Structures

Question 1

What is a truss?

Answer 1

An assembly of long/ thin members

Question 2

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

Answer 2

Tension

Question 3

For a truss if the force is negative that means?

Answer 3

Compression occurs

Question 4

What does the method of joints involve?

Answer 4

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

Question 5

What does the method of sections involve?

Answer 5

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

Question 6

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

Answer 6

It is statically indeterminate

Question 7

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?

Answer 7

The structure will deform slightly less

Question 8

What is a pinned support?

Answer 8

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

Question 9

At each joint how many force equations are there?

Answer 9

2 - horizontal and vertical forces

Question 10

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

Answer 10

2j - two times the number of joints

Question 11

If 2j>b+k what happens?

Answer 11

A mechanism forms- structural collapse

Question 12

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

Answer 12

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

Question 13

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

Answer 13

the number of members

Question 14

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

Answer 14

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

Question 15

What is a simple truss?

Answer 15

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

Question 16

What is a compound truss?

Answer 16

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

Question 17

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

Answer 17

Method of sections

Question 18

What is a complex truss?

Answer 18

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

Question 19

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

Answer 19

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

Question 20

What is strength?

Answer 20

Ability of structure to carry loads

Question 21

What is stability?

Answer 21

Structures remain stable

Question 22

What is stiffness?

Answer 22

Structures don’t deform excessively

Question 23

How is the capacity of a tension member determined?

Answer 23

allowable stress (normal) x area

Question 24

How is the capacity of a compression member determined?

Answer 24

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 )

Question 25

How do you calculate stiffness?

Answer 25

force/ displacement (elongation)

Question 26

How do you calculate stress?

Answer 26

force/area

Question 27

Thermal strain causes ---- axial stress

Answer 27

no (since the bar is free to expand)

Question 28

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

Answer 28

The strain due to the applied force

Question 29

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

Answer 29

distance from the bar

Question 30

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

Answer 30

Quadratic

Question 31

What is shear strain?

Answer 31

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

Question 32

What is positive sheer strain?

Answer 32

When the angle of the object decreases

Question 33

Is bearing stress typically uniform?

Answer 33

No - bearing stress is not typically uniform

Question 33

What is bearing stress?

Answer 33

Stress that occurs because of the contact between two surfaces

Question 34

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

Answer 34

normal and shear

Question 34

A shear stress of t(xy) describes what?

Answer 34

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

Question 35

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

Answer 35

the sum of the angles of deformation

Question 36

How is shear strain defined?

Answer 36

shear stress (t)/G

Question 37

What is poissons effect?

Answer 37

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

Question 38

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

Answer 38

one direction

Question 39

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

Answer 39

(oxx -oyy - ozz) /E

Question 40

What are typical values for v?

Answer 40

0 to 0.5

Question 41

What is bulk stress?

Answer 41

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

Question 42

How can you define volumetric strain?

Answer 42

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

Question 43

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

Answer 43

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

Question 44

What is volumetric strain as a function of bulk stress?

Answer 44

(1-2v) 3ob/E

Question 45

What is the material bulk modulus?

Answer 45

bulk stress/ volumetric strain

Question 46

How can you calculate the material bulk modulus?

Answer 46

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

Question 47

What does a bulk modulus of infinity mean?

Answer 47

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

Question 48

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

Answer 48

- 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)

Question 49

What is the euler-bernolli beam theory?

Answer 49

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