Full
Scale Straw Bale Vault Test
by David Mar, SE
Abstract
This test was conducted to evaluate
the effectiveness of a straw bale vaulted roof system, proposed for a
residential project in a seismically active area of California. The prototype
roof vault was a composite structure of straw bales, welded wire mesh
and stucco. The design to resist shear loads was based on a truss mechanism
borrowed from reinforced concrete design. Similarly, the design to resist
flexural loads was based on a bending model from reinforced concrete design.
The test consisted of loading a four foot segment of the vault to simulate
seismic effects. The structure remained stable as it was loaded well into
the plastic deformation range; carrying 94% of its ultimate load (over
1.26 g) with an average displacement ductility of 12.6. Testing was concluded
when the stroke of the rig was exhausted.
Introduction
Skillful Means Builders designed a residence of straw bale construction
to be built in Joshua Tree, California; a seismically active region located
near many active faults (see figure 1). The main feature of the design
is a great room with a vaulted roof. A test was developed for a prototype
vault to prove that the vaulted roof could be made only of straw bales,
welded wire mesh and stucco, and satisfy the building code strength requirements
to resist out-of-plane seismic loads (see figure 2). Testing was conducted
by Consolidated Engineering Laboratories. A previous design using more
traditional and better understood reinforced concrete shells for the roof
and wall structure had to be abandoned just before the start of construction
because of its tremendous cost. It was at this point that Skillful Means
Builders risked more time and money to test an as yet unproven, although
economically feasible composite design of straw, mesh and stucco. The
vault design was based on simple calculations using mechanisms adapted
from reinforced concrete design, with the straw replacing the concrete.
The specific goal of the test was first to elastically resist an equivalent
out-of-plane lateral load of 30% of the vaults weight. This would
satisfy the code strength requirement for portions buildings such as walls
(1994 UBC section 1630.2).
Fp = Z x I x Cp x Wp = 0.3 x Wp
; Z = 0.40 for zone 4, I = 1, Cp = 0.75
In recognition of the unconventional and pioneering nature of this design,
the vault structure should not only be strong enough, but also tough and
safe. The vaulted roof had to remain stable and absorb energy under intense
earthquake shaking. To this end, it was designed to plastically deform
and resist an equivalent lateral load of 100% of its weight (over 3 times
code) under ultimate conditions.
Design Mechanisms
Straw bale construction is potentially well suited to resist to severe
seismic loads. The bales themselves are light, and bulky, and thus the
assemblages tend to be stable. There is also anecdotal evidence that bales
can absorb significant amounts of energy through deformation. Tests of
unplastered bales and wall assemblies conducted at the University of Arizona
hint at the potential of bales to absorb energy. As part of an overall
design methodology, straw bale structural systems should have an initial
strength and stiffness to resist service level seismic and wind forces
without damage. This initial resistance is primarily by the stiff stucco
shells. However, under extreme excitations from large earthquakes the
stucco shells will most likely crack and degrade. As straw bale structures
soften, they need to maintain strength, and absorb seismic energy through
plastic deformation. To accomplish this, the structures should have a
"ductile core."
As mentioned above, the composite straw design mechanisms hypothesized
are adaptations of conventional reinforced concrete mechanisms, with straw
substituting for concrete. Like concrete, bales work well in compression.
The major difference is in the strengths of the reinforcement relative
to the straw and the concrete. In well proportioned concrete structures,
the primary reinforcement is weaker than the concrete, and as such, the
steel within the assemblage is designed to yield and absorb energy prior
to the onset of brittle concrete crushing. The design intent for straw
assemblies is that straw should be weaker than the reinforcing mesh. If
the steel mesh and stucco surrounds and traps the bales, the deformation
and energy absorption can occur within the straw rather than within the
reinforcing steel. The tested vault segment incorporates this basic idea
and proved that a well-detailed composite straw, mesh and stucco structure
can be made strong, ductile and safe.
Shear Mechanism
The out-of-plane shear mechanism of the vault is a truss or strut and
tie mechanism, similar to that used to design reinforced concrete beams
(see fig. 3). Each bale is contained within a rectangular cell of steel
mesh and stucco on the outside and inside of the vault and wire and grout
between the courses. Under shear loads, the cells distort to form a parallelogram.
As two diagonal corners of the cell come together, the trapped bale is
compressed, forming a diagonal strut, just as concrete does. The wire
cross ties, acting in tension, link the straw struts, similar to reinforcing
steel stirrups in a concrete beam. The curtain of mesh in tension (shown
on the outside of the vault in the diagram) acts as longitudinal reinforcing
does on the tension face of a concrete beam. The stucco shell and straw
in compression (shown on the inside of the vault in the diagram) act as
concrete does on the compression face of a concrete beam. The key elements
in this detail are the wire cross ties, which must exist to allow the
straw compression struts to form. A free body cut through the section
shows the shear force to be equal to the tension force in the wire ties.
This equals the component of the straw diagonal strut which is perpendicular
to the vault surface. During the entire test, there were only small measurable
shear distortions at the ends of the vault (see figure 19 and displacement
data from stations 1,2,3 and 6,7,8, figures 13 and 14).
Bending Mechanism
The out-of-plane bending mechanism hypothesized for the vault is also
adapted from reinforced concrete beam design (see figure 4). The straw
bales and the stucco are like concrete in that they can only carry compression
loads; they cannot resist tension loads. Tension loads are carried by
the wire mesh. Bending loads are resisted by a couple formed with tension
in the mesh and compression in the stucco and straw. Changes in tension
and compression along the vault length (shear flow) are carried by the
shear mechanism described above.
When the load causes tension in the inside layer of mesh there needs
to be special anchorage details to prevent the mesh from delaminating.
The mesh follows the bale profile; straight along the width of the bales
and changing direction or kinking at the joints. When the mesh goes into
tension, it tries to straighten at the joint and delaminate. The wire
cross ties, used for shear, are also sized to prevent the delamination.
The cross ties are wire ties which anchor dowels at the bale joints placed
outside and trapping the mesh layers.
The Prototype Vault Design
The vault is semi-circular with an inside diameter of 12 feet and an outside
diameter of 16 feet (see figures 5 and 6). In the actual building, the
vaulted roof will spring from bond beams 10'-8" above grade. There
were 16 bales stacked in the prototype segment, to form a section 4 feet
wide and 2 feet thick. In the actual structure the bales will be placed
in a running bond. The outer and inner layer wire mesh is a 2 inch by
2 inch by 14 gage welded wire fabric. An additional layer of 18 ga. chicken
wire mesh was placed inside the vault to facilitate the overhead placement
of stucco. Cross ties consisting of four 12 gage wire ties (12 inches
on center) were placed between the bales. These ties were doubled back
around #4 reinforcing dowels which trapped the inside layer of mesh, and
twisted around #4 dowels which trapped the outside layer of mesh. See
figures 7 and 8 for reinforcement details. The wedge-shaped space between
the bales was grouted with mortar. Two of the normal three coats of cement
stucco were placed on the vault; a 1 inch thick scratch coat and a 1/4
inch thick brown coat. The plasterer was specifically instructed to do
a poorer than average job, so that the test results would not depend on
good craftsmanship. The structural layers of mesh were anchored around
the test rig. An additional mesh stirrup was added around the first bale
on each side of the vault. This mesh stirrup or shear cleat was designed
to catch the leading corner of the first bales and prevent them form sliding
off of the test rig, or the bond beam in the case of the actual structure.
Four ½ inch diameter all-threaded rods with two 2x4 wood clamps
also anchor the first bales to the rig or bond beam. These test details
were incorporated into the actual building design.
The design weight of the 4 foot wide vault segment was 5,980 pounds.
The seismic loads were based on this weight. Of this, the straw weighed
1,440 pounds, the stucco shells weighed 2,288 pounds and the grout wedges
weighed 2,252 pounds. This is 66 pounds per square foot of arch, based
on the centerline length.
The Test Rig
The test rig, shown in figure 9, was designed to simultaneously
pull in and push out the arch to simulate extreme seismic or wind lateral
loading. The loading pattern is closest to forces created by wind pressure
and suction, and more severe and destabilizing than the lateral inertial
loads from an earthquake. The inward and outward acting load vectors intersect
at the center of the base of the vault. They generally canceled out their
vertical force components and created a net horizontal shear force.
The rig had two 6x6 wood rails, 4'-6" apart, with connecting cross
pieces of 6x6 topped with plywood under the ends of the vault. Matching
diagonal 6x6 wood masts were anchored at the center of the base of the
vault. They went around the vault and supported a steel pipe at their
top. Rigging, consisted of wire rope, shackles, turnbuckles, all-thread
rods and a calibrated hydraulic jack, connected a pair of stiff pipes
which loaded the vault. The jack used was a hydraulic ram made by Enerpac,
model RCP-55 (see figures 19 and 20). The rigging pulled one pipe,
located outside the vault, in towards the center of the base. The wire
rope went through central eye bolts and around the pipe at the top of
the masts, to the second pipe located inside the vault. When the jack
was loaded it contracted, to simultaneously pull in and pull out the vault.
Under the loading pipes, wood saddles in contact with the vault were used
to spread out the loads (see figures 19 and 20). Slack and shortening
in the rigging due to the plastic deformations in the vault were taken
up by the turnbuckles.
Inward acting loads were directly read from the gage of the calibrated
hydraulic jack. The outward acting loads were measured from the gage readings
of a tension load cell made by Enerpac, model TM-5, linked in series just
beyond the point of loading (see figure 19). The tension load cell is
a device which can measure tension in the rigging. It was placed just
beyond the point of loading so that its readings were of the actual force
delivered to the vault. The differences between the hydraulic jack loads
and the tension load cell readings are the friction losses in the system.
The deflections of the vault were measured via scales at 10 stations
(see figure 9). The scales were connected to the fixed rig and the relative
movement was the deflection in the vault. The trailing end of the vault
was behind the direction of the net shear load, while the leading end
was in front. Stations 1, 2 and 3 allowed measurements of translation
and rotation at the trailing end of the vault, and stations 6, 7 and 9
allowed measurements of corresponding values at the leading end of the
vault. Station 5 allowed measurements of the diagonal displacement inwards
to the center of the vault. Station 10 allowed measurements of the diagonal
displacement outwards from the center of the vault. Station 9 allowed
measurements of the horizontal translation at the top of the vault. Station
4 was not used.
The Test Results
Bill Rothacher and Doug Stark of Consolidated Engineering Laboratories
(CEL) conducted the tests. The hydraulic jack and the tension load cell
were calibrated in accordance to ASTM standards prior to testing (see
calibration curves, figures 10, 11 and 12). The jack 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. After several uneventful cycles, the vaults elastic
limit was reached 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 vaults elastic limit was equivalent
to a lateral load of 50% of its weight. This value satisfies the code
minimum seismic resistance of 30% of the vaults weight for out-of-plane
loading and far exceeds any code wind requirements. The vault was loaded
for seven more cycles, well into its plastic and energy-absorbing range,
until the throw limitations of the test rig were reached. The vault could
not be collapsed by the test, even at the extreme distortions of 11.9
inches diagonally in at a point load (station 5), 6.6 inches laterally
at the apex (station 9), and 6.3 inches diagonally out at the other point
load (station 10). The maximum loads were reached at cycle 6 with a jack
load of 6,700 pounds. This corresponds to an equivalent lateral load of
126% of the vaults weight (1.26g). This exceeded the targeted minimum
ultimate lateral load strength of 100% of the vaults weight and
provides a safety factor of 4.2 for minimum code requirements. As a measure
of the vaults toughness, it achieved an average displacement ductility
of 12.6 (ductility is the ratio of ultimate displacement over yield displacement).
This is a good result, even for structures of conventional materials such
as steel, reinforced concrete and plywood. The complete test results are
in figures 13 and 14. The load deformation curves for stations 5, 9 and
10 are printed in figures 15, 16 and 17, respectively. Photos of the vault
test are shown in figures 18 through 23).
The test revealed the ductility and toughness of composite bale systems.
At an early cycle during the test, the exterior stucco shell, under the
concentrated load pulling diagonally inward, completely collapsed at one
edge of the saddle (see figures 20, 22 and 23) . The stucco shell was
distorted to the point where it could not resist compression loads. From
this point onward in the test, the bending couple on this side of the
vault was solely between the straw and the tension mesh. Figure 21 shows
stucco cracking at the tension mesh. The structure continued to carry
increasing loads. This begs the question: can other weaker skins be effective
in a composite straw-bale system?
Conclusion
The test successfully proved that the prototype vault could meet, not
just the strength requirements of the building code, but also satisfy
the spirit of code in regards to toughness, energy absorption, stability
and safety. It also successfully demonstrated that one could predict behavior
and achieve good structural performance using composite straw-bale, stucco
and wire mesh construction designed using simple engineering principals
and mechanisms that include the straw as a key component.
References
Bou-Ali, Ghailene, "Straw Bales and Straw Bale Wall Systems,"
summary of results from a Masters Thesis at the University of Arizona.
International Conference of Building Officials, Whittier, California,
1994 Uniform Building Code.
International Conference of Building Officials, Whittier, California,
Maps of Known Active Fault Near-Source Zones in California and Adjacent
Portions of Nevada.
King, Bruce, P.E., Buildings of Earth and Straw, Sausalito California,
Ecological Design Press, 1996.
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