Civil Engineering Reference
In-Depth Information
the top and base of the tower, he replaced the area of members in the lattice with solid
surfaces with the same enclosed area. In the middle section where the tower
Figure 11.1
Drag coefficients for square
towers with flat-sided members.
solidity is lower, he assumed a frontal area equal to 'four times the actual area of iron'.
These very conservative assumptions, of course, resulted in a very stiff structure with no
serviceability problems in strong winds.
Eiffel constructed a laboratory at the top of the tower and carried out various scientific
experiments, including measurements of the deflection of the tower, using a telescope
aimed vertically at the target at the top. Some of these measurements were later analysed
by Davenport (1975). These indicated that the effective drag coefficient used in the
design was approximately 3.5 times that required to produce the measured deflections,
and currently used in design for a tower with a solidity of about 0.3 (see Figure 11.1).
Later on the tower, Eiffel, perhaps concerned with the over-conservatism of his
designs, carried out some experiments on wind forces on simple plates.
Development of high-voltage power transmission, and radio and television
broadcasting, from the 1920s onwards promoted the efficient use of steel for lattice tower
construction.
11.2.2
Tall chimneys
In the nineteenth and early twentieth centuries, most factory and power station chimneys
were of masonry construction. With the known weakness of masonry joints to resist
tension, these structures would have relied on dead load to resist the overturning effect of
wind loads. Although undoubtedly many of these failed in severe windstorms, Kernot
commented in 1893 that: “…there are thousands of such chimneys in existence, many
in very open and exposed situations, which, apart from the adhesion of the mortar,
would infallibly overturn with a pressure of not more that 15 pounds per square foot”
