Lighthouse Structures

LIGHTHOUSE STRUCTURES The structures of lighthouses may be divided into two classes, (a) those on rocks, shoals or in other situations exposed to the force of the sea, and (b) the more numerous class of land structures.

Towers.—In determining the design of a light house tower to be erected in a wave-swept position consideration must be given to the physical features of the site and its sur roundings. Towers of this description are classified as follows: (I) Masonry and concrete structures; (2) Openwork steel and iron-framed erections on pile or other foundations; (3) Cast iron plated towers; (4) Structures erected on caisson foundations.

(I) Masonry towers are generally preferred

for erection on wave-swept rocks affording good foundation, and have also been constructed in other situations where adequate foundations have been made by sinking caissons into a soft sea bed. Smeaton's tower on the Eddystone rock is the model upon which most later designs of masonry towers have been based, although many im provements in detail have since been mace. In situations of great exposure the following principles in design should be observed: (a) The centre of gravity of the tower structure should be as low as possible. (b) The mass of the structure superimposed at any horizontal section must be sufficient to prevent its displacement by the combined forces of wind and waves without dependence on the adhesion at horizontal joint-faces or on the dove-tailing of stones introduced as an additional safeguard. (c) The structure should be circular in plan throughout, this form affording the least resistance to wave stroke and wind pressure in any direc tion. (d) The lower portion of the tower exposed to the direct horizontal stroke of the waves should, for preference, be con structed with a vertical face. The upper portion should be either straight with uniform batter or continuously curved in the verti cal plane. External projections from the face of the tower, except in the case of a gallery under the lantern, should be avoided, the surface throughout being smooth. (e) The height from sea-level

to the top of the tower should be sufficient to avoid the obscura tion of the light by broken water or dense spray driving over the lantern. (f) The foundation of the tower should be carried well into the solid rock and secured to it. (g) The materials of which the tower is built should be of high density and of resistant nature. (h) The stones used in the construction of the tower, at any rate those on the outer face, should be dove-tailed or joggled one to the other in order to prevent their being dislodged by the sea during the process of construction and to afford additional stability. Of late years, cement concrete has been used to a con siderable extent for maritime structures, including lighthouses, either alone or faced with masonry. Reinforced concrete has also been employed.

(2) Many examples of openwork steel and iron lighthouses exist. Some typical examples are described hereafter. This form of design is suitable for situations where the tower has to be carried on a foundation of iron or steel piles driven or screwed into an insecure or sandy bottom, e.g., on shoals, coral reefs and sand banks or in places where other materials of construction are exceptionally costly and where facility of erection is a considerable advantage.

(3) Cast-iron plated towers have been erected in many situa tions where the cost of stone or scarcity of labour would have made the building of a masonry tower excessively expensive.

(4) Cylinder or caisson foundations have been used for light house towers in numerous cases where such structures have been erected on sand banks or shoals. A remarkable instance is the Rothersand tower. Two attempts have been made to sink a cais son in the outer Diamond Shoal off Cape Hatteras on the Atlantic coast of the United States, but these endeavours have proved to be futile.

Famous Wave-swept Towers.

The following are brief de scriptions of some of the more important wave-swept towers in various parts of the world.