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…