Seismic Response of Innovative Straw Bale Wall Systems and System Identification

Transcription

1 Seismic Response of Innovative Straw Bale Wall Systems and System Identification Charly Champion Home Institution: Georgia Institute of Technology NEES Institution: University of Nevada, Reno PIs: Darcey Donovan, P.E., and Dr. Sherif Elfass, P.E. Abstract The objective of this research project is to determine the performance of clay plastered load bearing, thin, straw bale wall assemblies under in-plane cyclic loading, and the performance of a small full-scale straw bale house using shake table simulation. This study focuses on system identification based on bi-axial shake table tests of a x x full-scale straw bale house. Structurally safe building methods are largely unaffordable for the poor in developing countries such as Pakistan. In response, Pakistan Straw Bale and Appropriate Building (PAKSBAB) is developing unique earthquakeresistant straw bale building methods that are affordable, energy efficient, and utilize local labor and indigenous renewable materials. Between each increasing amplification of the Canoga Park record, white noise tests were conducted and, from these, natural frequencies determined. In the X direction the natural frequency decreased from. Hz to. Hz and in the Y direction the frequency decreased from. Hz to. Hz. Greater degradation is expected in the Y direction due to the veranda wing walls. After natural frequencies were determined, modal damping ratios were calculated for accelerometer.-b-y using the half-power bandwidth method. The damping ratio decreased from.% to.% in the X direction while in the Y direction it decreased from.% to.% before increasing to.%. These results can be attributed to the cantilever design of the veranda wing walls. To conclude system identification, the mode shape was determined for the first natural frequency,. Hz, before the. Canoga Park test. This mode shape indicates a greater response to ground movement from the wing walls than the rest of the house. Based on these data, the veranda wing walls have a significant impact on wall behavior and should be more closely examined to maximize structural performance.

2 Introduction On October,, the northern mountainous region of Pakistan was struck by a. magnitude earthquake which killed an estimated, people and rendered more than million homeless due to poor building construction. Structurally safe building methods are largely unaffordable for the poor in developing countries such as Pakistan. In response, Pakistan Straw Bale and Appropriate Building (PAKSBAB) is developing unique earthquake-resistant straw bale building methods that are affordable, energy efficient, and utilize local labor and indigenous renewable materials. PAKSBAB is a nonprofit organization established in, whose mission is to adapt, apply and transfer straw bale and other appropriate building methods to protect and improve the lives of the rural poor, especially those living in earthquake and extreme weather regions of the developing world. Through our Community Participation Program they help build houses for those in need, and engage the beneficiaries in the construction process. The building material cost for our typical x house is $, with a men / month construction duration. To date houses have been constructed in Pakistan s North West Frontier Province [PAKSBAB]. Although straw bale structures have been used throughout history, very little earthquake performance analysis has been performed on them [King ]. While this research is to act as a baseline for performance analysis, the data collected will definitely affect the building methods and designs used by PAKSBAB in Pakistan, and will eventually be submitted as data for developing a comprehensive building code for straw bale wall systems. Objective The objective of this research project is to determine the performance of clay plastered load bearing, thin, straw bale wall assemblies under in-plane cyclic loading, and the performance of a small full-scale straw bale house using shake table simulation. The sitefabricated bales were not as wide as those used in a typical straw bale building, and the fishing net reinforcement and gravel bag foundation were non-conventional. Therefore these tests were necessary to establish the capacity of this unique system. The overall research plan consists of components:. Component field tests and material tests. Wall assembly tests of varying configurations. Bi-axial shake table tests of a x house. Report of findings and seismic design and construction recommendations This particular research focused on the full-scale house test performance, specifically system identification using white noise tests. Methodology Construction Method For the full-scale house test, a straw bale structure was built at by by foot plan. As Figure shows, the house is a simple square base design with attached veranda wing walls and window cutouts.

3 Figure. Full-scale straw bale house layout. Construction begins with field produced straw bales using a field jack and locally resourced straw, as shown in Figure. Construction method was a load bearing design consisting of straw bales resting on a soil cement encased gravel bag foundation. Exterior opposing bamboo pins were used to keep the straw bale walls plumb during construction and to provide out-of-plane support. Fishing net was installed under the foundation, stretched up both sides of the walls and nailed to the top plates. The roof consisted of wooden I-joists insulated with light straw clay (straw tossed in a mixture of clay and water) and covered with corrugated metal roofing. Gravel bags were placed at the top of walls to simulate psf snow load (% of psf). Before the walls were finished with clay plasters and lime wash, the house appears as in Figure. Figure. On-site fabrication Figure. Test house under construction

4 Testing Method Shake table tests were performed on a bi-axial shaking table at the University of Nevada Reno. The objective of the shake table tests was to assess the seismic response of a x x full-scale house constructed with heavy detailing. The input motion was the Canoga Park Topanga Canyon record of the Northridge, California earthquake, Mw.. The house was subjected to eight levels of seismic shaking, beginning at % of the recorded ground acceleration and increasing at % increments. The house withstood % times the Canoga Park record, or.g, twice the acceleration of the Canoga Park record. Between each increasing amplification of the Canoga Park record, standard white noise tests were conducted. System Identification Due to the time limits on this portion of the overall research project, a brief system identification was completed, generating an overview of house performance over the course of the seismic testing. A Matlab based program, TF Multi, was used to calculate and output transform functions with applied Hamming filter to all raw data for all the white noise tests conducted on the straw bale house. From these functions, natural frequencies were determined and modal damping was calculated for a single accelerometer using the half bandwidth method. Finally, a sample mode shape was generated. Results Natural Frequency In the X direction the natural frequency decreased from. Hz to. Hz and in the Y direction the frequency decreased from. Hz to. Hz as shown in Figure and Table. Greater degradation is expected in the Y direction due to the veranda wing walls. X Direction Y Direction... White Noise Test (amplification of Canoga Park Record in g) Figure. Natural Frequency in the x and y directions

5 Table Natural Frequencies for all white noise tests Natural Frequencies (Hz) White Noise X Direction Y Direction Modal Damping Ratio After natural frequencies were determined, modal damping ratios were calculated for accelerometer.-b-y using the half-power bandwidth method, which is illustrated in Figure. This accelerometer was located on the south west side doorway, perpendicular to the veranda wing wall. The damping ratio decreased from.% to.% in the X direction while in the Y direction it decreased from.% to.% before increasing to.%, as shown in Table. These results can be attributed to the cantilever design of the veranda wing walls. Accelerometer. B Y Amplitude of Transfer Function..... Figure. Points of interest for Modal Damping Ratios Table Modal Damping Ratios for Accelerometer.-B-Y in x and y directions Modal Damping Ratios for. B Y (%) White Noise Test. X Direction... Y Direction...

6 Mode Shape To conclude system identification, the mode shape was determined for the first natural frequency,. Hz, before the. Canoga Park test. This mode shape indicates a greater response to ground movement from the wing walls than the rest of the house, as shown in Figure. This mode shape was developed by plotting TF amplitudes for accelerometers located throughout the structure, as calculated in Table. Figure. Mode Shape for white noise test before. Canoga Park Record Table s used in drafting Mode Shape s for Mode, f =. Hz / T =. sec White Noise. Accelerometer A A. B. B. B. B C C X Y Physical Performance In this load bearing straw bale house design, straw bales, plaster skins, and fishing net act in unison as a low tech composite material. The plaster skins are the stiffest elements, providing compressive strength and transmitting the loads to the foundation. As the plaster cracks and spalls during an earthquake, it provides damping and dissipates energy. Once the plaster has been significantly damaged, with a corresponding decrease in stiffness, the ductile straw bale core acts as a backup mechanism and helps to resist the

7 vertical and lateral loads. The fishing net provides tensile strength and ductility. At the foundation interface it provides shear and overturning resistance, allowing the building to displace to a certain extent, then pulling it back to its original position [PAKSBAB]. During the full-scale house test spalling, cracking, and base displacement occurred, as shown in Figure ; however, the structure remained standing and the damage was classified as reparable. Figure. Damage to the house after.g (% Canoga Park) Conclusion Based on these data, the veranda wing walls (see Figure ) have a significant impact on wall behavior and should be more closely examined to maximize structural performance. While the straw bales and gravel bag foundation accomplish enough damping for the structure to remain standing, amplification of the veranda wall movement due to its cantilevered design increase the damaging effects of an earthquake. Careful consideration should be applied when redesigning the wing walls. Further stiffening would have negative effects on the beneficial damping properties of the straw bales, thereby reducing stiffness performance during an earthquake throughout the entire structure. Reduction of the veranda wing wall extension or complete removal of the wing walls from future plans could possibly enhance performance, at the expense of comfort and aesthetics.

8 Even with the behavior of the veranda wing walls, the straw bale house performed extremely well up to.g or times the Canoga Park record. This stability under an intense earthquake demonstrates the effective nature of this load bearing straw bale wall system. With further development performance could improve and reductions in damage to the plaster are possible. Figure. South west veranda wing wall This high level of earthquake performance as well as the other benefits of straw bale structures including fire resistance, natural insulation, sustainable materials, and inexpensive production price [King ] suggests that these structures are viable as low cost housing, especially in underdeveloped areas such as northern Pakistan. Further straw bale education in principle and application as well as construction programs are needed. Research on seismic performance and system performance should continue to enhance future structural performance of straw bale houses. This test was successful, but more information is needed to capitalize on the potential of load bearing straw bale systems.

9 Acknowledgements Funding was provided by EERI s Special Projects and Initiatives Endowment Fund as well as the NEES REU program. The experimental work was conducted using the NEES facilities at the University of Nevada, Reno (NEES@UNR) as a designated shared-use project. Special thanks to Robert Nelson, Patrick LaPlace, and Gokhan Pekcan for use of their software and coding knowledge. Reference Anil K. Chopra, [] Dynamics of Structures Theory and Applications to Earthquake Engineering Third Edition, Pearson Education, Inc., Upper Saddle River, NJ Bruce King, [] Design of Straw Bale Buildings. Green Building Press, San Rafael, CA Cale Ash, Mark Aschheim [], In-Plane Cyclic Tests of Plastered Straw Bale Wall Assemblies, University of Illinois, Illinois. Pakistan Straw Bale and Appropriate Building (PAKSBAB),

10 Appendix Sample Transform Function Plots and Phase Angles for White Noise before x Canoga Park in the X Direction Top Acc A X Top Acc A X Phase Angle.... Top Acc A X Top Acc A X Phase Angle.... Top Acc. B X Top Acc. B X Phase Angle.... Top Acc. B X Top Acc. B X Phase Angle.... Top Acc. B X Top Acc. B X Phase Angle....

11 Sample Transform Function Plots and Phase Angles for White Noise before x Canoga Park in the Y Direction Top Acc A Y Top Acc A Y Phase Angle.... Top Acc A Y Top Acc A Y Phase Angle.... Top Acc. B Y Top Acc. B Y Phase Angle.... Top Acc. B Y Top Acc. B Y Phase Angle.... Top Acc. B Y Top Acc. B Y Phase Angle....