Since it is assumed that all of the compression occurs in the slab, the only work done by the concrete in the rib is to transfer the tension in the steel to the slab, to resist the shearing and web stresses, and to keep the bars in their proper place. The width of the rib is somewhat determined by the amount of reinforcing steel which must be placed in the rib, and whether it is desirable to use two or more rows of bars instead of merely one row. As indicated in Fig. 104, the amount of steel required in the base of a T-beam is frequently so great that two rows of bars are necessary in order that the bars may have a sufficient spacing between them so that the concrete will not split apart between the bars. Although it would be difficult to develop any rule for the proper spacing between bars without making assumptions which are perhaps doubtful, the following empirical rule is frequently adopted by designers: The spacing between bars, center to center, should be two and a-quarter times the diameter of the bars. Fire insurance and municipal specifications usually require that there shall be two inches clear outside of the steel This means that the beam shall be four inches wider than the net width from out to out of the extreme bars. The data given in Table XVIII will therefore be found very convenient, since, when it is desired to use a certain number of bars of given size, a glance at the table will show immediately whether it is possible to space them in one row; and, if this is not possible, the necessary arrangement can be very readily designed. For example, assume that six 7/8-inch bars are to be used in a beam. The table shows immediately that the required width of the beam (following the rule) will be 14.72 inches; but if, for any reason, a beam 11 inches wide is considered preferable, the table shows that four 7/8-inch bars may be placed side by side, leaving two bars to be placed in an upper row. Following the same rule regarding the spacing of the bars in vertical rows, the distance from center to center of the two rows should be 2.25 X .875 = 1.97 inches, showing that the rows should be, say, two inches apart center to center. It should also be noted that the plane of the center of gravity of this steel is at two-fifths of the distance between the bars

Table XVIII

Required Width of Beam, Allowing 21/4 x d, for Spacing Center to Center, and 2 Inches Clear on Each Side

Formula: Width = (n-l)2.25d + d + 4 = 2.25 nd - 1.25d + 4

No. of Bars

1/2-lN.

5/8-In.

3/4-In.

7/8-lN.

1-IN.

l 1/4-lN.

2

5.62

In.

6.03

In.

6.44

In.

6.84

In.

7.25

In.

8.06

In.

3

6.75

,,

7.44

,,

8.13

,,

8.81

,,

9.50

,,

10.87

,,

4

7.87

,,

8.84

,,

9.81

,,

10.78

,,

11.75

,,

13.68

,,

5

9.00

,,

10.25

,,

11.50

,,

12.75

,,

14.00

,,

16.50

,,

6

10.12

,,

11.65

,,

13.19

,,

14.72

,,

16.25

,,

19.31

,,

7

11.25

,,

13.06

,,

14.87

,,

16.68

,,

18.50

,,

22.12

,,

8

12.37

,,

14.46

,,

16.56

,,

18.65

,,

20.75

,,

24.94

,,

9

13.50

,,

15.87

,,

18.25

,,

20.62

,,

23.00

,,

27.75

,,

10

14.62

,,

17.28

,,

19.94

,,

22.59

,,

25.25

,,

30.56

,,

above the lower row, or that it is eight-tenths of an inch above the center of the lower row.