Prestressed concrete beams - upward load 1

[LR11]

Unfamiliar with prestressed concrete beams.

Is it acceptable to apply upwards loads to prestressed concrete beams? The amount of upward load would be limited to the weight of the beam, the moment/shear on the beam may be close to zero along the length.
Reason is that destabilizing wind loads on separate structure above would give uplift.

Also, just out of interest, I've heard about stress relaxation with prestressed beams.
Is it common that maintenance is required over time, or is it typically a case of set-and-forget, allowing for some loss of prestress?

 

[Agent666]

You can always put conventional reinforcement in the top of the beam for the uplift case. Have not done a lot of PT design, but there is no need to actually limit the uplift load to the weight of the beam. Provided the cross-section analysis works you could have the beam in negative flexure with the post tensioning at the bottom for example.

For the design it's a case of making sufficient allowances for the long-term behaviour that leads to a loss of prestress (creep, shrinkage, etc). Depending on where you are in the world, you'll either have unbonded strands, or bonded. Typically they are set and forget unless you specifically make provisions for re-stressing in the future, with bonded strands they are grouted so this is not possible.

https://engineervsheep.com

 

[LR11]

Thanks for confirming.
I though potentially the strands may need a certain amount of tension in them.

 

[rapt]

If you are using Low relaxation Strand, the loss of force due to relaxation is only a few percent. That should be allowed for in design. Normal Relaxation strand and Prestressing Bar have much higher relaxation losses (about 3-4 times as much) which also can be allowed for.

The strands will still have most of their tension in them.

Agree with Agent666 on his other comments.

 

[LR11]

Thanks for additional comments.