huangyhg

cut prestressed concrete dbl-t joist
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,


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

i interned at a very large precaster and they always used straight tendons.

thanks structuraleit,
based on your comments, which makes sense, cutting the joist is then very possible with little or no side effects...
regards,

maybe you should x-ray the web to make sure.
ba
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
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

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