huangyhg

hanging from precast double tee stem
i am thru bolting the stem of a precast prestressed double tee to support a hanger. i am trying to calculate the shear breakout force of the bolt but am wondering if the bottom of the tee goes into tension and thus reduces the allowable shear.
can i use 2 x sqrt of f'c per 11.4.1 for the min. allowable?
how high up on the the stem would you recommend the bolts to be? i know this depends upon the breakout force....
i might try using an app d approach. i would try to at least get it a few inches above the strands. how high above depends on the load.
through bolting a precast tee stem is not typical.
first, the location of the through bolt will need to be above the strands and any longitudinal reinforcement. this must be coordinated with the precaster (this is not an option and if you don't do it you may need to replace the tee or worse). you will probably find that you will need to through bolt about 12" from the bottom of the stem. at this point you will probably not need to concern yourself with breakout, but if you do...
consider that you are now above the main structural reinforcement of the precast element and it is hard to believe that any load that can be sustained by the tee would cause failure at that location.
this is a common detail for existing buildings with double tees.
in my 10 years experience with precast parking garages, the only through bolting that was done was through the flange for bollards or through the spandrel panels for rare instances.
what are you hanging that requires a through bolt through the stem? usually any piping and conduits can be managed with an expansion bolt. even with expansion bolts i still prefer to limit their connections to the flange.
this is for hanging piping in industrial buildings. for new buildings, we would have sleeves cast thru the webs, but for existing buildings, one needs to drill thru the stems and support hangers with u-straps.
to answer your question, yes the stem goes into tension at the middle of the span under full loading. unless other criteria was listed, a precaster will allow some tension at the center span.

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