reinforcement 12

8/12/2019 Reinforcement 12

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VOL. V - PART 2

DATE: 31Mar2006

SHEET 1 of 8

PRESTRESSED CONCRETEREINFORCEMENT

GENERAL INFORMATIONFILE NO. 12.01-1

GENERAL INFORMATION:

This section specifies the general practices and requirements regarding the use of reinforcingsteel for the design and fabrication of precast prestressed concrete members. For specificpractices and requirements (sizes, spacings and misc. details), refer to the appropriate section ofthis chapter.

All prestressed and non-prestressed reinforcement used in the design and fabrication ofprestressed concrete members shall conform to the requirements of Sections 223, 405 and 406of the current edition of the VDOT Road and Bridge Specificationsand as specified herein.

PRESTRESSED TENDONS:

Prestressed tendons shall conform to the requirements of Section 223 of the VDOT Road andBridge Specificationsand as specified below.

Prestressed tendons shall be uncoated, seven-wire, low-relaxation steel strands conforming tothe requirements of ASTM A416 (AASHTO M203), Grade 270.

For strand properties and design strengths, see File No. 12.01-3.

The use of stress-relieved strands or substitution of stress-relieved strands for low-relaxationstrands shall not be permitted.

Prestressed concrete members shall be designed using1/2 diameter strands unless specified

otherwise in this chapter.

Strands for prestressed members shall be distributed uniformly across the width of the member ina 2 X 2 grid pattern for strands up to and including 0.6 diameter. The grid pattern shall be laidout symmetrically about a vertical axis through the centroid of the member cross section.

For computation of prestress losses, see File Nos. 12.01-4 thru -6.

The use of debonded strands in prestressed members shall not be permitted.

NON-PRESTRESSED REINFORCEMENT:

Non-prestressed reinforcement shall conform to the requirements of Sections 405 and 406 of theVDOT Road and Bridge Specificationsand as specified below.

Deformed reinforcing bars shall conform to the requirements of ASTM A615, Grade 60.

Plain steel bars when used as dowels shall conform to the requirements of ASTM A36.

Spiral wire ties shall conform to the requirements of ASTM A82 (AASHTO M32).


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VOL. V - PART 2

DATE: 01Jun2005

SHEET 2 of 8


GENERAL INFORMATIONFILE NO. 12.01-2

NON-PRESTRESSED REINFORCEMENT CORROSION PROTECTION:

For non-prestressed reinforcement requiring corrosion protection, refer to the appropriate sectionof this chapter or the standard detail sheet for the member.

Non-prestressed reinforcement requiring epoxy coating or galvanization shall conform to therequirements of Section 223 of the VDOT Road and Bridge Specifications.


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VOL. V - PART 2

SEVEN-WIRE GRADE 270 LOW-RELAXATION STRANDS

Nominal Areaof

Strand

UltimateStrength of

Strand

YieldStrength of

StrandNominal

Diameter ofStrand

sA

ss Af' sy Af

ss Af0.75'

sy Af0.90

(in) (in2) (lbs) (lbs) (lbs) (lbs)

3/8 (0.375) 0.085 22950 20660 17210 18590

7/16 (0.438) 0.115 31050 27950 23290 25160

1/2 (0.500) 0.153 41310 37180 30980 33460

1/2 (special) 0.167 45000 40500 33750 36450

0.60 0.217 58590 52730 43940 47460

The1/2 (special) strand shown above is not listed in ASTM A416. It is a

1/2 diameter strand

designed to have a minimum breaking strength of 45,000 lbs. and a minimum yield strength of40,500 lbs.

'sy f0.90f =

= yield stress of prestressing steel (AASHTO 9.1.2).

ss Af0.75' = required tensioning force per strand immediately prior to release (after losses due

to anchorage set and other factors). This force shall be entered in the table onthe standard beam detail sheet as the Prestress Force Per Strand(AASHTO 9.15.1).

sy Af0.90 = maximum tensioning force per strand for short periods of time prior to seating tooffset seating and friction losses (AASHTO 9.15.1).

DATE: 31Mar2006

SHEET 3 of 8


STRAND PROPERTIES AND DESIGN STRENGTHSFILE NO. 12.01-3


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VOL. V - PART 2

PRESTRESS LOSSES:

The loss of prestress can be defined as the difference between the initial stress in the strandsand the effective stress in the strands after losses have occurred. The loss of prestress cangenerally be attributed to the cumulative effects from the following sources:

Elastic Shortening (ES)

Relaxation of Prestressing Steel (CRs)

Shrinkage of Concrete (SH)

Creep of Concrete (CRc)

Anchorage Set (seating or slip) caused by movement of strands due to chuck seating inwedge-type anchorages

Other Factors such as casting bed and form deformations, and temperature effects, ifany.

Producers of prestressed concrete members will normally make the necessary adjustments to theprestressing (jacking) force to compensate for the losses due to anchorage set and other factors.

COMPUTATION OF PRESTRESS LOSSES:

Prestress losses excluding friction for normal designs shall be computed using the approximatemethod contained in Article 9.16.2 of the AASHTO specifications as supplemented herein. Forunusual or complex designs, more detailed methods (time-dependent analysis) may be used. Inno case shall the total prestress losses excluding friction be less than 25,000 psi for bothpretensioned and post tensioned members.

For the design of prestressed concrete members prestress losses excluding friction shall be

computed at the following stages in the life of the member:

Transfer (release) of Prestress

fs(at transfer) = CRs(18) + ES

where CRs(18) =( )

i

y

i

r

f0.55

f

f

K

t24log

*

where CRs(18) = loss due to relaxation of prestressing

steel at 18 hours (psi)

t = time in days (0.75 days) for whichrelaxation is calculated

DATE: 31Mar2006

SHEET 4 of 8


PRESTRESS LOSSESFILE NO. 12.01-4


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VOL. V - PART 2

if = initial stress at the beginning of the

relaxation loss period

= 0.75fsfor low relaxation strands

*yf = yield stress of the strands

= 0.90fsfor low relaxation strands

rK = 45 for low relaxation strand

Service Load

fs (at service) = SH + ES + CRc+ CRs

where fs(at service) = total loss excluding friction (psi)

SH = loss due to concrete shrinkage (psi)

ES = loss due to elastic shortening (psi)

CRc= loss due to creep of concrete (psi)

CRs= loss due to relaxation of prestressing

steel (psi)

For estimating elastic shortening loss (ES) to be used in the above equations,Article 9.16.2.1.2 of the AASHTO specifications gives the following equation:

ES = cir

ci

sf

E

E AASHTO Eq. 9-6

where Es= modulus of elasticity of prestressing steel

= 28 x 106psi

fci= strength of concrete at release

Eci= modulus of elasticity of concrete at release after losses

= 'cifw332

3

fcir= average concrete stress at the c.g. of prestressing steel due to

prestressing force after losses and dead load weight of the beam

immediately after transfer.

DATE: 31Mar2006

SHEET 5 of 8




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VOL. V - PART 2

The value of fcirmay be approximated as follows:

ftransfer= stress in prestressing steel at transfer after losses

ftransfer=

+

+

+

bm

2s

bms

ci

s

bm

sbmci

s

si

I

e

A

1.0A

E

E1.0

I

eME

E

(18)CRf

*

Therefore fcircan be calculated as follows:

fcir=bm

sbm

bm

2sstransfer

bm

stransfer

I

eM

I

eAf

A

Af **

+

For estimating creep of concrete loss (CRc), Article 9.16.2.1.3 of the AASHTO

specifications notes the following equation:

CRc= 12fcir 7fcds AASHTO Eq. 9-9

where fcds= concrete stress at the c.g. of the prestressing steel due to all

dead loads except dead load weight of the beam.

The value of fcdsmay be computed as follows:

fcds=( ) ( )

comp

sbcbDL

bm

sDL

I

eyycompM

I

ecompnonM ++

DATE: 31Mar2006

SHEET 6 of 8




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VOL. V - PART 2

DRAPING OF STRANDS:

Prestressed members shall be designed with strands having either a straight or a doubledraped (harped) profile along the length of the member.

Draped points (hold-downs) shall be located at 0.4L and 0.6L.

The number of draped strands permitted shall not exceed 14 ( 7 rows of 2 strands ).

COMPUTATION OF HOLD-DOWN FORCES:

Strand hold-down devices are rated (maximum safe working load) by uplift force per strand aswell as by total uplift force per device. The magnitude of these forces shall be investigated toensure the availability of hold-down devices to carry these loads. In the event the actual upliftforce per strand or total uplift force per hold-down device exceeds the maximum safe workingloads of 4 kips and 48 kips respectively, the designer shall verify the availability of a hold-downdevice to carry the required uplift forces with local fabricators of prestressed members. Only asa last resort may these forces be split into two or more locations straddling the 0.4L and 0.6Lpoints.

Definitions:

Fpull=Maximum pretensioned force per strand immediately prior to transfer.

Fv= Uplift force per strand at drape point.

Fh= Horizontal force per strand at drape point.

V = Distance from c.g. of strands at midspan to c.g. of strands at end of beam.

H = Distance from end of beam to drape point.

Friction Factor = Increase in force due to friction losses (1.05 for swivel devices and 1.15for non-swivel devices).

DATE: 01Jun2005

SHEET 7 of 8


DRAPING OF STRANDS / HOLD-DOWN FORCESFILE NO. 12.01-7


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VOL. V - PART 2

Equations for calculating hold-down forces:

*s

'spull Af0.80F = (for low-relaxation strands)

FactorFrictionH

VFF

pullv

= 4 kips per strand

Total Uplift Force = Fvx(number of draped strands) 48 kips per device

DATE: 31Mar2006

SHEET 8 of 8


DRAPING OF STRANDS / HOLD-DOWN FORCESFILE NO. 12.01-8

reinforcement 12

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