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Re: Shortening existing concrete joist

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It would sound from your description that you don't have a negative moment
at the end of the span where the shortening will occur.  If my read is
incorrect and you do have a negative moment, then you will need to take a
close look at the top bars on the end that would be cut for proper
development and moment capacity.  You also should check your development
of the bottom should not be a problem with only cutting 3 feet,
but check never the less.  You may also want to check your moment
coefficients...if you are using the ACI moment coefficient, keep in mind
that they are only permitted for certain conditions (see section 8.3.3 of
ACI 318-99).

Hope that helps,


On Mon, 26 Feb 2001, Roland Bokma wrote:

> 	I have a condition where I would like to shortening the span of an
> existing concrete joist.
> Information:	Approx 19 ft (single span) 
> 		Bearing on loadbearing masonry walls
> 		Existing drawings only identify joists as an 8" channel
> slab, nothing else
> 		The building was built in the early sixties
> 		Shortening span approx 3 ft
> 		Floor use is not increased
> 		For my reference, I have a 1963 CRSI Design Handbook Vol II.
> 		No old geezers in the office
> 	I haven't run any numbers yet, but the joist should be good for
> moment and shear (no tapered ends).  The CRSI manual lists bottom and truss
> bars, which are both at the bottom of the beam at a distance equal to the
> span/7 from each end (in the manual).  As long as the maximum moment isn't
> within this distance, the concrete joist should be okay to cut.  Am I right?
> Is there anything else that I should check?
> 	TIA
> 	Roland Bokma, E.I.T.
> 	ProgressiveAE
> 	Grand Rapids, MI