If the framing plan is drawn accurately to scale, the position of concentrated loads can be determined by scaling. In the case of short girders with heavy loads, however, a slight error in deter mining the position of loads would appreciably affect the result; hence it is necessary to exercise caution in scaling the position, to avoid any chance of great variation from true measurement.
Beams and girders carrying elevator machinery should have the loads and their position determined with special care. To this end the layout of the company- installing the machinery should always be used in final calculation. This layout gives the loads at the different points; and therefore the exact position on the supporting beams, and the reaction on the girders, can be determined. As elevators are liable to cause a shock in sudden starting and stopping, it is customary to multiply the total load by two to allow for this shock.
In the calculation of the girder the laws of some cities allow a reduction amounting to a certain percentage of the live load, on the assumption that the whole area adjacent to a girder is not likely to be loaded to its maximum at the same time. This, how ever, should not be done in warehouses, nor where on the other hand the assumed loads are very light; and in any case it should be done with discretion.
Lintels. The size and character of lintel beams depends (1) on load to be carried, (2) on arrangement of openings over beams. (3) on practical considerations of construction.
If the wall is solid above the opening for a height greater than the span of the opening, the masonry, if of brick, will arch to some extent and thus relieve the lintel of a portion of the load. Practice varies in the proportion of load assumed to be carried. It is good practice to consider the weight of a triangular section of wall, of height equal to the span, as carried by the lintel. If there is only a small pier under the ends of such a lintel, however, this arch effect should not be considered, but the full load of masonry provided for. In very wide openings, also, the full load should be calculated on the lintel. The basis for assumption of arching effect is that brickwork can be corbeled out at an angle of about and support safely its own weight after final set in the cement has taken place. This assumption should not be made where the center of gravity of such mass of masonry will fall out side the supporting base. The figures below will illustrate this
principle.
Another assumption sometimes made is, that the wall span ning the opening is capable, as a beam, of carrying a certain por tion of the load, and that the lintel need be calculated only for the additional weight. This is necessarily dependent on the tensile strength of the mortar joints, which, although being considerable in an old wall, would be very slight in a new wall; and for new work, therefore, this assumption should not be made.
The arrangement of openings above the lintels often makes it necessary to provide for the full load of wall, because this load is carried in the direct line of piers to the lintels. Such cases are illustrated by the figures below.
The particular form of lintel will depend not only on the load, but on the way in which the metal must be distributed in order to carry the load. A very thick wall may necessitate a number of beams or other shapes to provide neces sary width on which to lay the brickwork. If the stone or terra cotta facing has to be supported, this also necessitates special shapes to meet the requirements. More over, if floor loads are to be carried, the size and shape will be largely fixed by this further condition. A lintel may, therefore.
consist of a number of different shapes of different sizes. The problems below illustrate types of condition ordinarily met with.
Beam Plates. Beams and girders carrying ordinary loads, usually have plates under the ends resting on the walls, in order properly to distribute the load on the masonry.
The method of determining the proper size and thickness of such plates is as follows: In Fig. 78, / = dimensions of plate in inches transverse to web of beam; dimension of plate in inches in direction of web of beam ; b = width of flange of beam.
The plate should cause the load to be uniformly distributed on the masonry over its whole area.
The portion of the plate not covered by the flange of beam is in the condition of a beam fixed at one end and free at the other. The formula for the moment, therefore, is: considering a strip 1 inch in direction of web of beam; but from the formula for beams, The safe bearing on masonry has been specified in the chap ter on Building Laws and Specifications.