Joints and connections – Interfitted members – Clamped members
Reexamination Certificate
1998-12-30
2001-05-22
Kim, Harry C. (Department: 3629)
Joints and connections
Interfitted members
Clamped members
C403S371000, C052S223130
Reexamination Certificate
active
06234709
ABSTRACT:
TECHNICAL FIELD
The present invention relates to post-tension anchor systems, in general. More particularly, the present invention relates to the structure of an anchor body for such post-tension systems. Furthermore, the present invention more specifically relates to the formation of the cavity within the interior of the anchor body. The present invention also relates to wedge-receiving cavities having radiused edges.
BACKGROUND ART
For many years, the design of concrete structures imitated 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 (vertical) load, is extremely 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 (horizontal) 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 100 feet can be attained in members as deep as three feet for roof loads. The basic principal 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 principal, but the reinforcing is held loosely in place while the concrete is placed around it. The reinforcing is then stretched by hydraulic jacks and securely anchored into place. Prestressing 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 in such post-tensioning operations, there is provided a pair of anchors for anchoring the ends of the tendons suspended therebetween. In the course of installing the tendon tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Metallic components within concrete structures may be come exposed to many corrosive elements, such as de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If this occurs, and the exposed portions of the anchor suffer corrosion, then the anchor may become weakened due to this corrosion. The deterioration of the anchor can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of by-products from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
FIGS. 1 and 2
illustrate various components of a typical post-tension assembly designated generally at
10
. System
10
includes a tendon
12
having an exposed end protruding from a sheath
14
. The end of the tendon
12
is typically fitted through an extension tube
16
. Extension tube
16
has a diameter slightly larger than sheath
14
such that one end
16
a
of tube
16
may overlie sheath
14
. The opposite end
16
b
of tube
16
fits over, and communicates with, a rear tubular portion
18
of an anchor
20
. Rear tubular member
18
includes an aperture (not shown) which communicates with a frontal aperture
22
. Frontal aperture
22
defines a cavity in which wedges
24
and
26
are received as shown in
FIG. 2
, below.
FIG. 2
illustrates an assembled view (in one-fourth cutaway perspective) of system
10
shown in FIG.
1
. As known in the art, tendon
12
is disposed through extension tube
16
and through anchor
20
. In one known embodiment, end
16
b
of extension tube
16
is force-fitted over rear tubular member
18
. The other end
16
a
of extension tube
16
is sealed to sheath
14
, by use of tape or other means.
After tendon
18
extends through frontal aperture
22
(see FIG.
1
), and assuming the far end of the tendon (not shown) is fixed in place, tension is applied to tendon
16
, typically by use of a hydraulic jack. While applying this tension, wedges
24
and
26
are forced in place on both sides of tendon
12
within the wedge cavity defined by aperture
22
. Once in place, teeth
24
a
and
26
a
of wedges
24
and
26
operate to lock tendon
12
in a fixed position with respect to anchor
20
. Thereafter, the tension supplied by the hydraulic device is released and the excess tendon extending outward from anchor
20
is cut by a torch or other known device. Wedges
24
and
26
thereafter prevent tendon
12
from releasing its tension and retracting inward with respect to anchor
20
. Moreover, this tension provides additional tensile strength across the concrete structure.
After years of work with the anchor body of the prior art, it was found that the cavity used in the anchor body created many problems. The cavity in the anchor body is of a constantly diminishing diameter extending from a forward end of the anchor body to a rearward end of the anchor body. This internal cavity of constantly diminishing diameter is formed during the casting of the anchor body. Unfortunately, the narrow diameter end of the cavity creates problems with the installation of tendons in a corrosion-resistant environment.
When the anchor body is used in the formation of intermediate anchorages, it is often necessary to move the anchor body over a very long length of sheathed tendon. If there is insufficient clearance b
Harrison & Egbert
Kim Harry C.
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