BUILDING CONSTRUCTION ( )

1. BHOL SUFIYAN [ ] 2. CHAVDA KRUNAL [ ] YEAR: 2 ND SEM: 3 RD CIVIL ENGINEERING DEPARTMENT PRESENTED BY : GUIDED BY : PROF. MISHRA INDU VADODARA INSTITUTE OF ENGINEERING

 Necessity of shallow foundation  Types of shallow foundation

 Establish the most economical design that safely conforms to prescribed structural criteria and properly accounts for the intended function of the structure  Evaluate subsurface investigation and laboratory testing data for reliability and completeness  Total cost of a foundation must include all costs associated with the foundations - Need for excavation support system, pile caps, etc. - Environmental restrictions - All other factors as applicable

 LOADS :- - Permanent and Transient - Codes specify load combinations  FOUNDATION LIMIT STATES:- - Ultimate Bearing capacity, eccentricity, sliding, global, stability, structural capacity - Serviceability Excessive settlement, excessive lateral, displacement, structural deterioration of foundation

 Tests for gradation and durability of fill at sufficient frequency to ensure that the material meets the specification  Compaction tests  If surcharge fill is used for pre-loading verify the unit weight of surcharge

 Check elevations of footing, particularly when footings are on embankment fills  Periodic surveying during the service life of the footing, particularly if the subsurface has soft soils within the depth of influence  Impacts on neighboring facilities  Use instrumentation as necessary

 The following considerations are necessary for deciding the location and depth of foundation - As per IS: , minimum depth of foundation shall be 0.50 m. - Foundation shall be placed below the zone of the frost heave - Excessive volume change due to moisture variation (usually exists within 1.5 to 3.5 m depth of soil from the top surface) - Topsoil or organic material - Peat and Muck - Unconsolidated material such as waste dump  Foundations adjacent to flowing water (flood water, rivers, etc.) shall be protected against scouring. The following steps to be taken for design in such conditions - Determine foundation type - Estimate probable depth of scour, effects, etc. - Estimate cost of foundation for normal and various scour conditions - Determine the scour versus risk, and revise the design accordingly

 IS: recommendations for foundations adjacent to slopes and existing structures - When the ground surface slopes downward adjacent to footing, the sloping surface should not cut the line of distribution of the load (2H:1V). - In granular soils, the line joining the lower adjacent edges of upper and lower footings shall not have a slope steeper than 2H:1V. - In clayey soil, the line joining the lower adjacent edge of the upper footing and the upper adjacent edge of the lower footing should not be steeper than 2H:1V.

 Other recommendations for footing adjacent to existing structures - Minimum horizontal distance between the foundations shall not be less than the width of larger footing to avoid damage to existing structure - If the distance is limited, the principal of 2H:1V distribution should be used so as to minimize the influence to old structure - Proper care is needed during excavation phase of foundation construction beyond merely depending on the 2H:1V criteria for old foundations. Excavation may cause settlement to old foundation due to lateral bulging in the excavation and/or shear failure due to reduction in overburden stress in the surrounding of old foundation

 Footings on surface rock or sloping rock faces - For the locations with shallow rock beds, the foundation can be laid on the rock surface after chipping the top surface. - If the rock bed has some slope, it may be advisable to provide dowel bars of minimum 16 mm diameter and 225 mm embedment into the rock at 1 m spacing.  A raised water table may cause damage to the foundation by - Floating the structure - Reducing the effective stress beneath the foundation Water logging around the building may also cause wet basements. In such cases, proper drainage system around the foundation may be required so that water does not accumulate.

 PERMANENT LOAD: This is actual service load/sustained loads of a structure which give rise stresses and deformations in the soil below the foundation causing its settlement.  TRANSIENT LOAD: This momentary or sudden load imparted to a structure due to wind or seismic vibrations. Due to its transitory nature, the stresses in the soil below the foundation carried by such loads are allowed certain percentage increase over the allowable safe values.  DEAD LOAD: It includes the weight of the column/wall, footings, foundations, the overlaying fill but excludes the weight of the displaced soil  LIVE LOAD: This is taken as per the specifications of IS:875 (pt-2) – 1987.

 Following combinations shall be used - Dead load + Live load - Dead Load + Live load + Wind/Seismic load  For cohesive soils only 50% of actual live load is considered for design (Due to settlement being time dependent)  For wind/seismic load < 25% of Dead + Live load - Wind/seismic load is neglected and first combination is used to compare with safe bearing load to satisfy allowable bearing pressure  For wind/seismic load ≥ 25% of Dead + Live load - It becomes necessary to ensure that pressure due to second combination of load does not exceed the safe bearing capacity by more than 25%. When seismic forces are considered, the safe bearing capacity shall be increased as specified in IS: 1893 (Part-1) (see next slide). In non-cohesive soils, analysis for liquefaction and settlement under earthquake shall also be made.

 For economical design, it is preferred to have square footing for vertical loads and rectangular footing for the columns carrying moment  Allowable bearing pressure should not be very high in comparison to the net loading intensity leading to an uneconomical design.  It is preferred to use SPT or Plate load test for cohesion less soils and undrained shear strength test for cohesive soils.  In case of lateral loads or moments, the foundation should also be checked to be safe against sliding and overturning The FOS shall not be overturning. less than 1.75 against sliding and 2.0 against overturning. When wind/seismic loads are considered the FOS is taken as 1.5 for both the cases.  Wall foundation width shall not be less than [wall thickness + 30 cm].  Unreinforced foundation should have angular spread of load from the supported column with the following criteria - 2V:1H for masonry foundation - 3V:2H for lime concrete - 1V:1H for cement concrete foundation The bottom most layer should have a thickness of atleast 150 mm.