proper form of a roof is that which puts the whole in equilibrium , so that it would stand in that shape , supposing all the joints to be flexible . Any departure from this form renders the component parts subject to strain , and bracing is necessary to be introduced to maintain the form adopted .… … two kinds of purlins employed , one 72 feet long and 3 feet deep , and the other 24 feet long and 6 feet deep , the latter serving to brace each two ribs of one pair together , while the former act as pure purlins , supporting the intermediate rafters . A special arrangement was made for bracing…
