1S36
THEORY AND PRACTICE OF ENGINEERING.
Book II.
apart, one over each pier, and one against each pediment, making altogether 32 pairs: afterthey were raised, and the supports taken away, it was found that the tie-beams deflected 2inches more than was calculated upon, and at the end of four months the deflection increasedto nearly 5 inches ; this occasioned the whole to be reconstructed, and the principals werethen placed only 13 feet apart.
The tie-beams were formed of two thicknesses of timber scarfed together, each 12 inchessquare, one laid upon the top of the other, forming an entire beam 24 inches deep and 12inches wide; at every 3 feet of its length were introduced iron bolts, 1 inch in diameter;
double keys of oak, driven into _ ^ _
notches formed at equal distances lr jj T ;■ jj l
between the two beams, assisted in -■ '—^—■ l"j «-» jt i—j-i—■
resisting the horizontal drawing - P— ^ ^
of the two extremities; the middle Fig. 2144.
of the tie-beams was drawn up13 inches by the middle or centre post, which is 34 feet in length, or £ of the total span,forming an angle of 21° 48'; above the chief tie-beam arc three others placed horizontallyat equal distances, which are tied together in a similar manner : a number of designs havebeen given by the most celebrated engineers of Europe for roofs of this kind, in whichiron-and timber are used to the best advantage.
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A
The rafters, if formed of several stout timbers, are capable of supporting considerableweight, when held firmly at iheir abutments, and the tie-beams strongly bound together,to prevent any drawing out, may be suspended to the principals: a series of iron straps orbands passed round the feet of the principal rafters, and secured to the tie-beams, will
Fig 2146.
cfFectuallv prevent any rising or spreading of the main timbers; but for still greatersecurity the introduction of an iron rod has been suggested in lieu of a timber tie-beam :