Static structures (e.g. – buildings) – With component having discrete prestressing means
Reexamination Certificate
2000-08-07
2002-10-29
Mai, Lanna (Department: 3635)
Static structures (e.g., buildings)
With component having discrete prestressing means
C052S223140, C052S223130, C052S223600, C052SDIG004
Reexamination Certificate
active
06470636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a detensioning apparatus and method for the repositioning of a clamp on a prestressed strand.
2. The Prior Art
The use of prestressed concrete is a common and vital construction technique with which high strength structures are formed. Prestressed concrete is an often preferred alternative to reinforced concrete for its increased strength and improved durability with respect to exposure to natural elements, such as moisture, freezing, thawing, and corrosion, and in particular is more resilient to cracking.
Formation of prestressed concrete structures involves the use of a form or “casting bed”. As shown in
FIG. 13
, the casting bed may be categorized as either a self-stressing prestress casting bed (Type 1—Diagram A) or an abutment anchorage prestress casting bed (Type 2—Diagram B). Please note that any dimensions or numerical values shown in
FIG. 13
are given by way of example, and are not intended to limit the invention, as described elsewhere herein.
The Type 1 self-stressing prestress casting bed of Diagram A is formed typically from a steel form, that is provided with heavy steel plates positioned at its ends. One plate is the “dead” end and the other is the “live” end, at which manipulation of the prestressing wires is accomplished. The Type 2 abutment anchorage prestress casting bed employs abutments embedded in the ground, to form the ends of the casting form. Again, one abutment acts as the dead end and the opposite abutment acts as the live end.
Steel strands (each comprising one or more wires) are passed through the steel plates or abutments, to extend through the form where the concrete is to be poured. The steel strands are typically tensioned for times ranging from 2 to 12+ hours, prior to pouring the concrete. The strands are secured in a stressed position by placing wedge-shaped steel jaw(s) around the circumference of each strand. The jaws forms a chuck that engages a conical bore of a strand chuck barrel, which is secured against the live end of the casting bed, in such a manner that the pulling force of the strand on the chuck is translated into compressive pressure on the strand to bind the chuck thereto. Tension may be applied by placing a second, “temporary” chuck on each strand, to the outside of the respective “permanent” chucks. Tension may be applied by a hydraulic ram, pushing against the temporary chuck, relative to the live end, to stretch the strand. The “permanent” chuck is then placed against or extremely close to the outer surface of the live end. The tension exerted by the hydraulic ram is released, enabling the strand to contract a small amount, sufficient to cause the jaws in the “permanent” chuck to engage the strand and be pulled tightly against the strand chuck barrel. The amount by which each strand is allowed to recover is less than the amount by which each strand was originally stretched, so that tension remains, pulling the “permanent” chucks against the live end.
After concrete is poured into the pre-cast concrete form structure, the concrete is allowed to cure. The “permanent” chucks are then removed from the strands.
On certain occasions, the stressed strands must be moved for realignment, or the amount of tension must be reduced, before the concrete is placed in the casting bed. Typically, the procedure of detensioning a stressed strand involves the practice of over-tensioning the strand; thereby stretching it and removing pressure from the steel chuck. In so doing, as a result of the high compressive pressures applied thereto, the chuck often binds to the strand chuck barrel's conical inner surface and subsequently to the prestressed strand. In the prior art chuck removal involves manual manipulation of the chuck before tension on the strand is released.
This method of detensioning a stressed strand is exceptionally dangerous. During the time at which the chucks must be removed, the strand is under extreme tension. Consequently, any failure of the strand while a worker is manually removing the chuck could potentially result in severe injury or death to a worker, or other damage. Furthermore, when the one or more jaws forming the chuck bind to the strand, the substantial manual manipulation by a worker required breaking the chuck free further increases the danger. Accordingly, significant danger, and effort became associated with the detensioning procedure.
U.S. Pat. No. 3,912,496 discloses a gripping device in which a pull tube extends outwardly from the device and allows for the engagement of a prying tool. The force exerted by a worker on the prying tool directly results in the axial movement of the chuck, thereby releasing the strand from the chuck's grip. However, this invention requires the positioning of the worker in close proximity to the strand while it is under extreme tension, thereby increasing the potential for serious bodily harm if strand failure were to occur while the worker manually pried the chuck out from the strand chuck barrel.
U.S. Pat. No. 3,478,396 teaches a gripping device that incorporates an externally threaded strand chuck barrel, in which a chuck fit into the conical bore, and a matching internally threaded collar. The threaded housing and collar allows for the re-tensioning of the strand by a worker manually rotating the threaded collar with respect to the threaded bushing.
U.S. Pat. No 5,594,977 describes the combination of engagement of threaded members that, upon rotation, forces a pull tube to axially withdraw the chuck from the conical bore thereby resulting in the release of the strand from the chuck's grip. These detensioning devices however are permanently integrated into the strand chuck housing, resulting in not only more complex and expensive devices, but also potential corrosion of the threaded members, thereby potentially rendering the invention difficult to use or requiring additional maintenance. Furthermore, the worker must manually manipulate these devices while the strand is under full load, thereby increasing the risk of potential physical harm to the worker from strand failure.
U.S. Pat. No. 3,910,546, and U.S. Pat. No. 3,965,542, both disclose a gripper device incorporating a chuck that can be physically pulled back by a pull rod, which engages channels in the rear edge of the chuck. Neither of these devices can be used in conjunction with a member that passes completely through the channel formed by the chuck, as would be the case with a tensioning strand in actual prestressed concrete construction.
U.S. Pat. No. 3,778,869 teaches the use of a detensioning screw that can be turned to advance or withdraw a pushplate against cylindrical lugs bearing outwardly against the chuck through which the strands pass. These lugs in turn force the chuck rearward from the conical bore, and thereby release the strand from them. However the detensioning aspect is permanently integrated into this device and therefore may not be removed for the purpose of detensioning other chucks.
U.S. Pat. No. 2,245,316 discloses an apparatus that facilitates the removal of a clamping chuck in tensile testing machinery. The angles of the conical bore of the strand chuck barrel are significantly larger than that of the chuck's outer surface so that when the force on a strand passing through the chuck is released, the radial forces exerted by the chuck forces the body with the greater taper, the strand chuck barrel, to be pushed away, whereby the strand is released from the chuck. However, this device does not consider the situation where an extended period of compressive pressure has bound the chuck to the inner conical surface of the strand chuck barrel and the prestressed strand.
U.S. Pat. No. 4,114,242 discloses an apparatus requiring physical interlocking between the chuck and an external tube. The apparatus involves a tube having an initial flare that engages with a groove on the inside surface of the chuck, and a second flare engaging a mounting plate, which, after the first flare is engaged
A Phi Dieu Tran
Dick and Harris
Mai Lanna
Rose Dallas R.
LandOfFree
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