Good design should not only be safe, but should aim to save construction costs, time and materials. The following procedures should help to achieve this and a ‘trained’ client will see the importance of financing this work with a realistic fee.
1. Determine the location of columns and foundation and the type of loads acting on them. (ex deal tax, live tax or wind tax)
The position of columns and load-bearing walls must be marked on the construction plan, as well as any other loads and bending moments caused. The loads should be classified into dead, imposed and wind loads, giving the appropriate partial safety factors for these loads.
2. Estimate the allowable bearing pressure of the soil using the soil survey report.
From a site survey study (if any), the strength of the soil at different depths or layers below foundation level should be examined to determine the safe bearing capacity at different levels. These values - or assumed bearing values (of any standards or codes) in the absence of a site survey – are used to estimate the allowable bearing pressure.
3. Determine the depth of the foundation
The invert level (bottom) of the foundation is determined by either the minimum depth below ground level which is unaffected by temperature, moisture content variations or erosion – this can be as low as 450mm in granular soils but depending on location and the soil conditions can be greater than 1 m – or due to the depth of cellar, boiler house, service channels or the like.
4. Calculate foundation area
The required foundation surface is determined from the characteristic (working) loads and estimated allowable pressure. This determines the preliminary design of the types or combination of foundation types. The selection is usually based on economy, speed and buildability of the structure.
5. Determining Vertical Stress Variation
The variation of vertical stress from depth is determined to check for possible overload of underlying weak layers.
6. Calculate settlement
Settlement calculations should be performed to verify that the total and differentiated settlements are acceptable. If these are unacceptable, a revised allowable bearing pressure should be determined and the foundation design changed to increase the surface area, or the foundations should be lowered to a deeper and stronger layer.
7. Cost Control
Before finalizing the choice of foundation type, preliminary cost calculations of alternative superstructure designs should be made, to determine the economics of increasing superstructure costs to reduce foundation costs.
8. Consider Time
Alternative safe designs should be checked for economy, speed and simplicity of construction. Speed and economy can conflict when building foundations – an initial inexpensive solution can extend the construction period. Time is often of the essence for a client who needs an early return on capital investment. An accelerated program of superstructure construction can be negated by slow foundation construction.
9. Variation in soil conditions
The design office must be prepared to adjust the design if excavations show deviations in the soil condition compared to the predictions from the soil investigation and survey of the site.