1322
THEORY AND PRACTICE OF ENGINEERING.
Book II.
When the upright posts are omitted, for the sake of obtaining a spacious room, it isnecessary to hold up the tie-beams, so that they are prevented from sagging; and wheretwo or more stories are required in a roof, as in theatres, every means must be taken torender them secure.
At the Palais Royal, in Paris is a remarkable roof of considerable span, with twofloors within it: the tie-beams are scarfed together, and hung up to the rafters by strongstirrup-irons. *
The roofs of the houses in Flanders are frequently contrived to contain three or morefloors, and examples exist of more than double that number : when the building is of aconsiderable span, and the roof true pitch , it is important that the space it contains shouldnot be entirely useless ; upright posts and partition walls generally sustain a portion of theload that such an arrangement must produce. In theatres and large assembly rooms,where the whole weight of the roof is laid upon the outer walls, difficulties often occur thatrequire considerable skill on the part of the constructor, to avoid the danger which ac-companies them. The first study should be directed to securing the feet of the principalrafters to the ends of the tie-beams, andwherever a joint occurs in the length of thelatter to hold it up, and prevent it fromsagging : the more securely the timbers areframed together and made to resemble anentire truss, the less is their liability to dis-arrangement, which often occurs throughhanging additional weight to the severalfloors contrived within it, or the bending orpushing out of the walls of the building.
Fig. 2114. ROOP AT THE PALAIS ROYAL .
In some instances, as at Louvois , the great weight of the roof is deposited below thetie-beam by an angular brace, at the foot of which is a tie-beam hung up to pendentives
attached to the principal rafters, where the collar that supports the king-post is framed in:another iron strap secures the collar to the post, and binds all firmly together.
Several methods have been adopted to brace the rafters together where tie-beams havenot the king-post to hold them up : in some examples, on one side is a stirrup-iron, and onthe other a brace; the latter is rather calculated to deposit weight than to uphold it,whilst the former, which performs both services, is to be preferred.