Influence lines (structural engineering)

In summary, the concrete highway bridge shown in Figure Q3a is supported on columns at B and E, and abutments at A and F. The bridge has joints at C and D, which can be modeled as pins. Figure Q3b shows an idealised model of the bridge deck. For the influence lines, the shear force across joint C has a value of 0 when x is less than 15m and when x is greater than 30m, and a value of (30-x)/15 when x is between 15m and 30m. The moments about joints C and D are assumed to be 0. The axial force in column E and the bending moment in the beam over column B for the
  • #1
haris89
1
0
The concrete highway bridge show in Figure Q3a is supported on
columns at B and E, and abutments at A and F. The bridge has joints
at C and D, which can be modeled as pins. Figure Q3b shows an
idealised model of the bridge deck.
Draw influence lines for:
i) the shear force across joint C;
ii) the axial force in column E;
iii) the bending moment in the beam over column B.

I attempted part (i) and got: when x<15m shear force at C=0, when 15m<x<30m shear force at C=(30-x)/15 and finally when x>30m shear force at C=0. I assumed the moments about joints C and D will equal 0 as they are pinned. Can anyone tell me if I'm correct?

Parts (ii) and (iii) are puzzling me though.
Could anyone give me some advice/tips on how to get an expression for the axial force in column E and bending moment over column B for the influence line?

[PLAIN]http://img294.imageshack.us/img294/1899/influenceline.jpg [Broken]
 
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  • #2
Haris, I believe you need to include the weight, 100kN, in your equations. Also, for part i, you should have 3 equations, one with the front axle on the center span, one with both axles and one with only the rear axle on the center span. Once you get that you might move on to the other two. You'll have to expend some pencil lead to complete them. S
 

1. What are influence lines in structural engineering?

Influence lines are graphical representations of the effect of a unit load moving along a specific point on a structure. They show how the reactions and internal forces of a structure vary as the load moves along the structure.

2. How are influence lines used in structural analysis?

Influence lines are used to determine the maximum and minimum values of reactions and internal forces at any point on a structure. They can also be used to determine the location of critical points, such as the point of maximum bending moment or shear.

3. What is the difference between influence lines and shear/moment diagrams?

Influence lines are used to find the maximum and minimum values of reactions and internal forces at any point on a structure, while shear/moment diagrams show the actual values of these forces at specific points along the structure.

4. How are influence lines affected by changes in the structure?

Influence lines are affected by changes in the structure, such as changes in support conditions or geometry. However, they can still be used to determine the maximum and minimum values of reactions and internal forces for the specific configuration of the structure.

5. What are the limitations of using influence lines in structural engineering?

Influence lines are limited to statically determinate structures, meaning structures where all reactions and internal forces can be calculated using equations of static equilibrium. They also do not account for secondary effects, such as deflections, and are only accurate for linearly elastic structures.

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