Reinforced concrete foundations are designed based on column loads and moments at base and the soil data. Following are the necessary data / steps need to be considered while designing of foundations: 1) Design basis of the foundations :
Codes Applicable (i.e. Indian (IS), US (ACI), British (BS) etc.
Grade of Concrete and Steel as per Design Basis Report and Technical Requirement.
2) Geotechnical Report :
No. of Boreholes in the plant area and on which particular Bore hole (BH) foundation is coming.
Presence of Ground water if any.
Design Soil Parameters : Net Allowable Bearing Capacity, Angle of Internal Friction, Soil Unit Weight, Subgrade Modulus, Coefficient of friction between concrete and soil.
Recommendation if any by Soil Investigation Agency : i.e. to go for isolated foundation / raft foundation / pile foundation.
3) Design Philosophy :
Description of Type of Structure for which foundation is need to be designed.
Possible worst Load Combinations both for serviceability and strength design as per the relevant codes.
Design Softwares i.e. Staad pro, SAP 2000 etc.
4) Design Loadings :
There are various types of loads which are acting on the structure and their simultaneous action in terms of Load Combinations are need to be taken at the base of the foundation ensuring safe design.
Type of Loadings : o
Dead Load: Self weight of the Building or structure , filling weight of the soil if any, equipment dead weight etc.
o
Live Loads: Live Loads on structures and substructure (i.e cable trench etc) are provided from the Design basis report and in case of unavailability from the design codes.
o
Wind Loads : Wind Loads on structures as per applicable codes i.e. IS 875, ASCE 07 etc. are calculated for the given design wind speed. Design wind speed is either recommended or c an be checked from the wind zone under which plant is coming.
o
Seismic Loads : Seismic Loads on structures are calculated as per Applicable Design codes i.e. IS 1893, UBC etc. Seismic zone is determined as per codes. Also, its important to determine which type of structural analysis i.e. (static or dynamic) is demanded as per zone. Normally for lower seismic zones, static analysis is done while for higher , dynamic is done. Different structural analysis, provide different seismic design loads, which really determines size of foundations.
o
Crane Load : Crane loads in case of buildings, industrial sheds etc are calculated. Crane data is provided from the vendor /crane manufacturer side.
o
Horizontal Soil Pressure : In case of retaining wall or cable tunnel wall etc, the lateral pressure due to soil is calculated and applied.
o
Surcharge : Surcharge is a live load provided normally in design basis report or can be calculated manually.
o
Further in special cases, if there are surrounding foundations, and the foundation which is to be designed is coming below their levels, then the effect of loads transferred from adjacent foundation is also need to be considered.
5) Size of foundation :
Under serviceability criteria, foundation is checked for overturning, sliding, bearing pressure and buoyancy for unfactored Design Loads in case of Isolated, Combined and Raft Foundations.
For Pile Foundations, serviceability checks are for compression, uplift and lateral stability.
Serviceability criteria determine the area and depth of foundation / Length of pile.
Also need to be checked as per the plant foundation layout or boundary limitations, are there any size limitations, depth limitations for foundation if any. Necessary modifications need to be done to avoid fouling of foundations and for ease of construction.
As per thumb rule, if for a building more than 50% of building area is coming under Isolated footing , then raft foundation is need to be provided.
In case of Load combination including either wind or seismic, Soil Bearing Capacity can be increased by 25% as per Design Codes which further led to optimize the size and depth of foundation.
While determining size of foundation, gross bearing pressure shall never be negative.
6) Strength Design :
Under Strength criteria, foundation is designed for factored loading i.e. (Factor of safety as per applicable Concrete Design Code) which determine the thickness of the foundation and its reinforcement.
Foundation thickness is determined from the shear criteria. i.e. one way shear and two way shear.
Strength reduction factors if any as per applicable design codes are need to be considered while designing of foundation.
During RCC design of foundation, foundation is to be designed for NET factored loading i.e. loading due to soil and self weight of footing shall be deducted. Considering gross loading leads to uneconomical design of foundations.
In case of raft, since maximum moment comes at the face of the column, thickness and reinforcement of the raft can be reduced by providing extra
thickness around the periphery of column and by providing extra reinforcement near column face satisfying design requirements and design codes criteria.
Also , while plate modelling in Staad, it is very important to correctly assess the output i.e. inplane stresses and moments , which really help in optimizing the reinforcement and also determines the critical locations.
Above approach is for general foundation design as per Limit state design. However, in case of Transmission Towers, loadings get different and foundation design is as per Working Stress.
