Civil Engineering Reference
In-Depth Information
Strength and stability
Timber frames in heritage buildings
were designed on a rule of thumb
basis founded on long experience.
Nearly all features were
overdesigned, promoting longevity,
and safety factors in consequence
were large though undetermined.
In modern timber frame, to a
considerable degree, frame and
cladding act together to provide the
required strength and stability to the
walls. Exploratory tests on single
frame elements and small areas of
brickwork indicate a substantial
strength contribution even from
unreturned brickwork
(141)
.
Based on the evidence of the BRE
studies, most system built timber
frame dwellings constructed between
1920 and 1975 have no serious
structural faults. In dwellings built
between 1945 and 1965, no instances
of the frames of dwellings being
compromised on structural stability
grounds have been
observed by BRE, although some
Calders have been subject to
differential settlement. Decay in the
structural frames of Scottwoods
could compromise structural
stability if nothing is done about the
decay observed. Most timber in the
structural frame is potentially
vulnerable to decay if the moisture
content remains above 22% for
comparatively long periods, and
some softwood species are at risk
at 20%.
In other systems the most common
defect affecting structure is nailing
error, observed in several systems.
Where this occurs to a significant
proportion of fixings in any particular
area, integrity may be reduced.
Misplacing is not easy to spot on
inspection unless suspicions are
aroused because of some visually
apparent distress in the structure; for
example following high winds.
Structural rigidity of the frame in
timber, low-rise post-1965 systems
is normally provided by the
plywood or fibreboard sheathing.
There have been some cases of
decay of this sheathing, particularly
in Frameform dwellings.
By far the commonest fault in this
category of dwellings has been the
lack of adequate provision for
differential movement between frame
and brick cladding. Although
structural performance is rarely
compromised, there have been cases
where transfer of roof loads has taken
place from frames to what should
have been non-loadbearing outer
leaves (Figure 3.35).
There have been cases of missing
wall ties, particularly at gables, and
some cases of walls, especially gable
peaks, being damaged by wind
suction. Spooners suffer particularly
in this respect, showing as bulging
wall panels. Replacement of wall
ties will be needed where
inspection (eg by optical probe)
reveals this deficiency.
The structural use of wood based
panels, for example for sheathing
purposes, is discussed in BRE
Digest 423
(142)
.
Case study
Cracking in the brick veneer of timber
framed housing
Following some disruption in the external
cladding of some timber framed houses, the
BRE Advisory Service was asked to
investigate. The front and rear elevations
were either brick or rendered blockwork
clad to first floor height, above which was
vertical tile hanging, with an aluminium cover
to the top of the brick and blockwork. The
gable ends were clad with brickwork or
rendered blockwork to the full height.
Cracking was reported some three years
after the dwellings were occupied. Also
there had been a considerable deterioration
to the brickwork, ie erosion of the mortar
and cracking in the mortar joints. There had
been cracking of the rendering on the
blockwork and an outward movement of the
gable end, such that the overhanging verge
on some dwellings was no longer present.
Some of the cracking was seen to be
typical of sulfate attack, which was
confirmed by laboratory analysis. However,
other movements in the external leaves
could not be explained by this mechanism
alone, and more investigation on site
revealed two further problems - a lack of
sufficient wall ties, and differential
movement between the timber frame, which
had shrunk, and the brick cladding. This
movement had not been allowed for by using
soft joints.
The gable end brickwork to one property
was removed to expose the wall ties,
building paper and plywood sheathing. The
first three layers of wall ties from the DPC
were of the chevron type fixed to the timber
frame studwork. Above this level, butterfly
ties had been used. They were fixed to the
studwork with one or two nails bent over the
wire section of the tie, which was then bent
to lay on the mortar joint. These ties were
quite inadequate for their intended use.
From the number of ties actually counted,
compared with what would normally have
been specified at 600 mm horizontal
centres and 450 mm centres vertically, only
30% of the required ties were actually
present.
Installation of extra ties was required to
restore the integrity of the structure. Since
the differential movement had all taken
place by the time of the inspection, re-
sealing the disrupted joints would be
sufficient to restore weathertightness.
Gap develops
over r oof wall
plate
Dimensional stability, deflections
etc
Infilling to the early oak frames with
brick nogging, in either herringbone,
stretcher or stack bond, was better
able to resist distortion of the frame
than wattle, but considerable
movements could still take place
without threat of collapse (Figure
3.36). Sometimes the wood was left
to weather naturally, and sometimes
was given a coat of tar, largely for
cosmetic reasons.
Local crushing of
lower edge of
projecting rafter
Figure 3.35
Roof loads transferred to the non-
loadbearing outer leaf
