pre-cast concrete wall panels

huangyhg

pre-cast concrete wall panels
we have a client with pc wall panels. they carry their weight and wind (out of plane) load only. they like to cut a 3 ft by 3 ft window in the panels. the design drawings delegated the design of the pc panels to the pre-caster. we cannot find the shop drawings. no one knows if the panels were reinforced with rebars or cables (pre-stressing) or combination of both.
let’s assume that the panels were pre-stressed. in discussing this matter in our office, one of our engineers feels that cutting the strands should not pose any problem. he had far more experience than i do in pre-cast concrete.
i have some reservations for several reasons and they are as follows:
1. the type of design and construction is unknown.
2. the location of the pre-stress tendons or cables is unknown, if any.
3. we will add steel members (wf beams and channels) to redistribute the load around the new window opening. however, i still have concern with cutting the pre-stressing steel.
am i too cautious? does anyone have similar situations like this? what pre-cautions should be taken?
regards,
lutfi
when i was in the bridge building industry, we had occasion to cut up and dispose prestressed concrete girders removed from dismantled bridges. it seems we notched the concrete with a pavement breaker, and cut the strands with an acetylene torch. as i re  
my experience with precasters is that they dont put anything in their beams/wall panels that doesnt belong.  if there are strands there, they're there for a reason.
how large is each individual panel?  i know of panels that were 35 feet by 12 feet and had prestressing strands.
more than likely, the strands in architectural precast concrete are bonded tendons, meaning they are bonded to the concrete and will not do anything like slip or fly out when cut.
daveatkins
my concern with cutting the strands would be that you're altering the strength of the precast piece in a negative way, but you're not reducing the load on it.  as slideruleera said, they cut the strands for demolition.  they didnt cut the strands and then put the road deck back on.
i think you may go in steps. to cut 3'x3', start with cutting a slot of 12"x1" or so. it will tell you something before you proceed.
ciao.
i agree with your concerns lufti.  most wall panels i have seen are not bonded, lately it is one continuous strand in a loop pattern within the panel and only one live end to be jacked.  if it is a non-bonded tendon, the thing that bothers me is even if you re-distribute the load around the opening with steel shapes, what are you re-distributing to...an unreinforced concrete panel?  i would try to squeeze in a steel frame behind the panel, anchor the panel to the frame, locate the live end of the tendon and stay away from it, then cut the hole.
i have done several buildings with precast panels which had no prestressing tendons, only mild rebar.  is there something odd about the size or shape which leads you to believe there are tendons in these panels?  in fact, this is the first i have heard of such a thing.  i could only imagine they must be very tall with a very long unbraced length to need such tendons.
that said, if you cut the tendons, i would agree that you are hurting the panel.  if i suspected tendons are in the panel, i would have the panel xray'd, or some other technique to locate the reinforcing.  then i would carefully chip into the panel to expose the reinforcing to determine type.  only then would i proceed.  
my cleints keeps saying it was pre-stressed. however, no other indicators.
lutfi