Presentation on theme: "Part1: Shollow foundations"— Presentation transcript:
1 Part1: Shollow foundations CONSTRUCTION SYSTEMS 2
2 Foundations Learning Outcomes At the end of this presentation, students would be able to describe the site in terms of:Legal titleLocal authority requirementsTopographical and survey termsPhysical featuresWhere would you choose to put down your foundations?Auckland volcanic mapImage:
3 Foundations Learning Outcomes At the end of this presentation, students would be able to describe the site in terms of:Legal titleLocal authority requirementsTopographical and survey termsPhysical featuresFoundations are bearing directly on the soil, thus the system must:Distribute vertical loads so the settling of a building is negligible oruniform under all parts of the building.Lifting Forces:Anchor the building‘s super-structure to prevent uplifting due to wind and earthquake forces.The foundation system for a building is the critical link in the transmission of building loads down to the ground
4 LOADS Typical real life answers: Building loads(Forces)Gravity LoadsLateral LoadsDead LoadsLive loadsBuilding massFixed contentFloor Live loadsRoof Live loadsWindEarthquakeSoil & groundwater pressureTypical real life answers:Was the District Plan consulted? – Uhm…I think Roy did that.Do we need resource consent? – Not as far as I know.Who are the consultants? – Check with Daisy ok?What’s the budget? – Around 6 and a half, just don’t go over!You want it when? – Yesterday, need you ask…So, do you also like pink windows? – Yeah right! (could go either way!)We say: “Have a go, just follow your nose”!And you say:Stuff that mate, I paid good money for this %$#& course!“But we are all lost and confused!” – later, bye…Please can I change stream/go back to first year?I hate you, you are such a bastard!So aren’t you going to teach us anything?Yes! Just ask a sensible question!
5 Foundation Settlement Patterns a building bears down on the supporting soil, some settlement is to be expected.The cause of settlement is typically due to a reduction in the volume of air voids in the soil.A properly designed and constructed foundation system should minimize settlementImage:
6 Foundation Settlement Patterns .Uneven or "differential" settlement can cause a building to shift out of plumb causing cracks in the foundation, structure, or finish.Extreme differential settlement can lead to failure of a building's structural integrity.Differential Settlement causes Frame Distortion or Sloping sites contains the risk of slip plane actionImage:
7 To get the district plan maps; Left:Bulb of Pressure showingratios decreasing outwardsRight:Bulbs of Shear Stress, againshowing reducing ratios asoutward distance increasesSloping sites contains the risk of slip plane actionISource: Stroud Foster p.35
9 Foundations Types SHALLOW Column or wallColumn or wallSHALLOWShallow foundations transfer the load to soil at the base of the substructureDEEPDeep foundations transfer loads far below the substructure> 3mThese foundations penetrate incompetent soil until a satisfactory bearing stratum is reached.
15 Stress Distribution in Soil Strip footings are placed under continuous walls, thus carrying a uniformly distributed load.The required width is related to: - the imposed load and- the bearing capacity of the soil.The strip footing may be stepped on a sloping site to minimise trenches and the amount of walling in the soil
16 Shallow Foundation:Foundation pad size dependent on:The soil strength or bearing capacityThe pad thickness is dependent on the column loading.The upper surface can be flat, stepped or sloping.The major reinforcing is placed at bottom layer, to compensate for the low tensile strength of the concrete
17 Forming a pad footing for isolated column To get the district plan maps;Forming a pad footing for isolated columnstep 1: Pocket excavated to firm bearing, Blinding layer (optional) and DPM placedStep 2: Formwork erected and steel cage dropped in placeNote cage in background
22 Strip Foundations & Pads: Combined Foundations: Shallow Foundations:Strip Foundations & Pads: Combined Foundations:Spread type foundations and are designed to overcome particular restrictions, thus are less common .For example:may be used to distribute loadsbetween unequally loaded columnsusing a tapered pad shapeImage:
23 Combined Foundations: For improved load distribution:Pad Footings may be combined by linking pads together with ground beams , in order to provide support to slab edges and/or loadbearing walls
24 Strip Foundations & Pads Cantilevered Foundations: Shallow Foundations:Strip Foundations & Pads Cantilevered Foundations:When some obstruction at the left column position prevents an adequate foundation being placed directly below the column,Image:
25 Formwork for a pad footing for an isolated column with linking tie beams to adjacent columns Rebar: 4/D500E32 longitudinal
26 Dual system: Pads & Slab on grade Pad footings and ground beams (in foreground) may be combined with reinforced slab on grade.Differential movement checks by Engineer: isolation joints?
29 Cellular Raft Basement walls forms Ribs Twin slabs: below and above /
30 Raft Foundations:The raft foundation avoids many of these issues by sitting on top of the ground and having a lattice of steel reinforced concrete ribs and beams to provide support and stiffness.Because they do not require excavation and are specifically designed for each building they can be more robust, quicker to build and often less expensive than traditional foundations.Image:
31 Raft Foundations:The RibRaft flooring system is an innovative method of concrete floor construction that's ideal for masonry construction and light commercial flooring.Image:
35 Seismic ForcesDuring an Earthquake the ground moves rapidly while buildings tryto resist displacement via their inherent inertiaMajor damage tend to be caused by the horizontal componentsof these imposed loads/
36 Seismic Forces Primary Factors Affecting Foundation Choice: Sub-surface soil (Geotechnical Profile)Ground water table conditionsBuilding structural requirementsSecondary Factors Affecting Foundation Choice:Construction access, methods and site conditionsEnvironmental factors (i.e. seismic zone)Building Codes and Regulations (i.e. setbacks)Impact on surrounding structures (subsidence)Construction schedule (lead times for piles)Construction risks (see piling video clips)/