What the Pros Know… or need to

Straw-Bale Arch Test

by David Mar, SE, with John Swearingen

CASBA Home Page .

FAQs About Straw Building.

Straw Building in California and Beyond.

Contact CASBA. One grey evening this spring, a group of CASBA members and others in Berkeley witnessed the first of what we hope will be a series of tests that will push, pull, poke and probe straw bales and bale structures. The results exceeded our expectations, and opened new possibilities for straw-bale design.

This first test was of a segment of a straw-bale vault that was designed by Skillful Means for the composer Lou Harrison, in Joshua Tree, California. David Mar, S.E. devised the structural system, with input from Dan Smith, Bob Theis, Kelly Lerner, John Swearingen and Bruce King. The week before the test date, Janet, Paul, Randall, Pam, Bob and John from Skillful Means assembled the rig, stacked and stuccoed the bales. Then engineers, builders, architects, children and a few homeless people gathered together, eager to watch the destruction of the test arch, and enjoyed the hospitality provided by DS&A while the rig was set-up.

The assembly we built and tested was a simple, inexpensive and lightweight composite of straw, stucco and wire mesh, without rebar pins. Our design goal was to meet specific strength requirements of the building code and also to satisfy the spirit of code regarding toughness and energy absorption. Our idea was to develop details which would adapt standard reinforced concrete internal load mechanisms to straw-bale assemblies. Although straw might be seen as a weak substitute for concrete, it actually has several potential advantages, such as light weight, bulk (for stability), and, we think, its ability to absorb energy.

The assembly we built and tested was a simple, inexpensive and lightweight composite of straw, stucco and wire mesh, without rebar pins. Our design goal was to meet specific strength requirements of the building code and also to satisfy the spirit of code regarding toughness and energy absorption. Our idea was to develop details which would adapt standard reinforced concrete internal load mechanisms to straw-bale assemblies. Although straw might be seen as a weak substitute for concrete, it actually has several potential advantages, such as light weight, bulk (for stability), and, we think, its ability to absorb energy.

The testing used a calibrated hydraulic jack, supplied by Consolidated Engineering Labs (CEL), to simultaneously pull in and push out the arch to simulate extreme seismic or wind lateral loading. Bill Rothacher and Doug Stark of CEL operated the jack and loaded the vault in 1,000 lb. increments.

After each load and measurement, the load was released to record the elastic recovery and the permanent plastic deformations (see graph). After several uneventful cycles, we reached the vault’s elastic limit, when the first signs of stucco cracking were heard, at 3,000 lbs. The vault bounced back elastically with no plastic distortion when the load was released.

The vault’s elastic limit was equivalent to a lateral load of 55% of its weight. This value satisfies the code minimum seismic resistance of 30% for out-of-plane loading and far exceeds any code wind requirements. We loaded the vault for seven more cycles, well into its plastic and energy absorbing range, until we reached the throw limitations of the test rig. We were never able collapse the vault, even at extreme distortions: 11.9 in. diagonally in at a point load, 6.6 in. laterally at the apex, and 6.3 in diagonally out at a point load.

The maximum loads were reached at Cycle 6 with a jack load of 6,300 lbs. This corresponds to an equivalent lateral load of 115% of the vault’s weight. This exceeded our minimum ultimate strength target for lateral load resistance of 90% of the vault’s weight.

The displacement ductility of the vault, a measure of toughness, (the ratio of ultimate displacement over yield displacement) was over 20, significantly greater than for mainstream materials such as steel, reinforced concrete or plywood, which commonly achieve ductility ratios between 4 and 12.

The shear mechanism, a truss or "strut and tie" assembly is illustrated at left. When shear forces act on the wall section, the straw, replaces concrete to form a diagonal strut which resists deformation. The wire cross-ties link the straw struts. The outside curtain of mesh, in tension, acts as longitudinal reinforcing (rebar) would in a beam. The stucco shell and straw on the inside of the vault acts as concrete does in a beam.

The key elements in this detail are the wire cross ties, which allow the straw compression struts to form. During the entire test, there were no measurable shear distortions--the assembly remained stiff.

The bending mechanism, shown at right, is also used in reinforced concrete beam design. This mechanism forms a bending couple by tension in the internal layer of mesh and compression in the external stucco and straw. Changes in tension and compression along the vault length are carried through the straw (shear flow). The wire cross ties between bales prevent the inner mesh from delaminating from the vault when the mesh tries to straighten out in tension.

During the test, the stucco shell completely collapsed at the point load. Although the couple was then solely between the straw and the tension mesh, the vault remained stable and continued to carry increasing loads. This raises the possibility of adapting this system to allow the use of weaker skins, such as lime plasters or even earth renderings.

This test was successful in its primary objective, to satisfy minimum building code requirements using simple and inexpensive materials. More importantly, the test showed that we can predict behavior using simple models, and achieve good, tough performance using fundamental engineering principles that include the straw as a key component.

	Straw-Bale Arch Test by David Mar, SE, with John Swearingen
CASBA Home Page . FAQs About Straw Building. Straw Building in California and Beyond. Contact CASBA.	One grey evening this spring, a group of CASBA members and others in Berkeley witnessed the first of what we hope will be a series of tests that will push, pull, poke and probe straw bales and bale structures. The results exceeded our expectations, and opened new possibilities for straw-bale design.
	This first test was of a segment of a straw-bale vault that was designed by Skillful Means for the composer Lou Harrison, in Joshua Tree, California. David Mar, S.E. devised the structural system, with input from Dan Smith, Bob Theis, Kelly Lerner, John Swearingen and Bruce King. The week before the test date, Janet, Paul, Randall, Pam, Bob and John from Skillful Means assembled the rig, stacked and stuccoed the bales. Then engineers, builders, architects, children and a few homeless people gathered together, eager to watch the destruction of the test arch, and enjoyed the hospitality provided by DS&A while the rig was set-up. The assembly we built and tested was a simple, inexpensive and lightweight composite of straw, stucco and wire mesh, without rebar pins. Our design goal was to meet specific strength requirements of the building code and also to satisfy the spirit of code regarding toughness and energy absorption. Our idea was to develop details which would adapt standard reinforced concrete internal load mechanisms to straw-bale assemblies. Although straw might be seen as a weak substitute for concrete, it actually has several potential advantages, such as light weight, bulk (for stability), and, we think, its ability to absorb energy. The assembly we built and tested was a simple, inexpensive and lightweight composite of straw, stucco and wire mesh, without rebar pins. Our design goal was to meet specific strength requirements of the building code and also to satisfy the spirit of code regarding toughness and energy absorption. Our idea was to develop details which would adapt standard reinforced concrete internal load mechanisms to straw-bale assemblies. Although straw might be seen as a weak substitute for concrete, it actually has several potential advantages, such as light weight, bulk (for stability), and, we think, its ability to absorb energy. The testing used a calibrated hydraulic jack, supplied by Consolidated Engineering Labs (CEL), to simultaneously pull in and push out the arch to simulate extreme seismic or wind lateral loading. Bill Rothacher and Doug Stark of CEL operated the jack and loaded the vault in 1,000 lb. increments. After each load and measurement, the load was released to record the elastic recovery and the permanent plastic deformations (see graph). After several uneventful cycles, we reached the vault’s elastic limit, when the first signs of stucco cracking were heard, at 3,000 lbs. The vault bounced back elastically with no plastic distortion when the load was released. The vault’s elastic limit was equivalent to a lateral load of 55% of its weight. This value satisfies the code minimum seismic resistance of 30% for out-of-plane loading and far exceeds any code wind requirements. We loaded the vault for seven more cycles, well into its plastic and energy absorbing range, until we reached the throw limitations of the test rig. We were never able collapse the vault, even at extreme distortions: 11.9 in. diagonally in at a point load, 6.6 in. laterally at the apex, and 6.3 in diagonally out at a point load. The maximum loads were reached at Cycle 6 with a jack load of 6,300 lbs. This corresponds to an equivalent lateral load of 115% of the vault’s weight. This exceeded our minimum ultimate strength target for lateral load resistance of 90% of the vault’s weight. The displacement ductility of the vault, a measure of toughness, (the ratio of ultimate displacement over yield displacement) was over 20, significantly greater than for mainstream materials such as steel, reinforced concrete or plywood, which commonly achieve ductility ratios between 4 and 12. The shear mechanism, a truss or "strut and tie" assembly is illustrated at left. When shear forces act on the wall section, the straw, replaces concrete to form a diagonal strut which resists deformation. The wire cross-ties link the straw struts. The outside curtain of mesh, in tension, acts as longitudinal reinforcing (rebar) would in a beam. The stucco shell and straw on the inside of the vault acts as concrete does in a beam. The key elements in this detail are the wire cross ties, which allow the straw compression struts to form. During the entire test, there were no measurable shear distortions--the assembly remained stiff. The bending mechanism, shown at right, is also used in reinforced concrete beam design. This mechanism forms a bending couple by tension in the internal layer of mesh and compression in the external stucco and straw. Changes in tension and compression along the vault length are carried through the straw (shear flow). The wire cross ties between bales prevent the inner mesh from delaminating from the vault when the mesh tries to straighten out in tension. During the test, the stucco shell completely collapsed at the point load. Although the couple was then solely between the straw and the tension mesh, the vault remained stable and continued to carry increasing loads. This raises the possibility of adapting this system to allow the use of weaker skins, such as lime plasters or even earth renderings. This test was successful in its primary objective, to satisfy minimum building code requirements using simple and inexpensive materials. More importantly, the test showed that we can predict behavior using simple models, and achieve good, tough performance using fundamental engineering principles that include the straw as a key component.