Cut Double Tee Stem

[mkberry]

Has anyone sucessfully seen a cut stem of a double tee? I have a double tee roof that needs to have the stems of the tees cut for an elevator. The stems will be supported where they are cut and the span is signifigantly reduced. I have a design that considers the structural integrity of the tee's to be non-existent after they are cut and I have steel framing designed to re-support this area.

However... if it turns out the existing tees have a straight strand pattern (Meaning the strands are in the same position throughout the length of the double tee, as opposed to a draped strand pattern) the structural property of the tee will remain un-changed and the shorter span will give that much more capacity.

Thoughts anyone?

 

[spats]

To my knowledge, drapes are only used for post-tensioned members, not prestressed members. Double tees are a straight pull.

If you cut a double tee too short, there may not be enough development length to maintain the prestress in the strands, and you could lose some or all of the prestress. That is the only potential problem I am aware of.

 

[mkberry]

Thanks Spats. Maybe "draped" is the wrong word. The strand patterns for these members (8DT-24) in the PCI handbook indicate either straight strands, or, if you need greater strength, strands that are closer to the top of the stem at the ends and close to the bottom of the stem in the middle. I have also heard this called "Harped" strands. I had a stem for another project x-rayed before so I could determine the strand pattern used. It had a "harped" strand pattern.

Per the PCI Tables, a straight strand pattern could have been used in my application, but it is close. I think I am going to go the x-ray route again to make sure. I can't ignore the cost savings to be able to salvalge the existing tees after they are cut. If it is a straight strand pattern, I will analyze the section as a simply reinforced tee beam (Not prestressed) to see if it works in its shorter span position.

A local precaster who works on Hollow core slabs says they make them 300 feet long and cut them to length for each project. I feel like this is a similar thing.

 

[hokie66]

I think you need to know the strand configuration to intelligently make your decision. The strands in a double tee could be either straight or deviated. I don't think that double tees are ever made like hollow core slabs, in continuous castings and cut to length.

 

[mkberry]

I didn't mean to imply the tee's were made like hollow core slabs, but rather, if the hollow core slabs are pre-stressed, cast in one long sheet, then cut to length; it is reasonable to expect a double tee with a straight strand pattern to be able to be supported in two places, have a small section cut out, and still have integrity in the un-molested areas.

 

[hokie66]

Yes, I agree.

 

[cooperDBM]

You should also consider that if the strands are straight (not harped) then some of them (usually near the bottom) may be debonded at the ends. This would have been used to vary the prestress eccentricity to reduce tensile stress at transfer. These debonded strands may become ineffective if the remaining length of beam is too short. Of course they may not be required with the shorter span.

Also, cutting the beam will probably eliminate most of the prestress camber. At the actual opening you can simple set the steel framing tight to the existing soffit to avoid movement at that point. This may still have some effect on the existing beam profile in the remaining spans if they're long enough. May be negligible but worth considering as it may result in some cracking.

 

[WpgKarl]

Double tees are either made with straight strands (and sometimes with debonding for cases with lots of strands) or with draped strands (typically done at mid-span).

Don't take the PCI charts as gospel, only as a guide - you are better to try and investigate which of your local producers may have made the DT's - you'd be surprised they may still have shop dwgs for the double tees.

Scanning the legs of the double tees is a good idea, even if you have the shop dwgs as sometimes production may change the strands and it gets handled by an internal memo.

You probably could still check the strands as stressed rather than reinforced - if your loads are all UDL loads and the span isn't too short (so the strands can't fully develop) I'd be ok with it.

 

[msquared48]

Hopefully, the tendons are bonded, and not unbonded.

Mike McCann
MMC Engineering

 

[hokie66]

Mike,

If you mean the entire length of the strands, I don't think double tees are ever made with unbonded tendons. The strands are often unbonded for short lengths for the reason given by coopDBM and to avoid high splitting stresses.

 

[mkberry]

Thank you all. I really appreciate the input. I will x-ray the strands at the ends of the tee and at the areas of cutting. Will the un-bonded strands be in a metal sleeve? I will obviously look at the ends of the cut to see if the strands are un-bonded, and monitor the tee's performance during construction.

 

[hokie66]

That I can't tell you. I think there are several ways of debonding short lengths.