Capability and limitations

Owned by Iain Whyte
Last updated: Apr 11, 2023
4 min read

The new feature of the program is a section relating to design for the influence of trees and removal of trees. The input allows for ys and yt, potential surface movement due to the tree, (induced suction change in addition to the normal design suction change) and optional ‘if tree is removed’. The design is in accordance with Appendix H Guide to Design of Footings for Trees and appendix CH Commentary to Guide to Design of Footings for Trees. Guidance to the design for trees using CORD is also provided in Appendix B of this manual.

  • The program allows for different footing types, stiffened Raft, Waffle Raft and Strip footing design for the classes of structures set out in AS2870-2011, Section 1.1.

  • The program is a design ''tool'' to assist the user in designing raft and waffle raft slabs and strip footings. It is intended to be used in conjunction with the design procedures and site management requirements set out in AS2870-2011. A thorough understanding of the importance of the geotechnical input data relating to the physical characteristics of the site foundation soils is considered a necessary prerequisite for the satisfactory use of the package.

  • The normal working limits for which CORD is recommended are:

  • Articulated superstructure, 20 mm ≤ ys ≤ 250 mm

  • Non-articulated superstructure, 20 mm ≤ ys ≤ 120 mm

  • CORD 8.0 includes the design calculation of footings for the influence of trees (tree drying effects) and the effects of tree removal or death. These are calculated by the program.

  • For ys < 50 mm the moment of resistance for centre heave (hogging) is the critical design case. The depth of footing required is very sensitive to the percentage reinforcement for this case. Designs where the footing depths are significantly less than those given in Section 3.0 of AS2870-2011, which have a history of satisfactory field performance, should be reviewed by a Qualified Engineer.

 

The package calculates the bending moments, ductility and moments of inertia for the footing cross section as a whole (per 1m width) for each rectangle chosen, both long and short direction, edge heave and centre heave.

The program also calculates for singe strip footings.

The following 3 criteria are then considered:

(1) Strength

Bending moment required < 0.8 x Mu

For tree removal option the following is also considered.
The edge heave bending Mu should not be less that the moment resistance Mu for the centre heaves.

(2) Ductility

1.2 x cracking moment < Mu

Except for tree cases, existing trees, new trees and trees removed then 1.5 x cracking moment < Mu for centre heaves and edge heaves

(3) Stiffness

Moment of Inertia required < effective footing beams moment of inertia (Branson formula)

 

The program is currently set up to analyse the load case (DL (Dead Load) + LL (Live Load)). Please enter a suitably modified value for LL (e.g. 0.5 LL).

Hence the following code assumptions are satisfied:

  • For long term computation of deflection –

    I(DL + LL) < Ieffective

  • For short term calculation of cracking or yield

    M(DL+LL) < 0.8xMu

 

If the choice of footing fails any of the above criteria then those particular inadequacies (or multiple inadequacies) are highlighted in red on the preliminary screen output. The user can then return to the design and modify parameters (most likely beam depth and quantity of reinforcement, but in fact any parameters throughout the whole design can be modified). The user iteratively changes the footing design until the requirements are met. New inputs or corrections may be done at any stage of the program. There is no need to end the program and restart.

In the program the following information has been used (as outlined in AS2870-2011, Section 4).

The cracking moment has been determined for sagging moments with 20 MPa concrete using a tensile strength of 2.7 MPa and for hogging moments 1.8 MPa. (AS2870-2011, 4.4(i)).

 

The concrete strength is defaulted as 20MPa and may be adjusted at input stage.

The effective total width of the flange in both tension and compression has been taken as follows:

  • Edge beams: Beam width + (Slab length) / 10

  • Internal beams: Beam width + (Slab length) / 5

  • --- but not greater than the distance halfway to the adjacent beam.

 

Concrete stiffness. The Youngs Modulus of concrete is calculated in accordance with the Concrete Structures Code (AS3600) formula.

The influence of creep is included through the simple deflection multiplier, 80% increase in deflections due to creep. This was reduced by 25% to allow for the influence of a delay, usually not less than 3 months between the construction of the slab and any design ground movement.

The total effect was that the value of Ec (maximum) was taken as 15.5 GPa for 20 MPa concrete. This value however assumes that the area of compression and tension reinforcement is equal in the relationship (2-1.2 Asc/Ast). The program allows for variations in the ratio of Asc to Ast. In the analysis section the required I values are calculated using Ec max. However, in the beam analysis section the I required values from the analysis are factored up to take into account a reduction in Ec from 15.5 GPa.