Advanced Methods of Structural Analysis

48 3 Multispan Beams and Trusses

3.2.2 Compound Trusses...........................................

A compound truss consists of simple trusses connected to one another. There are
different ways to connect simple trusses in order to construct a compound truss.
First approach uses three hinges, not arranged in line. Such an interconnection
can be considered as a simple triangle on the basis of its three rigid discs. In the
examples below, two of the trusses are represented by disksD 1 andD 2 (Fig.3.8).
Two immovable (pinned) supports may be treated as rigid diskD 3 (the earth).

• 
  

  •

  
  

  D 1 D 2 D
  1 D 2

  
  

C C

D (^3) D
3 is earth
Fig. 3.8 The generation of a rigid disk by means of three hinges
Another approach uses hingeCand a rodR, which does not pass through the
hinge (Fig.3.9).

• 
  


• 
  

  D 1 D 2

  
  

C

R

D 1 D

2

C

R

Fig. 3.9 The generation of a rigid disk by means of a hinge and a rod

A third method uses three rods, which are not parallel and do not all intersect at
the same single point (Fig.3.10).

D 1 D 2

R 2

• 
  


• 
  


• •


• 
  


• 
  

R 1

R 3

D 6 R 2 D 2

R 1

R 3

Fig. 3.10 The generation of a rigid disk by means of three rods

In addition, compound trusses can be composed of members that are themselves
also trusses (Fig.3.11). Such trusses suit structures carrying considerable loads, in
which the truss designs would require heavy solid members. In these cases it is often
more efficient to replace solid members with truss members.