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Re: cracking in waffle slab

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At 04:09 PM 1/27/97 -0600, you wrote:
>I am investigating an existing waffle slab that has
>numerous cracks visible from below.  The structure
>was built in the late 1950's, and the existing structural
>drawings indicate the joist reinforcing, but do not use
>the ?column strip, middle strip? distribution.  Our
>analysis indicates that the total amount of top
>reinforcing across a bay is about 10% low, but the
>total positive reinforcing across a bay is 20% high.  It
>seems that with moment redistribution, the overall
>load carrying capacity is adequate.  A recent core
>sample indicated a compressive strength of 3200 psi,
>with the original design strength being 3000 psi.
>The cracks appear to emanate from the corners of
>the solid heads, and mostly run transversely across
>the width of the structure.  There are also numerous
>cracks at the interface between the joist and the slab
>when viewed from below.  Unfortunately, most of the
>top slab is covered with asbestos tile, so we cannot
>see the crack patterns from above.
>The slab is surrounded on all sides by concrete
>foundation walls.  It is 3 bays @ 21' wide and 5 bays
>@ 21' long.  There is a steel framed structure above
>with moment frames that are supported on the
>foundation walls.  The structure is located in Fremont,
>CA.  An earlier report indicated that the cracks were
>due to seismic activity.
>Has anyone seen similar patterns of cracks in older
>waffle slab structures, and is it possible the cracking
>is due to seismic forces?  My concern is the low
>concrete strength, however, I feel the cracking does
>not indicate any structural distress.  Also, does
>anyone have experience with repairing cracks like
>these?  The earlier report indicated pressure injected
>epoxy, which seems excessive.
A few questions that might help:
1.	How is the slab connected to the foundation walls?
2.	Describe the foundation system supporting the walls and the columns.
3.	What is the occupancy type above and below the slab?
4.	Are the cracks recent relative to the age of the structure?

Paul G. Adams, PE
Frederic R. Harris, Inc.
New York, NY
212 973-2951