Advanced Methods of Structural Analysis

50 3 Multispan Beams and Trusses

influence lines for trusses will be based noton the repeated calculation of a required

factor (reactions, internal forces) for successive position of a unit load as it moves

across the span, but on thederiving of the functionfor the required factor.

For construction of influence lines of internal forces, we will use the method

of sections and method of isolation of joints. The following are some fundamental

features of the joints and cuts methods.

Using the joint isolation method,threetypes of position of a unit moving load on

a load chord should be considered:

1.A moving load at the considered joint
2.A moving load anywhere joint except the considered one
3.A moving load within the dissected panels of a load chord

Using the cuts method,threetypes of position of a unit moving load on load chord

should be considered:

1.A moving load on a left-hand part of dissected panel of load chord
2.A moving load on a right-hand part of dissected panel of load chord
3.A moving load within the dissected panel of load chord

Design diagram of the simple truss is shown in Fig.3.13. Influence lines for reac-

tionsRAandRBfor truss are constructed in the same manner as for reactions for

one-span simply supported beam.

h=4m,

d=3m

sinj=0.4061

RA RB

d

1

2

4

6

7

8

10

(^3911)
P= 1
5
12
h
l= 6 d
j
Fig. 3.13 Design diagram of the triangle truss
The following notation for internal forces will be used:Ufor bottom chord;O
for top chord;Vfor vertical elements; andDfor diagonal elements.
Influence Line for ForceO 4 - 6 (Section 1-1, Ritter’s Point 7, Fig.3.14a)
The sectioned panel of the loaded chord (SPLC) is panel 5-7. It is necessary to
investigate three position of unit load on the loaded chord: outside the panel 5-7
(when loadPis located to the right of the joint 7 and to the left of joint 5) and within
the panel 5-7. Load-bearing contour (or loaded chord) is denoted by dotted line.