CIVE2004 Structure II

Question 1:

Answer:-

Design Of Raft Foundation

Instructions

Building dimensions: Length – 26 m, Width – 21.5 m

Location: Adelaide, Australia

Ys = 35+(last student number digit x 4) 2 x 4 = 42

Construction type: Articulated Full Masonry

Instructions: Design waffle raft footing based on soil reactivity

Design according to AS 2870-2011

Choice of foundation type

Location has high reactive soils

Soil classification for Ys = 42, (40 < Ys < 60) H1 – Ground has high movements from deep-seated moisture changes

According to soil conditions and classification, the Waffle raft foundation footing is not suitable nor recommended. The large level of movements that the soil undergoes require a stiffened raft which resists the expansion better than a waffle raft foundation. Therefore, the design shall be for a stiffened raft foundation footing of the given dimensions.

Use of waffle raft foundation under these conditions using articulated full masonry shall require specified engineering design and practice outside the scope of AS 2870-2011 and other available guidelines, and design to be done according to advanced engineering principles of foundation slabs.

Design Of Stiffened Raft Foundation

Basic information

Footing size – Length (L) 26 m, Width (w) = 21.5 m

Heave size, Ys = 42, Class H1

Layout: No partitions, open layout

Assumptions and Data

Basic description: For stiffened raft, beams layout should not exceed 4 m spans in either direction. Adopt 4 m

Concrete and Structural Parameters

Modulus of Elasticity E_c = 10000 kN/sqm

Poisson’s Ratio = 0.16

Slab Thickness = 100 mm

Stiffening Beam Depth = 750 mm

Beam width of stiffening beam = 300 mm

Beam spacing adopted = 4 m longitudinally, 4 m transversely

Soil Parameters

Poisson’s ratio of the soil = 0.4 (range = 0.3-0.5)

Shape exponent (for Adelaide) m = 4

Edge moisture variation distance e_m = 0.2 all round

Modulus of Elasticity of soil E_s =

Differential Heave (Ys) = 42

Design according to soil reactivity as defined in AS 2870 – 2011

Depth of beam = 750 mm

Bottom reinforcement = 3N16

Top Reinforcement = 2N16

Maximum beam spacing = 4 m either side. Adopt 3.25 m longitudinally, and 2.6875 m transversely

Use the 300 mm thickness. Therefore, the footing is as illustrated in cad file pdf Drawing Number 001

Question 2:

Answer:

Location chosen (Fig 2): 4B Trinity Cct, Mawson Lakes SA 5095, Australia

Site Classification from TMK Database (Reference: 7903): H2-D, Ys = 62 mm, no trees and no deep clay (Fig 1)

Foot design in this location would be different from the previous site location (Ys = 42) in the following ways According to AS 2870-2011:

1. Use of suspended slab would be required for most of construction methods. Waffle or stiffened rafts are not allowed.
2. Design would be custom-made, according to engineering principles and experiment on the ground.
3. Use of Full masonry construction type is not allowed in this site.
4. Where construction is done using Clad frame, masonry veneer and articulated masonry veneer, stiffened raft may be used with a depth of about 1 meter.

Figure 1: Location

Figure 2: Soil Classification

Different construction types are classified according to their potential tolerance to differential movement. That is, some construction methods are stiff and rigid, and do not allow or tolerate large soil movements while others are flexible. Clad frame is the most flexible, and hence requires less reinforcement and beam depth.  Its foundation does not need to be deep for stability due to its tolerance. Full masonry is not flexible – it is the stiffest of all types. It is recommended that with a stiff construction type, the foundation has to be stiff and rigid, and deep into the ground to reduce movement, hence increased foundation depth and less tolerance of movement.

Question 3:

Discuss the difference in design principles between footing design based on soil reactivity and footing design based on bearing capacity

1. Soil reactivity methods are used more for less rigid raft footings, while bearing capacity method is used for stiff non-tolerant footings.
2. Soil movement allowance is provided when designing footings based on soil reactivity, while the footing is firm and soil is stable when designing based on bearing capacity.
3. In soil reactivity methods, footing is designed without reliance on the soil underneath while on bearing capacity, footing size and shape depends on the strength of the soil underneath it.
4. Reactivity methods are applied in clayey soils which cause large or considerable ground movements due to moisture changes. In bearing capacity methods, the moisture content of soil is not expected to significantly change the volume or position of soil and ground.
5. In soil reactivity methods, foundations are made more rigid, while in soil bearing capacity methods, the ground is stabilized and made more firm.

Question 4:

The ground condition is shown below along with the material properties for a building site. Design a footing based on bearing capacity equations for a column that will carry 1700 kN of load. Assume water table at ground surface and use a factor of safety of 3.0. Present the final design dimensions and footing depth using a final sketch.

Details summary:

Water table at ground level, FoS = 3.0

Soil type: Light brown clay

Unit weight of soil Ys = 17.5 kN/m3

C = 4.5 kPa

Gamma angle = 28^o

Soil depth = 15m

Design:

Design Load = 1700 kN (assume footing weight, negligible)

Assume a square footing. To find area of footing, we first calculate the ultimate bearing capacity ratio for the soil.

Bearing capacity of soil: Using Terzaghi’s method is:

qu = 1.2c’Nc+YDfNq+0.4YBNy where, for the soil whose Sigma is 28o,

Nc = 32, Nq = 18.58 and Ny = 15.7

Assume B = 2m

Thus qu = 1.2x4.5x32 + 17.5x2x18.58 + 0.4x17.5x2x15.7 = 933kN/m²

Safe bearing capacity = 933/3 = 311kN/m²

Plan Area = N/bearing capacity = 1700/311 = 5.47m²

Thus, assumed size is inadequate.

For a square footing, take dimensions 2.4m

qu = 1.2x4.5x32+17.5x2x18.58+0.4x17.5x2.5x15.7 = 1042.9kN/m2

Thus, new qsafe = 1042.9/3 = 347.63kN/m2

Area = 2.4x2.4 = 5.76m2

Self-weight of footing = Area x h x density of concrete (assume h = 0.6)

  = 5.76 x 0.6 x 24 = 82.94 (<assume 90kN)

Thus Total Loading = 1700+90 = 1790kN.

Therefore, bearing pressure under footing = 1790/5.76 = 310.76k<347.6kN/m² (ok)

checking for economic design:

% of difference between qsafe and bearing pressure:

(347.6 - 310.76) x 310.76/100 = 11.9% <15% (OK)

Thus use square footing 2.4m by 2.4m by 0.6mm high, 2m deep

References

AS2807 – 2011

Brandon, T (2017). Footings on Expansive Soils: Technical Factsheet. Chinhilla

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