Sheds for the reception of dutiable goods should be provided with a small office, or lock-up, in the interior for the use of the Customs'

Figs. 357, 358, 359, and 360.- Details of Shutter and Mechanism.

SECTION OF
SHUTTER.

Authorities. Public conveniences, including urinals and w.c., are useful adjuncts.

Lighting.--Single storey sheds are best lighted from the roof, either by glass tiles, skylights, or lanterns. Artificial light is also necessary for night time, and during the short days of winter. Gas may be burnt in the form of sunlights, as shown in fig. 372, suspended from the roof by chains, by means of which the frame can be lowered for cleaning purposes. Electricity is a common illuminant, and there are other systems, such as the Kitson light (burning petroleum vapour), the Lucigen light, acetylene, and others, into the relative merits of which it is unnecessary to enter here. The lower floors of sheds more than one storey in height, will necessarily derive their natural light from the sides, either through windows or glazed panels in the doors.

Shed Floors.—The nature of the material employed for the formation of shed floors is of some importance. The area may be paved, flagged, asphalted, tiled, concreted, or timbered, but it must be borne in mind that the dust arising from the wear of a stone surface is exceedingly detrimental to cargoes consisting of cereals. On the other hand, timber platforms are hardly suitable where there is vehicular traffic within the shed, and, from the point of view of fire prevention, their introduction is not to be commended. So-called asphalt floors, consisting of macadam bedded in tar, are flexible, and do not crack or fracture under concentrated moving loads, as sometimes occurs with floors of more rigid materials laid upon a yielding foundation, but their very plasticity is an objectionable feature in warm climates and in situations exposed to the direct heat of the sun's rays. Natural asphalt forms a smooth, hard, and durable surface. This and a granolithic surface, composed of equal parts of Portland cement and crushed granite, will be found to yield the least amount of dust from attrition. But the former is expensive, and the latter is only adapted to the conditions of ordinary trucking. Where vehicular traffic is heavy, a pavement of granite or whinstone setts, laid in cement on a bed of rock rubble and concrete, will generally be found the most serviceable.

Sheds of more than one storey should have upper floors of fireproof, or, at any rate, of fire-resisting material. For this purpose combinations of iron or steel and concrete are generally employed. And as this department of shed construction is of a very important character, some of the more prominent forms will be briefly noticed.

The first and earliest type was that formed of a series of iron girders connected by brick arches, the upper surface being levelled with concrete.

A later example (fig. 361) is that of a floor, formed by buckled iron plates, rivetted to the upper flanges of plate girders. A concrete covering forms a bed for Staffordshire blue tiles, 14 inches thick. In the instance selected for illustration the iron plates are 52 inches square.

A third form of floor, shown in fig. 362, consists of a series of rolled steel joists, 6 by 3 inches, bedded in concrete at a uniform distance apart of

26 inches. The upper surface is of granolithic concrete to a depth of 2 inches. The main joists are 12 feet apart.

The foregoing examples constitute very heavy types of floor, in proportion to their strength. With a view of minimising the amount of material, and reducing the cost of construction, various systems have been proposed in recent years, chiefly founded upon the intimate incorporation of iron or steel and concrete in one mass, and in such a way that each exercises its characteristic strength to the best advantage. One or two of the more important of these systems may advantageously be described, as there can be little doubt that the combination of these two fire-resisting materials is capable of effecting a great and useful saving in structural weight. 1/4" Staffordshire tiles

Monier System. The Monier system consists of a network of metal bedded in a concrete slab, the network being formed by two rows of bars or wires crossing one another at right angles. The lower row are the stressed bars. They are intended, in flat floors, to relieve the concrete of its tensile stress, and consequently are proportioned in number and size to the load to be carried and the amount of span. In arched floors they assist in taking up the compressive stress. The function of the upper row of bars is merely to distribute the pressure evenly, and they are generally made three-fourths of the diameter of the lower bars. The floor is divided into bays by a series of iron joists, upon which the network is laid. It is recommended that the width of the bays should not be too small. "Fairly large spans enable the supporting joists to be more economically designed, on account of a better proportion of depth to length being obtained." At the same time, the floor must not be made unduly deep or it will prove an arrangement of dubious economy. "The minimum thickness of the concrete, Beer on "The Monier System of Construction," Min. Proc. Inst. C.E., vol. cxxxiii.

under ordinary conditions, considered exclusively of any wearing surface, may be 1 inches for flat floors and interior roofs and 2 inches for arched floors and exterior roofs, while 3 and 8 feet may be considered as minimum spacings for flat and arched floors respectively. Arched floors are generally constructed with a rise of only one-tenth the span; the thrust, where much weight is supported, is therefore considerable. Provision for the thrust may be supplied by tie-rods in the end bays of a floor or by horizontal end girders suitably anchored to the walls-the latter method, where possible, being preferable. Further, when a series of arches succeed one another, care should be taken that their centre lines meet on the vertical centre lines of the girders which carry them, for a very small divergence will cause an appreciable tendency to twist. This tendency may be further guarded against by embedding the girders in concrete. It is customary with ordinary flooring arches (which probably partake more of the nature of a girder than an arch) to allow a series to finish with its end member resting simply on a brick corbel; this should not be attempted with Monier arches, but a shallow, wide joist should be used as a wall-plate."

Examples of floors constructed on the Monier system are reproduced in figs. 363 and 364, from Mr. Walter Beer's paper, from which quotations have been already made, and in which the student will find a very interesting investigation of the nature and amount of the stresses set up in the various parts. These stresses, which have engaged the attention of several eminent mathematicians, are too complicated for analysis in these pages.

Joints are formed by causing the ends of the bars to overlap by a certain amount, which depends on the tensile strength of the bars and the coefficient of adhesion between iron and concrete, the latter being about 300 lbs. per square inch of surface. After the bars have been laid the concrete is deposited in layers, not less than 1 inches thick, and well rammed. Thin slabs need a closer mesh than thick slabs, owing to there being greater liability to local failure.

* Beer on

"The Monier System of Construction," Min. Proc. Inst. C.E., vol. cxxxiii.

The lightness and slenderness of the floor call for the best materials and the most careful workmanship. The concrete should be composed of the best Portland cement, with an aggregate of broken brick or clean gravel and sand or crushed granite, in the proportion of 1 to 3. The best metal for the bars is hard steel; a soft iron does not possess a sufficiently high coefficient of elasticity. "Expanded metal," which is a network sheared out of a solid steel plate, may be used instead of disconnected bars.

Hennebique System.-This system differs from that just described more in detail than in principle. There is the same network of bars, but the meshes are larger, the bars thicker, and the parts are generally set obliquely with reference to the supporting beams. These beams are themselves constructed on the same system as the flooring.

Figs. 365 and 366 are the plan and section, respectively, of a bay of Hennebique flooring. It will be seen that the main beam is composed of

*

three vertical rows of bars, each row containing two bars, of which the lower one is straight and the upper curved. These bars are bedded in concrete of a rectangular section, adhesion between the parts being assisted by U-shaped clips of hoop iron, which enclose the bars and extend almost to the upper surface of the beam. The model is that of a trussed beam. The concrete takes the compressive duty; the bars are simply tension rods.

The ends of the bars are either turned up or split to a fish-tail to increase the hold.

The floor illustrated has its beams 8 feet 4 inches apart, centre to centre. The latter are 8 inches wide by 14 inches deep. The floor is 5 inches thick, and was tested to a uniform load of 183 cwts. per square yard.

Hope on "Construction in Fortified Concrete," Min. Proc. L. E.S., vol. xxii.