The stresses in all the vertical bars, excepting 11-9 and 18-2, may be written down in a similar manner, thus

The maximum stress in the bars 11-9 and 18-2 will occur when the reaction at the cross-girder is a maximum, and this, we have seen in Chapter XIV., is +209 tons for the live load, and the dead-load stress is + 54, so that the total stress is 26.3 tons.

The maximum stresses in the horizontal members may be found when the position of the engine-wheels and uniform load is known. We have seen that the maximum bending moment occurs when—

where the horizontal distance from the left abutment to the point about which the moments are taken. The following table shows the position of the engine loads and train which produce the maximum stresses in the top and bottom horizontal members, when one load is at the panel-point l' from the left abutment:—

The stresses may now be written on the various members of the truss, Plate II., Fig. 1.

The wind stresses in this truss will be considered in Chapter XIX.

Table LXVIII. may now be prepared, and the sectional areas of the various members determined. In this case Mr. Theodore Cooper's rules have been used for finding the safe intensity of working stress instead of the Launhardt and Weyrauch formulæ given in Chapter I. The figures explain themselves for the tensile stresses, but the straight-line formulæ given in Chapter XII. have been used for determining the working stress in compression.

The actual sections adopted have been taken from Carnegie's list of rolled sections.

The details may be further studied by referring to Plate II.

CHAPTER XIX.

WIND PRESSURE.

THE published information on this subject which may be consulted for more complete information includes the following:

Papers by Mr. C. B. Bender and Professor Gaudard, published in Proceedings of the Institution of Civil Engineers (Vol. LXIX. p. 80), with discussions by Sir B. Baker and other wellknown authorities.

"Experiments by Sir B. Baker at the site of the Forth: Bridge," published in Engineering, February 28, 1890.

Experiments by Mr. O. T. Crosby, published in Engineering, May 30, June 6, and June 13, 1890.

Paper by Mr. C. Shaler Smith, in Proceedings of the American Society of Civil Engineers, Vol. 10, p. 139.

When a horizontal force of wind strikes an inclined surface it causes a normal pressure upon that surface (Fig. 340), the intensity of which is expressed by the following formula, deduced from experiments made by Mr. Hutton for gunnery purposes.

FIG. 340.

Let P denote the horizontal pressure on an inclined surface, such as a roof inclined at an angle 0, Fig. 340. Let N denote the normal pressure. Then

Professor Unwin states that this formula gives almost the same results as Duchemin's, which is

The following table gives the values of N for different values of when P = 40 lbs. per square foot.