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.
Shapes of wrought-iron columns.
Figure 281 is more easily riveted up than 280, but is not quite as strong.
Figure 282 shows an elevation with wrought-iron base, and Figure 283 the side view of base. Architects are apt to use cast bases with wrought-iron work, but as a rule a wrought-iron base can (and should) be designed, which will not only be better adapted to wrought-iron construction, but will be cheaper and stronger, and has the merit that it can be riveted fast to the column or other construction.
Figure 284 shows a column made of four Z irons, with central plate. It has the great merit of being a strong column, and though of live parts, it requires only two lines of rivets. This column adapts itself excellently to building walls, as the horizontal wall girders, and floor girders are easily attached to it, and it readily holds the "filling-in walls " in place, without the use of anchors. Then, too, it can be easily covered with fireproof blocks. The writer recently erected in New York City, a fireproof office-building, ten stories high above the sidewalk, with only twelve inch thick brick walls all the way up, by using these columns combined with horizontal girders in all the walls.
Of course, in such construction, proper precautions must be taken to prevent the building from collapsing under wind pressure. In the case above referred to, in order to avoid cross-partitions, the wind-bracing was all done in the front and rear walls, and in the floor levels.
Figure 285 gives a combination column of two tees, riveted together with separators.
Figure 286 is made of four angle bars latticed together, a very light, but strong column.
Figure 287 consists also of four angle bars, which adapt themselves more readily to fireproofing, and require less riveting, but the column is not nearly as strong as the previous one. In the last case separators are used in place of lattice bars.
It would be quite impossible to give any curve tables for wrought-iron construction, but Table XLIX will greatly facilitate the calculation. It will be necessary in each case to find only the ratio of the length of column (in inches) divided by the radius of gyration, or the square of the length, divided by the square of the radius of gyration, and look up the value per square inch of cross-section, according to the condition of ends of column, and the assumed safe value for (c/f) The table is calculated respectively for 8000, 10000 and 12000 pounds per square inch values for (c/f) In buildings use the value
Explanation of Table XLIX.
12000 for wrought-iron.
To find the ratio look up the value of the square of the radius of gyration in the tables, or if it is not given, find the moment of inertia according to rules given in Table I on page 10 of Vol. I, and divide by the area, see page 9, Vol. I.
It should be borne in mind that where pieces are doubled or their number increased along the same neutral axis, the moment of inertia will be doubled or increased accordingly. But the square of the radius of gyration will remain constant, as it simply represents a ratio, and the area and moment of inertia increasing in the same amount, their ratio, which gives the square of the radius of gyration, will, of course, remain constant. In some cases, it will be easier to find the radius of gyration, instead of its square, and in such cases the second column in Table XLIX should, of course, be used. An example will best illustrate the use of Table.
A flat eye-bar of wrought-iron 1 1/2" x6" in a truss, is liable at times to be under compressive stress, what will it safely stand? The bar is 5' 6" long from centre to centre of eyes.
From Table I, section 2, we have
= (1 1/2)2/12 = 0,19
Example of use of Table XLIX.
the area will be a = 1 1/2.6 = 9 square inches, and l2 = 662 = 4356 The ends being eye-bars, we use, of course, in Table II, the value n for "both ends pin ends," or n = 0,00005 Inserting the values in Formula (3) we have for the safe compressive strain, w=9.12000/1+4356.0,00005/0,19 = 50232
Now had we used Table XLIX we should have had the ratio l2 = 4356/ 0,19 = 22926
The nearest value to this in the first column of the Table is 22500 and under the heading " both ends pin ends for a value of ( c/f) =
12000 pounds, we find 5655 which is the safe load per square inch on our bar, or the total safe strain,1 w = 9.5655 = 50895 pounds.
Which closely approximates the above result.
In our case it would have been easier to use the second column of Table XLIX, we should have had
and for the length Z=66 therefore the ratio l = 66/0,433 = 152
1 he nearest value, to this in the second column of Table XLIX is 150 which would give the same result as before.
There are several patent wrought-iron columns made, of which the "Phoenix" column is undoubtedly the best. It is made up of from four to eight segments, riveted together.
1 (Obtained by multiplying by the area of bar.)
Each segment somewhat resembles a channel with the web bent to a segment of a circle, instead of being straight.
Figure 288 shows one of the smaller columns, made up of four segments. For heavier columns each segment is rolled thicker, as shown in the Figure in outline. When it is necessary to have very heavy columns flat pieces are inserted between each flange, as shown in Figure 289. These columns can be readily covered with fireproof blocks to make a circular finish in buildings, and are largely used both for this reason, and on account of their great strength, (owing to all the metal being near the outer edge).
Table L gives the properties of these columns, as taken from the hand-book of the Phoenix Iron Co.
Column G is made in eight sections, column E in six sections, all the others in four sections.
In column A
3/8" rivets are used; in columns B1 and B2 1/2" rivets are used; while in the others 5/8f" rivets are used, until the shell becomes over 5/8" thick when 3/4" rivets are used.
The thickness given is the thickness of shell; the diameters are the "inside diameter" ; "outside diameter," (or inside diameter plus the two thicknesses of shell) ; and the "diameter over all," that is to outside of flanges. The weight per yard of these columns will, of course, be in pounds, ten times the given area. It will be not iced that the "radius of gyration" is given, and not its square, which will make the use of Table XLIX in connection with these columns much easier.
When calculating the load on a column it should be borne in mind, that if the girder or beam is continuous over the column, the loads will be equal to the reactions as given in Table XVII on pages 218 and 219 of Vol. I. If the girders or beams overhang columns and are built into the wall at the other end (such as gallery beams for instance) the respective loads on the column and wall and upward pressure on wall can be found from Formulae 116 to 119 inclusive.
Explanation of Table L.
Load on column.
If the load on the overhang is uniform its reaction would be the same as a similar amount of load concentrated at one-half the span of overhang.
If the beams or girders are inclined they can be calculated the same as already explained (in the previous chapter), when calculating transverse strains on rafters; and the amount of anchoring necessary to prevent pulling out, can readily be found by obtaining the horizontal thrust by the graphical method, as already explained.