## Table XIII. Loop Bars. Allowance For One Loop. American Bridge Company Standards

(All dimensions in inches)

American Bridge Company practice requires the smallest pin to be not less than three-fourths the width of the eye bar.

Bars of a square or circular section could, as in the case of bolts, have a screw thread cut on their ends and by means of nuts be connected to the other part of the structure, but such an operation would be costly since the bars are long and much of the section would be wasted for a great length. In such cases the bars are ordered 6 inches longer than required and this 6 inches is, after heating to a welding heat, upset or pushed in 6 inches, thus increasing the diameter of the bar at the end so that the diameter at the bottom of the screw-threads will be greater than the diameter of the original bar. This is done so that the bar will break in the body, and not at the joint.

The sizes of upsets for bars of various sizes are given in the handbooks. Let it be required to determine the size hole through which a l 1/8-inch bar with upset end would pass and the nut required. We find opposite the 1 1/8 the value 1 1/2, showing that the upset will be 1 1/2 inches. In another table opposite 1 1/2 is given the size and weight of a square nut, viz, 1 1/2 inches thick, 3 inches on the side, and weight 3.175 pounds. The use of square nuts is not to be encouraged, the hexagonal form being the better, on account of their lighter weight.

Instead of the rods being fitted with nuts and threads at their ends, they may, as mentioned above, be made into loop bars. Loop bars are welded, and for this reason are not to be desired since welds are never as strong as the original. However, the loop bar has 100 per cent excess through the pin, and in order to have an efficiency of 100 per cent it must have a weld with an efficiency of 50 per cent. Since such a weld is well within the limits of possibility, it is permissible to use loop bars in highway bridges or other structures where the impact is not great, and in counters, since here the pins are usually of such a diameter that they would be too great for an eye bar of the section of the counter. Table XIII gives information regarding loop bars. They must be made of wrought iron since steel does not weld well.

Clearances. It is very important that each member of a structure fit together well in the field; and it is equally important that the draftsman should so detail his work that the various parts of any particular member should, without further cutting than the first, fit together. Also the rivets should be so spaced and placed that they can be driven.

The rivet clearances have been mentioned under "Rivets and

Rivet Spacing" and will not be taken up here. It is sufficient to say that on the rivet clearances is where the novice makes the most of his mistakes.

Where the distance between the outer faces of several members placed together is to be computed, it is necessary, on account of the liability of plates to exceed their nominal thicknesses, and rivet heads their nominal height, to make certain allowances. The usual practice is:

(1) Between eye (or loop) bars allow 1/16 inch.

(2) Between an eye( or loop) bar and a built-up member 1/8 inch.

(3) Between two built-up members 1/4 inch.

Fig. 47. Joint Showing Clearance between Members.

For example, suppose it was required to compute the distance out to out of the members shown in Fig. 47. The clearance would be as indicated, and the distance D would be:

D = 2 (10/2 + 0.4+5/8 + 1/4+ 0.28+1/8 + 1 1/4+1/16+1 1/4) = 18.485= 18 1/2 inches

This value would be the grip of the pin which was used at this joint. The 0.4 inch and 0.28 inch in the above are the thicknesses of the channel webs, and the 5/8 inch is the height of a 7/8 inch rivet head.