Hardware, estimating, and mill design

by Unknown

STEEL-FRAME MILL BUILDINGS

43. There is a type of building which, while not distinctly mill construction as usually understood, is frequently used for one-story buildings, such as rolling mills, cement works, machine shops, foundries, rail yards, and buildings of this class.

The essential feature of these buildings is a steel-roof truss supported on steel columns, the columns being braced both to the truss and longitudinally of the building. It is usually the purpose in the design of such buildings to neglect everything but the necessary stability and the first cost. The steelwork, consequently, is of the lightest possible construction, usually designed for a unit fiber stress of from 18,000 to 20,000 pounds, and the covering of the sides of the building, together with window details, etc., is made only sufficiently good to keep out the weather.

44. Material for Roof Covering.—The roof covering of this class of building is either of slag on 2-inch spruce plank, spiked to nailing strips bolted on to steel purlins from beneath, with lagscrews, or of slate laid on 1-inch or 2-inch sheathing boards. Even galvanized iron is used for the roofing of some of the cheapest class of buildings, especially those which, owing to the process of manufacture, are subjected to a high temperature.

45. Construction of Sides of Building.—The sides of these buildings may be covered with either expanded-metal lath on metallic furring strips, plastered inside and out with cement mortar so as to form a fireproof and rigid screen wall about 2 inches in thickness; or, the walls may be 9-inch or 13-inch brick walls built part way up the height of the columns and leaving the columns exposed on the face; or, corrugated galvanized iron lapped 6 inches and secured either by riveting to metallic supports or nailed to wooden studding secured to the steel frames. Of these constructions, probably the first is the most expensive and also the most satisfactory.

Fig. 21

46. Partially Supported Steel-Frame Building.— In [Fig. 21], there is designated a type of construction that may be built for about $1 per square foot of the area covered. This consists of steel I beams, or angle-and-plate columns, used for column supports carrying the usual angle iron steel-roof truss. The roof is sheathed with 2-inch spruce tongued-and-grooved planking, covered with a good quality of roofing felt and slag, with a stop-gutter a at the edge. Owing to the fact that the steel columns are supported in a direction of their minimum radius of gyration by means of the brick walls, they can be made very light. The building illustrated has what is known as a saw-tooth roof. By this means, light is obtained on the side next to an adjacent and higher building by means of a sash b. This sash is usually made hinged or pivoted, to provide the necessary ventilation.

47. In [Fig. 22], there is illustrated, diagrammatically, the framework of a one-story skeleton-construction building. In the design of all such buildings, where there are no end gable walls, the several columns and trusses must be braced diagonally, as indicated at a, a, and frequently it is necessary to introduce a secondary system of horizontal bracing from one panel point on the lower chord to another, as indicated at b, b.

Fig. 22

In placing galvanized ironwork on the sides of steel-mill buildings, it is best to construct the necessary framework between the main supporting members of the building of light angles, or tees. These should be furnished punched with ⅜-inch or ⁵/₁₆-inch holes, to which the galvanized iron may be riveted, it being best to mark the galvanized iron in the field and punch it there. This may be done without much difficulty with the usual light gauge used for this purpose. It is sometimes necessary with this construction to flash around the window and door heads with IX tin.

DETAILS OF MILL CONSTRUCTION
AND DESIGN

STRUCTURAL FEATURES

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BEAM CONNECTION TO GIRDERS

48. In factory construction, the headroom is seldom available to support beams on the girders, as indicated in [Fig. 23 (a)]. It is usually necessary, in order to cheapen the construction of mill buildings, to keep the distance between the clear headroom and the finished floor level to the very minimum, and consequently the tops of the beams are most always brought flush, or nearly so, with the top of the girder.

A common construction is to use some of the various forms of wrought-iron hangers, as shown in [Fig. 23 (b)]. The type of hanger shown is a single stirrup, and is probably the best of any on the market; where beams enter the girder on both sides, the hanger is designed double. While it is popularly supposed that this hanger would readily fail by the bending of the metal at a, it is usually proportioned to safely carry any reaction imposed under ordinary floor loads. This hanger is obtained stamped out of steel plate or formed from bar iron.

(a)

(b)

(c)

Fig. 23

49. Where it is not desirable to use wrought-iron or steel hangers, a simple and inexpensive form of construction may be adopted as that shown in [Fig. 23 (c)]. Here the beam a is supported on a wooden strip b, which extends the full length of the girder, and is bolted near the bottom with through bolts. Such a construction provides sufficient strength for the support of the average factory floor, but its strength is difficult to figure with any degree of certainty, and some surer form of connection is generally considered preferable. In all instances, it is good practice to tie together the opposite floor-beams butting on a girder by means of an iron dog, or tie-plate, c.

50. In [Fig. 24 (a), (b), (c), and (d)] are indicated other methods of supporting the secondary floor-beams on main girders in the construction of factories. In [Fig. 24 (a)] is shown an I-beam girder supporting heavy timbers of a floor of slow-burning construction. It is always necessary in this construction to bring the top edge of the timbers above the upper flange of the I beam, and to span the space a thus created with a piece of timber for a tie and for the support of the floor planking. By providing this space between the ironwork and the wooden tie, any shrinkage that may occur in the secondary timbers will not cause the floor to ride on the top of the steel beam and thus make a ridge evident in the finished floor at this place. The timbers forming the secondary girders may either be supported on angle-iron brackets, or on angle irons extending the entire length of the girder. The latter method is only pursued when it is necessary to keep the end of the timber a few inches away from the steel beam, and the angle, consequently, being subjected to a greater bending moment, must have more resistance by increasing the width of the section of the bracket.

Fig. 24

Sometimes, the secondary beams are supported on double stirrup hangers, as shown in [Fig. 24 (b)]. When it is not desired to use steel beams, resort is frequently had to flitch-plate girders. They are, however, held in some disfavor by the building departments of the several cities, who do not consider that the combined strength of the timber and metal can be taken, and will only permit the strength of either the timber or metal to be used.

51. The building departments of several of the large cities stipulate that buildings of the second class, which includes factory construction, shall not have steel girders that are not fireproofed supporting brick walls or floors. When this construction is required, the secondaries must be supported as in [Fig. 24 (c)]. In this view is two angle brackets riveted or bolted to the steel beam, and extending through the concrete for the support of the wooden beams. While there is some danger of heat being transmitted to the beams through the projecting ends of these brackets, nevertheless it is considered better construction than that shown in [Fig. 24 (d)], where stirrups are used over the concrete fireproofing. In this latter construction, there is a liability of the stirrup bending at a, a, and crushing the concrete beneath. Where the reaction from the end of the girder is great, this undoubtedly is likely to occur, and such stirrups should be provided with a bearing plate on top of the concrete, so that their bearing at the edge will be distributed over a considerable area.

TRAVELING-CRANE LOADS

Fig. 25

52. Planning for Traveling Cranes.—In designing factories or mill buildings in which traveling cranes are to be installed, it is important to observe that the track of the crane can be properly supported, and also that there is sufficient headroom under the floor or roof construction to permit the trolley of the crane and the traveling mechanism of the crane girder to move underneath.

In [Fig. 25], there is shown the upper portion of a steel-mill building. The columns a support the girder carrying the runway of the crane. A convenient means of supporting the roof is to splice to this column a similar column b, which is incorporated in the design of the roof truss and rigidly braced with the truss by means of a knee brace at c. In the design of such a building, it is very important to determine the distances x and y required by the makers of the traveling crane. These distances x, y depend on the size of the crane, that is, whether it is designed to carry 5, 10, 15, or more tons. Usually from 9 to 12 inches is sufficient for the measurement x, while the measurement y varies from 5 to 8 feet.

53. Cranes Supported on Reinforced-Concrete Walls.—Frequently, in the latest types of construction, the runway for the crane is supported on reinforced-concrete walls, which construction is shown in [Fig. 26 (a)]. It will be observed that the pilasters supporting the crane are strongly reinforced in all directions from which stresses are likely to be created from the eccentric load imposed by the crane track.

Where cranes are supported on reinforced-concrete columns, as in [Fig. 26 (b)], it would be good practice to put additional rods in the far side of the column as at a, in order to supply a greater resistance to bending, and thus counteract the effect of the eccentric load produced by the reaction from the crane track. Where cranes handle heavy rails or cumbersome material that might, by swinging, impose a blow on the reinforced-concrete columns, it is good construction to protect the edge of the columns with an angle iron as indicated at b. This angle iron may be fastened in the forms and anchored by means of pronged anchors back into the concrete when it is tamped.

Fig. 26

Fig. 27

54. Detail of Track Construction.—Many crane failures have been due to the spreading of the track between supports. It is better, therefore, to supply considerable lateral rigidity to the beam supporting the track or traveling crane. Where loads are heavy and plate girders are used for the runway tracks, the flanges of the girder are sufficient for this purpose. Where I beams are used, however, for the support of the crane track, it is good practice to place on the top of them and rivet with countersunk rivets, spaced about 18 inches apart on each flange, channel irons as indicated at [a, Fig. 27]. By means of these channel irons, which are drilled with open holes b b, the rail c may be readily clamped in place by means of wrought-iron clips and bolts, and the rails nicely aligned and adjusted by wedging between these clips and the track.

55. Maximum Stress on Track Girders.—The principal calculation for the construction of the runway of cranes exists in determining the maximum bending moment. The maximum bending moment on a runway girder occurs when the wheels of the traveling crane are in the position indicated in [Fig. 28]. It will be noticed that the center of the girder is midway between the center of the near wheel and the center of the crane trolley, that is, the distance a is one-half the distance b. The following formula will give the maximum bending moment on a crane girder when the load is in the position indicated in [Fig. 28]:

M =	w(l - a)²
	2l

in which	M =	bending moment, in inch-pounds;
	w =	load on one wheel of crane, in pounds;
	l =	span of girder from center to
		center of support, in inches;
	a =	distance, in inches, marked in [Fig. 28].

Fig. 28

In order to illustrate the application of this formula, assume that the wheel load w equals 10,000 pounds; that the distance from center to center of supports of the runway girder is 15 feet, or 180 inches; and that the distance a is 12 inches. By substitution,

M =	10,000 × (180-12)²	= 784,000 inch-pounds
	2 × 180

From this bending moment may be found, by the methods given in Design of Beams, the proper size girder to use.