CAMBER BEAMS, those which are cut with an obtuse angle on the upper edge, forming a declivity each way from the middle of their length ; they are used in truncated roofs, where, after being covered with boards, the boards are again covered with lead, in order to discharge the rain-water towards each edge of the flat, or platform.
Cambered beams are employed in a multitude of situations where great strength is required. All beams which are so situate as to be subject to cross-strain should be cambered. Instances of cross-strain occur in bressummers, which are loaded with a wall, and of course are most affected by the gravity of its materials where the bearing is greatest, which will be in their mid-length. A weight applied in this man ner will have the effect of pressing the centre below the level of the ends of the beam, and thus fracturing the super incumbent wall ; and besides this, will tend to snap and tear asunder the timber ; and although, on account of its great scantling, such an event rarely, if ever, occurs, yet it strains the beam in the direction of its length, a test which timber should not be subjected to. Moreover, in all eases where beams of any great length are employed, the gravity of the timber itself will weigh them down midway, even where they are subjected to no additional weight, as in the case of the tie-beams of a truss. In all these instances the diffi culty may be obviated by cambering the timber upwards.
This method not only ensures that the beam shall be level after settlement, but entirely alters the nature and operation of the force ; for whereas previously the beams were strained or extended, this tension, by the employment of a camber, is changed into a pressure, so that the tendency instead of being to tear the particles asunder, and thus weaken or break the timber, is rather to press them more closely together, and render the beam firmer and more compact.
Further, all timber is liable to shrinkage by the evapora tion of the moisture which is always present in a greater or less degree, and thereby becomes of smaller dimensions than when first inserted in a building. This defect may be recti fied as far as the length is concerned, by cambering to such a degree, that when the wood is completely dry, it may fall into a horizontal position, or nearly so. The extent to which the beam should be bent is a matter of nice calculation, and the regulation of it must be left to experience. In trusses the camber of the tie-beam should not be too great, as if so, it will tend to thrust out and derange the principals. When bressummers occur one above another, the higher ones should be cambered to a greater extent than those below, the camber increasing in direct proportion to the number of bressummers beneath it.