Relaxation and prestress loss for a double tee beam

[csd72]

I am carrying out a check for additional loads induced in a double tee beam. The induced loads are only 2% increase in stress levels, but the local authority wants calculations to prove this.

I dont do post tensioning, but in this case I need to do a quick calculation to justify the overall stress. Buying the PTI handbook would be a little over the top.

Can someone please help me answer the following:

1. how do I calculate the level of relaxation/tension loss in the tendons.
2. What is the safety factor for the induced concrete stress.

Many thanks.

 

[civilperson]

Buy the book. PCI Handbook.

 

[NS4U]

Everything you need to know is right here:

http://www.pci.org/publications/bridge/

Chapter 9 has several prestressed examples, that should be enough to show how to calc prestress loss. As an even easier method, AASHTO has the extremely simple "Lump sum method" for calcing the loss.

As a sanity check, your losses should be on the order of 15-25%

 

[shin25]

csd72,

What kind of strands do you have? Stress relieved or Low relaxation? Based on this your relaxation loss will be different. Why only the relaxation loss, how about the other components of the loss? You need to consider them as well. To calculate the relaxation loss, you have to calculate the other components of the loss.

 

[csd72]

mh819,

Thanks, that is somewhat helpful.

shin25,

they are low relaxation strands.

I found a dusty old version of the handbook and I am just going to follow that. The local authority is just being pedantic, there is not really any issue with this (6kip) load. Unfortunately the only way I can prove it is by doing a full calculation.

Thanks everyone for your help.