Strawbale - Structures

While there are a variety of structural systems being utilized in bale buildings, they generally fall into two categories: load-bearing and frame and infill. In addition, there have been structural enhancements developed here in seismic country to turn the earthquake liability of the extra weight of bale walls into the seismic asset of greater seismic damping. These approaches incorporate composite designs with stressed skins, exterior ribbing, constrained bale design, and alternate geometries such as vaulting.

Load-Bearing

"Nebraska" Style, or load-bearing, is the classic structural type and intuitively simple. However, as they have evolved into current practice, load-bearing designs are simpler in material use, but more complex in structural function. Although generally appropriate only for single story structures, higher loads can be accommodated through significant pre-compression of the bales. This style of load bearing construction is especially popular in Arizona, Colorado, and Canada. In load-bearing construction, the use of bales most closely resembles masonry, as the bales are laid flat in running bond with door and window headers.

The elastic nature of bales is exemplified by their compression under load and over time. Many of the Sand Hills, Nebraska houses had hip roofs in order to equalize the loading onto the wall. The traditional approach to wall compression is to put the roof on and wait at least six weeks for the weight of the roof to compress the bales prior to plastering. Today, many systems are used to pre-compress the bales between the footing and the top of wall box beam or bearing plate. Using full height threaded rods every six feet is a direct and effective method, but awkward to install, especially in rice straw. A more common technique is strapping the bales with polyester or wire, often combined with some method of compression such as course-by-course "stomping" by foot or pressing on the wall with a backhoe. An ingenious and effective system developed by Bob Platts in Canada involves wrapping the stucco netting up and over the box beam, and pre-compressing with an air bladder along the top of the wall.

The disadvantage of a load-bearing system is the complexity of the structural action, including load and distribution limitations, as well as complex settlement patterns. In seismic areas, for two-story structures, or in high snow load areas where Building Department engineers want a calculable system, a load-bearing system makes for a more complex approval process.

Compression tests in Colorado have shown that the plastered bale walls act as a stressed-skin panel with surprising strength (up to 4,000 pound per square foot), far more than the bales or stucco skins separately. There is now considerable interest in the strength of earth plastered walls, and such tests are part of an ambitious testing program under the auspices of Bruce King's Environmental Building Network. The newly revised California Bale Ordinance allows up to 400 psf for a cement or lime-cement plastered bale wall.

Post and Beam   

There are many approaches to using a building frame to carry vertical loads with the bales as infill.  In general, adding a frame greatly simplifies the engineering and approval process, especially for two story or seismic conditions. The character of the walls (as well as the process) is affected by whether the bales become "infill" panels between supports, the bale walls are a continuous and intact "fabric" with added supports, or whether the bales are a "wrap" around an existing wall.

Bale Wall with Light Notched-in Posts  
The most popular system in California is a light system of 4x4 posts and 4x4 beams notched into a continuous bale wall, so that the frame is not exposed. With the frame flush to the face of the bales, it is easy to add strap or cable x-bracing, significant in seismically active California. We have found this approach, using 4x4 posts, interfaces very well with conventional wood stud framing. Typically this frame is erected first and the bales are notched around the frame. The frame is usually wood, 4x4 to 6x6 posts, but steel has also been used as a light moment frame (by Tony Perry in New Mexico).   
As an alternative, we have experimented with stacking the bale walls first, putting on the box beam and roof to compress the bales, and then notching in the posts afterwards.  It helps with compression, but we have found that getting a frame and roof up before the bales is more practical.

Bale Wall Infill Between Full-Width Supports
In this approach, the "posts" are usually full-width vertical supports such as concrete block, concrete, short 2x4 fin walls, or wood I-beams. The top beam is then a concrete tie beam, wood beam or box-beam. The post and beam approach is the most structurally straightforward in terms of dealing with the unusual aspects of bale walls, and the most conservative. The disadvantages include the necessity of more framing material, lack of continuity in the bale fabric, and typically a diminished "bale character" in the wall edges and alignment.

Bale Wall Alongside Exposed Timber Frame
This system is often typified by an exposed heavy timber frame with the bale wall running alongside the frame. While the bale walls are usually tied to the posts, they can also be independent, attaching only to the ceiling, allowing the walls to curve. In this approach, the bales are placed on edge. 
The general advantages to this approach is that the frame and roof can go up first and protect the bale raising, the frame can be engineered for a greater variety of loads, and the walls can have more architectural freedom of movement. The disadvantages are that a heavy exposed braced frame adds a significant additional layer of cost and labor. Also, overall lateral bracing has to come from the braced frame, and the bracing potential of the plastered bale walls is more difficult to incorporate.  
  
Bale Wall Wrapped Around an Existing Shell
This is a common approach to insulating an existing house, barn, or steel industrial building. The bales are typically wrapped outside the existing skin of the building, and then tied to it.

Strengthened Bale Approaches

Several interesting alternative strengthened bale approaches are emerging, such as enhanced stucco stressed skin panels, lighter infill posts, exterior ribbing stiffened baskets, and vaults. All these are "strengthened bale" approaches, in that the bales work with another material to become a composite structure.

Stressed Skin Panel with Stronger Concrete Skins
Increasing the strength of the concrete surfaces of straw bale walls enhances their stressed-skin panel nature. Several people have done this by substituting gunnite for stucco. Gary Black has developed a high strength system for particularly high walls such as those of multi-story buildings which uses 3 inch gunnite skins and innovative cross wall "spars" between the bales to tie the two skins together.

Strongly Enmeshed Bale Wall
This approach is based on the premise of enhancing the inherent seismic resistance of the flexible bales by treating them as if they were shock absorbers, with the mesh in tension constraining the bales in compression. In this scenario, the cement plaster would contain the bales in low to moderate earthquake forces, whereas under severe stress the stucco could crack, allowing the bales to flex and absorb force, but would be constrained by the mesh. In this system the mesh is heavier, 2x2x14 gauge, for example, and attachment/ embeddment at the foundation, wall tops and wall ends is especially important (David Mar's vault test). This approach can be used in load bearing structures, either with or without diagonal strapping, or with post and beam structures without diagonal strapping.

Exterior Bale Ribbing
Another approach is to gradually stiffen the bale wall with exterior bracing or ribbing. Many people are replacing the interior pinning with exterior pinning where it bonds with the plaster skin and has far better bracing effect. We have been using a system of exterior rebar or bamboo ribs to stiffen bale vault structures. The rebar ribs are at 16 in. centers, or two per bale, and are tied through the bales, creating a "basket" effect. The same approach holds promise for stiffening tall walls, replacing the interior pinning while improving the stressed skin panel action. 

Vaults
There has been a lot of excitement about vaults and domes built with bales. For us, it was a natural interest in extending the character of the bale wall up through the roof, rather than have to transition to a conventional wood framed roof. Using the analogy of masonry vaults and domes, bales seem a natural. However, the elastic nature makes the curved structures act more like baskets, with gradual stiffening with external ribbing. Vaults, like load bearing approaches, depend on compression, and require well-compacted bales. The vault has the promise of minimizing the use of wood framing while maximizing the effect of the super-insulating envelope. Dave Mar and John Swearingen have tested and built a cement skinned semi circular vault in Joshua Tree, which tested as a remarkably tough and flexible structure. 

Tests and Codes

Refer to the CASBA (California Straw Building Association) website, www.strawbuilding.org, for a summary of tests.  

Compression:
The allowable compressive stress for 3-string straw-bale walls in the codes is 400-800 pounds per lineal foot, (new Calif. Code allows 400 psf for plastered rice bales). 

In-Plane Lateral Load:
Two tests at San Luis Obispo on plastered bale walls show greater than Code "stucco" values: 
A) Minimal stucco netting stapled at edges: 1,500 plf ultimate for a 2-sided plastered wall.  With a 4x safety factor this is a 375 plf design strength.  The failure occurred at the stucco netting attachment with no cracking in the field of the plastered bale wall. These results compare favorably with 2x 180 plf Code allowance for stucco.  Because failure occurs first at the stucco netting attachment, we recommend a 4 ft. nailing pattern at sill plate, posts and beam.  
B) Enhanced mesh, 2x2x16 gauge mesh well-attached (2" on center all edges), increased in-plane lateral strength to approximately 4,000 plf.

Clearly, significant improvement can be achieved with heavier reinforcement and stronger attachment for a super stucco approach. Enhanced mesh and attachments should get the pounds per lineal foot per face up to 250 lbs. The ultimate strength using the constrained bales approach could be as high as 1,000 plf.  

The new California Bale Ordinance (SB332) now recognizes the seismic capacity of bale walls explicitly as 180 plf per face or 360 plf total with cement or lime-cement plaster, if designed by an architect or engineer.  We recommend that using bales for seismic value needs enhanced mesh detailing and stronger mesh.  We use 2x2x16 gauge welded wire mesh and wrap the mesh between doubled bottom plates at each side at the bottom of the wall, using anchor bolts with 2x2 washers at 2 ft. on center.

For lateral stress from wind, the inherent strength of the plastered bales is generally adequate for one-story buildings. However in seismic areas, such as in California’s Zone 4, enhanced bracing becomes necessary, as does mandatory structural analysis by an architect or engineer.  Methods of “X” Bracing include flat 3 ft. (12ga) steel straps, cables, and threaded rods.

Out of Plane Lateral Load:
The width of the bale walls is an advantage here, with an aspect ratio of a ten foot wall of 5:1 compared with a 6 ft. wall at 4:1.  In New Mexico, tests of a plastered bale wall handled up to a 100 psf load with less than 5% (1/2") deflection.

Here is where the bale wall's inherent stressed skin panel nature (even imperfect) helps.  As Bruce King has described, the typical bale wall is working very inefficiently compared to a perfect sandwich panel, but nevertheless, it is 20 times stronger than the unplastered bale wall. The Berkeley vault test provides a dramatic example of extreme out-of-plane stress.

Guidelines for Load Bearing Walls:
In unbraced bale walls, the Pima County code gives height/width guidelines of 5.6:1 for height, (10'6" for a 23" wall), and length/width limits of 13 (in California 15.7), for a maximum of (25' for a 23" wall).  Wall openings should be less than 50% of the wall length, and at least 4 ft. from corners. While California proscribes load bearing limits, it leaves seismic engineering up to architects and engineers, and the vast majority of California bale buildings are non-load bearing.