3 DESIGN OF STRUCTURES 211 
nditions which the roof has to satisfy, the type of building it has to 
2 the span, whether the ends are to be hipped or not, ete. 
oy type of roof covering, and arrange for a suitable support 
or it seeing that the proposed distance apart of the purlins works in 
the secondary bracing of the truss chosen. 
~The pitch should next be fixed. This depends largely on the type of 
covering to be used. 
BP iecide whether the tie rod is to be cambered or not. This depends 
a m the conditions of the case, whether the roof is to support a ceiling, 
hether head room is a necessity, etc. The advantages of a camber are, 
er struts, greater head room, and better appearance. Speaking 
generally, it is better to give the tie rod a small camber if possible. 
Draw an outline diagram of the truss. The proportions should please 
ne eye. 
___ Fix the distance apart of the principals. This will depend to some 
extent on the type of roof covering, purlins, etc. Usually it may be 
e from one-eighth to one-fourth of the span. The larger the interval 
dy the larger the ratio of the least lateral dimension of the struts to 
their length, and they will therefore be lighter in proportion to their 
petrength. Too large a pitch of principals involves heavy purlins and 
ine 3 the cost. 
_Find the loads upon the truss. These are: (1) The weight of the 
. (2) The weight of the covering. (3) The weight of snow upon 
the ay (4) The weight of the ceiling, if any, carried by the trusses. 
0 is often neglected, but in large roofs it should be allowed for. (2) 
1 be approximately estimated (see table, p. 210). Allowance should 
‘also be made for the weight of the purlins, rafters, ete. (3) can be 
taken at about 6 lbs. per square foot of area covered in the British Isles. 
) must be estimated approximately, the weight being carried from the 
A A aed of the roof. For wind pressure, see Arts. 179 and 180, 
p. 19 
The loads should now be divided up, and the resulting forces at each 
of the joints found. It is well to keep the loads at the joints due to the 
wind pressure separate from the others. 
___ Choose next the methods of support for the ends of the trusses. 
Usually one end is left free, to allow the principal to expand and contract 
with changes of temperature. The reaction at this end is then assumed 
to be vertical. 
7 Find the stresses in the ‘iain either graphically or analytically, 
or preferably by both methods. The dead load stresses should be found, 
» stresses with the wind pressure on one side and then on the other, 
d the three sets of figures should be combined, as shown on p. 196. 
“' ~The sections of the various members can now be ascertained by the 
ordinary rules. It is safer to assume that all the struts are hinyed at the 
ends. See also that members in which the stress reverses are capable of 
withste nding the reversed load, although it may be smaller than the 
normal load. Use a low working stress for these members. Use also 
per stress for members which are welded. 
_ Design the joints. Arrange sufficient rivets, bolts, or pins to take 
e stress from the bars on to the gussets. Where a gusset connects one 
r more ties or struts to a rafter, bear in mind that the total shearing 
