Structural Design

STRUCTURAL DESIGN Stone and Marble Staircases.—Modern stone and marble staircases are of two main types. In the first the actual exposed treads and rises form, themselves, the structural elements. In the other type the treads and rises are supported upon a separate structural base which may consist of a masonry vault, a steel framework or a reinforced concrete slab. In the first class, in which the steps themselves are structural, the staircase is usually built into a wall at one side. Each step rests for a small distance upon the step below and is built into the wall for a distance suffi cient to make the weight of the wall above prevent it from turning.

The undersides of the steps, where exposed, are frequently cut to form an inclined surface, following the slope of the stairs, and may be decorated with panelling or carved or other ornament. This type of stair can be used both for straight and curved runs, and is particularly applicable to monumental entrance halls, where the stair, in a triple run with two landings, or in a continuous, curved sweep, fills a recess opposite the door. In such staircases the railing is usually of metal with the uprights securely dowelled into the ends of the steps. Stairs somewhat similar can be built with the ends of the steps built into the walls on both sides, where a monumental run rises between two walls.

Wooden Staircases.

These can be divided into two classes, closed and open string stairs. In both the essential support is given by rough timbers called carriages, which follow the slope of the stairs and are cut roughly to the shape of the under side of the steps. In the case of closed string staircases both sides of the staircase are finished by a sloping, straight-sided plank, ap proximately an inch and a half thick, on the inner faces of which grooves are cut, whose upper and outer faces exactly fit the top and side of the tread and riser; the under and inner sides are, however, sloped, so that wedges may be driven in to force the tread and riser boards into position and hold them with perfect rigidity. Similar wedges are driven in between the rough carriages and the under sides of treads and risers. To give additional strength, the treads and risers are, themselves, rebated together, so that a small, projecting strip of the tread fits into a groove in the bottom of the riser, and a similar strip on the top of the riser fits into a groove on the bottom of the tread. The projecting por

tion of the tread is moulded and an additional small cover mould placed below to cover the joint.

In the open string staircase the strings at the end are cut to the shape of the steps, and the vertical cuts are usually mitred at 45°, so that the ends of the risers, also mitred, may form with them an invisible joint. In the ends of the treads dove-tailed sockets are left to receive the dove-tailed ends of the balusters; a moulding, exactly similar to the nosing, is then applied across the end of the step, and covers the bottom of the balusters. At times, bracketed and carved pieces are mitred at the ends of the risers and applied over the face of the string. It is this type of staircase that was the rule during the Georgian period in England and in the developed American colonial and early republic work. Where an open string stair is built against a wall, it is usually finished at the wall end by a closed or housed string, similar to that described, forming a base-board for the wall.

The structural strength for a wooden staircase is usually fur nished by framing a heavy member, called a header, at the top and bottom of every run; to these headers the rough carriages are spiked, and the newels, if they exist, firmly fastened. The newels themselves are designed to receive the finished stair strings, and also the finished string member that is occasionally used at the edges of a floor in the stair-well opening. In stair cases similar to those of the early 19th century in England and America, in which newels did not exist, except at the bottom, and in which stair rails and balusters were of the most extreme atten uation, and the whole frequently laid out with complex curves, strength was achieved by the most exquisite workmanship, whereby the careful framing of every part made the finished staircase an independently rigid structure. Frequently the curved plan itself furnished a certain amount of lateral rigidity. In mod ern staircases of this type additional strength is obtained by in serting iron or steel bands into the hand-rail; occasional balusters may be of steel, well fastened to the string.