DESIGN PROCEDURES FOR A BUILDING FOUNDATION
Good design must not only be safe but must aim to save
construction costs, time and materials. The following procedures should help to
achieve this and an ‘educated’ client will recognize the importance of funding
this work with a realistic fee.
1. DECIDE THE LOCATION OF COLUMNS & FOUNDATION AND
TYPE OF LOADS ACTING ON THEM
On the building plan, the position of columns and
load bearing walls should be marked, and any other induced loading's and bending
moments. The loads should be classified into dead, imposed and wind loading's,
giving the appropriate partial safety factors for these loads.
2. ESTIMATE ALLOWABLE BEARING PRESSURE OF SOIL USING
GROUND INVESTIGATION REPORT
From a study of the site ground investigation (if
available), the strength of the soil at various depths or strata below
foundation level should be studied, to determine the safe bearing capacity at
various levels. These values – or presumed bearing values (from any standards
or codes) in the absence of a site investigation – are used to estimate the
allowable bearing pressure.
3. DECIDE DEPTH OF FOUNDATION
The invert level (underside) of the foundation is
determined by either the minimum depth below ground level unaffected by
temperature, moisture content variation or erosion – this can be as low as 450
mm in granular soils but, depending on the site and ground conditions, can
exceed 1 m – or by the depth of basement, boiler house, service ducts or
similar.
4. CALCULATE FOUNDATION AREA
The foundation area required is determined from the
characteristic (working) loads and estimated allowable pressure. This
determines the preliminary design of the types or combination of types of
foundation. The selection is usually based on economics, speed and build ability
of construction.
5. DETERMINE VARIATION IN VERTICAL STRESSES
The variation of vertical stress w.r.t depth is
determined, to check for possible over-stressing of any underlying weak strata.
6. CALCULATE SETTLEMENT
Settlement calculations should be carried out to check
that the total and differential settlements are acceptable. If these are
unacceptable then a revised allowable bearing pressure should be determined,
and the foundation design amended to increase its area, or the foundations
should be taken down to a deeper and stronger stratum.
7. COST CONTROL
Before finalizing the choice of foundation type, the
preliminary costing of alternative superstructure designs should be made, to
determine the economics of increasing superstructure costs in order 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
in foundation construction – an initial low-cost solution may increase the
construction period. Time is often of the essence for a client needing early
return on capital investment. A fast-track programmed for superstructure
construction can be negated by slow foundation construction.
9. VARIATION IN GROUND CONDITION
The design office should be prepared to amend the
design, if excavation shows variation in ground conditions from those predicted
from the site soil survey and investigation.
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