Static structures (e.g. – buildings) – Footing or foundation type – Framework spans footings
Reexamination Certificate
1999-04-13
2001-07-10
Friedman, Carl D. (Department: 3635)
Static structures (e.g., buildings)
Footing or foundation type
Framework spans footings
C052S294000, C052S296000, C052S649200, C052S649100, C405S229000
Reexamination Certificate
active
06256954
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a fixed construction which comprises rigid piles and a monolithic concrete floor slab.
BACKGROUND OF THE INVENTION
Concrete industrial floor slabs usually rest via a foundation layer on a natural ground. Unevenly distributed loads on top of the floor slab are transmitted via the floor slab and the foundation layer in a more evenly distributed form through to the natural ground, which eventually bears the load.
Natural grounds of an inferior quality, e.g. characterized by a Westergaard K-value of less than 10 MPa/m, are first dug up and/or tamped down and leveled before the foundation is laid over it.
Due to the fact that a lot of natural grounds of good quality (characterized by a high Westergaard K-value) have already been taken for existing constructions, the number of natural grounds with inferior or even unacceptable quality (i.e. with a low Westergaard K-value) which are being considered for constructions is increasing. The bearing capacity of some grounds is so bad that digging up and/or excavating and/or tamping down would constitute an enormous amount of work and cost.
In such a case it is known to rest the floor slab on driven or bored piles. Placing a floor slab on driven or bored piles under load, however, creates very high negative peak moments in the areas above these piles and relatively much lower (about one fifth of the height of the peak moments) positive moments in the zones between the piles. Reinforcing floor slabs on driven or bored piles with uniformly distributed steel fibres would not be economical since the zones between the piles would have a quantity of steel fibres which is unnecessarily too high and which would cause trouble during the pumping and pouring of the concrete and would render the solution not economical.
This problem has been solved in FR 2 718 765 of applicant, by having the floor slab rest on a number of gravel columns. As has been explained therein, these gravel columns are not as rigid as common piles and compress relatively easily under a downward load (the compression modulus of gravel columns e.g. ranges from 0.2 to 0.4 MN/cm) so that the gravel columns function like a spring in a mathematical model, which means that the floor slab is no longer subjected to high bending deformations in the zones above the columns.
In the international application PCT/EP98/00719 of applicant a solution has been disclosed to the above-mentioned problem. The present invention involves an improvement of the invention disclosed in this international application.
SUMMARY OF THE INVENTION
The present invention provides an alternative reinforcement for concrete floor slabs resting on piles which saves weight of steel and which prevents introduction of high amounts of steel fibres into the floor slab. Another object of the present invention is to provide a reinforcement for concrete floor slabs resting on piles where the reinforcement functions as a tensile anchor for taking up shrinkage cracks.
Still another object of the present invention is to save time in constructing a concrete floor slab resting on piles.
In comparison with the invention disclosed in PCT/EP98/00719, the present invention provides a greater weight savings in steel and a greater and more reduction in time required to construct the concrete floor.
According to the present invention there is provided a fixed construction which comprises rigid piles and a monolithic concrete floor slab which rests on the piles. The rigid piles are arranged in a regular rectangular pattern, i.e. each set of four piles forms a rectangle. The floor slab comprises straight zones which connect the shortest distance between the areas of the floor slab above the piles. The width of such zones ranges from 50% to 500% the largest dimension of the piles. These straight zones run both lengthwise and broadwise. The term “lengthwise” refers to the direction of the longest side and the term “broadwise” refers to the direction of the smallest side. If, such as is often the case, the longest side is about equal to the shortest side, the terms broadwise and lengthwise are arbitrarily designated to the two directions.
The floor slab is reinforced by a combination of:
(a) fibres which are distributed over the volume of the floor slab;
(b) steel bars with a yield strength above 690 MPa and which are located in those straight zones, and preferably only in those straight zones, which means that outside these zones there is no substantial reinforcement except for the fibres under (a).
The term “rigid piles” refers to piles the compression modulus of which is much greater than the compression modulus of gravel columns and is much greater than 10 MN/cm. These rigid piles are driven or bored piles and may be made of steel, concrete or wood. They may have a square cross-section with a side of 20 cm or more, or they may have a circular cross-section with a diameter ranging between 25 cm and 50 cm. The distance between two adjacent piles may vary from 2.5 m to 6 m.
The term “yield strength” is herein defined as the strength at a permanent elongation of 0.2%.
By using this combination reinforcement constituted by fibres and a classical steel rod reinforcement which is only located in the critical points of the floor slab, it has proved to be possible to limit the total amounts of steel fibres in the concrete slab from about 120 kg/m
3
(=1.53 vol. %) until a concentration ranging from about 30 kg/m
3
(=0.38 vol. %) to about 80 kg/m
3
(=1.02 vol. %), or even lower.
A floor slab is an industrial floor with dimensions up to 60 m×60 m and more, and—due to the continuous rod reinforcement—carried out without joints, i.e. without control joints, isolation joints, construction joints or shrinkage joints.
The thickness of the floor slab may range from about 14 cm to 35 cm and more.
Of course, in order to cover large surfaces more than one such a jointless floor slab may be put adjacent to each other. With the present invention, i.e. with the combination of both fibres and continuous rods it has proved possible to eliminate expansion joints when constructing such a second (and a third . . .) jointless floor slab adjacent to the first one. This is done by reinforcing the transition zone from one floor slab to the other by means of a metal netting.
Preferably the floor slab “directly” rests on the piles. This refers to a floor slab which rests on the piles without any intermediate beams or plates. All reinforcement is embedded in the floor slab itself.
The fibres in the floor slab are preferably uniformly distributed in the concrete of the floor slab. The fibres may be synthetic fibres but are preferably steel fibres, e.g. steel fibres cut from steel plates or, in a preferable embodiment, hard drawn steel fibres. These fibres have a thickness or a diameter varying between 0.5 and 1.2 mm, and a length-to-thickness ratio ranging from 40 to 130, preferably from 60 to 100. The fibres have mechanical deformations such as ends as hook shapes, thickenings or undulations in order to improve the anchorage to the concrete. The tensile strength of the steel fibres ranges from 800 to 3000 MPa, e.g. from 900 to 1400 MPa. The amount of steel fibres in the floor slab of the invention preferably ranges from 30 kg/m
3
(0.38 vol. %) to 80 kg/m
3
(1.02 vol. %), e.g. from 40 kg/m
3
(0.51 vol. %) to 65 kg/M
3
(0.83 vol. %). So the amount of steel fibres in a concrete floor slab according to the invention is preferably somewhat higher than steel fibre reinforced floors on natural ground of good quality (normal amounts up to 35 kg/m
3
), but can be kept within economical limits due to the combination with the higher tensile steel rod reinforcement.
The other steel reinforcement in addition to the steel fibres, the steel rods'are preferably hard drawn and occupy maximum 0.4% of the total volume of the floor slab, e.g. maximum 0.3%, e.g. only 0.2% or 0.3%. The diameter of the steel rods ranges from about 3.5 mm to about 12.0 mm.
The minimum yield strength of the steel rods is
A Phi Dieu Tran
Friedman Carl D.
N.V. Bekaert S.A.
Shlesinger & Arkwright & Garvey LLP
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