pilesmakesmiles
Structural
- Jul 13, 2011
- 20
We're designing a piled tank for a sewage treatment works to Eurocode 2. The slab needs to be water retaining so thermal cracking is an important consideration.
Can anyone offer any advice on how to estimate the restraint factor (R) due to the piles?
I have come across 2 methods:
[ul][li]Work out the strain and shrinkage due to the temperature drop, then the stress, then the tensile force in the slab. Apply this force to the top of a pile and work out the deflection (assume horizontal spring stiffness=vertical spring stiffness). Plot two lines on a graph at R=0 and R=1 and where they cross is the restraint factor. This is the method suggested in CIRIA C660 based on a suspended slab between two walls - example attached.[/li]
[li]Work out the strain and shrinkage due to the temperature drop for 2 span sizes (not obvious how these spans are derived), calculate the force on a pile needed to create this deflection, use this force to work out the restrained stress in the slab, then the restrained strain in the slab and divide this by the strain with no restraint to get the restraint factor. (This is the example given in some course notes I have but I can't see where some of the numbers have come from).[/li][/ul]
However both methods give very small restraint factors (less than 0.05 for piles) however the second method says 'use 0.2' at the end of the calcualtion with no further explanation. The CIRIA report says that piled slabs should be designed as if they are end restrained, which suggests that the restraint is significant.
Can anyone offer any advice on how to estimate the restraint factor (R) due to the piles?
I have come across 2 methods:
[ul][li]Work out the strain and shrinkage due to the temperature drop, then the stress, then the tensile force in the slab. Apply this force to the top of a pile and work out the deflection (assume horizontal spring stiffness=vertical spring stiffness). Plot two lines on a graph at R=0 and R=1 and where they cross is the restraint factor. This is the method suggested in CIRIA C660 based on a suspended slab between two walls - example attached.[/li]
[li]Work out the strain and shrinkage due to the temperature drop for 2 span sizes (not obvious how these spans are derived), calculate the force on a pile needed to create this deflection, use this force to work out the restrained stress in the slab, then the restrained strain in the slab and divide this by the strain with no restraint to get the restraint factor. (This is the example given in some course notes I have but I can't see where some of the numbers have come from).[/li][/ul]
However both methods give very small restraint factors (less than 0.05 for piles) however the second method says 'use 0.2' at the end of the calcualtion with no further explanation. The CIRIA report says that piled slabs should be designed as if they are end restrained, which suggests that the restraint is significant.
