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Engineering science > Structures > Flashcards

Structures Flashcards

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1
Q

What is a truss?

A

An assembly of long/ thin members

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2
Q

In trusses if a force is positive the structure is under what?

A

Tension

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3
Q

For a truss if the force is negative that means?

A

Compression occurs

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4
Q

What does the method of joints involve?

A

Writing two force equations, starting with a joint with a single unknown force- this can be a heavily lengthy process

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5
Q

What does the method of sections involve?

A

Considering the whole structure - it all must be in equilibrium. Take a cut through a section. Use of rotation moments

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6
Q

If there are more restraints than required what would we say about the structure?

A

It is statically indeterminate

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7
Q

What is if there are more members than necessary, if an external load is applied compared to the required number of members to prevent collapse?

A

The structure will deform slightly less

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8
Q

What is a pinned support?

A

The structure is free to rotate about the joint, but no translation movement occurs

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9
Q

At each joint how many force equations are there?

A

2 - horizontal and vertical forces

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10
Q

How many total force equations are there when considering a structure?

A

2j - two times the number of joints

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11
Q

If 2j>b+k what happens?

A

A mechanism forms- structural collapse

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12
Q

b+ k = 2j is necessary but not /// for stability

A

sufficient (as it depends on the arrangement of the members)

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13
Q

What is b in the equation 2j = b+k?

A

the number of members

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14
Q

What is k in the equation 2j = b+k?

A

the number of restraints (forces split horizontal and vertical from supports)

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15
Q

What is a simple truss?

A

A stable statically determinate structure with the right number of supports ( most often a pin and a roller - providing three unknowns)

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16
Q

What is a compound truss?

A

The joining of two simple trusses at a single joint - an extra member is added

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17
Q

What method has to be used to find forces in a compound truss?

A

Method of sections

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18
Q

What is a complex truss?

A

Where there are overlapping members - even though it is statically determinate you can’t use method of joints or method of structures - instead use of simultaneous equations - eg 12 force equations are needed for 6 joints

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19
Q

Why are some trusses statically indeterminate (eg have redundant members?)

A

Extra members are needed to mitigate against the loss of a member

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20
Q

What is strength?

A

Ability of structure to carry loads

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21
Q

What is stability?

A

Structures remain stable

22
Q

What is stiffness?

A

Structures don’t deform excessively

23
Q

How is the capacity of a tension member determined?

A

allowable stress (normal) x area

24
Q

How is the capacity of a compression member determined?

A

Use of cross sectional area and length - long thin members are prone to buckling (sudden change in shape under load - we want to prevent this )

25
Q

How do you calculate stiffness?

A

force/ displacement (elongation)

26
Q

How do you calculate stress?

A

force/area

27
Q

Thermal strain causes —- axial stress

A

no (since the bar is free to expand)

28
Q

When a force is applied to a heated material to prevent thermal expansion, the strain due to thermal expansion is equal to what?

A

The strain due to the applied force

29
Q

Under self weight deformation stress and strain become linear functions of what?

A

distance from the bar

30
Q

What kind of function does elongation of a bar against distance from tension point become under self weight?

A

Quadratic

31
Q

What is shear strain?

A

The angle between two line segments when sheer stress is applied (typically defined in radians )

32
Q

What is positive sheer strain?

A

When the angle of the object decreases

33
Q

Is bearing stress typically uniform?

A

No - bearing stress is not typically uniform

33
Q

What is bearing stress?

A

Stress that occurs because of the contact between two surfaces

34
Q

At the infinitesimal level there is only what two types of stain?

A

normal and shear

34
Q

A shear stress of t(xy) describes what?

A

the surface is orthogonal to x and the stress acts in the y direction

35
Q

What is engineering shear strain defined as when strain happens in multiple directions?

A

the sum of the angles of deformation

36
Q

How is shear strain defined?

A

shear stress (t)/G

37
Q

What is poissons effect?

A

strain in the x direction leads to a negative strain in the y direction

38
Q

When applying poisons effect although strain occurs in both directions, stress only occurs in…?

A

one direction

39
Q

How do you work out strain in the x direction in a 3D model?

A

(oxx -oyy - ozz) /E

40
Q

What are typical values for v?

A

0 to 0.5

41
Q

What is bulk stress?

A

a special case where all normal stresses are the same and there is no shear stress (eg a cube underwater)

42
Q

How can you define volumetric strain?

A

v-v0/v
exx+eyy+ezz (sum of strains in x, y and z directions)

43
Q

What is the key concept linking volumetric strain and bulk stress?

A

the stress in all directions is the same (ob) hence you can rewrite the matrix calculation including this

44
Q

What is volumetric strain as a function of bulk stress?

A

(1-2v) 3ob/E

45
Q

What is the material bulk modulus?

A

bulk stress/ volumetric strain

46
Q

How can you calculate the material bulk modulus?

A

E/ 3(1-2v)
bulk stress/ volumetric strain

47
Q

What does a bulk modulus of infinity mean?

A

the material is incompressible (no matter the stress there is no strain)

48
Q

What are the steps in accomplishing a nice relationship between max stress, maximum bending moment, y max and moment of inertia?

A
  • plane sections remain perpendicular to the beams’ axis
  • hooke’s law - move to stresses
  • use of force equilibrium
  • moment equilibrium (and use of second moment of area)
49
Q

What is the euler-bernolli beam theory?

A

for an elastic beam where H is much smaller than L, considering small deflections and zero shear force, the plane section remains plane and perpendicular to the beam’s axis

Engineering science (2 decks)

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