Patent Application
20240159054
The present application relates to the field of bridge construction, and in particular, to a pre-loaded concrete-filled steel tubular columns and a method for fabricating the same.
At present, for fabricating a concrete-filled steel tubular column (CFST column) for use in a bridge, a steel tube is usually fabricated at first, and then concrete is poured into the steel tube to form the CFST combination column. However, in this method, after the concrete is solidified, a gap will occur between the steel tube and the concrete due to a shrinkage of the concrete during drying. In addition, in an early loading stage of a concrete-filled steel tubular pier, the concrete core has a lateral deformation smaller than that of the steel tube, so that confinement of the steel tube to the core concrete cannot be manifested at this stage. Further, as the temperature around the column increases, the steel tube is directly heated while the concrete receives less heat, such that a temperature difference is generated therebetween, which further increases the gap or void between the steel tube and the concrete. For the above reasons, the steel tube and the concrete cannot be closely combined with each other to work together during fabricating or use.
In view of this, a self-stressing concrete is introduced to solve these problems. However, it is difficult to control the self-stressing value of such concrete, and the expansion and deformation of the concrete will abate or even disappear over time, so that the expansion merely plays the role of compensating a shrinkage of the concrete, rendering the self-stress a short term-effective stress.
Another method, that is, a post-tensioning method, is also considered, which includes applying a pre-pressure to the concrete by steel bar tensioning after the concrete is completely solidified in the steel tube. However, this method suffers from a problem of limited deformation of the concrete under compression, failing to achieve desired steel tube confinement.
Therefore, there is still a need in the art for a technical means which can achieve accurately controllable, steady, and long-term steel tube confinement to concrete core.
In view of the above, the present application provides a pre-loaded, for example, hoop pre-tensioned, concrete-filled steel tubular columns and a method for fabricating the same.
In a first aspect, the present application provides a circumferentially pre-loaded, for example, hoop pre-tensioned, concrete-filled steel tubular column, which includes a steel tube formed from a plurality of steel sheets with two flanges formed at two opposite sides of each of the steel sheets, respectively, and defined with bolt holes therein; and a concrete core provided in the steel tube. Every two adjacent steel sheets are connected with each other by bolts passing through corresponding bolt holes in two adjacent flanges to form the circumferentially pre-loaded concrete-filled steel tubular column.
In this aspect, by combining a self-stress from the concrete core with a radial pre-pressure applied to the concrete core by tightening the bolts, an increased radial compression force is generated between the steel tube and the concrete core, so that the steel tube and the concrete core can be always closely combined with each other. Owing to the presence of a self-stress of the concrete core and the radial pre-pressure from the steel tube, the concrete of column can always be kept with a controllable, even, stable, and long-term radial compression stress, thus being endowed with better mechanical properties than a conventional concrete-filled steel tubular column.
In some embodiments, the steel tube is one selected from a group consisting of a circular steel tube, a square steel tube, and a regular hexagonal steel tube.
In some embodiments, the concrete core is a concrete cone made from self-stressing concrete. In some embodiments, the steel tube is a circular steel tube formed from four ¼ arc-shaped steel sheets. Alternatively, the steel tube is a square steel tube formed from four L-shaped angle steel sheets. Compared with a conventional concrete-filled circular steel tubular column, a conventional concrete-filled square steel tubular column, that is, without being subjected to circumferential pre-loading, has a lower bearing capacity, since there is an uneven confinement to the concrete core by walls of the square steel tube, for example, higher confinement at corners and lower confinement at side walls of the steel tube. However, in the present application, a combination of flanges, bolts and pre-loading, preferably, together with reinforcing ribs, can not only greatly increase the confinement of side walls of the square steel tube to the concrete core and address the problem of uneven confinement inherent to the conventional concrete-filled square steel tubular column, but also enhance a rigid of the side walls of the steel tube while avoiding a premature local buckling which otherwise would occur at side walls of the conventional concrete-filled square steel tubular column at a later stage of loading, so as to achieve a purpose of improving a short-term bearing capacity, a long-term bearing capacity, and deformation capacity of the square steel tubular column
Alternatively, the steel tube is a regular hexagonal steel tube formed from six 120°-angled angle steel sheets.
In some embodiments, a reinforcing rib is symmetrically provided at a side of either of two adjacent flanges facing away from the other flange.
In some embodiments, the bolts are gradually tightened by an increment of torque by firstly tightening a first pair of bolts in the middle of the column, then a second pair of bolts near the first pair of bolts and symmetrical around a middle point of the column, then a third pair of bolts near the second pair of bolts and symmetrical around a middle point of the column, and so on, until all the bolts are tightened. In this way, the column can be provided with relatively stable circumferential strain, especially pre-tensile strain, and relatively even pre-pressure on the concrete.
In some embodiments, the increment of torque is 10-50 N·m, for example, 20, 30 or 40 N·m.
In some embodiments, a steel wire mesh is welded on an outer surface of the steel sheet. Further, a fire-proof cement mortar is coated on the outer surface of the steel sheet.
In a second aspect, a method for fabricating a circumferentially pre-loaded concrete-filled steel tubular column according to the first aspect is provided, which includes the steps of cutting the steel sheets into desired size, and forming bolt holes in the steel sheets; bending the steel sheets into desired shapes with two flanges formed at two opposite sides of each of the steel sheets, in which the bolt holes are located in the flanges; connecting every two adjacent steel sheets by tightening bolts passing through corresponding bolt holes in two adjacent flanges, with a gap left between the two adjacent flanges, filling the gap with elastic rubber, and sealing the gap with an adhesive tape; filling the concrete into the steel tubes, and curing to solidify the concrete to form a concrete-filled steel tubular column; and gradually tightening the bolts by an increment of torque by firstly tightening a first pair of bolts in the middle of the column, then a second pair of bolts near the first pair of bolts and symmetrical around a middle point of the column, then a third pair of bolts near the second pair of bolts and symmetrical around a middle point of the column, and so on, until all the bolts are tightened.
In some embodiments, the increment of torque is 10-50 N·m, for example, 20, 30 or 40 N·m.
In some embodiments, the method further includes a step of welding a steel wire mesh on an outer surface of the steel sheet. Further, the method includes a step of coating a fire-proof cement mortar on the outer surface of the steel sheet.
Compared with conventional technologies, in the present application, a steel tubular column and a method combining chemical (self-stressing concrete) and physical (bolt tightening) means to produce a prestress in the column are provided, so that the concrete can be tightly adhered to the steel tubes during use. In this way, the present application can improve short-term and long-term mechanical properties of a bridge pier, while achieving the advantages of low construction difficulty, time and labor saving, and low construction cost. When being applied to a steel tube column pier, the present application can guarantee that the steel tube can always be combined and work together with the concrete, providing the steel tube column with relatively high strength, rigid and ductility.
The accompanying drawings are included to provide further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and serve to explain principles of the present embodiments together with the description. The elements of the drawings are not necessarily to scale relative to each other.
Like reference numerals designate corresponding similar parts.
Referring to
Each of the steel sheets 3 is formed with two flanges 1 at two opposite sides thereof for connecting the steel sheets 3 with each other. The flange 1 is defined with bolt holes for passing bolt therethrough to fix the steel sheets. A concrete core 6, for example, a self-stressing concrete core, is provided in the steel tube. Every two adjacent steel sheets 3 are connected with each other by bolts 5 passing through corresponding bolt holes in two adjacent flanges 1 to form the circumferentially pre-loaded concrete-filled steel tubular column.
In some other embodiments, a reinforcing rib 2 is provided between the steel sheet 3 and the flange 1, so as to enhance firmness of the steel tube.
In some embodiments, a rubber layer 4, for example, an elastic rubber pad, is provided between two adjacent flanges 1 to prevent concrete from leaking out of the steel tube during pouring.
For better understanding the principle and concept for implementing the present application, following examples are provided. However, for those skilled in the art, it should be appreciated that, the following examples are provided merely for the purpose of illustration, not intended to impose any limitation to the scope of the present application.
Steel sheets having a thickness of 2 mm and 4 mm were cut into a size of 350 mm*265 mm, and drilled with 6 bolt holes at two opposite edges, respectively. Then the cut steel sheets were bent into a C-shaped member, with two edge portions with bolt holes remained flat at two opposite sides to form flanges, respectively. Four C-shaped members were combined with each other to form a circular steel tube. A gap of 4 mm was left between every two flanges, filled with elastic rubber, and sealed with a butyl adhesive tape to prevent the concrete from leaking out during pouring.
An inner wall of the steel tube was cleaned. The steel tube was combined with an underside formwork, and sealed with a hot-melt adhesive at the joint. A self-stressing concrete was formulated according to Li Shuang-xi, et al. (Y1 in table 1, C40 Self-compacting Concrete Mix Proportion Design Of Expansion Pipe And Experimental Research, Concrete, Issue 5, 2011, p114-115), and poured into the obtained circular steel tube under vibration.
The concrete was cured for 28 days. The four steel sheets were fixed with each other by tightening bolts passing through the bolt holes, with a reinforcing rib being symmetrically welded at a side of either of two adjacent flanges facing away from the other flange, to form a bolt pre-loaded self-stressing concrete-filled circular steel tubular column.
Compared with a conventional two-flanged concrete-filled circular steel tubular column having only two flanges symmetrically provided at two sides of the circular steel tubular column, the circular steel tubular column according to this Example has a more uniform deformation during use, since there is an uneven deformation in the conventional two-flanged concrete-filled circular steel tubular column, such that there is a lower hoop strain at non-flanged sides and a higher hoop strain at flanges sides, as clearly shown in
Steel sheets having a thickness of 2 mm and 4 mm were cut into a size of 350 mm*265 mm, and drilled with 6 bolt holes at two opposite edges, respectively. Then the cut steel sheets were bent into a L-shaped angle steel sheet members, with two edge portions with bolt holes remained flat at two opposite sides to form flanges, respectively. Four L-shaped angle steel sheet members were combined with each other to form a square steel tube with flanges. A gap of 4 mm was left between the two flanges, filled with elastic rubber, and sealed with a butyl adhesive tape to prevent the concrete from leaking out of the steel tube during pouring.
An inner wall of the steel tube was cleaned. The steel tube was combined with an underside formwork, and sealed with a hot-melt adhesive at the joint. A self-stressing concrete was formulated according to Li Shuang-xi, et al. (Y1 in table 1, C40 Self-compacting Concrete Mix Proportion Design Of Expansion Pipe And Experimental Research, Concrete, Issue 5, 2011, p114-115), and poured into the square steel tube under vibration.
The concrete was cured for 28 days. The four steel sheets were fixed with each other by tightening bolts passing through the bolt holes, with a reinforcing rib being symmetrically welded at a side of either of two adjacent flanges facing away from the other flange, to form a bolt pre-loaded, self-stressing concrete-filled square steel tubular column.
Steel sheets having a thickness of 2 mm and 4 mm were cut into a size of 350 mm*265 5 mm, and drilled with 6 bolt holes at two opposite edges, respectively. Then the cut steel sheets were bent into a 120°-angled angle steel sheet members, with two edge portions with bolt holes remained flat at two opposite sides to form flanges, respectively. Six angle steel sheet members were combined with each other to form a regular hexagonal steel tube with flanges. A gap of 4 mm was left between the two flanges, filled with elastic rubber, and sealed with a butyl adhesive tape to prevent the concrete from leaking out of the steel tube during pouring.
An inner wall of the steel tube was cleaned. The steel tube was combined with an underside formwork, and sealed with a hot-melt adhesive at the joint. A self-stressing concrete was formulated according to Li Shuang-xi, et al. (Y1 in table 1, C40 Self-compacting Concrete Mix Proportion Design Of Expansion Pipe And Experimental Research, Concrete, Issue 5, 2011, p114-115), and poured into the regular hexagonal steel tube under vibration.
The concrete was cured for 28 days. The six steel sheets were fixed with each other by tightening bolts passing through the bolt holes, with a reinforcing rib being symmetrically welded at a side of either of two adjacent flanges facing away from the other flange, to form a bolt pre-loaded, self-stressing concrete-filled regular hexagonal steel tubular column.
A digital electronic torque wrench was used for applying loading to the columns A preloading was applied to the steel tube and the concrete by a load increment of 20 N·m within the regime of elastic deformation. In particular, the preloading was applied by 20 N·m, 40 N·m, 60 N·m, and 80 N·m, respectively. After one loading, the strain was measured and observed, and a next loading was not applied until the observed strain became stable.
In this example, 6 bolts were arranged for pre-loading the column. Regarding the order for tightening the bolts and the placement state of the column, two routes for tightening the bolts, that is, route (1) tightening the bolts from those in the middle toward those at two ends of the column, that is, by the first order shown in
The results showed that, in the route (1), there was relatively even hoop pre-strains from top to bottom of the column, showing that the route (1) achieved good effect of bolt tightening, as clearly shown in
Therefore, the route (1) was adopted in this Example, and torsional moment was applied by 20 N·m, 60 N·m and 80 N·m, respectively, to give different loading samples, as summarized in Table 1 below.
As can be seen from Table 1, whether the thickness of the steel tube is 2 mm or 4 mm, with the increase in torque, the hoop pre-strain ϵsh of the steel tube increases, while the radial pre-pressure P0 of the steel tube on the concrete also increases, so it can be concluded that, tightening the bolts and applying different levels of torque play an important role in the growth of the radial pressure P0 on the concrete core.
From the hoop pre-strains as obtained, a radial pressure applied to the concrete core by the steel tube can be approximately deduced, for example, by the steps shown in
(a) Assuming no vertical slip between concrete and steel tube, the longitudinal strains of the concrete (ϵcz) and the steel tube (ϵsz) were equal; so,
ϵcz=ϵsz (1)
(b) According to equilibrium theory, the vertical internal forces in the cross-section of the steel tube and the concrete were equal in magnitude and opposite in direction. In the expressions below, Ac and As are the cross-sectional areas of the steel tube and the concrete, respectively, while σcz and σsz are the normal stresses of the cross-sections in the steel tube and the concrete, respectively.
(c) According to the generalized Hooke's Theorem, the expression for concrete is, shown in
The expression for steel, is shown in
In the expression, ϵsh is known, and D is the diameter of the specimen. Ec and Es are the elastic modulus of concrete and steel, respectively, μc and μs are the elastic Poisson's ratios of the concrete and steel, respectively, and σsh is the hoop stress of the steel tube.
(d) The relationship between p0 and σsh (
Equations (1-6) were solved to obtain the longitudinal stress of the steel tube:
The hoop stress of the steel tube is given by
σsh=EsΕsh−μsσsh (8)
The p0 is given by (
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the present application. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the present application.
