Cut Prestressed Concrete DBL-T Joist

[L775]

Hi All,

Is it acceptable to cut an existing precast and prestressed double-T concrete joist?

The roof joists span about 35 feet. and I need to cut about eight (8) feet off on one end. the newly cut end will be supported on new load bearing and reinforced CMU wall.

do you for see a problem cutting a presstressed joist?

Regards,

 

[L775]

It seams to me that chipping away the joist from its end would make sense as opposed to cutting in in the middle...

 

[JStructsteel]

I think you have to check for the prestress development length. It would make sense its ok, you are reducing the moment. Any special bearing would be the other thing.

 

[WpgKarl]

You need to be very careful with this - sometimes the strands are draped (pushed together near the middle of the span and fanned out towards the end), so cutting can do some weird stuff to the member stresses.

Are there just uniform loads on the DT, or are there loads which cause a high amount of shear? In some cases, some shear reinforcing is added at each end, so now you may be cutting this off.

Make sure you check with all of the local precasters in town - they sometimes have original shop drawings of the double tees and they may be able to assist you in the analysis of the DT.

 

[vandede427]

yes, be very careful.

in the original design of the dt, the point of maximum applied moment was where the precaster built the dt to have the point of maximum moment capacity.

assuming uniform loads, by removing 8' from one end, you've now moved the point of maximum load 4' off center from the point of maximum moment capacity ESPECIALLY IF THE TENDON WAS DRAPED AT A V SHAPE.

 

[L775]

Thank you all for the insight. I know that for a uniform loading (roof joist) the tendons profile is most likely parabolic. do you think it is also possible that the profile could be harped in a v-shape, enventhough the loading does not have any concentrated load applied?
I am asking this question eventhough the answer might be obvious, because i do not have too much knowledge on the general practice of the prestressed beam producing plants. there might be a practical answer if that was not the case...

Regards,

 

[Lion06]

I interned at a very large precaster and they ALWAYS used straight tendons.

 

[L775]

Thanks StructuralEIT,
based on your comments, which makes sense, cutting the joist is then very possible with little or no side effects...
Regards,

 

[BAretired]

Maybe you should X-ray the web to make sure.

BA

 

[L775]

BAretired, the DT was X-rayed in the mid span for the full depth on the DT. The X-rays shoe clearly that all tendons (4 of them) are parallel to each others about 4" to 3" apart and they are straight throughout the DT span as suggested by "StructuralEIT" above.
The final question is:
Is chipping the DT using the following described procedure acceptable?

CHIPPING PROCEDURE:
After securing the DT with shoring, start chipping the DT from its end and progress with chipping gradually toward the mid span of the DT.

Thank you for your reply and considerations.
Has this been done before? My practical experience of prestressed members is limited but my theoretical and fundamental background is very strong.
Thank you again,

 

[msquared48]

If the tendons are fully grouted, the development should not be an issue.

However, due to the shorter span, the tendency for long term creep/deflection in the upward direction will be increased as the prestress force will not change.

Mike McCann
MMC Engineering

 

[jike]

The DT's in our area are straight tendons. I believe the longer ST's were harped or draped.

You should make a few calls to local precasters near the project site to verify.

What does chipping from the end accomplish? It sounds very labor intensive! Why can't it just be sawn after the new support is in place?

 

[msquared48]

As with any PS member, if you support it in the wrong location, the prestress force not changing from the original, it will have a tendency to deflect upward, and sometimes, literally explode upward. You need to run the numbers before you do this to make sure that this will not happen. If there is a tendency to do this, you may have to add extra dead weight to compensate.

Mike McCann
MMC Engineering

 

[BAretired]

Mike has a good point. Cutting a 35' span down to 27' changes the moment by a significant amount. The prestress force may be close to 70% more than is required. Better check it out.

BA

 

[hokie66]

Mike and BA are right. You really need to be able to analyze what the double tee will do after you cut it. There may be reinforcement in the top to resist the upward force, but also maybe not.

If you find that it is OK, jike's advice is good. Just saw it off. You may need to use multiple saw cuts to avoid binding the blade.

 

[msquared48]

You won't bind the blade if you cut from the top. You will just have to do it gradually to avoid sudden movements upward.

Mike McCann
MMC Engineering

 

[msquared48]

In thinking about this further, you need to build the new support to the desired new level, and gradually cut the top of the beam until it rests on the support, hopefully not having to entirely cut the top steel until it reaches the level of the new support. Otherwise, it WILL drop suddenly as the crack quicky propagates.

Mike McCann
MMC Engineering

 

[BAretired]

What crack quickly propagates? Why will it drop suddenly? Assume that you don't cut anything...you simply move the support eight feet over and release the previous support.

Now, assuming it has not exploded upward, cut off the eight feet at one end. Where is the problem?

BA

 

[msquared48]

I guess I'm assuming the elevation of the supports will be equal at each end. If that is not needed, then you are correct. I am also assuming that the beam is cambered up, creating a small vertical gap unless the supports are at different elevations.

Mike McCann
MMC Engineering