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Structural Failure

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We are investigating a failure in a building and designing modifications to
repair the structure.  We do not understand the cause of the failure and
would appreciate any thoughts.  We have several theories but have not come to
a conclusion.
The building is a two story structure approximately 40' wide and 200' in
length.  It is approximately 30 years old. There are no drawings available
and there are a total of 4 buildings built at the same time and in a similar
manner.  The exterior walls from grade to the second floor are 8" cmu and 4"
brick which supports the second floor.  The structure above the second floor
is a pre-engineered building - steel framed structure with metal siding.
There are concrete pilasters in the exterior wall that support the columns
(spaced at 25' oc) of the pre-engineered building.  The second floor is a
concrete and metal form deck supported by steel bar joists spaced
approximately 2' oc.  There is a bond beam with (2) #3 bars at the top of the
cmu with a 1/4" x 3" steel plate for the joists to bear on.  We do not know
if the joists are welded to the plate or if there are anchors on the plate.
It appears the steel bar joists provide a tie at the top of the wall to
resist the horizontal forces from the pre-engineered building frame.
The first floor is a slab-on-grade cast monolithically with the grade beam
and footings at the pilaster.  We have done some selective demolition to
expose the grade beam and footings.  The grade beam is 24" deep and the width
at the bottom of the grade beam varies from 6" to 12".  There is a brick
ledge approximately 12" below the top of grade beam (finished floor).  We
have not confirmed rebar yet.  There are footings at the pilaster.   The
footing thickness based on core borings is 8" (top of footing is the same as
finished floor).  No rebar was found.  There is welded wire fabric in the
floor slab - at the bottom of the concrete.  The size of the footing is 5' x
4'.  However, the outside face of the pilaster is 5" from the outside face of
footing/grade beam to allow the brick to pass by (causing an eccentric load
on the footing).
The floor slab has settled 2" along the edge near the masonry wall/pilaster.
There is a crack in the floor slab approximately 5' from the wall.  The width
of the crack is 3/8" minimum.  This occurs along approximately 1/2 the length
of the building on one side.  The remaining length shows some settlement but
no cracks.  The other side full length is some settlement, but no cracks.
There are step cracks in the brick between the pilasters.  Finally, at one
end of the building, the steel bar joist has cracked (failed) the top of the
pilaster.  The width of the crack is 3".  The rebar in the bond beam has
failed in tension at the pilaster.  
We have performed soil borings along the outside and on the inside where the
crack in the floor was the worst.  At the end of the building where the
pilaster failed, the depth of fill was approximately 18'.  At the other end
of the building there was 23' of fill.  However, the fill was well compacted
and the geotechnical engineer recommended an allowable soil bearing capacity
of 2,500 to 3,000 psf.  The blow counts on the inside were 15 to 18.
The load on the soil is 5 ksf (dead and live) and 3.2 ksf (dead).  If we
assume the grade beam transfers the load to the footings.
We have found no water sources.
There has been cracks in the masonry walls for sometime.  Cracks have been
caulked.  Last December a crack in the wall was visually inspected and the
width of crack was 1".  We measured it last week and it was 2-1/2".  There
were cracks in the concrete floor slab for sometime.  Nobody can define when
the cracks first occurred.
One theory is that this a long term settlement problem.  As the structure
(floor slab) settled, loads were redistributed and the floor slab started
carrying more and more load.  This caused more settlement and the walls moved
out causing an increase in axial forces on the joists.
We are thinking of modeling the floor slab, grade beam, and footings
supported by springs to represent the soil to determine how much load the
floor slab would support.
Your thoughts would be appreciated.  Why nothing for 30 years?
Gary Loomis, PE
Master Engineers and Designers, Inc.