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Re: Minimum reinforcement for structural integrity at slab-column connections

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Kevin,

No, if clause 13.11.5.3 (b) is used and extra reinforcement is supplied, it only needs to overlap with the normal bottom reinforcement, not lap with individual bars. Providing a length of 2 * ld gives the bar sufficient development to act as a dowel, independent of laps to other bars.
If 13.11.5.3 (a) is used, where the bottom reinforcement on both sides of the column laps, a smaller lap length is allowed because a proper lap is being provided.

I think the reason for this is that is has been shown in tests that the post punching failure performance of a column head is improved enormously if a small amount of bottom reinforcement is available across the shear failure plane to provide dowel action. This reinforcement does not have to be continuous with the bottom reinforcement, it simply has to provide dowel action at the bottom of the shear failure zone. I think this was found in the testing for Studrail if my memory is correct.

Of course, it would be nice to lap it to main slab bars if possible, simply for ease of placing and also for possible moment reversal effects.

Personally, I prefer to lap 50% of my bottom bars at supports even though codes do not always require it. There are a lot of situations where reversals of moment can occur (such as sway situations and support settlement) which we may not allow for in design which I think justifies this plus it also improves punching performance (most codes do not have rules similar to 13.11.5 from the Canadian code).

At 05:36 PM 23/12/2003 -0500, you wrote:
In the Canadian concrete code, this requirement may be met by one of a
couple of methods.  One of them implies additional reinforcement,
passing over a column or column capital, such that an overlap of 2 ld is
provided with the bottom reinforcement in adjacent spans.

Should the bottom reinforcement in adjacent spans be grouped so as to
touch or be close to the additional reinforcement ?    For example, if
the additional reinforcement is 4 15M bars on each face of the column,
should 4 of the 15M bars of the reonforcement of the adjacent spans be
aligned to pass over the column ?

I have never seen that on drawings.  Everybody in this region at least
(Quebec City) simply adds the extra bars over the column, extending 2 ld
each side.  They leave the adjacent bars at whatever spacing and
position they happen to be at.  Which means that there may be only 2
bars overlapped with the 4 additional bars.  It seems to me that that
would net satisfy the intention to provide post-punching failure
resistance.



Kevin Below, ing., Ph.D.
GÉNÉCOR CIVIL INC.
660, avenue Royale
Beauport, (Québec) G1E 1Y7
Tél. : (418) 666-0009 poste 272
Fax : (418) 666-0572

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