This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
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 60°, 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 outside the supporting base. The figures below will illustrate this principle.
Another assumption sometimes made is, that the wall spanning the opening is capable, as a beam, of carrying a certain portion 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 necessary 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. Moreover, if floor loads are to be carried, the size and shape will be largely fixed by this further condition. A lintel may, therefore.
The plate should cause the load to be uniformly distributed on the masonry over its whole area.
If R = the reaction at wall end, then R/b'l = the load per square inch on masonry.
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:
M = 1/2 p L2 p = R, and L = l-b b'l' 2 therefore M = 1/2x R x (l-b/2)2 b'l considering a strip 1 inch in direction of web of beam; but from theformulaforbeams,
M =fI/y; if, therefore, t = thickness of plate,
= 1/6fbt2; then, since y = t/2, therefore 6M/fb = t2 = 1/2 x R/b'l x (l-b)2/4 x 6/f, since b = 1 which reduces to t2 = 3/4 R (l-b)2/b'lf
If two or more beams spaced close together were used, then b in the above formulŠ would be the extreme distance between flanges of outside beams.
Anchors. Beams resting on brick walls are anchored to these walls. Some of the more common forms of anchors are shown by Figs. 79 to 86.
Separators. When two or more beams are used together to form a girder, they are bolted up with separators. These separators are either bolts running through spool shaped castings of the required length to fit between the webs of beams, or plate-shaped castings made to fit accurately the outlines of the beams and having width equal to the space between webs of beams. The object of these separators is two-fold; (1) to prevent lateral deflection of the beams under the loading; (2) to distribute the loads equally between the beams when the loads are not symmetrical on the two beams, and to cause the beams to deflect equally. The latter function is by far the more important one, and for this purpose the second form of separator is the only one that should be used. Beams over 12 inches deep have, as a general thing, two horizontal lines of separators; beams under 12 inches, one horizontal line.
Figs. 87 to 89 illustrate the different types of separator.
Calculations. To find the actual fibre stress on a given beam supporting known loads:
1. Length of span of beam, center to center.
2. Size and weight per foot of beam.
3. The amount and character of load on the beam.