Prestressed Beam Analysis
Prestressed Beam Analysis
(OP)
I'm currently analyzing a prestressed concrete beam for the purpose of issuing repair recommendations. The as-built drawings date back about twenty-five years, and I question the logic in part of the design.
The beam in question is simply-supported with sixteen 1/2" prestressing strands on the bottom and four 1/2" strands on the top in the compression region. All strands are tensioned to 28.5 kips.
The question I have is why would somebody place prestressing strands (instead of conventional rebar) in the compression region of a simply-supported beam? My background does not include a great deal of prestressed experience, but doesn't this counteract the benefits of prestressing the bottom strands to a certain degree?
The beam in question is simply-supported with sixteen 1/2" prestressing strands on the bottom and four 1/2" strands on the top in the compression region. All strands are tensioned to 28.5 kips.
The question I have is why would somebody place prestressing strands (instead of conventional rebar) in the compression region of a simply-supported beam? My background does not include a great deal of prestressed experience, but doesn't this counteract the benefits of prestressing the bottom strands to a certain degree?






RE: Prestressed Beam Analysis
It could be that the top strands were there to control tension cracks from the bottom strands (during strand detensioning), near the ends of the beam (where there is very little beam self weight moment to put the top of the beam into compression.)
If the lift points of the beam were in from the ends (say at beam length/5), then you would have tensile stresses from the beam self weight at the top of the section where you are lifting. The bottom strands would further add to these tensile stresses, so some top strand would have counteracted these tension stresses at the top of the beam.
Be careful with the type of strand (older strands were usually stress-relieved, which resulted in more losses - now they are always low relaxation to minimize this effect)
A couple of helpful websites FYI
http://www.pci.org/intro.cfm
http://www.blackmint.com/ (you can get a free trial version of the prestress beam design software, it uses ACI, Canadian and Australian codes)
RE: Prestressed Beam Analysis
Be careful about the type of strand and ultimate strength to calculate the loss of stress and strength.
You are better of finding code books, ACI or AASHTO that date back to that time.
RE: Prestressed Beam Analysis
RE: Prestressed Beam Analysis