Lesson test: Trusses

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Trusses

1.[2p]
Why does an idealised truss member carry axial force only?

Forces act on it at two points alone, and two forces can only balance if they are equal, opposite and along the line joining them

Correct

Because bars are much stiffer in tension than in bending

The answer is: Forces act on it at two points alone, and two forces can only balance if they are equal, opposite and along the line joining them

Because the pins are frictionless, so no force can act across the bar

The answer is: Forces act on it at two points alone, and two forces can only balance if they are equal, opposite and along the line joining them

Because triangles cannot transmit moments

The answer is: Forces act on it at two points alone, and two forces can only balance if they are equal, opposite and along the line joining them

2.[3p]
A three-bar truss has supports at (0, 0) and (12, 0) and an apex at (6, 8), all in metres, and carries 30 kN at the apex. What is the tension in the bottom chord, in kN?

CorrectNot quite: 11.25

3.[2p]
Three members meet at an unloaded joint and two of them are collinear. How many of the three carry zero force?

CorrectNot quite: 1

4.[2p]
A determinate planar truss has 10 joints, a pin at one support and a roller at the other. How many members does it have?

CorrectNot quite: 17

5.[1p]
A member that carries zero force under the analysed load case can safely be left out of the built structure.

True

The answer is: False

False

Correct

6.[3p]
Which assumptions turn a real framework into an analysable truss?

Select all that apply

Every joint is a frictionless pin that transmits no moment

Correct

All loads are applied at the joints

Correct

Members are straight, and their self-weight is small or split between their end joints

Correct

Every member has the same cross-sectional area

The answer is: Every joint is a frictionless pin that transmits no moment, All loads are applied at the joints, Members are straight, and their self-weight is small or split between their end joints

7.[2p]
Why is a moment point chosen at the intersection of two of the three cut members?

Both of those forces pass through it, so the remaining unknown stands alone in one equation

Correct

It is the only point about which moments sum to zero

The answer is: Both of those forces pass through it, so the remaining unknown stands alone in one equation

It gives the largest moment arm for the unknown force

The answer is: Both of those forces pass through it, so the remaining unknown stands alone in one equation

It is where the truss is most heavily stressed

The answer is: Both of those forces pass through it, so the remaining unknown stands alone in one equation

8.[3p]
In the six-panel Pratt truss of the lesson, match each member to what it carries.

End diagonal
Top chord at midspan
Bottom chord at midspan
Vertical at midspan

128 kN tension
zero
144 kN compression
100 kN compression

Show the answer

End diagonal: 100 kN compression Top chord at midspan: 144 kN compression Bottom chord at midspan: 128 kN tension Vertical at midspan: zero

9.[3p]
Why do the chord forces in a truss grow towards midspan while the diagonal forces shrink?

The chords carry the bending moment and the diagonals carry the shear, and those peak at opposite ends of the span

Correct

The chords are longer, so they attract more force

The answer is: The chords carry the bending moment and the diagonals carry the shear, and those peak at opposite ends of the span

The diagonals are in tension, which is always smaller than compression

The answer is: The chords carry the bending moment and the diagonals carry the shear, and those peak at opposite ends of the span

The supports absorb the chord force but not the diagonal force

The answer is: The chords carry the bending moment and the diagonals carry the shear, and those peak at opposite ends of the span

1.[2p] Why does an idealised truss member carry axial force only?

2.[3p] A three-bar truss has supports at (0, 0) and (12, 0) and an apex at (6, 8), all in metres, and carries 30 kN at the apex. What is the tension in the bottom chord, in kN?

3.[2p] Three members meet at an unloaded joint and two of them are collinear. How many of the three carry zero force?

4.[2p] A determinate planar truss has 10 joints, a pin at one support and a roller at the other. How many members does it have?

5.[1p] A member that carries zero force under the analysed load case can safely be left out of the built structure.

6.[3p] Which assumptions turn a real framework into an analysable truss?

7.[2p] Why is a moment point chosen at the intersection of two of the three cut members?

8.[3p] In the six-panel Pratt truss of the lesson, match each member to what it carries.

9.[3p] Why do the chord forces in a truss grow towards midspan while the diagonal forces shrink?

1.[2p]
Why does an idealised truss member carry axial force only?

2.[3p]
A three-bar truss has supports at (0, 0) and (12, 0) and an apex at (6, 8), all in metres, and carries 30 kN at the apex. What is the tension in the bottom chord, in kN?

3.[2p]
Three members meet at an unloaded joint and two of them are collinear. How many of the three carry zero force?

4.[2p]
A determinate planar truss has 10 joints, a pin at one support and a roller at the other. How many members does it have?

5.[1p]
A member that carries zero force under the analysed load case can safely be left out of the built structure.

6.[3p]
Which assumptions turn a real framework into an analysable truss?

7.[2p]
Why is a moment point chosen at the intersection of two of the three cut members?

8.[3p]
In the six-panel Pratt truss of the lesson, match each member to what it carries.

9.[3p]
Why do the chord forces in a truss grow towards midspan while the diagonal forces shrink?