Overview of Structural Design and Detailing of Large Diameter Drilled Shafts (Caltrans Practice)Amir M. Malek, PE, PhDSenior Bridge Engineer (Technical Specialist)

Office of Bridge Design ServicesCalifornia Department of Transportation

OutlineTypes of Large Diameter Shafts and Comparison Design Highlights and Review of LRFD Requirements Communications of Structural and Geotechnical Designers for LRFD of Shafts Highlights of Seismic Design and Detailing Requirements per Caltrans Seismic Design Criteria (SDC)Case Study

Applications and TypesUsed for high seismic loads also where small footprint is desirable Most effective where hard layer (rock) is reachableUsed with/without casing Types I & II per SDC classification Type-I : More ductile performance, advantageous for short columns Type-II : Easier post-event repair, shaft enlargement of at least 18 (24 under study) to contain inelastic action to the column (SDC 7.7.3.5)

Test of 6 diameter Type-I Shaft at UCLA

Test of 6 diameter Type-I Shaft at UCLA

Types of Large Diameter Drilled Shafts (Caltrans SDC)

LRFD & Seismic Design HighlightsStructural Designer provides Factored Loads for applicable Limit States Geotechnical Designer will provide tip elevations based on Compression, Tension, and Settlement also Factored Nominal Resistance for Service, Strength and Extreme Event Limit States (LRFD)Structural Designer performs Stability Analysis and provides tip elevation for Lateral LoadsStructural Designer analyzes, designs and details the shaft for Seismic Demands according to Caltrans SDCScour, Liquefaction and Lateral Spreading are considered in design (if applicable)

Review of LRFD RequirementsConsider Service, Strength and Extreme Event Limit States for Geotechnical and Structural Design of the ShaftFollow MTD3-1 for Communications and Transfer of Information between SD and GS as summarized in the following Tables

Preliminary Design Data Sheet(to be provided by SD)

General Foundation Information(to be provided by SD)

Foundation Design Loads(to be provided by SD)

Lateral Stability (BDA Chapter 12) Available Software: LPILE, W-FRAME, or SAP

General Seismic Design Highlights (Requirements that may be affected by size/type of the shaft)Geometrical/Structural IrregularitiesDemand and CapacityP- EffectDisplacement Ductility LimitationMinimum Local Displacement Ductility Capacity

Geometrical/Structural Irregularities: Balanced Stiffness of Bents (SDC 7.1.1) Balanced Frame Geometry (SDC 7.1.2)Demand vs. Capacity (SDC 4.1.1)P- Effect (SDC 4.2)Displacement Ductility Demand Limits (1.5-3/5 for bents supported by the shafts, per SDC 2.2.3)Minimum Local Displacement Ductility Capacity Limits (SDC 3.1.4.1)

Structural Analysis for Demand AssessmentUse Expected Material PropertiesDetermine Column/Shaft Plastic Moments from Section AnalysisUse Mo=1.2Mp Use Push-over Analysis and Find Shear and Moment Demands at Collapse

Demand Calculation (Single Column Bent)Mo

Seismic Demand Calculation (Multi-Column Bent)Type-IMo

Seismic Demand Calculation (Multi-Column Bent)Type-IIMo

Structural Design of the Shafts

MneType II >= 1.25 MDemand (SDC 7.7.3.2) VnType II >= VDemand (SDC 3.6.7)

Shear capacity is calculated as a ductile member using SDC 3.6 requirements (for Type-II assume d=1)

Detailing RequirementsNo Splice Zones (SDC 8.1.1) Plastic hinge region and areas of MD>MyUltimate Splices (SDC 8.1.2) Ductile members outside No Splice ZoneService Splice (MTD20-9) Capacity Protected Members like Bent Cap For Hoops and Spirals in Ductile Members Use Ultimate Splices, Except: No splices in spirals used in No Splice Zones (end anchorage has been used to improve constructability)

Case Study (Type-II)Top of the Pile Boundary Conditions:V & M (V=150 kips, M=3,750 k-ft)

Liquefied Layer

Scour Included

Case-I ResultsCompetentNot LiquefiedLiquefied (I)Liquefied (II)Top Deflection (in.)0.92 3.49.812.214.418.2Mmax (kip-in.)(x10-4)5.36.57.69.29.411.5Location of Mmax (ft)71432374242Vmax (kips)320268420517420519Location of Vmax (ft)173250505557Stable Length (ft)34546575

Summary (Method-I)

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