By increasing the depth to at least 4th of the length of truss, inserting counter braces in the two middle panels, and proportioning members to the respective strains to which they are liable; this plan is undoubtedly well adapted to rail road purposes in spans from 50 to 70 feet in length.

For greater spans than 70 feet for rail roads and 80' for common roads, higher trusses, with top connections and lateral bracing or tying, should undoubtedly be adopted.

CLXI. The bridge usually designated as Beardsley's Bridge, is identical with the one shown in Fig. 63, modified by the substitution of iron bottom chords, composed of two parallel rods (to each truss) in 5 pieces or parts corresponding in size with the stresses of chords under respective panels. The middle and largest part extending under the two middle panels, and the others, each under one panel only.

These pieces or parts, being connected by turn-buckles, or screw couplings, pass through cast iron shoes, into and against which the several braces toe and thrust; the shoes being prevented from sliding outward upon the rods, by the couplings.

The shoe should in all cases be so formed and located that the axes of action of chord, brace and vertical, meet at the same point, as it regards the intermediates. while as to those upon the abutments, the axes of chord and brace should meet over the centre of bearing upon abutments.

This arrangement (understood to have been the suggestion and device of Mr. Geo. Heath), gives very satisfactory results, and the only practical question with regard to it, as compared with the one with wooden

chords, seems to be merely one of economy and con venience. If suitable timbers for chords can be readily and reasonably obtained, it is thought to be quite as advantageous to use wooden chords.

THE HOWE BRIDGE.

CLXII. A very popular plan of wooden bridges, which has, in fact, superseded most others in New York and New England for rail-road purposes from the time of the introduction of the rail road system, is known as the Howe Bridge.

The trusses have upper and lower parallel chords, together with main and counter braces, of wood, tied vertically by wrought iron tension rods from chord to chord, the principle of action being the same as in the plan shown in Fig. 63.

The braces act upon the chords and verticals through the medium of cast iron shoes or skewbacks, with ribs or flanges let into the chords to a sufficient depth to sustain the horizontal thrust of braces, and with tubes, or hollow processes, square externally, and having round holes to receive the vertical bolts. These tubes project downward through the lower, and upward through upper chord, between the courses of timber composing the chord, being boxed into the timber on each side of the tube, so as to leave about an inch between adjacent courses for ventilation; the tubes, extending through the chords, reach an iron plate upon the opposite side, which serves as a washer, or bearing for the nuts of the suspension bolts.

By this means the vertical action of braces is brought directly upon the verticals, without a transverse crushing action upon the chord timbers.

The chords are formed of 3 or 4 courses of timber side by side, with a depth equal to two or three times the thickness; the joints in the several courses being so distributed that no two courses may have a joint in the same panel when avoidable.

Fig. 64, represents a side view in the upper, and a top view in the lower diagram, of a portion of the bot

tom chord. At t is represented a view of the tube of the skewback as it would appear with the outside chord timber removed; at m m, the seats of the main braces, and c, the seat of the counter brace. Over a, is a clamp, or lock piece, and bb' are transfer blocks, or packing pieces, to secure the joint, and transfer the strain from one to another of the chord timbers. The transfer blocks may be placed obliquely as at b, or straight, as at b'. The latter is the more usual, but the former leaves the greater section of timber at the point where the bolt holes occur.

The braces are usually placed with a horizontal about half as great as the vertical reach, and extending across one panel only. Counter braces used throughout, and the upper chord made of equal leugth with the lower, giving the truss a rectangular, instead of a Trapezoidal form.

Now, it is obvious that in a rectangular truss, as represented in Fig. 52, the end posts, and one panellength of the upper chord at each end, as well as one counter-brace, are entirely useless, as it regards sustaining weight of structure and load. It will readily be seen, moreover, that no counter-braces except those of the two middle panels, in the 8 panel truss, Fig. 52, have any sustaining action, unless the variable exceed 4 times the permanent load of the truss.

It is furthermore manifest that there is a large amount of surplus material in the portions of lower chord toward the ends; the tension of that chord being in the several panels, proceding from the end (in the case of Fig. 52), as 31, 6, 7 and 8. Hence, over onefifth of the material in a chord of uniform section, is in

excess.

But the greatest sacrifice of economy in the Howe Bridge as usually constructed, results from the steep pitch of the braces. For, while, as was seen [LXVI], braces act with about the same economy at an inclination giving a horizontal reach equal to the vertical, as when the former equals only one-half of the latter, that is, with hand h, it was shown in the succeeding section, that the action upon verticals was nearly twice as great in the latter, as in the former case. For instance, suppose Fig. 18 to represent a 16 panel truss, with thrust braces and tension verticals. Estimating successively the action upon verticals with diagonals crossing two panels, as in Fig. 18, and the same with diagonals crossing but one panel, we find the action over 85 per cent more in the latter than in the former

case.

With regard to chords, the horizontal effect is essentially the same in both cases, while the vertical thrust

of braces, being but little over half as great with the long, as with the short horizontal reach, may be sustained by the timber of the chord, thus obviating the necessity of tubes extending through the chord from the cast iron skewback; and furthermore, may enable the iron shoe to be dispensed with altogether, in many Hence would result a still further saving in expense, as well as in weight of structure.

cases.

Take, for example a brace 10" square, capable of resisting a thrust of 50,000lbs. in the direction of its length, and a vertical pressure of 35,000lbs. when inclined at 45°. Whether the end be cut as at d, e, or f (Fig. 64), it covers a horizontal area of 141 square inches, giving a square inch for every 250lbs. of vertical pressure. This does not much, if any, exeeed the capacity of timber for resisting transverse crushing, as estimated in section CXLIII, when acting upon a portion of surface so limited with respect to the whole.

Perhaps, however, the propriety of dispensing with the iron shoe, should not be too strenuously urged. But there seems to be little excuse for incurring the sacrifice of iron required in suspension bolts in case of the steep braces, over what is required with the greater inclination. The interference of bolts with braces, when the latter reach across two panels, is perhaps the greatest obstacle in the way of adopting the latter arrangement; and this may be managed by either passing the bolts through the intervening braces (which does not materially impair their strength, when supported at intervals by counter-braces), or between main and counter braces, as may seem most favorable in respective cases,

In view of the above considerations, the author can not avoid regarding the usual practice in the construction of Howe Bridges, as decidedly faulty.