I have to say ditto to Jordan's response. We had a very dry summer and
I received a call about a basement floor crack 3/4 inch wide that just
occurred over the summer.
Gary
Jordan Truesdell, PE wrote:
> Gary -
>
> Here's a stab in the dark - Is there any chance that the end of the
> building which has dropped has an excessive amount of expansive clay
> when compared to the rest of the fill? We've seen a lot of long term
> settlement issues get out of hand this summer in the western end of VA
> due to the drought.
> Jordan
>
>
>
> Gary Loomis wrote:
>> 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.
>>
>>
>>
>> ________________________________
>>
>>
>
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