1、Section 9 PRESTRESSED CONCRETE Part A GENERAL REQUIREMENTS AND MATERIALS 9.1 APPLICATION 9.1.1 General The specifications of this section are intended for de- sign of prestressed concrete bridge members. Members designed as reinforced concrete, except for a percentage of tensile steel stressed to im
2、prove service behavior, shall conform to the applicable specifications of Section 8. Exceptionally long span or unusual structures require detailed consideration of effects which under this Section may have been assigned arbitrary values. Notations = area of non-prestressed tension reinforcement = a
3、rea of compression reinforcement (Article = area of prestressing steel (Article 9.17) = steel area required to develop the compressive strength of the overhanging portions of the flange (Article 9.17) = steel area required to develop the compressive strength of the web of a flanged section (Arti- cl
4、es 9.17-9.19) (Articles 9.7 and 9.19) 9.19) = area of web reinforcement (Article 9.20) = width of flange of flanged member or width of rectangular member = width of cross section at the contact surface being investigated for horizontal shear (Arti- cle 9.20). = width of a web of a flanged member = l
5、oss of prestress due to creep of concrete (Ar- = loss of prestress due to relaxation of pre- = nominal diameter of prestressing steel (Arti- ticle 9.16) stressing steel (Article 9.16) cles 9.17 and 9.27) d = distance from extreme compressive fiber to centroid of the prestressing force, or to cen- tr
6、oid of negative moment reinforcing for pre- cast girder bridges made continuous = distance from the extreme compressive fiber to the centroid of the non-prestressed tension reinforcement (Articles 9.7 and 9.17-9.19) = loss of prestress due to elastic shortening (Ar- ticle 9.16) = base of Naperian lo
7、garithms (Article 9.16) = average concrete compressive stress at the c.g. of the prestressing steel under full dead load (Article 9.16) = average concrete stress at the c.g. of the pre- stressing steel at time of release (Article 9.16) = compressive strength of concrete at 28 days = compressive stre
8、ngth of concrete at time of initial prestress (Article 9.15) = average splitting tensile strength of light- weight aggregate concrete, psi = stress due to unfactored dead load, at extreme fiber of section where tensile stress is caused by externally applied loads (Article 9.20) fPC = compressive str
9、ess in concrete (after al- lowance for all prestress losses) at centroid of cross section resisting externally applied loads or at junction of web and flange when the centroid lies within the flange (In a com- posite member, fpc is resultant compressive stress at centroid of composite section, or at
10、 junction of web and flange when the centroid lies within the flange, due to both prestress and moments resisted by precast member act- ing alone.)(Article 9.20) = compressive stress in concrete due to effective prestress forces only (after allowance for all prestress losses) at extreme fiber of sec
11、tion where tensile stress is caused by externally applied loads (Article 9.20) dt ES e fCdS fcir fC ci fCt fd fpe 225 226 HIGHWAY BRIDGES 9.1.2 = guaranteed ultimate tensile strength of the = the modulus of rupture of concrete, as defined = total prestress loss, excluding friction (Article = effecti
12、ve steel prestress after losses = average stress in prestressing steel at ultimate load = ultimate stress of prestressing steel (Articles 9.15 and 9.17) = yield stress of non-prestressed conventional reinforcement in tension (Articles 9.19 and 9.20) = yield stress of non-prestressed conven- tional r
13、einforcement in compression (Article 9.19) = yield stress of prestressing steel (Article 9.15) = 0.90 f: for low-relaxation wire or strand = 0.85 f, for stress-relieved wire or strand = 0.85 fi for Type I (smooth) high-strength bar = 0.80 f, for Type II (deformed) high-strength = overall depth of me
14、mber (Article 9.20) = moment of inertia about the centroid of the cross section (Article 9.20) = friction wobble coefficient per foot of pre- stressing steel (Article 9.16) = length of prestressing steel element from jack end to point x (Article 9.16) = moment causing flexural cracking at sec- tion
15、due to externally applied loads (Article 9.20) prestressing steel, Atf, in Article 9.15.2.3 (Article 9.18) 9.16) bar = cracking moment (Article 9.18) = composite dead load moment at the section (Commentary to Article 9.18) = noncomposite dead load moment at the sec- tion (Article 9.18) = maximum fac
16、tored moment at section due to externally applied loads (Article 9.20) = nominal moment strength of a section = factored moment at section 3 +Mn (Articles = A,bd, ratio of non-prestressed tension rein- = AYbd, ratio of prestressing steel (Articles = A:/bd, ratio of compression reinforcement = factor
17、ed tendon force = statical moment of cross-sectional area, above or below the level being investigated for shear, about the centroid (Article 9.20) 9.17 and 9.18) forcement (Articles 9.7 and 9.17-9.19) 9.17 and 9.19) (Article 9.19) = loss of prestress due to concrete shrinkage (Article 9.16) = longi
18、tudinal spacing of the web reinforcement (Article 9.20) = noncomposite section modulus for the ex- treme fiber of section where the tensile stress is caused by externally applied loads (Article 9.18) = composite section modulus for the extreme fiber of section where the tensile stress is caused by e
19、xternally applied loads (Article 9.18) = average thickness of the flange of a flanged member (Articles 9.17 and 9.18) = steel stress at jacking end (Article 9.16) = steel stress at any point x (Article 9.16) = permissible horizontal shear stress (Article 9.20) nominal shear strength provided by conc
20、rete (Article 9.20) nominal shear strength provided by concrete when diagonal cracking results from com- bined shear and moment (Article 9.20) nominal shear strength provided by concrete when diagonal cracking results from exces- sive principal tensile stress in web (Article 9.20) shear force at sec
21、tion due to unfactored dead load (Article 9.20) factored shear force at section due to exter- nally applied loads occurring simultaneously with M, (Article 9.20) nominal horizontal shear strength (Article 9.20) vertical component of effective prestress force at section (Article 9.20) nominal shear s
22、trength provided by shear re- inforcement (Article 9.20) factored shear force at section (Article 9.20) distance from centroidal axis of gross section, neglecting reinforcement, to extreme fiber in tension (Article 9.20) friction curvature coefficient (Article 9.16) total angular change of prestress
23、ing steel pro- file in radians from jacking end to point x (Ar- ticle 9.16) factor for concrete strength, as defined in Ar- ticle 8.16.2.7 (Articles 9.17, 9.18 and 9.19) factor for type of prestressing steel (Article 9.17) = 0.28 for low-relaxation steel = 0.40 for stress-relieved steel = 0.55 for b
24、ars 9.1.3 DIVISION I-DESIGN 227 9.1.3 Definitions The following terms are defined for general use. Specialized definitions appear in individual articles. Anchorage Device-The hardware assembly used for transfening a post-tensioning force from the tendon wires, strands or bars to the concrete. Anchor
25、age Seating-Deformation of anchorage or seating of tendons in anchorage device when pre- stressing force is transferred from jack to anchorage device. Anchorage Spacing-Center-to-center spacing of an- chorage devices. Anchorage Zone-The portion of the structure in which the concentrated prestressing
26、 force is transferred from the anchorage device into the concrete (Local Zone), and then distributed more widely into the structure (Gen- eral Zone) (Article 9.21.1). Basic Anchorage Device-Anchorage device meeting the restricted bearing stress and minimum plate stiffness requirements of Articles 9.
27、21.7.2.2 through 9.21.7.2.4; no acceptance test is required for Basic Anchorage Devices. Bonded Tendon-Prestressing tendon that is bonded to concrete either directly or through grouting. Coating-Material used to protect prestressing ten- dons against corrosion, to reduce friction between tendon and
28、duct, or to debond prestressing tendons. Couplers (Couplings)-Means by which prestressing force is transmitted from one partial-length prestressing tendon to another. Creep of Concrete-Time-dependent deformation of concrete under sustained load. Curvature Friction-Friction resulting from bends or cu
29、rves in the specified prestressing tendon profile. Debonding (blanketing)-Wrapping, sheathing, or coating prestressing strand to prevent bond between strand and surrounding concrete. Diaphragm-Transverse stiffener in girders to main- tain section geometry. Duct-Hole or void formed in prestressed mem
30、ber to accommodate tendon for post-tensioning. Edge Distance-Distance from the center of the anchorage device to the edge of the concrete member. Efective Prestress-Stress remaining in concrete due to prestressing after all calculated losses have been de- ducted, excluding effects of superimposed lo
31、ads and weight of member; stress remaining in prestressing ten- dons after all losses have occurred excluding effects of dead load and superimposed load. Elastic Shortening of Concrete-Shortening of member caused by application of forces induced by pre- stressing. End Anchorage-Length of reinforceme
32、nt, or me- chanical anchor, or hook, or combination thereof, beyond point of zero stress in reinforcement. End Block-Enlarged end section of member designed to reduce anchorage stresses. Friction (post-tensioning)-Surface resistance be- tween tendon and duct in contact during stressing. General Zone
33、-Region within which the concentrated prestressing force spreads out to a more linear stress dis- tribution over the cross section of the member (Saint Venant Region) (Article 9.21.2.1) Grout Opening or Vent-Inlet, outlet, vent, or drain in post-tensioning duct for grout, water, or air Intermediate
34、Anchorage-Anchorage not located at the end surface of a member or segment; usually in the form of embedded anchors, blisters, ribs, or recess pockets Jacking Force-Temporary force exerted by device that introduces tension into prestressing tendons. Local Zone-The volume of concrete surrounding and i
35、mmediately ahead of the anchorage device, subjected to high local bearing stresses (Article 9.21.2.2) Loss of Prestress-Reduction in prestressing force resulting from combined effects of strains in concrete and steel, including effects of elastic shortening, creep and shrinkage of concrete, relaxati
36、on of steel stress, and for post-tensioned members, friction and anchorage seating. Post-Tensioning-Method of prestressing in which tendons are tensioned after concrete has hardened. Precompressed Zone-Portion of flexural member cross section compressed by prestressing force. Prestressed Concrete-Re
37、inforced concrete in which internal stresses have been introduced to reduce potential tensile stresses in concrete resulting from loads. Pretensioning-Method of prestressing in which ten- dons are tensioned before concrete is placed. Relaxation of Tendon Stress-Time-dependent reduc- tion of stress i
38、n prestressing tendon at constant strain. Shear Lag-Nonuniform distribution of bending stress over the cross section. Shrinkage of Concrete-Time-dependent deformation of concrete caused by drying and chemical changes (hy- dration process). Special Anchorage Device-Anchorage device whose adequacy mus
39、t be proven experimentally in the standardized acceptance tests of Division II, Article 10.3.2.3. 228 HIGHWAY BRIDGES 9.1.3 Tendon-Wire, strand, or bar, or bundle of such ele- ments, used to impart prestress to concrete. Transfer-Act of transferring stress in prestressing tendons from jacks or prete
40、nsioning bed to concrete member. Transfer Length-Length over which prestressing force is transferred to concrete by bond in pretensioned members. Wobble Friction-Friction caused by unintended devi- ation of prestressing sheath or duct from its specified pro- file or alignment. Wrapping or Sheathing-
41、Enclosure around a pre- stressing tendon to avoid temporary or permanent bond between prestressing tendon and surrounding concrete. 9.2 CONCRETE The specified compressive strength, f:, of the concrete for each part of the structure shall be shown on the plans. The requirements for f: shall be based
42、on tests of cylin- ders made and tested in accordance with Division II, Sec- tion 8, “Concrete Structures.” 9.4 GENERAL 9.3 REINFORCEMENT 9.3.1 Prestressing Steel Wire, strands, or bars shall conform to one of the fol- lowing specifications. “Uncoated Stress-Relieved Wire for Prestressed Con- crete,
43、” AASHTO M 204. “Uncoated Seven-Wire Stress-Relieved Strand for Pre- stressed Concrete,” AASHTO M 203. “Uncoated High-Strength Steel Bar for Prestressing Concrete.” ASTM A 722. Wire, strands, and bars not specifically listed in AASHTO M 204, AASHTO M 203, or ASTM A 722 may be used provided they conf
44、orm to the minimum requirements of these specifications. 9.3.2 Non-Prestressed Reinforcement Non-prestressed reinforcement shall conform to the re- quirements in Article 8.3. Part B ANALYSIS Members shall be proportioned for adequate strength using these specifications as minimum guidelines. Con- ti
45、nuous beams and other statically indeterminate struc- tures shall be designed for adequate strength and satisfac- tory behavior. Behavior shall be determined by elastic analysis, taking into account the reactions, moments, shear, and axial forces produced by prestressing, the ef- fects of temperatur
46、e, creep, shrinkage, axial deformation, restraint of attached structural elements, and foundation settlement. 9.5 EXPANSION AND CONTRACTION 9.5.1 In all bridges, provisions shall be made in the design to resist thermal stresses induced, or means shall be provided for movement caused by. temperature
47、changes. 9.5.2 Movements not otherwise provided for, including shortening during stressing, shall be provided for by means of hinged columns, rockers, sliding plates, elas- tomeric pads, or other devices. 9.6 SPAN LENGTH The effective span lengths of simply supported beams shall not exceed the clear
48、 span plus the depth of the beam. The span length of continuous or restrained floor slabs and beams shall be the clear distance between faces of sup- port. Where fillets making an angle of 45” or more with the axis of a continuous or restrained slab are built mono- lithic with the slab and support,
49、the span shall be mea- sured from the section where the combined depth of the slab and the fillet is at least one and one-half times the thickness of the slab. Maximum negative moments are to be considered as existing at the ends of the span, as above defined. No portion of the fillet shall be considered as adding to the effective depth. 9.7 FRAMES AND CONTINUOUS CONSTRUCTION 9.7.1 Cast-in-Place Post-Tensioned Bridges The effect of secondary moments due to prestressing shall be included in stress calculations at working load. In calculating ultimate strength moment and shear require- ment
