Prestressed Concrete Design PDF

Prestressed Concrete Design INTRODUCTION TO PRE- o In 1904, Freyssinet attempted to introduce permanent acting STRESSED CONCRETE forces...

Prestressed Concrete Design INTRODUCTION TO PRE- o In 1904, Freyssinet attempted to introduce permanent acting STRESSED CONCRETE forces in conc. to resist elastic Reinforced Concrete forces under loads and was named “Pre stressing”. Recall Reinforced Concrete knowledge: o Concrete is strong in compression but weak in tension o Steel is strong in tension (as well as compression) o Reinforced concrete uses concrete to resist compression and to hold the steel bars in place, and uses steel to resist all of the tension o Tensile strength of concrete is neglected (i.e. zero) o RC beam always crack under service load Principle of Prestressing Prestressing is a method in which compression force is applied to the reinforced concrete section. The effect of prestressing is to reduce the tensile stress in the section to the point that the tensile stress is below the cracking stress. Thus, the concrete does not crack! It is then possible to treat concrete as an Principle of Prestressing elastic material The concrete can be visualized to have 2 Stress in concrete section when the force systems prestressing force is applied at the c.g. o Internal Prestressing Forces of the section (simplest case) o External Forces (from DL, LL, etc...) These 2 force systems must counteract each other Pre-stressed Concrete What is Pre-stressed Concrete? o Internal stresses are induced to counteract external stresses.  Stress in concrete section when the Eugene Freyssinet (1879-1962) was the prestressing force is applied first to propose that we should use very eccentrically with respect to the c.g. of high strength steel which permit high the section (typical case) elongation of steel. The high steel elongation would not be entirely offset by the shortening of concrete (prestress loss) due to creep and shrinkage. First prestressed concrete bridge in 1941 in France First prestressed concrete bridge in US: Walnut Lane Bridge in Pennsylvania. Historical Perspective Built in 1949. 47-meter span. The concept of prestressing was invented centuries ago when metal bands were wound around wooden Classification and Types pieces (staves) to form a barrel. Pretensioning Posttensioning The metal bands were tighten under tensile stress, which creates compression External Internal between the staves allowing them to resist internal liquid pressure Linear Circular End- Non-End- Anchored Anchored Bonded Unbonded Tendon Precast Cast-In-Place Composite The concept of prestressed concrete is Partial Full Prestressing also not new. In 1886, a patent was granted for tightening steel tie rods in Classification and Types concrete blocks. This is analogous to modern day segmental constructions. Pretensioning vs. Posttensioning Early attempts were not very successful In Pretension, the tendons are tensioned due to low strength of steel at that time. against some abutments before the Since we cannot prestress at high stress concrete is place. After the concrete level, the prestress losses due to creep hardened, the tension force is released. and shrinkage of concrete quickly The tendon tries to shrink back to the reduce the effectiveness of prestressing. initial length but the concrete resists it through the bond between them, thus, compression force is induced in  concrete. Pretension is usually done with the tendon; therefore, it is non-end- precast members. anchored In Posttensioning, tendons are anchored at their ends using mechanical devices to In Posttension, the tendons are transfer the prestress to concrete; tensioned after the concrete has therefore, it is end anchored. (Grouting hardened. Commonly, metal or plastic or not is irrelevant) ducts are placed inside the concrete Partial vs. Full Prestressing before casting. After the concrete hardened and had enough strength, the Prestressing tendon may be used in tendon was placed inside the duct, combination with regular reinforcing stressed, and anchored against concrete. steel. Thus, it is something between full Grout may be injected into the duct later. prestressed concrete (PC) and reinforced This can be done either as precast or concrete (RC). The goal is to allow cast-in-place. some tension and cracking under full- service load while ensuring sufficient ultimate strength. We sometimes use partially prestressed concrete (PPC) to control camber and deflection, increase ductility, and save External vs. Internal Prestressing costs. Prestressing may be done inside or outside Linear vs. Circular Prestressing Prestressing can be done in a straight structure such beams (linear prestressing) or around a circular structure, such as tank or silo (circular prestressing) Bonded vs. Unbonded Tendon The tendon may be bonded to concrete (pretension or posttensioning with grouting) or unbonded (posttensioning without grouting). Bonding helps prevent corrosion of tendon. Unbonding Advantages of PC over RC allows readjustment of prestressing force at later times. Takes full advantage of high strength concrete and high strength steel End-Anchored vs. Non-End-Anchored o Need less materials tendons o Smaller and lighter structure o No cracks In Pretensioning, tendons transfer the o Use the entire section to resist prestress through the bond actions along the load o Better corrosion resistance  o Good for water tanks and nuclear plant Anchoring of Placing tendons against the Very effective for deflection control end abutments of jacks Better shear resistance Stages of Loading Cutting of the Applying tension to the tendons tendons Unlike RC where we primarily Casting of consider the ultimate loading stage, the concrete we must consider multiple stages of construction in Prestressed Concrete Typical stages of loading considered are Initial and Service Stages Initial (Immediately after Transfer of Prestress) o Full prestress force o No MLL (may or may not have MDL depending on construction type) Service o Prestress loss has occurred o MDL+MLL 2-METHODS OF PRESTRESSING CONCRETE Pre - tensioned concrete In pre tensioning system, the high- strength steel tendons are pulled between two end abutments (also called bulkheads) prior to the casting of concrete. The abutments are fixed at the end pre stressing bed Post - tensioned concrete Once the concrete attains the desires strength for prestressing, the tendon is Post-tensioning is a method of cut loose from the abutments. The pre reinforcing (strengthening) concrete or stress is transferred to the concrete from other materials with high-strength steel the tendons, due the bond between them strands or bars, typically referred to as During the transfer of prestress, the tendons member undergoes elastic shortening. If Post-tensioning applications include the tendons are located eccentrically, the office and apartment buildings, parking member is likely to bend and deflect structures, slabs-on-ground, bridges, (camber) sports stadiums, rock and soil anchors, The various stages of the pre-tensioning and water-tanks operation are summarized as follows ADVANTAGE OF BOTH SYSTEM Pre-tensioned Concrete Materials, Hardwares and Improving the performance of the Equipment for Prestressing building under various service Materials and Hardwares for Prestressing condition. Allow to carry a greater load or span a Concrete greater distance then ordinary reinforce Prestressing Steel concrete. Prestressing Hardwares It permits steel to be used at stresses Concrete several times larger than those permitted for reinforcing bars Mechanical properties of concrete that are relevant to the prestressed concrete Post-tensioned Concrete design: Large reduction in traditional o Compressive Strength reinforcement requirements as tendons o Modulus of Elasticity cannot distress in accidents o Modulus of Rupture Tendons can be easily "woven" allowing Concrete: Compressive Strength a more efficient design approach Higher ultimate strength due to bond AASHTO LRFD generated between the strand and For prestressed concrete, the concrete compressive strength should be from 28- Strong against compression 70 MPa at 28 days For reinforced concrete, the compressive Applications of Prestressed Concrete strength should be from 16-70 MPa at Bridges 28 days Slabs in buildings Concrete with f’c > 70 MPa can be used Water Tank when supported by test data Concrete Pile Thin Shell Structures Offshore Platform Nuclear Power Plant Repair and Rehabilitations Concrete: Modulus of Elasticity Shrinkage AASHTO (5.4.2.4) In a concrete element, shrinkage results to a Ec = 0.043 Yc1.5(f’c) 0.5 MPa decrease in volume when the concrete loses o Yc1.5 in kg/m³ moisture by evaporation. o f’c in MPa Prestressing Tendons For normal weight concrete, we can use o Ec=4800(f’c)0.5 MPa Prestressing tendon may be in the form of strands, wires, round bar, or threaded rods Concrete: Modulus of Rupture Materials Indicates the tensile capacity of concrete o High Strength Steel under bending o Fiber-Reinforced Composite Tested simply supported concrete beam (glass or carbon fibers) under 4-point bending configuration Common shapes of prestressing tensons fr = My/I = PL/bd2 AASHTO (5.4.2.6) o fr = 0.63 (f’c)0.5 MPa Time-Dependent Deformation of Concrete Time-dependent deformation of concrete resulting from creep and shrinkage result in a partial loss of prestress force and significant changes in deflection. Creep Creep – property of materials by which they continue to deform over considerable lengths of time at constant stress or loads The initial deformation due to load is the elastic strain, while the additional strain due to the same sustained load is creep strain. Creep strain for concrete has been found experimentally to depend on: o time o mix proportions o humidity o curing conditions o age of the concrete when first loaded Prestressing Steel Prestressing Strands Modulus of Elasticity o 197000 MPa for Strand o 207000 MPa for Bar The modulus of elasticity of strand is lower than that of steel bar because strand is made from twisting of small wires together. Prestressing Strands Prestressing strands have two grades o Grade 250 (fpu = 250 ksi or 1725 MPa) o Grade 270 (fpu = 270 ksi or 1860 MPa) Types of Prestressing Steel Types of strands o Stressed Relieved Strand There are three common forms in which o Low Relaxation Strand (lower steel is used for prestressed concrete prestress loss due to relaxation tendons: of strand) o Cold-drawn round wires o Stranded cable o Alloy steel bars Round Wires o The individual wires are manufactured by hot rolling steel billets into round rods o After cooling, the rods are passed through dies to reduce their diameter to the required size. o In the process of this drawing operation, cold work is done on the steel, greatly modifying its mechanical properties and increasing its strength o The wires are stress-relieved after cold drawing by a continuous heat treatment to produce the prescribed mechanical properties  o Available in Grades 235 o After cold-stretching, the bars (minimum ultimate strength are stress-relieved to obtain the 235,000 psi) to Grade 250 required properties (minimum ultimate strength o Available in diameters ranging 250,000 psi) from 5/8 in to 1 3/8 in, and in Grade 145 (minimum ultimate Stranded Cable strength 145,000 psi) and Grade o Fabricated with six wires wound 160 (minimum ultimate strength tightly around a seventh of 160,000 psi) slightly larger diameter o The pitch of the spiral winding is between 12 to 16 times the Importance of High Strength Steel nominal diameter of the strand o The same type of cold-drawn The lack of success of most early stress-relieved wire is used in attempts to PSC was the failure to making stranded cable as is used employ steel at a sufficiently high stress for individual prestressing wires and strain. o However, the apparent The time-dependent length changes mechanical properties are permitted by shrinkage and creep of the slightly different because of the concrete completely relieved the steel of tendency for the stranded wires stress. to straighten when subjected to Hardwares & Prestressing Equipment tension because the axis of the wires do not coincide with the Pretensioned Members direction of tension o Hold-Down Devices o Strands may be obtained in a Posttensioned Members range of sizes from 0.25 in to o Anchorages 0.60 in diameter ▪ Stressing Anchorage o Cable is stress-relieved by heat ▪ Dead-End Anchorage treatment after stranding o Ducts o Two grades are manufactured: o Posttensioning Procedures ▪ Grade 250 – minimum ultimate strength of 250,000 psi Pretensioned Beams ▪ Grade 270 – minimum ultimate strength of 270,000 psi Alloy Steel Bars o The high strength is obtained by introducing certain alloying elements, mainly manganese, silicon and chromium during the manufacture of the steel o In addition, cold work is done in making the bars, further increasing the strength Pretensioning Hardwares Hold-Down Devices for Pretensioned Beams Posttensioning Hardwares – Ducts Posttensioned Beams Posttension Hardwares o Stressing Anchorage o Dead-End Anchorage o Duct/Grout Tube Posttensioning Procedures Posttensioning Hardwares – Anchorages Tendons Holding Tank 13.0 paddle agitating Common shapes of prestressing tendons Drive Power Air 300 CFM, 100 Most popular (7-wire Strand) psi Physical Dimensions 96" L x 60" W Specifications x 63" H Weight 1800-2800 lbs. Equipment T6Z-08 Air Powered Grout Pump T7Z Hydraulic Jacks Pumps cement grout only, no sand. 32 Gallon Used for testing and prestressing anchor bolts. Mixing Tank. Mixes up to 2 sacks of material at Available with up to 5-1/8" center hole. Unit once and allows for grout to be pumped during comes with ram, pump, gauge, hoses, jack stand, mixing or mixed without pumping. high strength coupling, high strength test rod, Approximate size 50" long plate, hex nut and knocker wrench. Calibrations 30.5" high 52" wide Weight 560 lbs. T80 Post-Tensioning Jacks Production Rate 8 gallons per minute at 150 psi With the T80 series the enclosed bearing housing contains a geared socket drive to tighten the bolt hex nut during tensioning. Test jack Colloidal Grout Plant housing will accommodate up to a 9” deep The heavy duty, high volume Colloidal Grout pocket. Plant is favored for precision post-tension grouting. The unit features a high-speed shear mixer that thoroughly wets each particle and T8Z-18 Hydraulic Torque Wrench discharges the mixed material into a 13 cubic foot capacity agitating holding tank. A direct The hydraulic torque wrench is used for coupled progressing cavity pump delivers tensioning anchors in tight fitting locations slurries at a rate of up to 20 gpm and pressures where it would be difficult to use a hydraulic of up to 261 psi. The unit easily mixes and jack. The wrench is also recommended for use pumps slurries of Portland cement, fly ash, when setting the large diameter Spin-Lock bentonite, and lime flour. All controls are anchors. The torque wrenches are light weight conveniently located on the operator platform and can achieve a maximum of 8,000 ft-lbs. for easy one-man control. Pump Pump Type 31.6 progressing cavity Output/Pressure variable up to 20 gpm, 261 psi Colloidal Mix Tank 13.0 CF with Mixer bottom clean out Mixing Pump 2x3x6 diffuser-type centrifugal T8Z Torque Wrench Corrosion Protection For applying torque to the anchor bolt when setting the anchor. Methods of Corrosion Protection T8Z-04 Torque Multiplier (4:1) For use with T8Z Torque Wrench. Other sizes Epoxy Coating available Fusion bonded epoxy coating of steel bars to help prevent corrosion has been successfully employed in many applications because of the chemical stability of epoxy resins. Epoxy coated bars and fasteners should be done in accordance with ASTM Epoxy coating patch kits are often used in the field for repairing nicked or scratched epoxy surfaces. Pre-Grouted Bars T1Z & T2Z Long Fitting Tool Adapters Cement Grout filled corrugated polyethylene tubing is often used to provide an additional For driving hex nuts and setting tools, typically barrier against corrosion attack in highly with our Spin-Lock anchor systems. Works with aggressive soils. torque wrench or impact gun. Available with 1" or 1-1/2" square drive. Please specify square Hot Dip Galvanizing drive for compatibility with your equipment. Zinc serves as a sacrificial metal corroding preferentially to the steel. Galvanized bars have excellent bond characteristics to grout or K3F-26 Long Fitting Wrench Adapter concrete and do not require as much care in handling as epoxy coated bars. For applying torque to recessed anchor nuts that are under tension when using hydraulic jacks. Extruded Polyethylene Available in all anchor sizes. Williams strand tendons contain an extruded high density polyethylene sheathing around each individual strand in the free-stressing portion of the anchorage. The sheathing is minimum 60 mils thick and applied once the 7-wire strand has been coated with a corrosion inhibiting o Abutments compound. Field Splice for Bars Continuous corrosion protection can even be accomplished for the MCP Pregrouted anchors manufactured from Williams Form Engineering. To achieve the equivalent levels of corrosion protection the coupled sections of bar anchors can be wrapped in a grease impregnated tape that is further protected with heat shrink sleeving. This scheme is acceptable by most governing agencies and is specified in the PTI Recommendations for Prestresed Rock and Soil o Moulds Anchors. PRESTRESSING SYSTEMS PRESTRESSING SYSTEM Hardware & Devices o Hydraulic Jack o Prestressing Bed o Anchoring Device o Harping Device o Grouting Equipment o Ducts Pretensioning System o HOYER SYSTEM (Long Line Method) o Couplers PRESTRESSING SYSTTEM Pretensioning System o HOYER SYSTEM (Long Line Method) ▪ used for mass production ▪ in this system, the end abutments are kept sufficient distance apart,  and several members o MAGNEL BLATON SYSTEM are cast in a single line ▪ the shuttering is provided at the sides and b/w the members ▪ the end abutments have to be sufficiently stiff and have good foundations o SHORER SYSTEM ▪ in this system a central tube of high strength steel carries the prestress from surrounding wires and the entire assembly is placed in position and concreted ▪ after the concrete has GIFFORD UDALL SYSTEM attained sufficient strength, the tube is removed, and the prestress is transferred to concrete through bond ▪ the hole left by the tube is grouted POST TENSIONING o FREYSSINET SYSTEM  LEE McCALL SYSTEM an inner piston in the jack then pushes the plug into the cylinder to grip the wires MAGNEL BLATON SYSTEM introduced by a famous engineer, Prof. Magnel of Belgium in this system, the anchorage device consists of sandwich plate having grooves to hold the wires and wedges which are also grooved each plate carries eight wires and between the two ends the spacing of the wires is maintained by spacers a specially deviced jack pulls two wires at a time and anchors them GIFFORD UDALL SYSTEM originated in Great Britain and is widely FREYSSINET SYSTEM used in India the first post-tensioning method this is a single wire system in which introduced by the French Engineer each wire is stressed independently Freyssinet using a double acting jack in this system, high strength steel wires in this system, any number of wires can of 5mm or 7mm diameter, numbering 8 be grouped together to form a cable, and or 12 or 16 or 24 are grouped into a wires are locked into the tapered holes cable with a helical spring inside and a by means of anchor wedges cable is inserted in the duct the anchor wedeges are attached to the anchorage device consists of a concrete end shutters and form an efficient cast-in cylinder with a concentric conical hole component of the anchorage and corrugations on its surface, and a conical plug carrying grooves on its surface where steel wires are carried LEE McCALL SYSTEM along these grooves at the ends wires are pulled by Freyssinet double in this system, the steel bars with acting jacks which can pull through diameter of between 12 and 28mm, suitable grooves all the wires in the provided with threads at the ends, are cable at a time inserted in the performed ducts one end of the wires is anchored, and the after stretching the bars to the required other end is pulled till the wires are length, they are tightened using nuts stretched to the required length against bearing plates provided at the end sections of the member OTHER PRESTRESSING METHODS GENERAL PRINCIPLES OF ELECTRICAL PRESTRESSING PRESTRESSING in this method, reinforcing bars is coated GENERAL PRINCIPLES OF PRESTRESSED with thermoplastic material such as CPNCRETE sulphur or low melting alloy and buried in the concrete after the concrete is set, electric current CLASSIFICATION OF PRESTRESSING of low voltage but high amperage is According to the PRETENSIONING & passed through the bar sequence of casting the POST-TENSIONING electric current heats the bar and the bar concrete and applying elongate tension to the tendons According to the EXTERNAL & bars provided with threads at the other location of the INTERNAL end are tightened against heavy washers, prestressing tendons after required elongation is obtained According to the shape LINEAR & when the bar cools, prestress develops, of the prestressed CIRCULAR and the bond is restored by member According to the FULL, LIMITED & resolidification of the coating amount of prestressing PARTIAL CHEMICAL PRESTRESSING force prestressing can be applied by EXTERNAL PRESTRESSING embedding steel in concrete made of expanding cement the member is prestressed by external steel is elongated by the expansion of reaction offered by rigid abutments the concrete and thus gets prestressed necessary prestressing force can be applied by compressing the member by jacking steel in turn produces compressive stress against abutments in concrete after the prestressing is over, the space between the end of the beam and the abutment may be packed with concrete and the jack recovered INTERNAL PRESTRESSING tendon is provided from which the prestress can be applied LINEAR PRESTRESSING prestressing force is applied longitudinally along or parallel to the axis of the member CIRCULAR PRESTRESSING circumferential hoop or "hugging" stress on the cylindrical or spherical structure neutralizes the tensile stresses at the outer fibers of the curvilinear surface caused by the internal contained pressure used in liquid containment tanks, pipes and pressure reactor vessels PRETENSIONING BASIC CONCEPT OF PRESTRESSING done at the fabrication plant for production Consider a simply supported rectangular of precast members beam subjected to a concentric prestressing tendons are tensioned first before concrete is force, P placed the concrete is cast around the stressed tendon as the fresh concrete hardens, it bonds to the steel when the concrete has reached the required strength, the jacking force is released, and the force is transferred by bond from steel to The compressive stress on the beam cross concrete section is uniform and has an intensity of: 𝑃 𝑓= − 𝐴𝑐 Where Ac = cross-sectional area of a beam section Ac = width x total depth = bh Sign convention: (-) compression, (+) tension. If external transverse loads are applied to the POST-TENSIONING beam causing a maximum moment M at done at the construction site for the mid-span, the resulting stress becomes: construction of cast-in- place members 𝑃 𝑀𝑐 hollow conduits containing the unstressed 𝑓𝑡 = − − 𝐴 𝐼𝑔 tendons are placed in the beam forms before pouring the concrete 𝑃 𝑀𝑐 𝑓𝑏 = − + the tendons are tensioned after the concrete 𝐴 𝐼𝑔 has hardened and achieved sufficient strength Where ft = stress at the top fibers fb=stress at the bottom fibers Ig=gross moment of inertia of the section c=h/2  𝑃 𝑀𝑐 𝑓𝑏 = − + 𝐴 𝐼𝑔 Equation indicates since the support section of a simply that the presence of the prestressing supported beam carries no moment from the compressive stress 𝑃 is reducing external transverse load, high tensile fiber − stresses at the top fibers are caused by the 𝐴 eccentric prestressing force 𝑀𝑐 the tensile flexural stress to limit such stresses, the eccentricity of the 𝐼 prestressing tendon profile (termed the cgs line) is made less at the support section than the concrete's ability to withstand tensile at the midspan section, eliminated stresses is effectively compensated for by altogether, and a negative eccentricity above the compressive force of the prestressing the cgc line is used tendon 𝑃 𝑀𝑐 𝑓𝑡 = − − 𝐴 𝐼𝑔 equation indicates that the compressive stress at the top fibers of the beam due to prestressing are compounded by the application of the loading stress 𝑀𝑐 − 𝐼 to induce tensile stresses at the top fibers due to prestressing, the prestressing tendon is placed eccentrically below the neutral axis at mid-span If the tendon is placed at eccentricity, e, from the center of gravity of the concrete (cgc), a moment, Pe, is created and the ensuing stresses at mid-span become: 𝑃 𝑃𝑒𝑐 𝑀𝑐 𝑓𝑡 = − + − 𝐴𝑐 𝐼𝑔 𝐼𝑔 𝑃 𝑃𝑒𝑐 𝑀𝑐 𝑓𝑏 = − − + 𝐴𝑐 𝐼𝑔 𝐼𝑔