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Analysis and Design of Waffle Slabs
“Preliminary Findings”
FFSV – 10th October 2013
Siva Sivagnanasundram
Outline
Design of waffle slabs – AS2870 FE analyses and results on a typical waffle slabs Discussion and conclusions Future plans
Deemed to Comply Provision (AS 2870)
H2 site class waffle raft footing for articulated masonry veneer construction
• Normal moisture condition to be maintained
• Slab length can span up to 30m in each direction
• Edge beams has 300mm bottom width for rebate and 3N16 bar.
• Ys = 60 – 75mm and corresponding Ym = 42 – 53mm
Internal beam
Eg: Slab dimension 11.3mx11.3m
Ø16
Ø8 SL82 Mesh Reo 8530
0
110
1200
E = 15000 MPa (AS2870)
20MPa Concrete
Design by Engineering Principles (AS 2870)
H2 site class waffle raft footing for articulated masonry veneer construction Design by engineering principles (AS 2870) Section 4:
Estimating ‘Δ’ – permissible deflection: Column3,Table 4.1 Ys = 60 -75mm for H2
and
Design by Engineering Principles (AS 2870)
Ys = 75mm D = 460mm
Ys = 60mm D = 375mm
Design by engineering principles (AS 2870) Section 4:H2 site class waffle raft footing for articulated masonry veneer construction
and
Analysis and Design Using SLOG
Method Beam depth ‘D’ for Ys 60mm Beam depth ‘D’ for Ys 75mmDeemed to comply 385mm 385mmEng. principles 385mm 460mmSLOG 385mm 385mm (9kN/m) & 535mm (15kN/m)
H2 site class Waffle raft footing for articulated masonry veneer construction
• Ys = 60 – 75mm and corresponding Ym = 42 – 53mm
Progressive Heave under Waffles
Void Space
11.3m span, 4.75kPa UDL, Ym = 42 - 53mm and 0.5EIg stiffness for D = 385mm
300x1200mm contact area
Progressive Heave under Waffles
Dry expansive clay
Edge lift
e-distance heave
Initial
(A)
(B)
(C)
Progressive heave
42mm movement was measured at Braybrook with in 5 months
FE Analyses –Edge LiftH2 site class Waffle raft footing for articulated masonry veneer construction
0
20
40
60
80
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
Verit
cal m
ovem
ent (
mm
)
Slab span (11.3m)
Mound profile used in the FE model
Mound heave
FE Analyses –Edge Lift
11.3m span, 4.75kPa UDL, Ym = 42 - 53mm and 0.5EIg stiffness for D = 385mm H2 site class Waffle raft footing for articulated masonry veneer construction
For Ys = 60 & Ym = 42 Δ = 42 > 28mm
For Ys = 75 & Ym = 53, Δ = 53 > 28mm
Slab failed to satisfy the deflection criteria
Design of Waffle Slab for Edge Lift
7000
H2 SlabD = 385mmW* in service = 5.7kN/mM*max in service for 7m span = 29kNmMu = 31kNmΔ = 17mm < L/400 = 17.5mm
7m maximum span
01020304050607080
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
Verit
cal m
ovem
ent (
mm
)
Slab span (11.3m)
Mound profile used in the FE model
Initial mound heave
Mound heave under slab load
Slab profile
‘e’-distance Heave under Wafflese-distance = 2.6m
Mitchell (1980)
“For Melbourne condition maximum e-distance = 2.6m for any span”
FE Analyses – ‘e’-distance Heave
11.3m span, 4.75kPa UDL, Ym = 42 - 53mm and 0.5EIg stiffness for D = 385mm H2 site class Waffle raft footing for articulated masonry veneer construction
For Ys = 60 & Ym = 42 Δ = 22 < 28mm
For Ys = 75 & Ym = 53 Δ = 27 < 28mm
Slab satisfies the deflection criteria for H2
FE Analyses –Asymmetrical Moisture – Edge Lift
11.3m span, 4.75kPa UDL, Ym = 42 - 53mm and 0.5EIg stiffness, D = 385mm H2 site class Waffle raft footing for articulated masonry veneer construction
For Ys = 60 & Ym = 42 Δ = 42 > 28mm
For Ys = 75 & Ym = 53, Δ = 53 > 28mm
Slab failed to satisfy the deflection criteria
FE Analyses –Asymmetrical Moisture- ‘e’-distance heave
11.3m span, 4.75kPa UDL, Ym = 42 - 53mm and 0.5EIg stiffness, D = 385mm H2 site class Waffle raft footing for articulated masonry veneer construction
For Ys = 60 & Ym = 42 Δ = 42 > 28mm
For Ys = 75 & Ym = 53, Δ = 53 > 28mm
Slab failed to satisfy the deflection criteria
Discussion and Conclusions
For H2 slab AS2870 deemed to comply provision (D = 385mm)is un-conservative compared to the engineering principledesign (D = 460mm)
Ym = 53mm was common in recent field inspections in thewestern suburbs. Category 2 damage to internal walls andcornices was noticed.
Previous mound models are based on slab in full contact withthe soil at the edges. It appears for waffle slabs the previousmound models are adopted without any modifications.
FE Results and Conclusions 7m span H2 deemed to comply slab satisfies the edge lift for
deflection and strength.
Symmetrical heaves H2 deemed to comply slabs with span > 7m will not satisfy the
deflection limit under edge lift. H2 deemed to comply slabs with span > 11.3m will not satisfy the
e-distance heave based on the mound model (Mitchell 1980).
Asymmetrical heaves H2 deemed to comply slabs with span > 7m will not satisfy the
deflection limit under edge lift. 11.3m span H2 deemed to comply slab fails under both edge lift
and e-distance asymmetrical heave.
Review of Footing Design -Future Work Perform time-dependent FE climate interaction analyses to
derive moisture profiles under the slabs.
Develop non-linear soil stiffness curves to represent the soil typeand moisture variation.
Perform further soil-slab interaction analyses using commerciallyavailable software to generate reliable soil mound profiles.
Thank you…