Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.153
If the effective prestress force is less than 40% of the tensile strength of the
flexural reinforcement, or if a more accurate method is preferred, the value of V c
should be taken as the smaller of the shear forces causing inclined flexure-shear
cracking V ci or web-shear V cw, but need not be smaller than b wd.
1.7 �ƒ� c
V � 0.6 �ƒ� bd� V � VM /M (9.132)
ci c w d i cr max
V � (3.5 �ƒ� � 0.3 ƒ ) bd� V (9.133)
cw c pc w p
where Vd � shear force at section caused by dead load
Vi � shear force at section occurring simultaneously with Mmax Mmax � maximum bending moment at the section caused by externally applied
factored loads
Mcr � cracking moment based on the modulus of rupture (Art. 9.51)
bw � width of web
ƒpc � compressive stress in the concrete, after all prestress losses have occurred,
at the centroid of the cross section resisting the applied loads,
or at the junction of web and flange when the centroid lies within the
flange
Vp � vertical component of effective prestress force at section considered
In a pretensioned beam in which the section h/2 from the face of the support
is closer to the end of the beam than the transfer length of the tendon, the reduced
prestress in the concrete at sections falling within the transfer length should be
considered when calculating Vcw. The prestress may be assumed to vary linearly
along the centroidal axis from zero at the beam end to the end of the transfer
length. This distance can be assumed to be 50 diameters for strand and 100 diameters
for single wire.
(‘‘PCI Design Handbook,’’ Precast/Prestressed Concrete Institute.)
9.110 BOND, DEVELOPMENT, AND GROUTING
OF TENDONS
Three- or seven-wire pretensioning strand should be bonded beyond the critical
section for a development length, in, of at least
2
Ld � (ƒps � ⁄3ƒ se)d b
(9.134)
where dp � nominal diameter of strand, in
ƒps � stress in tendons at nominal strength, ksi
ƒse � effective stress in tendons after losses, ksi
(The expression in parentheses is used as a constant without units.) Investigations
for bond integrity may be limited to those cross sections nearest each end of the
member that are required to develop their full strength under factored load. When
bonding does not extend to the end of the member, the bonded development length
given by Eq. (9.134) should be doubled.
Minimum Bonded Reinforcement. When prestressing steel is unbonded, the ACI
318 Building Code requires that some bonded reinforcement be placed in the precompressed
tensile zone of flexural members and distributed uniformly over the
tension zone near the extreme tension surface. The amount of bonded reinforcement