Static structures (e.g. – buildings) – With component having discrete prestressing means – Anchorage
Reexamination Certificate
2000-10-16
2003-10-14
Friedman, Carl D. (Department: 3635)
Static structures (e.g., buildings)
With component having discrete prestressing means
Anchorage
C024S122600, C403S371000, C403S374100, C052S223600
Reexamination Certificate
active
06631596
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to post-tensioning systems. More particularly, the present invention relates to encapsulated anchor systems which serve to maintain the tendon of the post-tension system in a corrosion resistant condition. More specifically, the present invention relates to an anchor as used in conjunction with a corrosion protection tube for such post-tension anchor systems.
2. Description of Related Art
For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly used in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of tensioning the cable in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of each cable for applying a predetermined amount of tension to the tendon, which extends through the anchor. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable at the anchor plate and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
A problem that affects many of the anchorage systems is the inability to effectively prevent liquid intrusion into the area of the unsheathed portion of the tendon. Normally, the unsheathed portion will extend outwardly, for a distance, from the anchor. In normal practice, a liquid-tight tubular member is placed onto an end of the anchor so as to cover the unsheathed portion of the tendon. The tubular member slides onto and over the trumpet portion of the encapsulated anchor so as to be frictionally engaged with the trumpet portion of the anchor. The opposite end of the tubular member will include a seal which establishes a generally liquid-tight connection with the sheathed portion of the tendon.
Unfortunately, various experiments with such systems have indicated that such “frictional engagement” between the liquid-tight tubular member and the trumpet portion of the anchor is inadequate for preventing liquid intrusion to the unsheathed portion of the tendon. In common practice, workers at the construction site will not attach the tubular member to the trumpet portion of the anchor in a suitable manner. As such, liquid will eventually migrate through the connection between the trumpet portion of the anchor and the end of the tubular member. In other circumstances, because of the stresses placed upon the tendon, the tubular member will become disengaged from the end of the trumpet portion of the anchor. In still other circumstances, workers will step on the tubular member during the installation of the anchorages such that the tubular member becomes dislodged from the trumpet portion of the anchor. In all of these circumstances, the “frictional engagement” between the tubular member and the trumpet portion of the anchor provides an inadequate connection.
The present inventor has developed corrosion protection tubes for more efficient engagement with post-tension anchors. U.S. Pat. No. 5,839,235, issued on Nov. 24, 1998, to the present inventor, teaches a corrosion protection tube with a snap-fit engagement for a post-tension anchor system. The snap-fit engagement creates a tight connection between the trumpet portion of the anchor and the tube. Although these tubes perform better than the “frictional engagement” used in prior art, experience has shown that the connection of the anchor to the tube still requires the use of a separate collar to enclose the point of connection between the anchor and tube. The snap-fit arrangement of the corrosion protection tube of the prior art does not provide an adequate connection to prevent liquid intrusion. Additionally, it can be somewhat difficult to insert the arrowhead-shaped tube into the interior of the trumpet portion of the anchor. Inspection can also be difficult.
U.S. Pat. No. 5,788,398, issued on Aug. 4, 1998 to the present inventor, describes another type of connector seal for an anchor and a corrosion protection tube of a post-tension system. This connector is formed of an elastomeric material and a seal formed interior of the connector. The body of the connector has a first receptacle formed on one end thereon for attachment to the end of the anchor. The body has a second receptacle formed at an opposite end thereof for attachment to the end of the corrosion protection tube. The seal is positioned between the first receptacle and the second receptacle so as to form a liquid-tight seal with a surface of the tendon passing therethrough. The first receptacle is an o
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