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When considering long-term prestressed girder camber (for a bridge), what are the implications (if any) of long-term downward deflection on girder performance? Should prestressing be designed to ensure that long term camber is always upward?

I'm using the PCI multiplier approach to calculating long term deflection.

Utah DOT Structures Design and Detailing Manual page 14-30 https://drive.google.com/file/d/0B8QRVMpaE6oYWTg3WU5ISDhkRGM/view

Utah DOT Camber

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The questions in your title and in your body-text are slightly different.

Prestressed girder camber should usually be upwards. This is because one usually applies sufficient prestressing to cancel out (or overcome) all possible dead loads and some fraction of live loads. This means that without live loads the prestressing will probably be larger than the applied dead loads, even if one only considers the bending moment component and ignores the compression component of the prestress. The girder's total camber will therefore be upwards in the absence of live loads.

That being said, it is theoretically possible (though a terrible idea, economically speaking) to prestress a beam such that the camber is downward. Just prestress the beam along its centroidal axis, such that the prestress overcomes the tensile stresses by simple compression (making sure not to rupture the beam by compression). In such a case the prestress would not generate an upwards camber and the girder's total camber would remain downwards.

Now, onto the second question: there is no problem in having long-term downwards camber. That just means that your prestress' bending moment component is smaller than the dead loads' total bending moment. So long as your compression component is taking up the slack in the long-term, there's no problem (assuming it's not so dramatic as to cross the acceptable deflection limits in the code, of course).

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  • $\begingroup$ Which code provisions are you referring to for "acceptable camber limits in the code"? $\endgroup$ – CableStay Aug 24 '15 at 18:59
  • $\begingroup$ Oh, whoops, I meant "acceptable deflection limits". $\endgroup$ – Wasabi Aug 24 '15 at 19:08
  • $\begingroup$ But, do you know where deflection limits under permanent loads are set for prestressed girders? I've only found deflection limits for live load (AASHTO 2.5.2.6) , and deflection limits for hollow-core slabs (PCI hollow core slab 2.4.2) $\endgroup$ – CableStay Aug 24 '15 at 19:22
  • $\begingroup$ @CableStay, well, I can't really help you with that because I'm Brazilian, so I'm not exactly an expert with regards to American codes. A quick Google informed that apparently ACI doesn't have deflection limits for dead loads, which surprises me. I don't know how accurate that is or if other relevant American codes do. Other international codes (Eurocode 2 and Brazilian codes, for example) usually place limits around L/250 for maximum deflection (dead load + prestress + live load deflections). $\endgroup$ – Wasabi Aug 24 '15 at 19:44
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The PCI manual (3rd Edition, Section 8.7.1) states:

It is not recommended that prestressing levels be increased in order to reduce or eliminate long-term downward deflection that might be predicted if the multipliers in Table 8.7.1-1 are used.

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