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POST AND BEAM SYSTEMS
Straw bale buildings are not the same as conventional buildings. This
is an advantage and precisely why I build them. I have
heard of people trying to create conventional frames and then
add the bales in. These people try to convince me that
it will save time to build the home conventionally and simply
use the bales as insulation. Invariably, this does not
work. The best thing to do is use a system that works
with the bales you have on site. For most non-loadbearing
homes, this is a post and beam or modified post and beam frame.
The frame of the home plays a number of important roles. In
non loadbearing construction, it is the structural strength
of the home. It provides a load path for the roof and
floor systems. It plays a role in the lateral shear strength
for the building. In all, it is the system that holds
the house up. That said, the bales themselves play a
large role in the strength of the structure too; however, most
engineers are only now learning to give appropriate values
to the strength of the bales and as such, the frame is often
considered to be the structural system.
SPECIAL CONSIDERATIONS
To accommodate the engineering requirements associated with
straw bale construction, the frame is changed in a number of
ways. First of all, there needs to be enough room for
adequate nailing of the lateral shear welded wire mesh around
all the doors and windows. To accommodate this, an additional
trimmer may be required depending on the inclusion of exterior
trim. In addition, there needs to be enough nailing surface
along the base of the wall systems for the welded wire mesh.
For that reason, I use 4 x 4 mudsills in straw bale construction.
The 4 x 4 sills are also stronger and resist breaking during
both uplift and lateral stress structural testing.
Knowing that we will be using plaster on the interior and
exterior of the building, I like to strengthen my window trimmer
details to offset the movement of the wood framing around windows
and doors. A little extra detail in these areas can help
prevent cracks in the plaster, something that no owner likes
to see. By pinching the header and sill framing members
between the trimmer sections, the corners are stronger and
able to resist movement during the drying of the lumber in
the frame.
FRAMING LAYOUT
Unlike conventional construction, the framing does not land
on four foot centers and in four foot modules to accommodate
plywood sheets. Instead, the layout of the framing is
designed to fit the size of the bales and support the calculated
loads. The more accurate you are with framing to the
bale sizes, the less notching you will have to do during the
baling stage. It is also important to pay attention to
the header and plate heights around the house. The goal
is to fit the bales tightly under the top plates to help squeeze
the bales and strengthen the wall. The window and door
header heights should be lined up so that numerous layers of
stuffing are not required to continue the running bond of the
bales.
ROOF DESIGNS
Roof
designs do not have to change with straw bale construction;
however, some roof designs take more work to incorporate into
a bale home. For example, a gable roof design will mean
the creation of more custom bales than a hip roof. Roof
designs can also help alleviate the darkness sometimes created
by the thick straw bale walls. By utilizing an open roof
design or dormers, more natural light can reach the room, thus
brightening a room that may otherwise seem dark due to window
size and orientation.
TOE UPS
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Part
of the framing process involves the creation of toe ups. Toe
ups are an absolute requirement with straw bale construction. As
mentioned earlier, they provide adequate nailing surface for
the welded wire mesh. In addition, they separate the
bales from the concrete and create a moisture break. Should
a water line or appliance ever break and spread water through
out the house, the toe ups will keep the bales dry and protected. The
toe ups are made from pressure treated 4x4 material, gravel,
and tar paper. They also provide a base for the
bale anchors, something required by code. The bales
need to be attached to the toe ups which are, in turn, attached
to the concrete. This is a way to anchor the entire system
to the foundation. I no longer use rebar impalers for
this purpose as required by many code books, but a series of
nails applied to the toe ups. The nails create a much
stronger connection for the bales and do not interrupt the
separation of bales from concrete the way a rebar section buried
in the concrete does. All of the building departments
I have worked with have easily accepted the change.
Tomorrow we'll cover the essential topic of making custom
bales
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