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Part1: Shollow foundations

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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 title Local authority requirements Topographical and survey terms Physical features Where would you choose to put down your foundations? Auckland volcanic map Image:

3 Foundations Learning Outcomes
At the end of this presentation, students would be able to describe the site in terms of: Legal title Local authority requirements Topographical and survey terms Physical features Foundations are bearing directly on the soil, thus the system must: Distribute vertical loads so the settling of a building is negligible or uniform 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 Loads Lateral Loads Dead Loads Live loads Building mass Fixed content Floor Live loads Roof Live loads Wind Earthquake Soil & ground water pressure Typical 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 settlement Image:

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 action Image:

7 To get the district plan maps;
Left: Bulb of Pressure showing ratios decreasing outwards Right: Bulbs of Shear Stress, again showing reducing ratios as outward distance increases Sloping sites contains the risk of slip plane action ISource: Stroud Foster p.35

8 Foundations Types: The Range
Shallow Deep

9 Foundations Types SHALLOW
Column or wall Column or wall SHALLOW Shallow foundations transfer the load to soil at the base of the substructure DEEP Deep foundations transfer loads far below the substructure > 3m These foundations penetrate incompetent soil until a satisfactory bearing stratum is reached.

10 Shallow & deep foundation

11 Shallow Strip Wide strip Narrow strip Pads Isolated Continuous
Foundations Types: The Range Shallow Strip Wide strip Narrow strip Pads Isolated Continuous Cantilever Raft Solid Slab Beam & Slab Cellular

12 Strip Footing

13 Pads, Isolated Pads

14 Rafts

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 capacity The 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 column step 1: Pocket excavated to firm bearing, Blinding layer (optional) and DPM placed Step 2: Formwork erected and steel cage dropped in place Note cage in background

18 Portal Frame Pad:

19 Portal Frame Pad:

20 Strip Footing Deep Strip Footing Rafts

21 Portal frame Footing:

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 loads between unequally loaded columns using a tapered pad shape Image:

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?

27 Precast Wall panel footing:

28 Raft Foundations: Raft Foundations Solid Slab Raft:
solid slab of reinforced concrete load bearing are thickened Beam & Slab thickness > 300mm Weak soils, beams down Cellular Raft depth > 900mm a hollow cellular raft. Image:

29 Cellular Raft Basement walls forms Ribs Twin slabs: below and above
/http://www.firth.co.nz/media/33148/Ribraft%20Technical%20Manual.pdf

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:

32 Cellular Raft /http://www.thecubedubai.com/index.php?constructionworksupdate

33 Raft Foundations: It provides an insulated floor of high strength and durability Offers potential savings in construction time (and cost?) Image:

34 Cellular Raft

35 Seismic Forces During an Earthquake the ground moves rapidly while buildings try to resist displacement via their inherent inertia Major damage tend to be caused by the horizontal components of these imposed loads /http://www.firth.co.nz/media/33148/Ribraft%20Technical%20Manual.pdf

36 Seismic Forces Primary Factors Affecting Foundation Choice:
Sub-surface soil (Geotechnical Profile) Ground water table conditions Building structural requirements Secondary Factors Affecting Foundation Choice: Construction access, methods and site conditions Environmental 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) /http://www.firth.co.nz/media/33148/Ribraft%20Technical%20Manual.pdf


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