Beams and Girders

If two or more beams spaced close together were used, then b in the above formuhe 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 sepa rators 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 twofold; (1) to pre vent lateral deflection of the beams under the loading ; (2) to dis tribute the loads equally between the beams when the loads are not symmetrical on the two beams, and to cause the beams to de flect 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 : Operations : 1. Find from the tables in Cambria the moment of in ertia of the beam.

2. Figure the bending moment clue to all the concentrated loads, and the uniform load in inch-pounds.

A more direct method would be to find the value of S (see Cambria) and dividing Al by S which would give the required fibre stress.

To find what load, uniformly distributed, will be carried by a given beam at a given fibre stress.

Data required : 1. Length of span, center of bearings.

2. Allowed fibre stress.

3. Size and weight per foot of beam.

3. Find the value of the beam in bending-moment foot pounds by dividing the result obtained under operation 2 by 12.

4. Find the value of W in the formula in which W = the total load in pounds uniformly distributed which the beam will support: M= the bending moment in foot-pounds ; and = length of span in feet.

To find the size of beam required to carry a system of known loads at a given fibre stress.

Data required : 1. Length of span, center to center.

2. Allowable fibre stress.

3. The amount and character of load on the beam. Operations: 1. Figure the bending moment in inch pounds due to Nil the concentrated loads, and the uniform load.

2. Divide the bending moment in inch pounds by the specified fibre stress, and the result will be the required section modulus, S.

3. Select from Cambria a beam having the required value of S.