Prestressed tilt wall panels and new openings 1

[02hokie]

I have been searching through the archives and found a few similar posts with some helpful information, but the situation we are in is somewhat different. We have been asked by an out-of-town architect / engineer to visit a local retail strip center which is undergoing a renovation. The building was evidently built in the 1960's, and surprisingly to me was built with field-cast concrete tilt panels which are pre-stressed. The panels are approx. 15' wide and 20' tall. No original documentation exists.
Along the front wall the scope is pretty straight forward - they are cutting a series of 4' wide window openings 8' on center. I am not too worried about that with the bonded tendons, outside of attempting to approximate the strength of the existing panel and that which will remain. However on the rear wall we are cutting two openings, and as you see in the photos the GPR shows us we have the good fortune of having the pull point located almost dead center in our proposed opening. I am told that it is just not possible to move the man door into the existing opening, of course my thinking is that it'd be cheaper to reconfigure the space but that is another thread.
I have limited experience with PT concrete, and most of my hands-on experience is from bridge work in college co-ops. I am comfortable with the fact that if the tendons are bonded, it is OK to cut them. My questions are:
-Am I nervous for no reason about cutting out the bottom of the panel at the pull point? If the tendons are bonded, it really doesn't matter where you cut it, correct?
-Any guidance or tips that can be offered about how to go about coming to a reasonable comfort level regarding the strength of these panels with no way of determining the prestress force would be appreciated. It seems the simplest engineering approach would be to put in a new beam/column line to take roof loads and analyze the wall panel for shear as a 6" unreinforced shear wall (or nominally reinforced), but I'm sure this would not be a popular route unless it's the only route.

 

[AELLC]

That panel will have zero strength after cutting that much out - yes, you need new beam/columns or reinforce panel edges with steel channel etc.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[02hokie]

Yeah, already planning on ringing the openings w/ channel or angle. Thanks much for your response.

 

[JAE]

I think I'd be wary of the "bonded" post tensioned condition -especially when it is old. You don't know for certain the quality of the grouting, etc.

Cutting through these types of walls would lead me to simply supplement the openings as though the concrete was facade only and not structural. That might mean running full height columns up the sides of the openings and bolting to the wall along their lengths.

 

[CELinOttawa]

Agree with AELLC and JAE.

I've done a lot of work with site cast tilt-up, and regardless of details and available documents, I don't trust any panel to be modified without full strength strong-backing. Full stop.

 

[Ingenuity]

02hokie,

STOP - the photos indicate UNBONDED PT. These are NOT bonded/grouted tendons.

Note the green-colored HDPE sheath and grease. If you cut these you will loose the tendon capacity over it entire length, and any cutting may present safety/danger issues for site personnel.

Seek the assistance of a local PT contractor to advise you of possible options.

 

[CELinOttawa]

Nice catch Ingenuity.... Must be the beer; I looked at the running grease and thought "yup, unbounded" and went right back to the post about BONDED tendons... *sigh* A star for you!

 

[02hokie]

Of course Ingenuity. That seems obvious. Now. We were discussing some other ideas at the site but they all involved a specialty contractor as you say. Thanks very much.

 

[Ingenuity]

02hokie,

No problems. That is the great thing about Eng Tips - always an engineer in "cyber space" that can assist, without all the obscure "advice" that happens otherwise on the Internet. I have benefited many more times than I have contributed.

Re your 1960's estimate of the project construction date. Based upon the use of what appears to be an extruded HDPE sheath to the UNBONDED PT, I would estimate that this building was constructed in the early 1970's. Prior to HDPE sheaths, there was heat-sealed, push-through and kraft paper sheaths for strand, and kraft paper for button-head tendons. Non-extruded sheaths were prevalent into the early 70's.

Hope this helps.

 

[02hokie]

You all have been a great help. Thank you