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Descriptions of design process for tall building foundation with a case study
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Soil-Foundation-Superstructure
Interaction for Tall Buildings on
Combined Pile-Raft Foundations
Konstantinos Syngros, Ph.D., P.E.
Alan Poeppel, P.E. Langan Engineering & Environmental Services, Inc.
Langan International
OVERVIEW
• Definitions
• Process of Foundation Design for Tall Buildings
• Soil Modeling For Structural Design
• Case Study
• Final Remarks
DEFINITIONS
Foundation Differential Settlements
• Differential settlements due to gravity loads
• For tall towers even small differential settlements can offset the top by meters and overstress the structure
• A special study is needed to determine differential settlements
TOWER
Raft Foundation Differential
Settlements
Soil Modeling for Structural Design
Structural Approach
Winkler Springs Model
Geotechnical Approach
Continuum Model
Raft Foundations
• Raft foundations are considered when:
– Individual footings cover more than about half of the building area.
– Total and differential settlements are too high.
RAFT (a.k.a. MAT)
COLUMN LOCATIONS
• A raft consists of a reinforced-concrete slab that supports the columns and walls of a structure, and distributes the loads to the underlying soils.
Combined Pile-Raft Foundations
• Combined Pile-Raft foundations are considered when:
– Settlements and differential settlements are too high when using only raft foundations. PILES/
DRILLED SHAFTS
Piled Raft - Overview
• Combination of raft + piles – Raft to provide adequate
bearing
– Piles to reduce total and differential settlements
• Piled-raft consists of a limited number of piles strategically located below heavily loaded area (e.g.: shear walls)
Piled Raft - Overview Kuala Lumpur,
Malaysia Brooklyn, N.Y., US Brooklyn, N.Y., US
Kuala Lumpur,
Malaysia
Settlement Comparison between Raft only and Piled-Raft
• Project: Brooklyn, N.Y., US
A
B
C
D
E A
B
C
D
E
Raft Only
Max ∆/L = 1/335
Piled-Raft
Max ∆/L = 1/630
1 in
2.5 in 0.8 in
1 in
0.9 in
0.8 in
1.5 in 0.7 in
0.8 in
0.8 in
Process of Foundation Design for Towers
Project Team for Towers
• Developer
• Architect
• Structural Engineer
• Geotechnical Engineer
• Local Consultants
• Peer Reviewers
• Specialty Foundation Contractors
+
Developing Foundation System • Geotechnical Investigation
• Structural Engineer Provides Initial Tower Loads
• Initial Studies (Bearing Capacity, Settlement) by Geotechnical Engineer
• Refinement of Studies (Determine Constructability, Iterative Process between Structural + Geotechnical Engineer)
• Peer Review Process
• Contractor Selection Process
• Local Authorities Approval Process
Iterative Process General Steps - RAFTS COLUMN LOADS
WINKLER SPRINGS, FOUNDATION SETTLEMENTS+ STRESSES
REVISED FOUNDATION SETTLEMENTS+STRESSES
NEW WINKLER SPRINGS, NEW FOUNDATION SETTLEMENTS+STRESSES
Structural Engineer
Geotechnical Engineer
Iterative Process – Combined Pile-Rafts
In addition to Raft Settlements, require convergence on
• Pile Loads
• Pile/Subgrade Load Sharing
• Column/Wall Loads
Soil Modeling for Structural Design
Soil Modeling for Structural Design
Structural Approach
Winkler Springs Model
Geotechnical Approach
Continuum Model
Elastic
Space
Inelastic
Space
Determination of Soil Springs (K-value)
• The K-value (a.k.a. Winkler Spring or Modulus of Subgrade Reaction) is the ratio of pressure over displacement
• K = p/δ
Foundation Response
• Significant differences in response between Winkler and Continuum models.
– Contact pressure beneath “rigid” foundations
– Settlements beneath “flexible” foundations
Foundation Response – Rigid Raft
Settlements
Stress Distribution
Structural Approach
Winkler Springs Model
Geotechnical Approach
Continuum Model
Foundation Response – Flexible Raft
Settlements
Stress Distribution
Winkler Continuum
Foundation Response
• Solution:
– Stiffness of outermost springs should be increased to account for increase in soil rigidity.
– For “stiff” soil, outer springs may increase 30-40%
– For “soft” soil, they may need to increase 2-3 times!
Determination of Modulus of Subgrade Reaction (K-value)
• The K-value can be obtained through:
• Method A: Plate load tests
• Method B: Tables of typical values /
correlations (based on plate load tests)
• Method C: Modeling of the loaded foundation
Determination of Modulus of Subgrade Reaction (K-value) – Method C
• Estimate settlements and bearing pressures at many points within the raft
• Determine the K-value from:
– K = p/δ (pressure/settlement)
• Draw K-value contours
• If piles are present, pile springs can be determined as pile force/pile settlement at the pile locations
CASE STUDY
Tower in Kuala Lumpur, Malaysia
Geotechnical Investigation
• Geotechnical Borings
• In-situ Testing (i.e., SPT, Pressuremeter Tests)
• Laboratory Tests (Index properties, Triaxial, 1-D Consolidation, etc)
• Geophysical Methods
Model Parameter Selection
Determine critical parameters such as
• Raft thickness
• Locations, geometry of piles
• Soil Modulus of deformation
• Method of construction
• Pile/Soil interface strength
Hand Calculations
Finite Element Modeling Input
• Geometry (raft, piles, stratigraphy)
• Material Properties (raft, piles, stratigraphy)
• Element Selection (Volume, area, line elements)
• Boundary Conditions
• Loading (Construction Staging, Self Weight, Tower Loads)
17
0m
RAFT: ~ 40m x 60m x 4m PILES D=2m, L=75m
Typical Soil/Rock Constitutive Models Soil
• Elastic (E, , unit weight)
• Mohr-Coulomb (friction angle, cohesion)
• Modified M-C/Soil Hardening
• Soft Soil/ Modified Cam-Clay
Rock
• Elastic (E, , unit weight)
• Mohr-Coulomb (friction angle, cohesion)
• Hoek-Brown
Typical Structure Models Raft
• Elastic (E, , unit weight), Volume Elements
• Plate Elements (thickness)
Piles
• Volume Elements (w or w/o interface)
• Beam Elements (w or w/o interface)
• Embedded Beam Elements w interface
This is the best but model may become too big for the computer to handle
Sufficiently accurate and can handle large number of piles.
Intermediate situation
Typical Loading Conditions • Soil/Foundation Self Weight
• Column/Wall Loads as
–Point Loads
– Line Loads
–Pressure Loads
• Foundation Construction Stages
Settlement Results
~100mm
~35mm
Settlement Results
40 m
70 m
Soil Subgrade Stresses
Vertical Strains (Block Behavior)
Pile Settlements
Pile Axial Loads
Pile-Soil Relative (Slip) Displacement
Soil/Pile Springs Determination
• Determine Pile Springs (Reaction/Settlement)
• Determine Area Springs (Pressure/Settlement)
• Determine Pile/Raft Load Share
• Provide the above to Structural Engineer
Pile Loads + Settlements
25
27
26
25
30
27
27
24
39
24
28
27
24
27
25
24
26
27
38
24
25
26
29
41
25
30
28
24
29
23
23
25
24
25
27
37
25
31
28
27
30
2631
32
25
25
40
36
32
28
2740
2736
25
24
31
32
23
26
30
27
29
29
25
24
2630
2526
26
27
2625
33
32
30
Pile Head Axial Load (MN)
Pile Loads
Raft Settlements
Soil/Pile Springs PILE SPRINGS
AREA SPRINGS
283
337
366
304
351
324
361
261
497
268
334
437
346
312
310
306
365
298
454
338
322
285
351
475
278
461
343
265
499
261
253
280
289
282
297
465
304
516
448
296
336
297344
499
367
264
486
502
456366
425466
379476
306
296
397
488
457
279
372
301
460
332
298
316
279370
292308
335
405
423288
436
406
385
Pile Springs (MN/m)
Winkler Springs Adjustment
Based on Structural Engineer’s
• Settlements
• Subgrade Stresses
• Pile Loads
• Pile/Raft Load Sharing
• Wall Loads
Re-run Foundation Model + adjust Area and Pile Springs appropriately
Comparisons: Iteration 1
Pile Loads > 20% Settlements >20%
Pile Loads > 20% Settlements >10% Wall Loads>10%
Comparisons: Iteration 2
Pile Loads > 5% Settlements >5% Wall Loads >10%
Comparisons: Iteration 3
Pile Loads > 5% Settlements >5% Wall Loads <10%
Comparisons: Iteration 4
Pile Loads ~5% Settlements ~5% Wall Loads <10%
Comparisons: Iteration 5
Shear-Wall Load Changes
RAFT EDGES RAFT CENTER
FINAL REMARKS
• The iterative process gives insight on the redistribution of column/wall loads.
• The redistribution is more pronounced when the superstructure is “stiff” compared to the foundation.
• The redistribution may necessitate “stiffening” the foundation.