This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
If Dead Lond
Intermittent Loads (off-and-on continuously.)
If in one direction only.
If in opposite directions.
Wrought Iron and Steel.
Wrought Copper and Brass, also
Cast metals :
Brass, Lead, etc.
By combining this table with the safe stresses given in Tables IV and V, that is, taking whatever part of the safe-stress there given for dead loads, that the nature of the load demands, we can obtain the safe-stress under any manner of loading. Where stresses in opposite directions take place, the material will yield in the direction of the weakest stress.
Thus, if we have a bar of American cast-iron, according to Table IV it will be safe under a steady (static) compressive strain of 15000 pounds, per square inch, or a steady tensional strain of 2500 pounds, per square inch.
If the compressive strain were constantly and entirely removed and then put on again, but without shock, it would be safe, according to Table XXXII, to use but 1/3 of this amount of 5000 pounds, per square inch; while under the same circumstances only 833 pounds would be safe in tension. If the strain were constantly removed and then put on again with shock, that is suddenly, or kept moving, only one-sixth would be safe or 2500 pounds per square inch, in compression, and 417 pounds in tension.
If the strain were alternately compression and then tension, but put on without shock the same strains would be safe; the safe strength of the bar would, therefore, be measured by the weaker of the two and would be only 417 pounds, per square inch. If the strains alternated between compression and tension and besides this were dynamic (put on suddenly) only one twelfth would be safe or 1250 pounds in compression and 209 pounds in tension, both per square inch. The strength of the bar under these circumstances would, therefore, be only 209 pounds, per square inch.
How to use Table xxxii.
For wrought-iron we have the same safe-stress, whether in tension or compression ; for a dead, constant load we should use then, from Table IV, 12000 pounds, per square inch; from Table XXXII we should have for intermittent loads, in one direction only, that is of the same nature, but put on-and-off continuously, but without shock, two-thirds or 8000 pounds, per square inch. If put on-and-off suddenly 4000 pounds, per square inch. The same if constantly reversed from compression to tension but done slowly and without shock. If constantly reversed and it is done suddenly or with shock the safe strain per square inch would be only 2000 pounds.
When a strain on a beam is never completely removed, but changes constantly from a larger to a smaller strain, both however in the same direction, the effect is not so great as where both strains are at times constantly removed.
In such cases the effect can be found from the following Formula:
w = (w1 - w11).x+w11
Where v = the corresponding dead load, or constant strain in pounds, per square inch, to produce the same effect as one alternating between two unequal loads or strains w1 and w11, but both in the same direction.
Where w1= the larger of the alternating loads or strains, in pounds, per square inch.
Where w11= the smaller of the alternating loads or strains, in pounds, per square inch.
Where x = 3,0 for cast-iron, and=1 1/2 for wrought-iron and mild-steel.
The above condition would frequently happen in the case of warehouse floors, bridges, and other places where there is a constant dead load, which at times is increased by other loads to be carried.
If the additional load is put on suddenly, or is a moving load, it should be doubled. In that case to, would be equal to the doubled dynamic load plus the static load, and w11 would be equal to the static load.
In all designs of metal structures great care should be taken to design all parts, not only of practicable shapes and sizes, but of dimensions that will not involve increased cost.
As a rule cast-iron, wrought-iron and steel are estimated at a certain price per pound. If the sizes are not unusually small nor unusually large the price will be the usual one. If, however, the sizes are very light the price will be greatly increased, for two reasons. The cost of preparatory work, working drawings, office expenses, models, etc., will be practically the same as for heavier work; the handling and labor will be very nearly the same, and in very light construction all this must be borne by fewer pounds and the price is consequently greater. On the other hand, if the pieces are unusually heavy, or large, or long, they may require special rolling or casting, and may require special cars and freight arrangement, special trucks, derricks, etc., or they may be very difficult to manufacture and involve much loss, many misgoes, etc.
The architect should, therefore, as far as possible design so as to use standard sizes.
In castings as well as in mill-work the standard will vary more or less with the parties doing the work. It will be impossible, therefore, to give here any universal standard. A few hints, however, may help the architect in economical designing.
For hollow-castings (columns) Planat gives the following as the French standard, the thicknesses being the minimum or smallest thickness possible for the length.
lons not less than
long not less than
long not less than
long not less than
long not less than
The practice of American iron-works is to regulate the thickness with the diameter rather than the length. Thus our iron-works make columns under 6" diameter about 1/2" thick, those over 6" diameter from 5/8" thick and upwards.
For castings made to carry weight the thickness should be more. In columns the maximum thickness should depend upon the possibility of making the core stiff enough to keep its place in the centre.
Large diameter with thinner shell (within reasonable limits) will give the most strength for the amount of material used, thick castings being much weaker, as already shown.
In rolled-work, beams, channels, tees, etc., of unusual length should be avoided. Most sections are rolled to 30 feet in length without extra charge; beyond this there is an additional charge per pound, for every extra five feet or fraction thereof in length, if the section is heavy, up to 40 or 50 feet which is the limit for beams and channels, or up to 90 feet for angles. Then, too, very long pieces (over 30 feet long)-involve being carried on two cars, which means extra freight charges, and very heavy pieces might be refused by many railroads. Where great length is desired it will be better, as a rule, to make it up of two pieces, the extra material required for the "splicing" being, as a rule, much more economical than the cost of manufacturing and handling long and heavy pieces.