Introduction To Prestressed Concrete - Jim Richardson
CE 537, Spring 2014Introduction to Prestressed Concrete1/7In prestressed concrete, compressive stresses are applied to the concrete prior to loading. Underservice loads, the entire cross section is essentially in compression, which takes advantage of concrete’sconsiderable compressive strength but minimal tensile strength. Therefore cracking under service loadsis minimized or eliminated (depending on the design). Also, since the entire concrete section resistsstress (unlike in “normally” reinforced concrete sections where most of the section is in tension andtherefore unused) service load deflections are greatly reduced.Prestressed concrete components are constructed in two different ways. In pretensioned concretecomponents, the prestressing steel is tensioned before the concrete is placed, as illustrated in Figure 1below. The prestressing tendons are typically un‐sheathed so that the concrete bonds to the tendons.Pretensioning frameanchored to groundPrestressing steel is tensionedPPretressing tendonsPConcrete is placedPrestressing force is transferred to the concreteFigure 1. Construction sequence for Pre‐tensioned concrete beamIn post‐tensioned concrete, the concrete is prestressing steel is tensioned after the concrete is placed,as illustrated in Figure 2 below. The prestressing tendons are sheathed or placed in ducts so that theconcrete and tendons are unbounded.Since concrete deforms under sustained loads (creep), prestressed concrete was not practical until theadvent of high‐strength prestressing bars and strands. Typical prestressing strands have a tensilestrength of 270 ksi, compared to 90 ksi for grade 60 rebar. With high‐strength prestressing, large initialprestress forces can be applied to the concrete, so that when the concrete shortens over time andcauses a decrease in prestress force, there is still sufficient prestressing force applied to the concrete tokeep the cross section in compression under service loads. Typical stress‐strain curves of prestressingstrands are shown in Figure 3.
Introduction to Prestressed ConcreteCE 537, Spring 20142/7The sizes of prestressing reinforcement are listed in the appendix of the ACI code and are shown inTable 1 below. Seven‐wire Grade 270 ½‐inch diameter strand is very common, and Grade 270 0.60 inchdiameter strand is becoming more popular.post‐tensioning cable(sheathed)Concrete forms are erected andprestressing strands are positionedformexistingcolumnshoringConcrete is placedCables are tensionedFigure 2. Construction sequence for Post‐tensioned concrete beam
CE 537, Spring 2014Introduction to Prestressed ConcreteFigure 3. Stress‐strain curves of 7‐wire strands (from PCI Manual).3/7
CE 537, Spring 2014Introduction to Prestressed ConcreteTable 1. Diameters and areas of typical prestressing reinforcement (from ACI 318‐11).4/7
CE 537, Spring 2014Introduction to Prestressed Concrete5/7Design of prestressed concrete beams consists of selecting the depth of the concrete beam and theamount and layout of prestressing tendons to: prevent or limit flexural tensile cracking in the concrete under service loadslimit service‐load deflectionsprevent flexure failure under over‐load (ultimate strength) conditionsIt’s possible to use too much prestressing resulting in excessive compressive stress in the concrete(usually at locations with high service‐load stresses) and/or resulting in excessive tensile stress in theconcrete (usually at locations with little or no service load stresses).Chapter 18 of ACI 318 on prestressed concrete presents limits on stresses in prestressing and concrete,which are summarized in Figure 4 below. Please refer to Chapter 2 (p 19) in ACI 318 for explanations ofnotation.The stress on the concrete due to prestressing is calculated using the equations from strength ofmaterials for normal stresses due to an eccentric axial load (see Figure 5). The effect of the eccentricaxial load is equivalent to the axial load applied at the centroid of the concrete section plus a momentequal to the axial load times the eccentricity. The normal stress at any location in the concrete crosssection can then be calculated using the familiar formula:! P My AI
CE 537, Spring 2014Introduction to Prestressed Concrete6/7Prestressing Steel: The following criteria are specified by ACI for the prestressing steel (Section 18.5.1,pg 287):Max stress due to jacking force min( 0.94 fpy , 0.80 fpu )Max stress at transfer min( 0.82 fpy , 0.74 fpu )Stage1. Concretestresses attransfer of PTforce toconcrete(ACI 18.4.1,pg 290)2. Concretestresses underservice loads(ACI TableR18.3.3,pg 291)2. Deflectionsunder serviceloads(ACI Table 9.5b,pg 129)3. Flexure strengthunder ultimateloadsDesign Criteriaat endsmax tensionmax comp.f c'i3 f c'i0.7 f c'i0.6 f c'i6Sustainedloadsmax tensionmax comp.max ΔLL elsewherewSWPAll loads12 f c'i0.45 f c'i0.60 f c'iL360max Δafter erection L240φ Mn Muφ M n 1.2 M cr(ACI 18.7 and 8, pp 294 - 296)Figure 4. Relevant design criteria in ACI 318‐11P
CE 537, Spring 2014Introduction to Prestressed ConcreteFigure 5. Calculation of concrete stresses due to prestressing7/7
Introduction to Prestressed Concrete 1 / 7 In prestressed concrete, compressive stresses are applied to the concrete prior to loading. Under service loads, the entire cross section is essentially in compression, which takes advantage of concrete’s considerable compressive
Lecture 24 – Prestressed Concrete Prestressed concrete refers to concrete that has applied stresses induced into the member. Typically, wires or “tendons” are stretched and then blocked at the ends creating compressive stresses throughout the member’s entire cross-section. Most Prestressed concrete is precast in a plant.
prestressed concrete members, the bonding action between the prestressing element and the concrete is a result of friction as well as adhesion and mechanical resistance. In the fabrication of a pre-tensioned prestressed concrete member the prestressing strand is first tensioned to the desired stress level. Concrete is then cast about the strand and
Precast/Prestressed Concrete Institute, Design Handbook-Precast and Prestressed Concrete: Code of Standard Practice for Precast/Prestressed Concrete (PCI MNL-120) Structural Material Specifications Concrete Foundations (Pile Caps and Grade Beams): 6,000 p
As prestressed concrete became widely produced and adopted, a second concept was formulated, commonly known as the ultimate strength theory. Under that concept, prestressed concrete is treated as a combination of h\Ph strength con crete and high strength steel, with concrete to carry the compression
stressed concrete columns with the exception that our knowledge of re-inforced concrete columns may be utilized when dealing with pre-stressed ones. Three approaches are possible for the design of prestressed concrete columns: 1. Rational and analytical ap-proach. 2. A new set of code design clauses to be set up specifically for prestressed .
a. prestressed box beam: 5 to 7 ksi b. prestressed I-beam: 5 to 7 ksi c. prestressed bulb-tee beam: 6 to 8 ksi An exception to the range shown above will be allowed for a higher strength if the higher strength can be documented to be of significant benefit to the project, it can be effectively
DPNS ASTM C823-12:2021 New Circulation 91.100.30 BPS/TC 5 Concrete, Reinforced Concrete and Prestressed Concrete Standard Test Method for Measuring Length of Concrete Cores DPNS ASTM C1542 / C1542M:2021 New Circulation 91.100.30 BPS/TC 5 Concrete, Reinforced Concrete and Prestressed
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