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Re: Shortening existing concrete joist

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While I would agree with Charles that the original designer may not have used
this manual or perhaps any other manual, I would suggest an argument that
doesn't
seem to have much acceptance here in the states, but is given some mention in
the
Canadian code.  That is the issue of past performance.  If the building is
fairly old,
as you say, and has performed adequetly in the past (ie. no excessive
deflections/
cracking, etc) it can be argued that excessive underdesign would have resulted
in
a service load failure of some sort.  The Canadian code (appendix to the code,
actually)
does give a rational basis for evaluating existing structures.  Since I don't
have it in
front of me, however, this appendix MAY assume that as-built conditions are
known.
Regardless, if there are no cracks or deflections, and you are not developing a
moment
over top of your support, I would feel comfortable shortening the joist because
you
are only reducing your moment (assuming similar loadings), even though you are
shifting the maximum moment away from its original location.

Craig Slama


Charles Espenlaub wrote:

> What makes you think that the reinforcing would comply with the 1963 CRSI?
> I would have some non-destructive testing done to verify the actual rebar
> sizes and locations, whether there are trussed bars or not.
>
> Just because CRSI has that sized joist in it does not mean that the designer
> reinforced it per CRSI.  Perhaps the designer used heavier reinforcing for
> loads not shown on the drawings, or all straight bars for ease of
> construction, or not enough bars because he did not know what he was doing.
> just because there was a standardized design table available at the time,
> does not mean the designer used it.  Test the in-place joists.
>
> Charles F. Espenlaub, III, P.E.
> Martin-Espenlaub Engineering
> Tel  215-665-8570
> Fax 215-561-5064
>
> -----Original Message-----
> From: Roland Bokma [mailto:bokmar(--nospam--at)progressiveae.com]
> Sent: Monday, February 26, 2001 5:27 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: Shortening existing concrete joist
>
>         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
>         www.progressiveae.com
>
>