Structural Analysis (MindTap Course List)
Structural Analysis (MindTap Course List)
5th Edition
ISBN: 9781133943891
Author: Aslam Kassimali
Publisher: Cengage Learning
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Chapter 8, Problem 1P
To determine

Draw the influence lines for the vertical reactions at supports A and C.

Draw the influence lines for the shear and bending moment at point B.

Expert Solution & Answer
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Explanation of Solution

Calculation:

Apply a 1 k unit moving load at a distance of x from left end A.

Sketch the free body diagram of beam as shown in Figure 1.

Structural Analysis (MindTap Course List), Chapter 8, Problem 1P , additional homework tip  1

Refer Figure 1.

Find the equation of support reaction (Ay) at A using equilibrium equation:

Take moment about point C.

Consider moment equilibrium at point C.

Consider clockwise moment as positive and anticlockwise moment as negative.

Sum of moment at point C is zero.

ΣMC=0Ay(28)1(28x)=0Ay(28)28+x=028Ay=28xAy=1x28        (1)

Find the equation of support reaction (Cy) at C using equilibrium equation:

Apply vertical equilibrium equation of forces.

Consider upward force as positive (+) and downward force as negative ().

Ay+Cy=1

Substitute 1x28 for Ay.

1x28+Cy=1Cy=11+x28Cy=x28        (2)

Consider Equation (1).

Find the value of influence line ordinate of reaction Ay at support A.

Substitute 0 for x in Equation (1).

Ay=1028=1k/k

Similarly calculate the influence line ordinate of reaction Ay for different value of x and summarize the result in Table 1.

x (ft)Ay(k/k)
01
140.5
280

Draw the influence line diagram for the vertical reactions at support A using Table 1 as shown in Figure 2.

Structural Analysis (MindTap Course List), Chapter 8, Problem 1P , additional homework tip  2

Consider Equation (2).

Find the influence line ordinate of reaction Cy at support C.

Substitute 28 for x in Equation (2).

Cy=2828=1

Similarly calculate the influence line ordinate of reaction Cy for different value of x and summarize the result in Table 2.

x (ft)Cy(k/k)
00
140.5
281

Draw the influence line diagram for the vertical reactions at support C using Table 2 as shown in Figure 3.

Structural Analysis (MindTap Course List), Chapter 8, Problem 1P , additional homework tip  3

Find the equation of shear force at B of portion AB (0x<14ft).

Sketch the free body diagram of the section AB as shown in Figure 4.

Structural Analysis (MindTap Course List), Chapter 8, Problem 1P , additional homework tip  4

Refer Figure 4.

Apply equilibrium equation of forces.

Consider upward force as positive (+) and downward force as negative ().

ΣFy=0

AySB1=0SB=Ay1

Substitute 1x28 for Ay.

SB=1x281=x28

Find the equation of shear force at B of portion BC (14ft<x28ft).

Sketch the free body diagram of the section BC as shown in Figure 5.

Structural Analysis (MindTap Course List), Chapter 8, Problem 1P , additional homework tip  5

Refer Figure 5.

Apply equilibrium equation of forces.

Consider upward force as positive (+) and downward force as negative ().

ΣFy=0

SB1+Cy=0SB=1Cy

Substitute x28 for Cy.

SB=1x28

Thus, the equations of the influence line for SB are,

SB=x28 0x<14ft        (3)

SB=1x28 14ft<x28ft        (4)

Find the value of influence line ordinate of shear force at various points of x using the Equations (3) and (4) and summarize the value as in Table 3.

x (ft)SB(k/k)
01
1412
14+12
280

Draw the influence lines for the shear force at point B using Table 3 as shown in Figure 6.

Structural Analysis (MindTap Course List), Chapter 8, Problem 1P , additional homework tip  6

Refer Figure 4.

Consider clockwise moment as positive and anticlockwise moment as negative.

Find the equation of moment at B of portion AB (0x<14ft).

MB=Ay(14)(1)(14x)

Substitute 1x28 for Ay.

MB=(1x28)(14)(1)(14x)=14x214+x=x2

Refer Figure 5.

Consider clockwise moment as negative and anticlockwise moment as positive.

Find the equation of moment at B of portion BC (14ft<x28ft).

MB=Cy(14)(1)[14(28x)]=14Cy14+(28x)=14Cy+14x

Substitute x28 for Cy.

MB=14(x28)+14x=x2+14x=14x2

Thus, the equations of the influence line for MB are,

MB=x2 0x<14ft        (5)

MB=14x2 14ft<x28ft        (6)

Find the value of influence line ordinate of moment at various points of x using the Equations (5) and (6) and summarize the value as in Table 4.

x (ft)MB (k-ft/k)
00
147.0
280

Draw the influence lines for the moment at point B using Table 4 as shown in Figure 7.

Structural Analysis (MindTap Course List), Chapter 8, Problem 1P , additional homework tip  7

Therefore, the influence lines for the vertical reactions at supports A and C and the influence lines for the shear and bending moment at point B are drawn.

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