CPCI_Structural_Design_n

CPCI_Structural_Design_n - PRECAST PRESTRESSED PRECAST...

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Unformatted text preview: PRECAST / PRESTRESSED PRECAST STRUCTURAL DESIGN PRINCIPLES PRINCIPLES 1 PRECAST PRESTRESSED CONCRETE Concrete is compressed by means of high-strength tendons to Concrete eliminate or reduce net tension. The resulting structural system is referred to as “Prestressed Concrete.” Concrete.” Prestressed Concrete (P/C) Pretensioned P/C Post-Tensioned P/C 2 PRECAST PRESTRESSED CONCRETE PRETENSIONED (P/C) Strand Formwork Bulkhead 1. Stretch (tension) the prestressing strands Concrete 2. Pour the concrete 3. Cut the strands loose after sufficient strength gain 3 PRECAST PRESTRESSED CONCRETE POST-TENSIONED (P/C) Duct 1. Cast the concrete Formwork 2. Transport to the job site and place on the supports Anchor Strand Hydraulic Jack 3. Stress the strands against hardened concrete 4 PRECAST PRESTRESSED CONCRETE Higher prestressing may be necessary in Higher high tension regions high Moment Diagram + e 5 PRECAST PRESTRESSED CONCRETE e 6 PRECAST PRESTRESSED CONCRETE POST-TENSIONED (P/C) POST-TENSIONED q q Loss of prestress is the reduction of tensile stress in prestressing tendons, either immediately after the transfer of stress or with time. Common prestress losses are: Anchorage Seating Loss Elastic Shortening Loss Friction Loss – Creep Loss – Shrinkage – Steel Relaxation q q The total loss in normal density concrete elements typically range between 200 MPa to 350 MPa (approximately 15% to 30%). The value of 240 MPa (21%) is used in practice as a rough estimate for preliminary design. 7 PRACTICAL CONSIDERATIONS AT THE PRACTICAL PRELIMINARY DESIGN STAGE PRELIMINARY Precast concrete is basically a “simple-span” structural system, Precast though continuity is needed in certain elements and can be achieved with properly conceived connections achieved connections 8 PRACTICAL CONSIDERATIONS AT THE PRACTICAL PRELIMINARY DESIGN STAGE PRELIMINARY Sizes and shapes of precast members are often a function of Sizes production, hauling and erection considerations. production, 9 PRACTICAL CONSIDERATIONS AT THE PRACTICAL PRELIMINARY DESIGN STAGE PRELIMINARY Maximum economy is achieved with maximum repetition of Maximum standard sections. When possible, bays should be laid out to fit the modules of the components selected components 10 PRACTICAL CONSIDERATIONS AT THE PRACTICAL PRELIMINARY DESIGN STAGE PRELIMINARY It is often feasible to cast wall panels and columns in It multi-storey units. Economy is achieved with fewer pieces to handle. Economy 11 PRACTICAL CONSIDERATIONS AT THE PRACTICAL PRELIMINARY DESIGN STAGE PRELIMINARY Selection of floor-to-floor dimensions should consider the Selection practical span-to-depth ratios of horizontal framing elements practical Typical span-to-depth ratios for prestressed elements Hollow core floor slabs Hollow core roof slabs Stemmed floor slabs Stemmed roof slabs Beams 30 to 40 40 to 50 25 to 35 35 to 40 10 to 20 12 PRACTICAL CONSIDERATIONS AT THE PRACTICAL PRELIMINARY DESIGN STAGE PRELIMINARY It is important to select the “right” lateral load resisting system METHODS USED TO RESIST LATERAL LOADS (in approximate order of economy) Shear walls (precast, cast-in-place, masonry) Cantilevered columns or wall panels (for low-rise) Steel or concrete X-bracing (for mid-rise buildings) Moment resisting frames (consider restraining forces caused by volume changes) 13 DESIGN OF PRECAST/PRESTRESSED DESIGN STRUCTURAL ELEMENTS STRUCTURAL DESIGN FOR FLEXURE DESIGN AND SHEAR AND Beams and Girders Beams and Girders DESIGN FOR FLEXURE Hollow Core Floor Hollow Core Floor Solid Floor Solid Floor Slab Bridge Girder Single and Double Tee’s Single and Double Tee’s Bridge Girder Beam Section Beam Section 14 DESIGN OF PRECAST/PRESTRESSED DESIGN STRUCTURAL ELEMENTS STRUCTURAL DESIGN FOR FLEXURE AND AXIAL LOAD Columns Walls 15 DESIGN OF REINFORCED AND/OR DESIGN PRECAST /PRESTRESSED STRUCTURAL ELEMENTS STRUCTURAL DESIGN FOR DESIGN STRENGTH AND SERVICEABILITY STRENGTH Consider: Ultimate Limit State Serviceability 16 ULTIMATE LIMIT STATE (Safety Against Collapse) Factored Resistance > Effect of Factored Loads Mr ≥ Mf Pr ≥ Pf Vr ≥ Vf Tr ≥ Tf 17 FACTORED LOAD COMBINATIONS 1. Without earthquake loads: αD D + γ ψ ( αL L + αW W + αT T ) 2. With earthquake loads: 1.0 D + g ( 1.0 E ) for storage and assembly: for 1.0 D + g ( 1.0 L + 1.0 E ) for all other loads: 1.0 D + g ( 1.0 L + 1.0 E ) 1.0 18 LOAD FACTORS Load type Dead load Dead loads resisting overturning uplift and stress reversals Live loads Wind loads Temperature, creep, shrinkage and differential settlement Factor aD = 1.25 aD = 0.85 aL = 1.50 aW = 1.50 aT = 1.25 19 MATERIAL RESISTANCE FACTORS Materials Cast-in-place concrete Precast concrete cast in a certified plant Reinforcing bars Prestressing tendons Structural steel Factor φ c = 0.60 φ c = 0.65 φ s = 0.85 φ p = 0.90 φ a = 0.90 20 CONNECTIONS Connections are designed for dead and live gravity loads, wind, earthquake, and soil or water pressure. They are also designed for loads that are caused by volume changes and forces required to maintain stability. Connections are often designed for: Compression Tension Flexure Shear Torsion 21 CONNECTIONS BEARING PADS Bearing pads are used to distribute vertical loads over the bearing area. They typically belong to one of the following categories: – Commercial grade elastomeric (Neoprene) pads. – Structural grade chloroprene (Neoprene) pads. – Laminated fabric (multiple layers of cotton) bearing pads. – Pads reinforced with randomly oriented fibers. – Tetrafluorethylene (TFE, Trade Name: Teflon) pads – Multi-polymer plastic bearing strips 22 CONNECTIONS Properties of a single layer bearing pad free to slip 23 CONNECTIONS LOAD BEARING WALL PANELS Panel vertical tie using hollow structural section 24 CONNECTIONS LOAD BEARING WALL PANELS Double tee to shear Double wall connection wall 25 Contact CPCI for more information Web: www.cpci.ca Members: www.precastsearch.com CPCI email: [email protected] 26 ...
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This note was uploaded on 03/10/2009 for the course ENGR BCEE 345 taught by Professor Drgala during the Winter '07 term at Concordia Canada.

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