6-Influence lines for statically Determinate structures.pdf

STRUCTURAL ANALYSIS
(Determinate)
Soran University
Faculty of Engineering
Department of Civil and Environmental Engineering
Yousif J. Bas
(2023-2024)

CHAPTER 6
Influence
lines for
statically
Determinate
structures

Influence lines for statically
Determinate structures
CHAPTER OBJECTIVES
▪ To show how to draw the influence line for a statically
determinate structure, and provide specific applications to
floor girders and bridge trusses.
▪ To determine the maximum live shear and live moment in a
beam or bridge girder that supports several concentrated
loads.
R. C. Hibbeler, Structural Analysis, 10th Edition, 2017 (Yousif J. Bas)

Chapter Outline
▪ Influence Lines
▪ Influence Lines for Beams
▪ Influence Lines for Floor Girders
▪ Influence Lines for Trusses
▪ Maximum Influence at a Point due to a Series of Concentrated
Loads

Influence Lines
▪ If a structure is subjected to a live or moving load, however, the
variation of the shear and bending moment in the member is best
described using the influence line
▪ An influence line represents the variation of either the reaction,
shear, or moment, at a specific point in a member as a
concentrated force moves along the member
▪ One can tell at a glance, where the moving load should be placed
on the structure so that it creates the greatest influence at a
specified point
▪ the magnitude of the associated reaction, shear, or moment at the
point can then be calculated from the ordinates of the influence-
line diagram

Influence Lines
▪ Influence lines play an important part in the design of bridges,
industrial crane rails, conveyors, and other structures where loads
move across their span
One should be clear of the difference between Influence Lines &
shear or moment diagram
➢ Influence line represent the effect of a moving load only at a
specific point
➢ Shear or moment diagrams represent the effect of fixed loads at
all points along the axis of the member

Influence Lines
Procedure for Analysis
▪ Tabulate Values
▪ Influence-Line equations

Influence Lines
Tabulate Values
▪ Place a unit load at various locations, x, along the member, and at
each location use statics to determine the value of the function
(reaction, shear, or moment) at the specified point
▪ If the influence line for a vertical force reaction at the support of a
beam is to be constructed, consider the reaction to be positive
when it acts upward on the beam.
▪ If a shear or moment influence line is to be drawn for a point, take
the shear or moment at the point as positive according to the
same sign convention used for drawing shear and moment
diagrams.

Influence Lines
Influence-Line Equations
▪ The influence line can also be constructed by placing the unit load
at a variable position x on the member and then calculating the
value of R, V, or M at the point as a function of x.
▪ In this manner, the equations of the various line segments
composing the influence line can be determined and plotted.

Influence Lines
Example
Construct the influence line for the vertical reaction at A of the
beam

Influence Lines
Example
Construct the influence line for the vertical reaction at B of the
beam

Influence Lines
Solution
Tabulate Values

Influence Lines
Solution
Influence-Line Equations

Influence Lines
Example
Construct the influence line for the moment at point C of the beam

Influence Lines for Beams
Once the influence line for a function (reaction, shear, or moment)
has been constructed, it will then be possible to position the live
loads on the beam so that they produce the maximum value of the
function
Two types of loadings will be considered:
1. Concentrated force
2. Uniform load

Concentrated force

Uniform load

Example
Determine the maximum positive shear that can be developed at
point C in the beam shown in Figure due to a concentrated moving
load of 4 kN and a uniform moving load of 2 kN/m.

Influence Lines for Floor Girders
▪ the floor loads are transmitted from the slab to the floor beams,
then to the side girders, and finally to the supporting columns.

▪ The influence line for a specified point on the girder can be
determined using the same statics procedure
▪ In particular, the value for the internal moment in a girder panel
will depend upon where point P is chosen for the influence line
▪ Magnitude of 𝑴𝑷 depends upon the point’s location from end of
the girder
▪ Influence lines for shear in floor girders are specified for panels in
the girder and not specific points along the girder

Example
Draw the influence line for the shear in panel CD of the floor girder.

Solution
The unit load is placed at each floor beam location & the shear in
panel CD is calculated. Finally a segment of the girder is considered
& the internal panel shear 𝑽𝑪𝑫 is calculated.

Solution

Example
Draw the influence line for the moment at point F for the floor girder.

Influence Lines for Trusses
▪ The loading on the bridge
deck is transmitted to
stringers which in turn
transmit the loading to floor
beams and then to joints
along the bottom cord
▪ We can obtain the ordinate
values of the influence line
for a member by loading each
joint along the deck with a
unit load and then use the
method of joints or method
of sections to calculate the
force in the member

Example
Draw the influence line for the force in member GB of the bridge truss

Solution

Example
Draw the influence line for the force in member CG of the bridge truss

Maximum Influence at a Point due to a Series
of Concentrated Loads
▪ The max effect caused by a live concentrated force is
determined by multiplying the peak ordinate of the
influence line by the magnitude of the force
▪ In some cases, e.g. wheel loadings, several concentrated
loadings must be placed on structure
▪ Trial-and-error procedure can be used.

▪ Shear
➢ Consider the simply supported beam with associated influence line
for shear at point C
➢ The max +ve shear at C is to be determined due to the series of
concentrated loads moving from right to left
➢ Critical loading occurs when one of the loads is placed just to the
right of

6-Influence lines for statically Determinate structures.pdf

▪ Shear
➢ Case 2 yields the largest value for VC and therefore represents
the critical loading
➢ Investigation of Case 3 is unnecessary since by inspection such
an arrangement of loads would yield a value of (𝑉𝐶)3 that
would be less than (𝑉𝐶)2

▪ Moment
➢ Consider the beam, loading & influence line for the moment at
point C

▪ Moment
➢ When the loads of Case 1 are
moved to Case 2, it is observed
that the 9 kN load decreases
∆𝑀1−2
➢ Likewise, the 18 kN and 13.5
kN forces cause an increase of
∆𝑀1−2

▪ Moment
➢ The max moment at C is therefore,

6-Influence lines for statically Determinate structures.pdf

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