Civil Engineering Reference
In-Depth Information
When during the 6th century BC classical Greek temple design was standardised
as a very sizeable long building with a cella say 10 m-12 m broad × 40 m-50 m
long no way could be found to fashion a gabled roof on timber which spanned
freely these dimensions, either longitudinally or transversally. A ridge beam was
the basic requirement and the obvious way to support it was by a row of columns
along the central axis of the building. But this, of course, was specii cally destruc-
tive to the design. To avoid this, therefore, the Greek designers spanned the cella
transversally with horizontal beams and set up upon each at its centre a short post
which supported the ridge beam, thus leaving the long axis of the cella clear. If
however the breadth of the cella was over 5 m-6 m, then again no timbers could
be found to span this dimension freely. h e solution was to break the transverse
dimension of the cella into three parts by providing two lateral ranges of columns.
h en over these three aisles three horizontal beams could be set of manageable
spans (i.e. ca 4 m). h is solution met all requirements. It supported the ridge beam;
it provided interim support to the raking spars via the lateral colonnades, and the
horizontal beams supported a horizontal ceiling which concealed the heavy wooden
spars ef ecting the double pitched roof. However the strength of this system rested
entirely on the resistance in tension of the horizontal beams at mid span where
they were placed in bending by the props of the ridge beam.
To this device an alternative system was available in antiquity which, according
to statical analysis, could employ wooden members to roof over very extended
spaces. h e device was that of the wooden truss and the question is how early this
device was used. It was certainly known in Hellenistic building and may be known
during classical times in South Italy (Hodge, pp. 38-44
et pass
).
h e principle of the truss is based on the fact that a triangle does not deform
under stress—one side cannot deform individually. All sides share in sustaining
the load, thus each member of the truss can be of slighter section. Furthermore
the ei ciency of the truss can be increased by insetting cross members into the
basic triangle thus articulating it into a series of compound triangles, each of which
is correspondingly non-deformable. Another way of visualising the statics of the
truss is to regard it as a gable. If the statical behaviour of a gable loaded by a roof
is examined, it will be found that the lines of force (stress) are not transmitted
uniformly downward throughout the area, but follow around the margins of the
triangle leaving the interior as a neutral zone. A truss may then be considered as
a fretted out gable retaining only the parts of the structure under stress (Sharma
& Kaul, pp. 304-16)
Bearer
beams &
trusses in
Classical
Greek
rooi ng
134a
134b,
136
3.
Reinforcing
Wood because of its strength in both compression and tension (i.e. it can serve
either as a tie or a strut), its ready availability, and above all, its workability is
