Connections to the Posts. If the truss rests upon posts at the end, sufficient rivets must be driven through the posts and the end connection plates to take up the end reaction, which (see page 38) is 24.970 pounds. Since the rivets are field rivets, this will require 24 970 3 750 = 7. This number is to be used in case the posts are built in masonry walls. In case the truss has knee-braces and the walls of the building consist of steel framework, the reaction due to the wind must be added to the above.

Fig. 79. Slotted Hole for Truss of Fig. 76.

Fig. 79. Slotted Hole for Truss of Fig. 76.

15. The Stress Sheet. This should also be somewhat of a general drawing, showing the details. It should give an outline sketch of the building, showing bays, the distances between trusses, and the bracing in the plane of the top and bottom chords. See Plate I, p. 43, which is a stress sheet of the truss designed in Article 14. While not necessary, it is very convenient to have the required number of rivets noted upon the stress sheet.

16. The Detail Drawing. The stress sheet, in the matter of sizes, gives general dimensions only. It would be impossible for the shop men to make a truss from the stress sheet.

The shop or detailed drawings must be prepared by the draftsman. These drawings must show the exact number of rivets, and their positions, the dimensions of every plate, member, and purlin. The placing of the dimensions so that it will be unnecessary to add or subtract in order to get another desired dimension, is quite an art, and can be attained only through experience or from the study of correctly detailed work. Plates II and III give the shop drawings for a roof truss and the bracing. These are made according to the latest and best practice, and a thorough study of them will be a help to an intelligent design of the trusses.

All members and plates which are to be riveted together in the field should be given a mark. This mark should be painted on the member or plate, and also marked on the Marking or Erection Diagram (see Plate IV). This diagram is a sketch, with the pieces in their proper position and the correct mark placed upon them. For example, if it is desired to rivet into place the first panel of the lower lateral system, the men look on the marking diagram and see that plates Pl7' Pl8, and Pl9, and the laterals BLl, BL2, and BL3 are required. They would then go to the place where all the trusses are piled up, and pick out the plates and members with these marks upon them. They would then rivet Pl7 at L0, Pl8 or Pl9 at L2, then BLl, then B L 3, and finally B L 2, all of which are shown on the Marking or Erection Diagram.

Plate II. Working Shop Drawing of a Roof Truss.

Plate II. Working Shop Drawing of a Roof Truss.

Plate III. Working Shop Drawing Showing Detail of Top and Bottom Lateral Bracing of a Roof Truss.

Plate III. Working Shop Drawing Showing Detail of Top and Bottom Lateral Bracing of a Roof Truss.

Plate IV. Marking or Erection Diagram.

Plate IV. Marking or Erection Diagram.

Care should be taken to give each piece that is different from others in any way whatsoever, a different mark. For instance, the purlins are the same size, and differ only in length and on account of the fact that one has holes in the bottom flange (see Plate III).

Plate IV gives the roof marking and erection diagram for the roof trusses of Plates II and III. Note that the roof truss on Plates II and III is not the same as that of which Plate I is a stress sheet.

17. Estimate of Cost. A rough estimate of the cost of steel in the roof may be obtained by multiplying the weight of the purlins, in pounds, by 2 cents; then adding to this the result obtained by multiplying the weight of all the steel in the trusses and the bracing by 3 cents. This will give the cost of the steel work in place with two coats of paint. This will give the cost closely enough for an Engineer's estimate; but should a Contractor desire, to bid, a detailed estimate should be made as indicated in the remainder of this article.

The cost of the roof covering may be approximately determined according to the prices given in Article 5, but may be more accurately obtained by asking a Contractor for a figure which his experience will indicate as correct.

Paint of various kinds may be bought in open market. Table XII gives some of the kinds used in painting structural steel, together with the amount of surface one gallon will cover.

Table XII. Surface Covered Per Gallon Of Paint*

Paint

Square Feet

1 coat

2 coats

Iron Oxide (powdered) ..........

600

350

" " (ground in oil).......

630

375

Red Lead (powdered)........

630

375

White Lead (ground in oil).......

500

300

Graphite " " " ..............

360

215

Black Asphalt..........

515

310

Linseed Oil ............

875

. . .

*Pencoyd Handbook, 1893. p. 893.

One gallon of paint will give two tons of structural steel the first coat, or 2 tons the second coat. The cost of one coat of paint in the shop is 45 cents, and two coats after erection $1.80 per ton of structural steel.*

The detailed estimate of the cost of steel includes several items which are given in Table XIII. In each case the weight of the steel on which the work is done must be multiplied by the unit-cost, and the sum total of all the costs will be the total cost of the entire roof or building. Table XIII gives the various operations which go to make up the cost, and also the unit-costs. Note that the costs vary com siderably. This table is given as a rough guide. In order to analyze intelligently the cost in this manner, great experience or access to the cost records of some structural steel company is necessary.

Table XIII. Analysis Of Cost Of Roof Trusses And Mill Buildings

Operation

Cost per Ton

Raw Material

$37.00 to $-40.00

Work done at Rolling Mills (mill work)

3.00 "

7.00

Work done in Bridge Shops

Columns

14.00 "

20.00

Trusses

12.00 "

25.00

Girders

12.00 "

25.00

Work done in Drafting Room

Purlins

.30 "

1.00

Trusses and Buildings

2.00 "

8.00

Painting

1.50 "

3.00

Shipping (depends upon freight rates)

Erection

5.00 "

15.00

It is not to be supposed that all of the operations indicated in Table XIII are made on one piece. Usually pieces which have mill work done on them require no shop work. In such cases a saving of freight.is effected, since the material may be shipped directly from the mill to the place of erection.