Straw Bale Construction Reaches New Dimensions of Style
Tipping Mar Architects, Berkeley, CA
EDnote: there are 1000 ways to build a house, there are just as many subsurface habitations. Here is a view of using haybale construction under ground or above ground or some of each.
The potential ductile performance of die composite straw-bale construction under earthquake effects was demonstrated by loading a four foot thick vault segment well Into Its plastic range. Consolidated Engineering laboratories conducted the test under the direction of Tipping Mar + Associates. Transverse seismic loading was conservatively simulated by using a calibrated hydraulic ram to contract a pulley system to simultaneously push the vault in and pull it out with diagonal loading. Successively greater loads were applied and released.
Deformations were recorded and after each such loading cycle as 10 stations on the vault.
The resulting lateral load and translation forms a backbone pushover curve. The vault remained elastic up to resultant shear of 2700 lbs (0.5g). The ultimate strength was 6700 lbs (1.26g). The translational displacement at the stop of testing 6.5inches, corresponding to a displacement ductility of over 16. Partway into the testing the stucco skins failed and the vault reached its highest strength and deformation with only the mesh and straw active. The toughness and energy absorbing capacity of theencapsulated bale system was tremendous. The test was only halted when the rig throw-capacity was reached.
Peer Review and Pushover Analysis
Wiss, Jainey, Elstner Assoc (WJE) verified that the structure complied with both the letter and intent of the building code, after conducting a peer review of the structure to satisfy the skeptical Building Official.
WJE performed a pushover analysis of the vaulted roof using the Capacity Spectrum Method with a backbone Curve based on75% of the backbone curve obtained in the prototype test (phi-factor of 0.75). The performance results evaluated under four earthquake levels are as follows:
EQ I Minor Event (87% chance of being exceeded in 50 yrs , 25 yr. return) The vault responds elastically.
EQII Moderate Event (50% chance of being exceeded jn 50 yrs, 70 yr. return)
Ductility demand 1.5 to 1.7. corresponding to a displacement at 0.4 inches. Relatively minor damage Is expected.
EQ III Design Basis Ground Motion (10% chance of being exceeded in 50 yrs, 475 yrs. return)
Ductility demand 2.5 to 3.5, corresponding to a displacement of 0.75 Inches. Moderate Damage is expected, however the vault is well within the life-safety requirements of the code.
EQIV Max Considered Earthquake - (2% chance of being exceeded in 50 years, 2000yr return)
Ductility demands 3.5 to 5 corresponding to a displacement of 1 inch (note: if the ATC 40 damping limit of 25% is imposed, the ductility demand is 9 and the displacement is 2.25 inches). Although the vault would have significant damage, it easily satisfies collapse prevention goals when considering the tested ductility of 16.