Prestressed Beam Analysis 1

[craigmcg]

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?

 

[WpgKarl]

It does counteract the bottom prestress a bit (moves the center of gravity of force upwards.) However, from a stress point of view, there is a Force/Area component and adding top strand will give the overall section more compression.

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)

 

[shin25]

Wpgkarl has said it right.

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.

 

[Lion06]

WpgKarl is pretty much on the mark. We have had had Precasters add prestressing strands to the center of precast columns (adding compression to a compression member) to prevent cracking at service loads (The architect didn't want to see any cracking - go figure).

 

[craigmcg]

Thanks to everyone for their input. Luckily enough, the partial set of drawings I have do in fact indicate the S.R. strands were used.