DESIGN OF STRUCTURES 235 
hold the rain, as does the LJ form, unless specially drained. The two 
halves of this form of boom are usually connected together by light 
- secondary bracing, as shown in Figs. 353 and 365. 
Fig. 355 shows a convenient form of boom for a light girder. It 
consists of four angles held apart by short plates at intervals. 
he cross section of the boom is proportioned 
to the stress which it has to carry, exactly asin a == Ee 
_ plate web girder. The graphical method shown 
Fig. 331, p. 227; may be used to obtain the TE: 
length of the flange plates, angles, etc. 7g: rn phy 
If the lateral dimensions of the compression - 
boom are small compared with its length, it should be examined as a 
strut hinged at the panel points. Generally, this is unnecessary. 
209. Joints in Boom Plates.—The joints in the horizontal flange 
plates are formed exactly as in the case of plate web girders, and similar 
calculations are necessary. The flange angle joints are also similar, being ’ 
constructed with round back covers. The joints in the stringer plates are 
usually butt joints with double covers, as shown in Fig. 349, sufficient 
rivet section being used to develop the full net strength of the plate. A 
grouped joint for a boom with vertical flange plates is shown in Fig. 353. 
210. Riveting in the Booms.—The riveting in the booms should be 
of a regular uniform pitch. As far as possible the rivets should be 
arranged to break pitch across the width, particularly in the tension boom, 
so as not to weaken the boom more than is unavoidable. A 4 inch pitch 
is the most common, but the pitch should not exceed 6 inches where 
there is a likelihood of water getting between the plates, nor in any case 
should the pitch be more than sixteen times the thickness of. the outside 
plate in the compression boom. 
Since the stress in the boom is transferred from the web bracing on to 
the stringer plate, and thence through the flange angles to the flange 
plates, sufficient rivets must be placed through the flange angles to 
transfer this stress on to the flange plates within a reasonable distance 
along the length of the flange. | 
211. Web Bracing.—The web bracing is constructed of the ordinary 
rolled sections, used singly or in combination. For ties, flats are most 
commonly used. If, however, lateral stiffness is desired, or if the stress 
is likely to be reversed, a channel or other suitable section may be 
employed. If the reversed stress is small in amount, two flat bars con- 
nected by secondary bracing, as shown in Fig. 357, may be used. Each 
of the flat bars must, however, be capable = 
of carrying one-half of the load when con- 2% * * 
iron distance pieces. 
For a light tie a single flat bar is best, \ * 
359, 360, 362, 364, and 365. A strong and light strut is formed by 
connecting together two or more simple struts by secondary bracing, as 
sidered as a column bending between the yj 
points of secondary support. This condition j; 
e 
and for a light strut a single angle or tee 
bar is most suitable (Fig, 356). ct cit 
usually determines the spacing of the cast- 
The usual sections suitable for heavier struts are shown in Figs. 358, 
