Strength of materials

Question 1

What are the 4 types of material strength?

Answer 1

Compressive strength, tensile strength, flexural strength, shear strength.

Question 2

If we were to design a steel cable for a bridge, which will only ever be in tension, what value of area do we require for the cable?

Answer 2

Area = Applied tensile force / Material stress capacity

Question 3

What are the 2 main things factors of safety make allowances for?

Answer 3

Substandard materials, and the deterioration of materials

Question 4

For steel, as the exact yield point of the curve is no always defined, how do we find it?

Answer 4

We use a 0.2 percent proof stress, which is defined as the stress to give a 0.2% strain

Question 5

In the structural design of steel, what are the main reasons ductility is utilised?

Answer 5

The redistribution of stress, reduced crack propagation

Question 6

What happens to steel at low temperatures?

Answer 6

May lose its ductility and be susceptible to fracture

Question 7

Why should care be taken when specifiying hot-rolled steel elements with large thicknesses?

Answer 7

Because as thickness increases, there is a greater risk of disconuities and imperfections within the grain structure, making the steel more prone to fracture at lower temperature

Question 8

How do we design a steel element in relation to its number of cycles?

Answer 8

Find the lower limit (where the applied stress with relation to number of cycles is horizontal

Question 9

What is the strength of concrete in tension compared to its compressive strength?

Answer 9

Its compressive strength is 10 times its tensile strength

Question 10

What are the 2 reasons concrete is only designed to resist compressive loads?

Answer 10

Far weaker in tension, and has a more brittle tensile fracture

Question 11

What are the 3 important factors that influence the strength of concrete at the design stage?

Answer 11

Properties of raw materials used, mix proportions, and the water/cement ratio.

Question 12

What are the 2 key factors that influence the strength of concrete at the construction stage?

Answer 12

Quality of compaction and curing

Question 13

Why is curing needed in concrete construction?

Answer 13

To maintain sufficient moistures and temperatures while the concrete is hydrating, if the concrete is dried to quickly cement grain will form and impermeable layer

Question 14

How is curing carried out?

Answer 14

By keeping in close contact with the surface area, e.g. spraying the surface, can be 3-7 days

Question 15

How is the strength of concrete measured?

Answer 15

Cube samples of concrete are tested at 7 days (when 50% of the 28 days strength should be achieved.

Question 16

How many days is the ‘characteristic strength’ of concrete based on?

Answer 16

28 Days

Question 17

How is the lack of tensile capacity in concrete structures overcome?

Answer 17

Through the addition of a steel reinforcing bar

Question 18

How does a steel reinforcing bar within a concrete structure help?

Answer 18

Transfer of shear stress between the two materials

Question 19

What makes masonry units and concrete similar?

Answer 19

Both far stronger in compressive strength

Question 20

How is the compressive strength of masonry units tested?

Answer 20

By crushing the units between steel platens

Question 21

As ‘masonry’ is made up of masonry units and mortar, what is the overall ‘characteristic strength’ dependant on?

Answer 21

The strength of the units and mortar

Question 22

Timber is an anisotropic material, what does this mean?

Answer 22

The compressive strength parallel to the grain is far stronger than perpendicular to the grain

Question 23

How do natural defects influence the strength of timber?

Answer 23

Timber comes with various natural irregularities, which can influence its strength. A knot affects the strength of timber

Question 24

Timber is a viscoelastic material, what does this mean?

Answer 24

Behaves as both a liquid and solid when deforming

Question 25

For large, sustained loads, how much can the strength of timber be reduced by?

Answer 25

As much as 50%, which could result in unexpected failure

Question 26

In order to account for the time and moisture-dependant behaviour of timber, what are the modification factors governed by?

Answer 26

The material form, the possible effects of exposure and duration of the applied load

Question 27

As timber is a natural material, what does this therefore mean?

Answer 27

It is variable

Question 28

As timber is variable, what must it be?

Answer 28

Graded, so the weak and defective pieces can be rejected

Question 29

How is timber graded?

Answer 29

Through visual inspection or machine strength grading

Question 30

What is machine strength grading of timber?

Answer 30

The bending stiffness of timber is checked through passing through a series of rollers, through which a timber grade can be achieved

Question 31

What is visual inspection grading of timber?

Answer 31

A trained inspector will examine each piece of timber checking for quantities and size of knots, distortions or other damage, through which the element is then accepted/rejected.

Question 32

What is engineered timber?

Answer 32

Timber that has been engineered to meet ‘precise’ design requirements, such as increased size and increased strength

Question 34

Stress - strain graph for different materials

Answer 34