5. Always avoid using special shapes and weights if time of delivery is any consideration, even at the expense of a little extra weight, unless special arrangement is made in advance as to the delivery which can surely be made It is inure important to avoid the delay that would hinder progress in all branches of the work on a building through waiting for a few pieces of steel, than it is to save a few pounds by the use of special shapes and weights.
Since all steel designs are dependent upon the use of the fore going shapes, it will be seen that it is necessary to refer constantly to tables containing their dimensions and other characteristics called " properties." This term " properties " covers all the character istics which determine strength, and which are illustrated by the tables.
The different steel companies issue different books, but the properties for all standard shapes are practically the same.
Before proceeding to a discussion of the use of tables, a caution should be given for the future guidance of the student. There is always danger in using tables, diagrams, and prepared by someone else. The danger is from two sources : (1) the information given may not be correct; and (2) the person using the data may, through failure to understand fully the basis on which they were prepared, use them where they are not applic able.
As regards the first point, the more authoritative the book in which the information is given, the greater is the probability that it is correct. Not everything in print, however, is reliable.
The second point is even more important, because in the case of almost every table, diagram, or formula, there are certain limi tations to its use, and certain cases to which, without a full under standing of these limitations, it is liable to be applied incorrectly.
From the outset the student should form the habit of investi gating the derivation of tables and diagrams and the basis of for 'nuke in order that he may use them intelligently. The basis and application of the fundamental formulae can be understood without necessarily retracing all the steps in their derivation. There are many special formulae given which are simply modifications of the fundamental formulae adapted to special cases, and such formulae should never be used without tracing their derivation from the fundamental formuloe.
Safe Loads. Table I gives the total loads, uniformly distrib uted, which can be safely carried by the different sections of beams and channels for spans varying by one foot.
The manner in which the problem of the safe load will gener ally come up is : Given a certain load per linear foot of beam, and 'a certain span, to find the required size and weight of beam. In this case
the total weight is obtained by multiplying the clear span by the weight per foot and adding the weight of the beam. As it is necessary to know the size of the beam before its weight can be added, this operation must first be neglected, and the size provi sionally determined from the tables showing what sections will carry the superimposed load. Then acid the weight of the selected beam, and again refer to the table to see if the capacity has been exceeded by the addition of the weight of the beam. If it has, a different section must be taken.
It is important to note that there is in general a difference between the length of spans used in computing the total load carried and that used in the table. These tables are compiled from results given by the use of the regular beam formula, which has been explained, and in this formula the length of span is the length between centers of bearings. It is this length which should be used in referring to the tables.
In some cases there would be practically no difference, as in the case of a beam framed between two steel girders. If, however, the beam were built into brick walls, the span used for computing total load would be the length between inside faces of walls, whereas the span used in tables would be from center to center of bearing plates.
Another point to be noticed in the use of these tables is that they are based on the supposition that the beam is supported by adjacent construction against lateral deflection. As will be more fully noted later on, long members under compression fail by deflecting sideways. In order, therefore, to be able to carry the full load indicated in these tables, the top flange of the beam or channel must be held against side deflection. This may be accom plished in a variety of ways. If the beam is in a floor or roof, the fireproof arches and the rods will generally provide the necessary support ; or, if it is in a building not fireproof, the wood beams or the planking will also accomplish this. If, however, the beam was used in an unfinished attic, and the ceiling construction was at the bottom flange, leaving the rest of the beam exposed, the load must be reduced as indicated by the auxiliary table of proportionate loads. The load would also have to be reduced in the case of a beam carrying a wall with no cross framing at the level of the beam. It is, therefore, of the first importance to know exactly how the loads are carried by the beam, and in what relations other parts of the construction stand to the beam.