Initial and effective prestress
Initial and effective prestress
(OP)
It is commonly believed that effective prestress will always be less than initial prestress because of the different losses that occur after concrete has hardened. I am working on a pre-tensioned beam design and using the software Concise Beam for this purpose. The results show an INCREASE in from initial prestress and shows effective prestress is greater than initial prestress. How is this possible? What can be the reason behind this?






RE: Initial and effective prestress
I am not familiar or know of "Concise Beam" software - are you a registered user of the software, or is it shareware? Is there user support available?
Seems like this may be your first time doing a pretensioned beam design. I would encourage you to do it by hand calculations to gain some experience and 'feel' for the numbers and concepts. There are lots of textbooks that go into detailed calcs and theory on this subject.
RE: Initial and effective prestress
Likely to be an error in inputting some of the design parameters. Check loads, properties, creep & shrinkage parameters etc.
RE: Initial and effective prestress
RE: Initial and effective prestress
RE: Initial and effective prestress
On the text data that you provided, below the Prestressing Steel Tendon data table it states:
"Calculated Losses: Initial = 2.8%, Final = -0.1%"
You can NOT have a negative final loss. You need to check your input data and amend the final losses so the magnitude (and sign) is greater than your initial losses.
RE: Initial and effective prestress
RE: Initial and effective prestress
I am not sure what you mean when you state that the results are "OK" is you reduce the span to 6m. What results are you referring?
RE: Initial and effective prestress
By OK I mean that the effective prestress becomes < than initial prestress and the % loss is a positive value
RE: Initial and effective prestress
You should check with their technical support to confirm this.
RE: Initial and effective prestress
If CooperDBM is correct, it is a matter of terminology. His is implying they are reporting the stresses under the permanent load condition not the Effective Long Term Prestress.
RE: Initial and effective prestress
Pj(jacking) = 642.5 kN ==> seems right at 70% of UTS
Pi(transfer) = 825.1 kN, ===> this is 30% HIGHER than jacking!!!
Pe(effective) = 1046.4 kN @ x = 4.500 m, ===> this is more than the UTS of 5 strands!!!
RE: Initial and effective prestress
RE: Initial and effective prestress