much like many STEM fields. However, there has been slow adoption of this technology in some of the STEM fields such as civil/structural engineering. Engineering is a very diverse field with various sub-disciplines that utilize math and science to create everything from robots, computers, and even buildings. Unlike mechanical and electrical engineering, civil/structural engineering severely lacks the ability to engage students with hands-on activities or first-hand experiences with structures in the classroom. Structural engineering, a sub-discipline of civil engineering, encompasses the design and analysis of structural elements to support loadings including the occupants, structural contents as well as the seismic and wind demands. These structures can range from office buildings to high-rises and even bridges. A common thread amongst all of these structures is that the majority of structural components must be observed on construction sites which is rarely accessible to students. Additionally, due to the impracticality of holding onto a structural element, students must blindly accept course lessons on how structural elements behave under loadings.

To fill this gap in engineering education, the lead author initiated an exploratory project to design and 3D print structural elements for research and educational purposes. As an initial study and as part of a senior design project, a team of eight undergraduate (UG) students along with one graduate student researcher (GSR) from San Francisco State University (SFSU) worked on designing a scaled tall building or tower using 3D printed isolators as their seismic protection system. The learning objectives for this study were for the students to: 1) identify components of seismic isolators, 2) understand the theory, 3) design and adapt isolator designs, and 4) observe performance of their isolator design through a scaled test. This study required the students to utilize engineering knowledge and practices to design and construct the tower using easily accessible materials and tools. The tower was constructed per the 2018 Earthquake Engineering Research Institute (EERI) Seismic Design Competition rules and guidelines defining the design details and the two ground motions applied. As the students did not plan to enter into the EERI SDC, they designed beyond the guideline restrictions to test the use of seismic isolators. The tower was experimentally tested using the Shake Table II by Quanser available at SFSU with and without isolators. This project built upon the students’ background in structural analysis, design, and mechanical vibrations. However, their interests in structural protection expanded beyond these courses leading to their pursuits of applying seismic isolation, a graduate level subject matter. This project exposed the students to seismic isolation theory and dynamics while exploring the practicality of using 3D printed components. The following sections will detail the design and testing process of the isolators along with the student assessment and lessons learned from this initial study. 2. Background Although 3D printing is not fully adopted at the academic level, industries at various levels have not only adopted it but are beginning to utilize it at unprecedent levels. The architecture field has found the use of 3D printing to be a new and effective means of bringing their designs to life. For example, Henning Larsen Architects (HLA) based out of Denmark took advantage of 3D printing to make scaled prints of their creations for designers and clients as shown in Figure 1 [1]. The firm found the technology extremely useful and effective due to improved design communication with a visual in-hand while increasing productivity by reducing the amount of time needed to build scaled structures out of balsa wood or other materials. Other firms such as Foster + Partners, Skidmore, Owings, & Merill, Zaha Hadid Architects, Rem Kolhass, and Rietveid Associates also promote 3D printing for architectural purposes [1]. There are numerous articles available addressing and encouraging the use of 3D printing in the architecture field [2], [3], [4]. Due to the flexibility of the technology, designers can quickly make changes to visual designs and re-use the materials which was quite difficult using the more traditional practices. Applications for architectural purposes range from traditional structures to industrial plants such as the one shown in Figure 2.

Figure 1. 3D Printed Concert and Conference Centre Designed by HLA [1].

Figure 2. 3D Printed Model of a Processing Plant for Linde AG [2].

Architects use 3D printing to create scaled models, however, the construction industry is taking this technology to the next level by printing full-scaled structures. Today’s market is seeing everything from 3D printed homes to bridges. ICON, a Texas based company, debuted last year their line of sustainable homes all created through rapid prototyping. The 1,000 square foot home shown in Figure 3 can be built for only $4,000 within 24 hours [5]. Homes such as these are part of a larger effort to provide low-income housing in developing countries. But this sets the precedent for home construction in urban areas such as the San Francisco Bay Area which is experiencing a housing crisis and is in desperate need of small, cheap, and quickly available housing. Printing of homes [6], [7] and even army barracks [8] are being tested worldwide. Beyond homes, even more complicated structures such as bridges are also entering the market of rapid prototyping.

Figure 3. Example of ICON’s 3D Printed Homes [5].

Just this month in Shanghai, a team of architecture students at Tsinghua University designed and produced the longest 3D printed concrete bridge as shown in Figure 4 [9]. This pedestrian bridge which is modeled after traditional Chinese structural design was created in 450 hours which is a fraction of the time a traditional construction would take. Pedestrian sized bridges have become a focus for many initial studies. There are examples in Spain [10] and the US [11] which are taking advantage of traditional construction materials but constructing in a new way.

Figure 4. World’s Longest 3D Printed Concrete Bridge in Shanghai [9].

As shown above, industry is embracing this technology leading to new creative uses and a need for a future workforce with a strong background and skill to take these designs to the next level. These projects alone can be quite awe-inspiring to any young adult. However, if we take a step back, we can evaluate what is needed to achieve these amazing projects. They all require visualization, problem solving, critical thinking, and usage of technology. For this reason, it is essential as educators that we recognize not only the ability to utilize this new technology to motivate and inspire our students but acknowledge the responsibility to prepare them for the demands of their future careers. Considering these industry examples, it is important to understand the basis of basic structural elements and the ability for students to relate to them. In this project, the 3D printed elements selected were friction pendulum isolators. Seismic isolators or bearings are a popular means of protecting a structure against earthquake forces and have been used for over a century with the greatest progress made in the past 50 years. It relies on the introduction of an isolator, i.e., a highly flexible layer between the structure and the ground, increasing its fundamental period producing rigid body motion. This design is effective for improving structural integrity. The numerous isolators available are categorized as elastomeric or friction isolators. Elastomeric isolators are made of a rubber-like material that uses elasticity and plasticity to dissipate energy, while friction isolators use the frictional coefficient of the surface for energy dissipation. Both types of isolators have evolved over the past 50 years with advancements in material technology and manufacturing processes. Seismic isolators increase the structure’s fundamental period and decreases accelerations (lowering forces). The highly flexible isolation layer accommodates the increase in displacement. As a result, seismic isolation integrates performance objectives into its design by

restricting the response to ensure structural integrity [12], [13], [14]. Isolators as shown in Figure 5 below are rarely visible to the public eye just like the majority of structural members. But unlike typical beams and columns, isolators are found in far fewer numbers especially here in the United States. Not only are these elements too large for physical demonstration in the classroom but are extremely difficult to access. As a result, courses which are typically restricted to graduate level students are very theoretically based with very little hands-on activities for students.

(a) (b)

Figure 5. Examples of Seismic Isolators: (a) Lead Plug Rubber Bearing [15] (b) Friction Pendulum Bearing [16].

In the classroom, there are efforts underway to increase the number of hands-on activities for students in engineering. Much of our work is misconstrued with being nothing more than drawings and calculations. However, the best engineers know not only design theory, they understand the product as a whole, from design inception to production and use. This is especially pertinent to seismic isolation which requires an understanding of structural dynamics to optimize a system. 3D printing is a matter of discussion in the American Society of Engineering Education with a number of papers being produced in the past several years chronicling the applications for industrial, mechanical, and electrical engineering uses [17], [18], [19]. Each of these applications have highlighted the ease of use and benefits of the technology in the classroom. In terms of seismic isolators, researchers out of University of Oklahoma are also working on 3D designs [20]. In their work, they have created 3D printed rolling pendulum bearings for uni-directional excitation. To take this work a step forward, this project conducted the initial tests on isolators that would allow for bi-directional motion. (Note: this paper only covers the uni-directional excitation studies for this product development.) For the above reasons, this work seeks to advance the use of this technology in the civil/structural engineering discipline and provide future and current students with improved means of learning about structural systems. 3. Project Details and Results

This study involved numerous structural components and tasks distributed to the project team. The structural system consisted of two components: superstructure (tower) and seismic isolators. For these two components, they were not only modeled computationally but also at a scaled level requiring work to occur in parallel to meet the project deadline. The superstructure was entirely

designed by the UG team members per the SDC EERI design requirements. While the superstructure was under design, the GSR worked on the prototype design of the seismic isolators and provided insight into the 3D printing process. Then, under the supervision and guidance of the GSR and lead author, the UG team took the prototype isolators tested and adapted the design for use in their final structural design bringing together the two components of the structural system. The following sections detail the work associated with each structural component with emphasis on the 3D printed seismic isolation system. 3.1 Superstructure The structure as shown in Figure 6a is a scaled model of a full-scale multi-use structure to be located in downtown Los Angeles. This structure had a pentagon shaped floor plan as shown in Figure 6b. The design was inspired by Taipei 101, Salesforce Tower, and previous competition submissions. The tower design went through a number of iterations based on the need for a seismically strong yet constructible design per the EERI SDC guidelines. The superstructure was modeled and dynamically analyzed using SAP2000. Construction of their final model resulted in a scaled 20-story balsa wood tower that weighed 26lb. The tower was tested on the Shake Table II as a fixed base or non-isolated structure using two scaled uni-directional ground motions provided by EERI for different return periods to aide in the design process.

(a) (b)

Figure 6. (a) Seismic Tower Made of Balsa Wood (b) Pentagon Shaped Floor Plan. 3.2 Initial Seismic Isolator Design As previously discussed, there are a myriad of seismic isolators available today. In terms of this project, the authors focused on friction pendulum bearings. As shown in Figure 5b, these isolators consist of sliding surfaces with a moving disc sitting between these surfaces. Full-scale isolators are designed based on the desired structural period along with the weight of the overall

structure defining the radius of the curved surface. The sliding surfaces are typically lined with a propriety material that provides a specific level of friction while allowing for smooth movement of the moving disc. In the case of this project, this was the first attempt to create scaled friction pendulum isolators using 3mm PLA 3D printing filament. As the authors were unsure of the scaling effect on response, this initial study applied a trial and error approach with prototype isolators. The first isolators went through several design iterations as the design/print process was understood by the GSR who had a background in seismic isolation. One major consideration was the ability to provide an effective sliding angle while still maintaining system stability and an effective enclosure for the moving disc. This refined the prototype design based on constructability and practicality knowing the limitations of the 3D printer nozzles and performance demands on the sliding surface. The tower was equipped with a wooden base plate with an isolator beneath each corner. The prototype isolator system consisted of 8 plates with a concave surface (Figure 7) and a moving disc. Two base plates would sit on top of each other with the moving disc inside to complete the isolator (Figure 8). The sliding surfaces were 3.3 inches in diameter with a 0.75 inch moving disc (Figure 9). These isolators would allow a structural displacement up to 2 inches. These isolators were designed using Fusion 360 and then printed on an Ultimaker 2+ 3D printer.

Figure 7. 3D Printed Sliding Surfaces (Interior View).

Figure 8. Completed Isolator Assembly.

Figure 9. Interior View of Isolator Assembly.

3.3 Small-Scale Shake Table Testing This initial isolator design was tested on the SFSU shake table using two scaled uni-directional ground motions provided by EERI for different return periods, Ground Motion 1 (GM1) and Ground Motion 2 (GM2). The isolators were attached by screwing the base plates to the wooden pedestals and gluing the plexiglass base to the bottom plate of the structure (Figure 10 and 11).

Figure 10. Screwed Base Plates.

Figure 11. Isolated Tower Assembled and Installed on Shake Table for Testing.

The first round of experimental testing produced a number of promising results. Firstly, the scaled structure exhibited the period shift expected with the inclusion of an isolated system. The structure’s fundamental period increased from 0.2s to 0.38s. (In a full-scale structure, this increase would present itself more significantly.) Secondly, the students visually observed rigid body motion in the isolated structure which was quite apparent against the fixed base response. However, the isolators’ radius was not sufficient for the ground motions applied. The team observed the structure reached the maximum allowable displacement of the isolator causing the moving disc to come in contact with the lip of the sliding surface. This led to the structure to begin rocking due to an accentuated overturning moment. As a result, it was clear that the isolators needed to be redesigned to accommodate a more appropriate displacement. The update in design gave the students an opportunity to practice the design process by identifying the key parameter influencing the response. In this case, this was achieved by closely examining the response spectrum of the stronger of the two ground motions. The team identified the necessary displacement of the structure for desired performance and found the corresponding structural period (T) from the response spectrum curve. Knowing the period, the students could solve for the system’s natural frequency (ω) using equation 1:

(1) 𝜔" =%&'

From basic structural dynamics, the total needed stiffness (ktot) was solved for using equation 2 where Mtot is the total structural mass:

(2) 𝑘)*) = 𝜔"%𝑀)*) Finally, from friction pendulum theory, the radius of curvature (R) could be determined using equation 3 where Wtot is the total structural weight:

(3) 𝑅 = -./.0./.

Through this series of calculations, this led to an increase in the sliding surface’s diameter to 5 inches (Figure 12). The original set of isolators were sanded down for a smoother surface removing the raised lines of filament present due to the 3D printing. However, due to time constraints, the team was not able to sand down the printing lines slightly altering the results.

Figure 12. New Isolator Design Increased Diameter of the Sliding Surface.

The isolated structural system was again tested using the same ground motions as before. As shown in Figure 13, the floor accelerations were improved at the top floor of the structural system with the inclusion of isolation for GM1 and GM2. The increased diameter of the isolator prevented edge impact, however, the printing ridges on the isolators were an issue. As the ridges were not sanded down due to time, the isolator’s moving disc could not smoothly traverse across the surface. As a result, Figure 13 also shows there were high frequency responses picked up due to the additional surface topography in the isolation motion. This did impede the visual observation of improved response with the new set of isolators, however, the data showed an improvement compared to the fixed base model. The response from GM2 saw lower accelerations compared to GM1 which can be attributed to the relationship between the structure’s modal behavior and the input motion. As this paper covers the learning experience of this UG team from the design and testing process, the detailed discussion of the technical results go beyond the breadth of this paper and can be found in a future complementary paper. Overall, even with the isolator surface smoothness issue the 3D printed elements performed well and presented the desired structural responses.

Figure 13. Acceleration Time History Results for the GM1 and GM2 Comparing Fixed Base and Isolated Responses at the Structure’s Top Level.

4. Student Assessment At the completion of this project, the student participants completed feedback forms to identify the effectiveness in achieving the outlined learning objectives. The questions aimed to evaluate the students’ skills and knowledge before and after the study. Overall, all the UG students began with little to no knowledge of seismic isolation with one student stating they knew “only a small portion” but “didn’t understand how it worked”. The students were exposed to the subject matter in a myriad of ways including reviewing literature, designing the isolators, creating a computational model, and testing the structural system. By the end of the study, they felt more confident in discussing the subject matter and felt they had a moderate level of knowledge related to what isolation is and its theory. But out of all the study activities, it was the designing

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and testing that made the biggest impact. Being able to see first-hand the structural response of the prototype system and having to adapt the design for the final isolator design pushed the students to think critically about the design approach. This was evident in one student’s statement, “I gained a better comprehension on how the Base Isolators are designed and expected to behave when used. This was due to the fact that we designed our own base isolators…”. Another student remarked, “I learnt more by watching the experiment being carried out and understanding the practical applications of the isolators.” With the prototype isolators, the students’ began to see the improvement in response isolation can provide a stiff structure with the presence of rigid body motion. But they also learned to identify behavior they observed in the test in the recorded data such as the impact of the interior moving disc with the isolator edge. Due to the time restraint of the study, the students did not conduct a full validation study of their computational model against the experimental results. However, they noted behavior such as overturning moments in the experimental test that did not appear in the computational simulations. This was a crucial component in the learning process as they learned the connection with and the need for experimental testing. This was reflected in one response, “We also knew that the isolation would have an effect on the structure response however, we didn’t expect it to have a huge impact. The results of our data was shocking to all of us.” Lastly, by using the isolators in the experimental test, the students had excellent insight into ways that these 3D devices can be improved for future applications as discussed in the next section. Overall, the study was effective in bringing a rather theoretical subject matter such as seismic isolation to life and increasing students’ ability to relate to the subject matter and begin to build upon the foundational knowledge.

5. Conclusion and Lessons Learned The use of the readily available, inexpensive 3D printing technology provided the authors an opportunity to learn about seismic isolation and observe first-hand the testing of a scaled isolated structure. A project such as this would be far beyond the expanse of a typical senior design project due to cost and production limitations of seismic isolator manufacturers. Additionally, the hands-on component of the students designing, printing, and testing the isolators themselves provided a more extensive learning experience compared to allowing the design and production to be conducted by the manufacturer. Overall, this initial study presented a number of lessons learned that will inform future endeavors to expand this work. Firstly, the printing of the isolators is a very time consuming activity. Each isolator took approximately two full days of printing. This print time was due to the use of a fine nozzle head and slow print speed to ensure a quality product. Secondly, the 3D prints require an amount of finishing for maximum performance. As mentioned, the original set of isolators were sanded down using store bought sand paper to remove the ridges produced by the layers of filament in the printing process. However, this again is a time consuming activity which does not have a high level of quality control. It is very difficult to ensure each isolator is sanded to the same level of finishing. The results, on the other hand, show that without this step the isolation can produce high frequency content. However, more investigation is needed to determine the magnitude of influence a finished versus un-finished surface has on the overall response. Also, there needs to be a consideration of liners for the 3D printed isolator base made from a metal or alloy. Thirdly, the overall design process needs to be further refined. Knowing the isolation

theory at a full-scale level, this project tested the ability to use this theory for a scaled project. Ultimately, it did prove rather effective. Lastly, the students made a very good observation in terms of the experimental testing. The shake table and equipment available only excited and recorded data in one direction. However, the isolation system is designed to provide and show movement in both horizontal planes of motion. As a result, the students were not able to capture the full picture of data due to these limitations. One suggestion for future projects would be the use of a higher level 3D printer. A more advanced printer would help reduce production and finishing time producing a more effective isolation system for testing. Through this project, students gained insight and experience in the design, construction, and testing process involved with seismic isolation. It also set the precedent for the use of 3D printed structural elements and has opened the door for further research in the use of 3D printing technology in civil/structural engineering curriculum to overcome the challenges of traditional classroom lessons.

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