DESIGN OF STRUCTURES 233 
“equal to the vertical component of the sum of the safe loads in all the 
jagonals cut. Vertical stiffeners similar to those used in plate girders 
‘are usually introduced, and for similar reasons. 
So far the types of open web girders referred to have parallel booms. 
7 large spans it may be more economical to curve one, or even 
a of the booms. Girders with curved booms are more expensive per 
ton than corresponding ones with straight booms, and should not be 
employed when parallel booms are suitable. Most of the types of web 
4 bracing already referred to can be used in girders with curved booms. 
yA girder with the top boom of parabolic form and the other straight 
P (Fig. 344) is termed a bow-string girder, from its similarity to a bow and 
string. Such a girder, carrying only a uniform dead load, would, theoreti- 
 eally, require no diagonal bracing in the web. Since all bridges have to 
support both non-uniform and rolling loads, in practice diagonalisation 
becomes necessary, as shown in Fig. 344. This form may be inverted, 
_ when the “bow” becomes a suspension chain. 
The type sometimes called the bow and chain girder is shown in Fig. 
345. It is the bow and inverted bow or suspension chain girder types 
_ combined, the object being to neutralise the thrust of the arched bow by 
a 
“5 _ 
‘ N 
Fia. 344. Fia. 345. 
_ the tension in the chain. Practically, it is a girder with two curved 
booms. The web bracing is usually of the type shown. The bridge 
floor is carried by suspension rods, as shown. 
Fig. 346 illustrates a type of truss common in America for large 
, NNN K 
CN 
Fig. 346. Fig. 347. 
spans. The web bracing is of the lattice type. Fig, 347 shows the 
Linville type of truss applied to a large span. 
A type of girder common in English railway practice is illustrated i in 
the worked example, Art. 227, pp. 248-258. 
207. Counterbracing. —The function of the ties and struts which 
_ form the bracing of an open web girder being to take the shear stress, it 
_ follows that if this shear stress be reversed in direction at any part of the 
’ girder, the ties become struts and the struts become ties at that part. 
Now, it was shown in Art. 107, p. 100, that a travelling load added to 
~ the dead load will have the effect of reversing the direction of the shear 
stress over a portion of the girder. In a plate web this reversal of 
_ stress is of little or no consequence, but in an open web it is obvious that 
But eithe must be made for it., The struts will as a rule act well as ties, 
ut either the ties must be designed to carry the compressive stress, that 
__ is, to act as struts, or else special members must be introduced to carry 
_ the reversed stress. These special members, which are diagonal ties 
