Civil Engineering Reference
In-Depth Information
fuselage and wings. Some aircraft hangars were of timber construction and utilised record
breaking large span small timber section trusses with bolted joints.
After the Second World War shortages of materials resulted in a licence being required for
all new building works, making economy in use of paramount importance. Imported ma-
terials such as timber were very much at a premium and TDA was given the task to find
ways of economising on the country's use of timber. Quite correctly they identified the
roof structures of buildings as a high volume user of timber and developed a design for a
domestic roof using principal trusses constructed of small timber sections connected with
bolts and metal connector plates. The roof used purlins and common rafters similar to the
systems previously discussed. These trusses became known as 'TDA' trusses, and with
some minor modifications are still in use today. It appears that some of these designs were
available shortly after the Second World War but were first published as a set of standard
design sheets around 1950.
The designs were based on existing truss shapes but were not engineered in the sense that
structural calculations were prepared for each design. Load testing on full size examples
of the truss was used to prove their adequacy and from these tests other designs deve-
loped.
Standard Design Roofs
The first designs produced were known as 'A' and 'B' types, dealing with 40º and 35º
pitches respectively. They covered spans up to 30 ft (9 m).
House design fashion changed during the later 1950s and early 1960s, demanding lower
roof pitches. 1960 saw the introduction of the TDA type 'C' range for pitches between 22º
and 30º. Spans were also increased up to 32 ft (10.8 m). Around 1965 the types 'D', 'E' and
'F' ranges were published; these later designs using a slightly different truss member lay-
out went down to 15º pitch and up to 40 ft (12 m) span. Further designs used trusses
spaced at 6 ft (1.8 m) centres and had some degree of pitch and span flexibility within spe-
cified limitations.
