mod3

Question 1

ULS

Answer 1

calculate the collapse load for the foundation

- most straightforward method is to use plastic analysis

Question 2

SLS

Answer 2

determine the limiting working loads the foundation can support without deforming or settling excessively

Question 3

Advantage of SLS Analysis: dividing the failure load by some “factor of safety”

Answer 3

stress-strain behaviour not required in detail for either ULS or SLS

Question 4

disadvantage of SLS Analysis: dividing the failure load by some “factor of safety”

Answer 4

• previous knowledge and experience of the soil and circumstance required
• foundation settlement not calculated at all
• may lead to over design ($$) or under design (death)

Question 5

Advantage of SLS Analysis: using stress-strain response of soil at working loads to determine expected elastic deformations

Answer 5

elastic methods give reasonable approximation of soil deformation

Question 6

Disadvantage of using SLS Analysis: stress-strain response of soil at working loads to determine expected elastic deformations

Answer 6

rather complicated - good understanding of soil theoretical behaviour is required

Question 7

Advantage of advanced analysis: geotechnical numerical modelling

Answer 7

• a fully modelled soil-structure interactive response

Question 8

Disadvantage of advanced analysis: geotechnical numerical modelling

Answer 8

• extensive knowledge of soil parameters required and analysis effort can be relatively time-consuming and costly

Question 9

Advantages of ULS: Plasticity Method

Answer 9

• do not need the full stress-strain relationship of the soil (only soil strength)
• if we use the critical state strength, we do not need to know about the soil’s initial state or stress history

Question 10

Disadvantage of ULS: Plasticity Method

Answer 10

• plasticity methods do not give information about deformations

Question 11

Assumptions in Plastic Analysis

Answer 11

1. Soil Failure

2. Post-yield behaviour

Question 12

In the case of elasticity based methods we use:

Answer 12

a) equilibrium of stresses and forces
b) a constitution law relating stress and strain
c) compatibility of strains and displacements

Question 13

In the case of plasticity based methods we use:

Answer 13

1) a lower bound solution which satisfies:
a) conditions of equilibrium
b) a law based on material behaviour to ensure this falure criterion is not violated
2) an upper bound solution involving:
c) a kinematically admissible mechanism

Question 14

lower bound “safe” theorem

Answer 14

if you can determine a set of stresses in the ground in equilibrium with external loads, and which does not exceed the soil strength anywhere in the ground, the structure will NOT collapse

Question 15

upper bound “unsafe” theorem

Answer 15

if you take any compatible mechanism of slip surfaces and consider an increment of movement, and if you show that the work done by the soil resisting movement equals the work done by the external loads, the structure MUST collapse

Question 16

kinematically admissible mechanism

Answer 16

upper bound kinematically admissible mechanism for plastic materials are made up of straight lines or circles