CA2233025A1 - Retrofitting existing concrete columns by external prestressing - Google Patents

Description

Retrofitting Ex~g Concrete Columns by External Prestressing, DISCLOSURE
Reinforced concrete structures subjected to earthquakes may experience significant inelasticity in their c;ritical regions. These regions must be designed for improved deformability to dissipate seismic induced energy. Beams and other flexure dominant elements often possess the required ductility, especially when premature shear failure is prevented, and the potential hinging region is confined by closely spaced closed hoops. During a strong earthquake, columns are generally responsible for overall strength and stability of the entire structure.
Concrete columns are often subjected to significant compression, while experiencing inelastic deformation reversals caused by seismic induced inertia forces. Presence of high axial compression reduces member ductility, and may lead to brittle and explosive failures. Therefore, new concrete columns are required to be confined by properly designed transverse reinforcement for improved deformability.
While the beneficial effects of confining concrete columns and prevention of premature shear failure have been recognized during the last three decades, many aspects of these concepts are still being researched. Research findings have been gradually introduced into building codes for improved performance of new structures. However, there exists an enormous inventory of existing buildings and bridges that had been designed prior to the development of proper seismic provisions, as stated in cun-ent building and bridge codes. It is economically not feasible to replace the entire existing infrastructure with new and improved structures. Therefore, retrofitting existing structures is the only solution to the problem of seismically deficient structures.
Performance of existing bridges during recent earthquakes indicates serious design deficiencies in concrete columns. The majority of bridge failures in the 1994 Northridge Earthquake can be attributed to lack of shear and/or confinement reinforcement in columns.
Similarly, a large number of building failures during past earthquakes have been attributed to poor column behaviour, especially due to lack of shear/confinement reinforcement. An estimated number of bridges that require column retrofitting in the State of California alone is over 3000. The only viable technique used currently for retrofitting concrete columns against shear and flexure is steel j acketing. Other retrofitting techniques, utilizing fiber reinforced plastic (FRP) material, mostly in the form of "fiber wrap" are being researched. While these techniques appear to be effective in improving seismic performance of columns, they tend to be very expensive, labor intensive and cumbersome.
A new retrofitting technique has been developed to improve strength and deformability of concrete columns through external prestressing. The technique is easy to apply, and mostly utilizes existing construction procedures. It is based on well established principles of mechanics and experimental verification.
BACKGROUND OF THE INVENTION
Concrete columns are used in buildings, bridges and other structures to support axial compression while also resisting flexural and shear stresses. They are often reinforced with longitudinal and transverse steel reinforcement. The longitudinal reinforcement contributes to axial and flexural resistance. The transverse reinforcement has three functions. These include;
i) improving shear (diagonal tension) capacity, ii) preventing or delaying buckling of longitudinal reinforcement in compression, and iii) confining concrete to improve strength and deformability of concrete. While the amount of longitudinal reinforcement affects flexural and axial strength, it does not play a significant role on column deformability. However, the transverse reinforcement plays a vital role on column shear strength and deformability.
Columns are often required to be designed for excess shear capacity to prevent premature shear failure, which is regarded as a brittle form of failure. Hence, in a properly designed concrete column, brittle shear failure never precedes ductile flexural failure. Therefore, columns are designed for sufficient transverse reinforcement, in the form of ties, hoops, overlapping hoops and crossties.
The same transverse reinforcement also improves flexural performance if placed with sufficiently small spacing. Closely spaced transverse reinforcement provides a reinforcement cage which confines compression concrete. Concrete in compression develops a tendency to expand laterally due to the Poisson's effect. Lateral expansion generates transverse tensile strains and longitudinal splitting cracks, which eventually result in failure. The presence of closely spaced transverse reinforcement controls the development of splitting cracks and delay the failure of concrete. Lateral expansion of concrete is counteracted by passive confinement pressure exerted by reinforcement.
The resulting confinement action enhances both the strength and deformability of concrete. These improvements directly translate into flexural strength enhancement, as well as a very significant increase in inelastic deformability.
Columns subjected to strong earthquakes are likely to develop inelasticity, especially in the first story building columns and bridge columns. Concrete cover (shell concrete) often spalls off in this range of deformations, exposing longitudinal reinforcement. The transverse reinforcement serves to restrain the longitudinal reinforcement against buckling, beyond the spalling of cover concrete.
Well confined column core continues resisting a large number of inelastic deformation reversals caused by strong earthquakes, if the longitudinal bars are prevented against buckling.
It is clear from the above discussion that the transverse reinforcement plays a significant role on inelastic deformability of concrete columns. While properly designed transverse reinforcement is required by building codes in all new columns, its function can be fulfilled by external prestressing in old and existing columns which may not possess adequate transverse reinforcement inside the column. Retrofitting through external prestressing has the added advantage of providing active lateral pressure, in addition to the passive pressure that develops during lateral expansion of concrete. Active lateral pressure delays the formation of diagonal shear cracks in columns, and limits widths of such cracks, improving aggregate interlock, and consequently increasing concrete contribution to shear resistance. The pressing tendons also contribute to the component of shear resistance provided by reinforcement. The end result is a significant increase in overall shear capacity of the reinforced concrete column.
The active pressure also provides increased confinement to the entire column concrete, as opposed to the core concrete in the case of internal hoops and ties. This enhances the mechanism of concrete confinement, and results in improved strength and ductility of concrete, which translates into a substantial increase in overall column deformability. External prestressing also contributes to the stability of longitudinal reinforcement in compression, restraining these steel bars against buckling.
It has been found that external prestressing enhances; i) column shear capacity, ii) concrete confinement and hence column deformability, iii) stability of longitudinal reinforcement and hence column deformability.

DESCRIPTION OF INVENTION
The invention described in this section involves a new retrofitting technique to increase strength and deformability of concrete columns during seismic and similar extreme events, including impact loading due to explosions. The technique is based on external prestressing, through high-strength prestressing tendons. It is applicable to columns of circular and non-circular sections. The applicability to circular and square columns has been verified by large-scale and full size concrete column tests in the Structures Laboratory of the University of Ottawa. Because the principles remain the same for columns of other geometric sections, including rectangular and hexagonal shapes with and without column tapers and column capitals, the technique is also applicable to columns of non-circular and non-square cross-sectional geometry.
The prestressing is done by means of hydraulic devices (hydraulic j acks) commonly used in construction industry for the purpose of prestressing concrete. The prestressing tendons can be of 1 S any material that is acceptable for the purpose of prestressing concrete.
The most common forms of prestressing tendons include prestressing wires of sizes 6, 7, and 8, with nominal diameters of 6.35 mm, 7.00 mm and 8.00 mm, respectively, as well as seven-wire-strands of sizes between 6 and 16, with a range of nominal diameters between 6.35 mm and 15.47 mm, respectively.
The grade of prestressing tendons should be 600 MPa and above. The common forms of wires used include Grades 1550 MPa to 1760 MPa, and seven-wire-strands used include Grades 1720 MPa to 1860 MPa. The prestressing is provided through discrete hoops, at a centre-to-centre spacing not to exceed h/2, where h is the smallest cross-sectional dimension of circular, square and rectangular columns.
For other shapes, h corresponds to the cross-sectional diameter of an equivalent circular column having the same cross-sectional area as the non-circular column. Tests have shown that column performance improves with a reduction in spacing. Excellent performance was observed when the spacing was reduced to h/4. The initial prestress in tendons should be at least high enough to obtain a snug-tight tendon, which will maintain its position during the service life of the columns, without sliding and slipping. The performance improves with the level of stress in tendon. A very good performance was obtained in columns with at least 100 MPa stress in prestressing tendons. The performance was excellent when the stress level was increased to 340 MPa.
An anchorage device is used to anchor the ends of individual tendons. The anchor must be strong enough to resist at least the tensile force in tendon. It has to have a proper geometry so that the centroid of the tendon on one side of the anchor coincides with that of the other side, to minimize twisting of the anchor. Anchors, similar to that developed by Dywidag for cylindrical tanks, can be utilized for this purpose. This is shown in Fig. 1, as applied to circular columns. Alternatively, a 5 continuous anchor can be used, as developed part of the current invention, for better control of anchorage locations and improved esthetics. The following section provide more specific information on application to circular and non-circular sections.
Circular Columns: The lateral pressure provided by external prestressing of circular columns results from hoop tension. Hoop tension produces almost uniform lateral pressure, which overcomes tendency of concrete to expand laterally under shear and/or compression. The circular geometry lends itself to the development of uniform external pressure through hoop tension. Individual tendons, stressed and anchored as described in the previous section, can be employed within the critical region of a column. Figure 1 illustrates one such application. Figure 2 shows the photograph of an actual column specimen, with external prestressing, during a laboratory testing.
Non-Circular Columns: Non-circular columns can not develop uniform lateral pressure if the column is to be externally prestressed, having points of contact only at the sharp corners of non circular geometry. This is typically the case in square, rectangular and hexagonal sections. In this case, the concrete is likely to crush at the corners due to stress concentration. Even a little crushing of concrete is sufficient to relieve the prestressing in strands, making external prestressing ineffective. This crushing can occur at the stress level required to attain good performance. The crushing can become even a more important issue during seismic excitation, when the laterally expanding concrete push against the prestressing tendon. Furthermore, lateral forces applied at four discrete points of square and rectangular geometry may not be sufficient to confine concrete, although they may prevent premature shear failure, assuming the sharp corners are not damaged.
Therefore, a new hardware was developed for prestressing columns of rectilinear geometry to increase the number of contact points to achieve a more uniform lateral pressure and eliminate the problem of stress concentration. This is illustrated in Fig. 3, and is discussed in the next section.
New Hardware: Transverse prestressing concrete columns requires a new set of hardware that is suitable for circular and square geometry. Specifically, the prestressing strands need to be anchored after tensioning by means of anchorage devices. Because the existing technology has not been applied to columns, anchorage devices have not been developed for these members. The search for existing anchors resulted in finding one such anchor developed by Dywidag for use in new construction of cylindrical tanks and silos. This device was altered and used in circular columns, as part of this invention, at the University of Ottawa. Figures 1 and 2 illustrate a circular column retrofitted through external transverse prestressing, utilizing modified Dywidag anchors. The Dywidag anchors were altered by cutting the ends to make the anchord shorter and bending the anchors so that the curvature of the anchors approximated the curvature of the circular column.
Though these anchorage devices proved to be effective from technical point of view, they were not esthetically pleasant, since they were intended for new construction where the devices would not be visible. Furthermore, handling individual units were found to be inconvenient.
Therefore, a new anchorage device was subsequently developed, consisting of a continuous unit, made out of a hallow steel section, as illustrated in Figure 3.
Square and rectangular columns could not be prestressed using the existing technology, as explained in the previous section. Therefore, a new hardware was devised, consisting of hallow steel sections (HSS) with welded circular raiser pieces which also guide the tendons to apply lateral forces. Other raisers can be utilized provided the raisers result in a change of slope to the tendon so that a normal force is applied against the column as illustrated in Figure 4. Figure 4 shows a schematic view of the steel hardware. Figure 5 illustrates a square column retrofitted using the new hardware.
Critical Regions of Columns: The new invention improves performance of columns that are critical in shear and/or in compression. Columns tend to become more critical in shear when the shear span (distance between the point of inflection and maximum end moment) is low.
Short columns attract higher shear forces when they attain their flexural capacity, due to the reduced lever arm. Therefore, these column may fail prematurely in shear, through the formation of diagonal tension cracks. If shear is the failure mode, then the column has to be externally prestressed within the entire high shear region. In building and bridge columns, where seismic induced shear forces are applied at the slab level, the entire column may be subjected to high shear forces, and must be retrofitted along the entire height. Retrofitting these columns is necessary only if the transverse reinforcement placed inside the column is not adequate to prevent shear failure prior to the development of ductile flexural yield mechanism.
When flexural compression is the governing mode of behavior, as in the case of the majority of columns with intermediate to long height range, the retrofitting needs to be applied only in the high moment regions where the formation of plastic hinges is anticipated during earthquake excitations.
These columns are designed to have sufficiently high shear capacities so that flexural yielding occurs prior to any distress due to shear. Columns framing into relatively stiff members at the ends attract high bending stresses near the ends. They often bend in double curvature, experiencing high flexural and flexural compressive stresses at the ends. Columns with flexible members at one end, or bridge columns that have hinge connections at bridge bearings behave in the cantilever mode, developing a zero moment point (point of inflection) at one end, while developing high end moments at the other end. These columns are subjected to single curvature, and may experience hinging only at one end.
Experiments have shown that column hinging region occurs near the ends, within a distance not exceeding twice the cross-sectional dimension "h", defined earlier. Therefore, it is recommended to provide external prestressing at columns ends, except the end where the point of inflection, if any, occurs, within a distance not less than 2 times h or 1/4 of the clear column height, whichever is greater. This prevents brittle failure of columns due to flexural compression.
Concentrically loaded columns are very rare in practice. However, if they do occur due to unusual structural configurations, these columns must be retrofitted along the entire height, to prevent brittle compressive failure of concrete.
Appendix "A", "B", "C", "D" and "E" are part of the description of the invention and are to be referred to for a fuller understanding of the invention.

Claims (2)

1. The method of retrofitting concrete columns by external pre-stressing substantially as described herein.

2. The apparatus of retrofitting concrete columns by external pre-stressing substantially as described herein.