Design fundamentals - The Hempcrete Topic: Designing and Building with Hemp-lime - page 501

Racking strength
Hempcrete's monolithic structure provides not only insulation and a substrate for plasters
and renders but also a great deal of additional rigidity to the structural frame (see Chapter
7, page 91 ) . This rigidity, or racking strength, is the ability of the structure to resist move-
ment as a result of external lateral forces, such as from wind. The higher you build, or
the more exposed the site is, the greater the racking strength required. Some binders, for
example Tradical ® HB, have been tested for racking strength, and this information should
be available from suppliers. All but the smallest and simplest of frames will require some
braces to provide racking strength within the frame design, if only to keep the frame up-
right until the hempcrete has been placed, but thanks to the rigidity provided by the hemp-
crete, the bracing required will be less substantial than it would have needed otherwise.
It should be remembered, however, that the racking strength provided by the hempcrete is
only as good as the workmanship of the hempcrete installer. If this was poor, i.e. with little
attention paid to the placement of the hempcrete at a correct and consistent density and
tightly against the structural members, then the racking strength it provides might be less
than expected.
Chapter 22 looks in more detail how racking strength can be achieved (see page 319 ).
Nature of the material
The fact that hempcrete is a plant-based material will pose a challenge to most UK archi-
tects and designers, who may be familiar with the use of timber and thatch in buildings
but, with the possible exception of straw bale, are unlikely to have come across the use
of organic materials to construct walls. The use of a lime, or lime-based, binder will also
be unfamiliar to most of those involved in conventional masonry construction, and brings
other factors that need to be taken into consideration.
There are a few simple but necessary differences in the design principles that should be
applied when using hempcrete as opposed to conventional building materials. Taking these
into account should prevent the construction of buildings that are destined to work badly
because the basic nature of the material was not properly understood at the design stage.
The key factors are explored below.