tion, and allows of forming a proper joint at B. It should be borne in mind that the strength of a truss depends largely upon the way in which the pieces are joined together, and that a truss may fail, simply through bad and improper joints.

Fig. 7 shows a truss used in one of the old buildings in London over a room sixty feet wide. It is built of oak, and has wooden

ties in place of iron tie-rods; but in principle it is the same as the truss shown in Fig. 6. The actual dimensions of the various pieces of the truss are given in the figure.

post roof-truss, adapted to the suspension of a floor from the joints of the truss. As it was necessary to have the centre rod to support the floor beneath, it then became necessary to put in the braces B, B. The braces CC would only come into play when one of the extreme rods was loaded, and none of the others.

These braces are called "counterbraces." The manner in which the foot of the principal rafter is secured to the tie-beam is rather uncommon,

and an enlarged detail of it is shown in Fig. 10.

This truss is

from the Museum of Fine Arts, St. Louis, Mo., Messrs. Peabody &

Stearns, architects, Boston, Mass.

from forty to eighty feet, a truss such as is shown in

Fig. 11 is one of the best forms to adopt, where a pitch roof is desired.

The struts should be largest towards the centre, and the tie-rods also.

The main rafter, on the contrary, and the tie-beam, have the greatest strain at the joint A. Figs. 12 and 13 show details of joints A and C.

The trusses which have thus far been given are the simplest forms of modern trusses for spanning openings up to sixty or

SUITABLE FOR SPANS UP TO CO FT.

10x 12"

The form of truss generally employed for flat roofs is that shown in Figs. 14 and 15. This truss may be adapted to any span from

not to be less than one-eighth of the span, and, if possible, should be made one-seventh, as the higher the truss, the less will be the strain on the chords.

It should be noticed, that in this truss the braces are inclined in the opposite direction to that in which they are placed in the

1 The two horizontal pieces are called the "chords; " the top one, the upper chord, which is always in compression; and the bottom one, the lower chord, which is always in tension.

trusses previously shown. The distances between the vertical rods should be so arranged that the braces shall not make an angle of more than forty-five degrees with a horizontal line.

Fig.16

DETAIL OF JOINTS "A "B." FIG. 15.

Fig. 16 shows the best method of forming the joints, A, A, A, B, B, B, etc. (Fig. 15), although not very frequently used in rooftrusses. For spans over forty feet, the tie-beam should be made up of plank bolted together, as shown in Fig. 3, unless it is possible to have the tie-beam in one piece. This is a good form of truss for theatres, and large halls where there is a horizontal ceiling.

Counter-Braces.

If it is desired to load the truss at any point other than the centre with a concentrated load, as, for instance, suspending a gallery by means of rods from the roof