[Strawbale] Load Bearing Multi-storey Straw Bale Tower House Castle

Chris Mowatt ChrisMowatt at i12...
Mon Jul 28 12:49:48 CEST 2003

Four Storey Straw Bale Tower House Castle at The Free Energy Centre,
Kildonan Farm, Isle of Arran, Scotland

Has anyone any experience of building multi storey straw bale buildings?

We want to build a four-storey tower house castle. This would be in the
style of the traditional Scottish four storey tower house castle. It would
be about twenty metres in height and include a conventional pitched roof and
simulated battlements. The aim is to use load bearing straw bale
construction methods. We have spoken to the local authority planning
department and building control and their response was encouraging.

This idea presents many challenges. There are very few two storey straw bale
houses and, so far as I know, no four storey buildings. Our site at Kildonan
Farm is a very exposed location subject to strong winds, heavy rain and
extremely variable and unpredictable weather! Not the ideal place for
construction of a straw bale building! However, challenges are what makes
life interesting and to succeed would be very noteworthy.

Our thoughts are to build everything at ground level and raise the completed
sections on jacks and using the weight of the completed structure to
compress the straw bale walls during construction. The build would take
place in something like the following sequence:

    1. Construct the roof and fourth storey wallplate at ground level
    2. Position Jacks and jacking props to lift from wallplate jacking
    3. Lift building 375mm, i.e. one straw bale course (350mm) plus some
    4. Prop building from wallplate propping points
    5. Relieve pressure on jack (but not release entirely - so the jacks can
act as a fail safe should the props fail)
    6. Lay one course of bales
    7. Repressurise the jacks
    8. Lower props 50mm
    9. Lower building until the load is resting on the bale wall and
compressing the wall
  10. Lift building back up to 375mm mark
  11. Raise props
  12. Release jacks so props are taking the full weight
  13. Reposition the jacking props (i.e. increasing length by 350mm)
  14. Repeat operations 3 to 13 until ten courses (i.e. one storey has been
  15. Lime render and lime wash completed storey
  16. Assemble integrated wallplate, floor joist matrix and flooring for
next storey
  17. Repeat operations 2 to 16 until final storey (ground floor) walls are
  18. Remove jacking system

There are a number of advantages in this approach
    1. Construction can take place in safety and comfort close to ground
    2. There will be no need for scaffolding.
    3. The weight of the raised structure can be used to compress and
stabilise the straw bales of the storey under construction during wall
raising and during rendering.
    4. There will be no need for a wire or thread type tensioning system to
compress and stabilise the walls.
    5. Only the storey being worked upon needs to be protected from the
    6. The straw bale walls will be protected from the weather by the roof
structure from outset.
    7. It will be spectacular if successful.

The disadvantages are
    1. Expense of a specialist high precision jacking system (20000 GBP,
28000 Euro,  30000USD, 45000 CAD) for a one-off lift.
    2. Potential danger of partial or complete collapse during the jacking
    3. Untried on straw bale building.
    4. Need for strong jacking points and good load spreading from these
jacking points.
    5. It could be spectacular if it fails.

We plan to address these disadvantages in the following way.

    1a. We are investigating the design of a lifting system using cheap mass
produced components. The inherent flexibility and relative lightness of a
straw bale building means that a specialist high precision lifting system
may be unnecessarily precise. Moreover, one of the beauties of building with
straw bales is the simplicity of construction. We do not want to use
sophisticated high precision high cost tools if at all possible. We want our
solution to be suitable for use throughout the world. It should be possible
to make the jacking procedure simple, safe and accurate.
    1b. It may be possible to recoup some of the cost by selling the lifting
equipment after use.
    2a. The jacking system will be remotely operated and people will be
excluded from the building zone during the lift and anytime the jacks are
under pressure.
    2b. There will be multiple jacking points (sixteen or more) will be used
to maximise load spread and minimise the impact of the failure of one piece
of equipment.
    2c. All jacks will be independent of each other, so no one piece of
equipment will be critical to the lift.
    2d. A fail safe will be built into the system to ensure the maximum drop
in the case of failure is 50mm (2 inches).
    2e. Pressure to each jack will be manually controlled, but using
synchronised pumping. Observers using binoculars to monitor progress against
measuring sticks will ensure the lift progresses smoothly and evenly.
Observers will relay instructions to the people operating control valves
using mobile site radios (walkie talkies).
    3. Jacking buildings is common place for underpinning operations and
many large buildings are routinely jacked during construction. The relative
lightness and flexibility of straw bale buildings make them ideal for
    4. The intention is to build an integrated wall plate and floor
structure with extended timber I beams to provide external jacking points
(to be subsequently removed). This structure will be designed to spread the
load and provide sufficient strength for the jacking points.
    5. Failure will be a learning experience. No publicity is bad publicity!
Jacking operations are, although exciting in concept, unexciting in
practice. The lift takes place very slowly under controlled circumstances.
Catastrophic failure is virtually unheard of. Any failure would be localised
and small scale. Such failure may lead to the project being abandoned if a
problem cannot be resolved, but it is unlikely to lead to front page headlin

We are also considering laying the straw bales on their sides, rather than
in the conventional way. There seem to be some a number of advantages of
doing this.
    1. The compressive and shear strength of the straw bale, when part of a
stressed skin sandwich, i.e. when rendered with lime render, is stronger
when the straw are in vertical alignment (the way nature intended). This
advantage is contrary to the findings of some early research on the subject,
but that research was on the straw bales themselves not the stressed skin
(rendered) straw bale sandwich.
    2. The walls are thinner giving greater internal floor space.
    3. The insulation properties of the straw bale are improved (contrary to
what one would expect).
    4. The straw bale is less permeable to moisture in the same way a thatch
is not very permeable to moisture.
    5. There aren't thousands of straw ends sticking out into the render or
stucco to invite moisture in by capilliary action.
    6. The intercourse bonding may be improved by allowing straws from
neigbouring courses to interleave with each other when compressed.
    7. Fewer straw bales are required giving a small cost saving.
    8. Fewer straw bales are required giving a reduction in wall weight.

Points 2 and 6 may also contribute to the disadvantages:

    1. A thinner wall concentrates the weight on a smaller area requiring
stronger foundations.  The effect of this is reduced because the weight of
the straw bales is lower owing to there being fewer of them. However, since
a fair proportion of the weight of a straw bale building is made up of roof,
wall plates, joist timbers etc this unaltered weight would still be bearing
on a smaller area of foundations.
    2. Interleaving of straw stems when under compression may cause bulging
and may even result in the strings or bindings failing.
    3. Lime render (the most suitable form of weather protection for our wet
and windy climate) will not adhere as well to vertically aligned straw.
    4. There is not much research, practical experience or knowledge on
building in this way.

We feel the disadvantages can be addressed in the following ways.

    1. Stronger foundations. We intend to use self-draining stone
foundations up to a height of 500mm above ground level (to minimise splash
damage to straw bales). These will be tapered to increase the area of ground
upon which the weight is bearing.
    2. We will experiment with the behaviour of straw bales under
compression and decide whether the bindings need to be suplemented.
    3. We will experiment with spray methods of applying the lime render
using different consistencies of render applied in different thicknesses to
see if the render can be driven into the straw to provide a deeper bed of
adhesion. If this is unsuccessful we will try using modern bonding agents
such as PVA and SBR to improve adhesion.
    4. We are searching for any knowledge or exerience on the subject.

We hope to be able to show that tall buildings can be built swiftly, safely
and economically using load bearing straw bale construction methods.

We would be grateful for any advice or information anyone can offer.

Kind Regards

Beth and Chris Mowatt
Kildonan House
Isle of Arran
KA27 8SD
Scotland, UK
Tel. +44 1770 820 324

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