Precast Prestressed U Beam bridge 1

[aun]

A 120' u54 beam to carry out HS-25 loading is vertically rotated to parallel with roadway surface, 2% cross-slope. The problem is that its neutral axis of the composited beam is needed to parallel to the roadway or not? if so, I think that calculation would be much more difficult. ( I do not have a computer program.) And, I read a book showing that haunch area is included to determine the composited section while the other doesn't . Sugestion would be greatly appreciate.

 

[aun]

I'd like to add one more question. I just recognize. It is that what is the difference between bond and unbond strand in term of allowable stress? I understand that the bonded strand will have more allowable tensile stress than the unbonded strand. what is debond meaning?

 

[ishvaaag]

For accuracy, a structure should be modeled as closely as possible as how it is going to be built.

It is very easy to get moments of inertia, section and center of gravity by use of Autocad's 12 module AME. There are also others free but may require a platform such Excel or Mathcad, and of course paid software.

Respect unbonded tendons, they don't follow tightly the deformation of the concrete plane sections orthogonal to the beam axis. Repeated small scale friction under repeated loadings and slippage appear; also the tendon does not enjoy the protection of concrete making it (might be) more susceptible to mechanical and even corrossion damage. This itself might be cause enough for lowering the allowable stress for such tendons.

 

[Ingenuity]

aun,

If I am understanding your post correctly, you have a precast pre-tensioned concrete girder bridge where the girder section is a U-section (or a trough section) with a deck slab that is composite with the girders. Several parallel girders will make up the full width of the bridge. span is 120 feet, girder depth is 54"?

A 2% deck cross-fall will have negligble effect on the composite section properties. You can include it simply enough but i would not worry.

For a precast pre-tensioned section you need to underake several calculations as non-composite and composite action. Presuming that the girders are self supporting across the span and do not have falsework or temporary shoring, then non-composite action shall be applicable up until the time that the deck slab has gained sufficient strength to be indeed composite with the precast section - e.g. camber and transfer calcs, weight of wet deck concrete, construction LL prior and during concrete placementetc. After the concrete deck gain strength then composite action is applicable and all future loads will be resisted via this action (ie the haunch effect of deck slab).

Bonded strands are those strands (pretensioned in this case) that are fully bonded to the concrete. Debonded (or "slipped" strands) strands are occasionally used to control stresses at transfer at the ends of the girder. Some DOT's do not permit debonded strands (increased corrosion is likely) so alternatively some of the strands can be harped (or "kinked&quot at strategic location within the girder length to reduce the transfer stresses at the girder ends. A debonded strand has ZERO tension force (it is able to slip) at the locations where it is debonded. At other locations the stressed strands (most commonly 0.5" - 7 wire strand ) is stressed to 70% or 75% of UTS. But you need to account for prestress losses - both long term and short term - to arrive at the "effective prestress" force.

Are you familar with prestressed concrete design? For example, you need to undertake service stress calculations and do superposition of stresses for non-compsoite and composite actions, and you also need to check strength (flexural and shear) and also estimate deflections, and also longitudinal shear check etc.


HTH

 

[aun]

Ingenuity
this girder is not permited to use drape strand.
In such case that the pretensioned girder section at end of beam before stress losses has tensile stress excessing the allowable tenstress,(3*sqrt(fc')). To decrease the excessive tensile stress, a number of strands will be debond over some length and be bond again where the tensile stress from pretension is equal to the allowable stress.
PS. I like your suggestion that I don't have to rotate the girder.

THANKS

 

[Ingenuity]

aun,

what is the reason why you cannot use harped strands? your precaster does not have the equipment nor bed to do it?

 

[aun]

DOT does not permit the use of drape strands in this U girder. Otherwise, it is mentioned to use debond.

 

[aun]

If there is a number of debond strands at end of girder, how are those strands developed the design stress at midspan??

 

[ishvaaag]

From the inner debonded point the strand will start a development length till it gains full prestress value. The debonded region and this transfer length (plus development length) at the position will have to be covered by passive reinforcement.

 

[Ingenuity]

aun,

refer to ACI318 12.9 or PCI 5th Edition...for actual code equations for calculating the transfer development length, ld.

to determine the transfer length for a stress in the strand of fse, you require a transfer length of

fse*db/3

to develop the design strength of the strand you need more length:

ld=fse*db/3 +(fps-fse)*db

where:

fps: nom strength of strand
fpe: effective stress in strand, after losses
db: dia of strand (or prestressing bar, or wire)

say for instance that you have calculated that you need to slip (debond) some of the strands for 60 inches from the girder ends to satisfy your transfer tensile stresses, then from this point of 60 inches until the transfer development length (ld) you have an value of steel stress that can be developed that varies from ZERO at 60" to full value of fps at a distance ld. Obviously, the force is ZERO from 0" to 60" on the debonded strands too.

the length ld has to be doubled when you use debonded strands, or if you have a design which includes tension at sevice loads in precompressed tensile zones.

you also need to be careful in short span elements where you may not develop the design force required due to premature slip. your span (120') is not short, so you should be okay here.

when you are calculating moment capacities at various location you need to check the level of contribution from strands that have been debonded if these strands are not fully developed. i prefer to use strain compatibility.

HTH