Advanced Methods of Structural Analysis

3.4 Trusses with Subdivided Panels 57

The elements of trusses with subdivided panels can be arranged into three groups

as follows:

1.The elements belonging to the main truss only. The internal forces in these ele-
ments are not influenced by the presence of secondary trusses. Such elements are
labeled #1 in Fig.3.15c.
2.The elements belonging to the secondary system only. The internal forces in these
elements arise if a load is applied to the joints of the secondary system. Such
elements are labeled #2.
3.The elements belonging simultaneously to both the main and the secondary
trusses. The internal forces in such members are obtained by summing the inter-
nal forces, which arise in the given element of the main and also of the secondary
trusses, calculated separately. Such elements are labeled #3.

3.4.2 Baltimore and Subdivided Warren Trusses..................

The Baltimore truss is widely used in bridge-building for cases involving partic-

ularly long spans and large loads. The classical Baltimore truss is a Pratt truss

strengthened by two additional members in each panel (members 2-4 and 3-4 in

Fig.3.16a). The modified Baltimore truss is a Pratt truss strengthened bythreead-

ditional members in each panel (members 3-4, 2-4, and 4-6 in Fig.3.16b). In both

cases, truss 1-2-3-4 should be treated as a secondary truss. This triangular truss is

supported by joints 1 and 3 of the main truss. In both cases, the vertical load, which

is applied to the loaded chord and acts at joint 2, leads to vertical reactions of the

secondary truss. These reactions are transmitted as active loads on the joints of the

samechord of the main truss.

b

4

3 2

6

1

(^35)
2
1
4
a
5
Fig. 3.16 Baltimore truss
In order to reduce the length of the compressed member of the upper chord,
member 2-4 is used in case (a) and additional member 4-6 in case (b).
Analysis of such trusses can be performed by considering the entire structure
as a combination of a Pratt truss (the main truss) and a set of additional triangular
trusses: one of them is additional truss 1-2-3-4.
There are three groups of elements: the ones that belong to the main truss only
(bars 3-5 and 4-5), the ones that belong to the secondary truss only (bars 2-4, 3-4,
and 4-6); and others that belong to both the main and the secondary trusses (bars
1-4, 1-2, and 2-3).