There are two general classes of centers - those which act as a truss; and those in which the support, at the intersection of braces, rests on a pile or footing. Trusses are used when it is necessary to span a stream or roadway. Sometimes the length of the span for the centering is very short, or there are a series of short spans, or the span may be equal to that of the arch. The trusses must be carefully designed, so that the deflection and deformation due to the changes in the loading will be reduced to a minimum. By placing a temporary load on the centers at the crown, the deformation during construction may be very greatly reduced. This load is removed as the weight of the arches comes on the centers. For the design of trusses, the reader is referred to instruction papers or other treatises on Bridge Engineering and Roof Trusses.

The lagging for concrete arches usually consists of 2 by 3-inch or 2 by 4-inch plank, either set on edge or laid flat, depending on the thickness of the arch and spacing of the supports. The surface on which the concrete is laid is usually surfaced on the side on which the concrete is to be placed. The lagging is very often supported on ribs constructed of 2 by 12-inch plank, on the back of which is placed a 2-inch plank cut to a curve parallel with the intrados. These 2 by 12-inch planks are set on the timber used to cap the piles, and are usually spaced about 2 feet apart. All the supports should be well braced. The centers should be constructed to give a camber to the arch about equal to the deflection of the arch when under full load. It is therefore necessary to make an allowance for the settlement of centering, for the deflection of the arch after the removal of the centering, and for permanent camber.

The centers should be constructed so that they can be easily taken down. To facilitate the striking of centers, they are usually supported on folding wedges or sand-boxes. When the latter method is used, the sand should be fine, clean, and perfectly dry, and the boxes should be sealed around the plunger with cement mortar. Striking forms by means of wedges is the commoner method. In Fig. 1G9, a shows the type of wedges generally used, although some-

Fig. 169. Wedges Used in Placing and Removing Forms.

Fig. 169. Wedges Used in Placing and Removing Forms.

RESIDENCE OF MR. E. S. MEYER, CLEVELAND, OHIO

RESIDENCE OF MR. E. S. MEYER, CLEVELAND, OHIO.

Watterson & Schneider, Architects, Cleveland, Ohio. Materials: First Story of Brick; Second Story and Roofs of Shingles. Built in 1904. Cost, $7,000. For Plans, See Opposite Page.

FIRST FLOOR PLAN

FIRST FLOOR PLAN.

SECOND FLOOR PLAN

SECOND FLOOR PLAN.

FIRST AND SECOND STORY PLANS OF RESIDENCE OF MR. E. S. MEYER. CLEVELAND, OHIO

Watterson & Schneider, Architects, Cleveland, Ohio.

Table XXI

Safe Load in Pounds Uniformly Distributed for Rectangular Beams, One Inch Thick, Long =Leaf Yellow Pine

Allowable fibre stress. 1,200 pounds per square inch; factor of safely, 6; modulus of rupture, 7,200 pounds per square inch.

Safe loads for other factors of safety may be obtained as follows: New safe load =

Safe load from table X 6

New factor

Span-in

Feet

Depth of Beam in Inches

Deflection Coefficient

4

5

6

7

8

10

12

14

16

4

533

833

1,200

1,633

2,133

3,333

4,800

6,533

.20

5

427

667

960

1,307

1,707

2,667

3,840

5,227

.31

6

356

556

800

1,089

1,422

2,222

3,200

4,356

.44

7

305

476

686

933

1,219

1,905

2.743

3,733

.61

8

267

417

600

817

1,067

1,667

2,400

3,267

.79

9

237

370

533

726

948

1,481

2,133

2,904

3,793

1.00

10

213

333

480

653

853

1,333

1,920

2,613

3,413

.1.24

12

178

278

400

544

711

1,111

1,600

2,178

2,844

1.78

14

152

238

343

467

610

952

1,371

1,867

2,438

2.42

16

133

208

300

408

533

833

1,200

1,633

2,133

3.16

18

119

1S5

267

363

474

741

1,067

1,452

1,896

4.00

20

107

1G7

240

327

427

607

960

1,307

1,707

4.94

22

97

157

218

297

388

606

873

1,188

1,552

5.98

24

89

139

200

272

356

556

800

1,089

1,422

7.12

26

128

185

251

328

513

738

1,005

1,313

8.35

28

119

171

233

305

476

686

933

1,219

9.68

30

111

160

218

284

444

640

871

1,138

11.12

To find the safe load for beams of hemlock from the above table, the above values must be divided by 2; for beams of short-leaf yellow pine and white oak, the values must be divided by 1.2; for white pine, spruce, eastern fir, and chestnut, the values must be divided by 1.71.

times three wedges are used, as shown by b in the same figure. They are from one to two feet long, 6 to 8 inches wide, and have a slope of from 1 to 6 to 1 to 10. The centering is lowered by driving back the wedges; and to do this slowly, it is necessary that the wedges have a very slight taper. All wedges should be driven equally when the centering is being lowered. The wedges should be made of hardwood, and are placed on top of the vertical supports or on timbers which rest on the supports. The wedges are placed at about the same elevation as the springing line of the arch.

Tables XXI and XXII can be used to assist in the design of the different members of the centers for arches.