Prof. Kid well also tested several styles of keyed beams, with the result that a compound beam keyed and bolted together, as shown in Fig. 488, was found to be the most efficient form that it is practicable to build.

It was found that with oak keys it was possible to obtain an efficiency for spruce beams of 95 per cent., while the deflection varied from 20 to 25 per cent, more than would be expected in a solid beam.

By using cast iron keys the deflection was found to be but little, if any more, than with a solid beam. The keys must be wedge-shaped, as shown in Fig. 489, so that they can be driven tightly against the end wood.

Fig. 488    Compound Keyed Beam.

Fig. 488- - Compound Keyed Beam.

Prof. Kidwell recommends that for ordinary purposes an efficiency of 75 per cent. be allowed when oak keys are used and 80 per cent, when the keys are of cast iron. The width of oak keys should be twice the height of the key. Numerous small keys closely spaced gave better results than fewer large keys. In the centre of the span a space equal to about one-quarter of the length of the beam should be left free of keys, bolts, etc. In his report, Prof. Kidwell also gives formulas for the number and spacing of the keys.

As compound beams, if used, would probably be built of either 8, 10, 12 or 14-inch timbers, the author has prepared the following table giving the maximum safe load that may be allowed for keyed beams 16, 20, 24 and 28 inches in depth, put together as in Figs. 488 and 489, and also the number of keys required on each side of the centre.

Safe Distributed Loads in Pounds for Compound Keyed Beams.

16 and 20-inch beams to have 1 x3-inch oak keys, -inch bolts, 3-inch washers. 24-inch beam to have 2x4-inch oak keys, 7/8-inch bolts, 3-inch washers. 28-inch beam to have 2x4-inch oak keys, 7/8-inch bolts, 3-inch washers.

Size of Beam.

Span of Beam in Feet.

20

24

28

30

32

36

qXI6

White Pine......

1152

960

823

768

720

• • • •

Spruce..........

1344

1120

960

896

840

• • • •

Oregon Pine....

• •...

1440

1234

1152

1080

• • • •

Georgia Pine....

160O

1371

1280

1200

• •

1x20

White Pine......

1800

1500

1285

120O

1125

. • • •

Spruce...............

• • • •

1750

1500

1400

1312

• • •

Oregon Pine....

• • • •

2250

1928

18OO

1687

I500

Georgia Pine....

• • • •

• • • ■

2142

2000

1875

1666

1x24

White Pine......

• •

2l60

1851

1728

1620

I440

Spruce..............

• • • •

2520

2l60

20l6

I89O

1680

Oregon Pine....

• • • •

• • • •

2777

2592

2430

2l60

Georgia Pine....

• • • •

• • • •

3085

2880

2700

2400

1x28 -

White Pine......

• • • •

. • • •

2520

2352

2205

1960

Spruce..........

• • • •

• • • •

• • • •

2744

2572

2286

Oregon Pine.

• • • •

• • • •

. .

3528

3307

2940

Georgia Pine....

• • • •

• • • •

• . •

3920

3675

3266

To find safe loads for any given thickness of beam, multiply the load in the table by breadth of beam in inches.

For centre loads, take one-half those in table.

Beams should not be used for shorter or longer spans than those for which safe loads are given, except that 28-inch beams may be used up to 40 feet.

Number of Oak Keys Required Each Side of Centre.

For Beams of

White Pine.

Spruce.

Oregon Pine.

Ga. Pine.

16-

inch beams

1x3-

inch keys.

7

8

11

12

20-

"

1x3

"

9

11

13

15

24-

"

2 X4

"

8

9

12

13

28-

"

2x4

"

9

10

12

14

Minimum Spacing of Keys.

1x3-

inch keys

..........................

11

inches.

11

inches.

9

inches.

9

inches.

2 X4 -

"

.........................

15

"

15

"

11

"

11

"

2x4-

"

.........................

17

"

17

"

13

"

13

II

The breadth or thickness of compound beams should be not less than two-fifths of the depth. The number of keys required is not affected by the length or breadth of the beam, if the beam is figured for the full safe load.

In spacing the keys (Fig. 489) they should not be closer than the minimum spacing given in the table. For beams loaded at the centre, the spacing of the keys should be uniform from X to Y, Y being one-eighth of the span from the centre. If the distance between the keys, centre to centre, works out less than the minimum spacing, the safe load should be correspondingly reduced or the thickness of the beam increased.

266 Keyed Beams 200368

Fig. 489.

For beams uniformly loaded, the first four or five keys from the ends should be spaced for minimum spacing, and the spacing of the remaining keys increased toward the point Y. When the ratio of depth to span is greater than 1 to 16, the inner keys may be a little more than one-eighth of span from centre for distributed loads.

Fig. 488 shows the proper spacing for a 20-inch spruce beam of 28 feet span and for a Georgia pine beam of 30 feet span, and the following table gives the proper spacing for spruce beams (figured from the end of the beam) of longer span. For other woods and spans the spacing should be made as near like these as the fixed conditions will permit.

The sizes of bolts and washers to be used are given in the heading. If the beam is not over 10 inches wide the bolts may be arranged as for the spruce beam, Fig. 488; if 12 inches wide or over the bolts should be staggered as shown for the hard pine beam. In a very wide beam the bolts might be spaced as in detail b, Fig. 489.

Spacing of keys in inches (commencing at end) for distributed load:

16-

inch

Spruce

Beam,

32

feet span.

10, 12, 12, 16, 19, 24, 32.

20-

"

"

"

32

"

10, 11, 11, 11, 12, 12, 12, 13, 15, 18, 24.

24-

"

"

"

36

"

13. 15, 15. 15. 15. 16, 18, 20, 30.

28-

"

"

"

36

"

15, 17.,17, 17, 17, 17, 17, 17, I7, 17.