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IRONWORK REPAIRS IN TIMBER-FRAMED BUILDINGS.
Why do timber frames
2.3 Loading failure
- timber unable to take load or is later overburdened.
Loading failures are
relatively uncommon in early timber-frames. The sections are over-engineered
with nearly square sections and the armature of the external cladding and
the roof battens normally prevents any failure under normal load. Loading
failures usually occur after the timber has been sufficiently weakened by
decay, shrinkage or by reduction in section by removal of timber.
A typical loading failure
would result in racking along one or more axes. The loosened pegged joints
allow sufficient rotation for the timbers to move under their own weight.
The interior of the granary at Cressing Temple. The pegs in the butt purlin
roof have all loosened over the years since it was erected in 1623. The building
has badly racked and successive timber windbraces and iron tie rods have been
inserted to arrest the movement.
2.4 Movement - thermal
- weather/ wind - accidental - changing loads
(new roof/ wall materials).
Movement accounts for
most joint failures in timber framed buildings. Even in a building that is
well protected from the buffeting of the weather it will be subject to movement
from the changing forces of the wind, rain and snow on its roof structure.
Appendix 4 contains details of a series of observations of the normal movement
of a timber-framed building. It can seen that the cyclical movement is constant
No building is free from
thermal movement. External temperatures and heat generated from within by
heating systems, cooking, electrical appliances and body heat all lead to
constant thermal movement of the frame as its elements expand and contract.
Extremes of changing temperature can actually lead to catastrophic failure
from the build up of internal stresses within a localised point. (Fig 2.10).
A cracked beam in The Rectory, Kelvedon Hatch. The calamitous failure (highlighted)
was caused by its proximity to a powerful modern stove which, over a period
of 20 years, heated and cooled the timber to beyond its endurance.
The walls and floors
will also be subject to the movement induced by the occupants of the building
walking, opening and closing doors, moving and placing furniture and other
activities. Accidental impact loads will also affect the frame in varying
degrees, from a slight knock to a vehicular impact to a falling tree.
Changing the loads that
the building was designed for will also lead to movement. Inserting a new
floor or firestack, re-roofing, extending or remodelling will all induce movement.
Subsidence or ground heave may affect the frame. The modern practice of underpinning
always leads to movement in a timber framed building usually signified by
cracking render. (For this reason Historic Buildings Advisors in Essex will
invariably oppose underpinning). Changing the external cladding from one type
to another will also lead to movement. (Fig. 2.11).
A tableau in the Wheat Barn at Cressing Temple shows how the barn was originally
walled with vertical boards. Around 1620 AD the walls were completely dismantled
and infilled with brick nogging. A significant amount of ironwork was then
introduced to arrest the subsequent movement.
Decay and Erosion