Static structures (e.g. – buildings) – With component having discrete prestressing means – Anchorage
Reexamination Certificate
2000-09-05
2003-02-04
Horton, Yvonne M. (Department: 3635)
Static structures (e.g., buildings)
With component having discrete prestressing means
Anchorage
C052S223800, C052S699000, C024S122600, C029S452000, C249S043000, C403S374100, C403S371000
Reexamination Certificate
active
06513287
ABSTRACT:
TECHNICAL FIELD
The present invention relates to dead-end anchorages. More particularly, the present invention relates to methods and apparatus which are used so as to mechanically secure the end of a tendon within an interior cavity of an anchor. The present invention also relates to dead-end anchorage forming mechanisms in which a compressive force is applied to the end of the tendon.
BACKGROUND 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, concrete design began to evolve. Concrete has the advantages of costing less than steel, of not requiring fireproofing, and of having 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 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, comprising a mixture of water, cement, sand, and stone or aggregate and having proportions calculated to produce the required strength, is set, 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 five 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.
In typical post-tension systems, the tendon is received between a pair of anchors. One of the anchors is known as the “live end” anchor, and the opposite end is known as the “dead-end” anchor. The “live end” anchor receives the end of the tendon which is to be tensioned. The “dead-end” anchor holds the tendon in place during the tensioning operation. Under typical operations, a plurality of wedges are inserted into an interior passageway of the anchor and around the exterior surface of the tendon. The tendon is then tensioned so as to draw the wedges inwardly into the interior passageway so as establish compressive and locking contact with an exterior surface of the tendon. This dead-end anchor can then be shipped, along with the tendon, for use at the job site.
One technique for forming such dead-end anchors is to insert the end of a tendon into the cavity of the anchor, inserting wedges into the space between the tendon and the wall of the cavity and then applying a tension force onto another end of the tendon so as to draw the wedges and the end of the tendon into the cavity in interference-fit relationship therewith. This procedure is somewhat difficult since the tendon can have a considerable length and since the use of tension forces can create a somewhat unreliable connection between the wedges and the tendon. Experimentation has found that the application of compressive force onto the end of the tendon creates a better interference-fit relationship between the wedges, the end of the tendon and the wall of the cavity of the anchor.
FIG. 1
shows one such type of compression system for the forming of a dead-end anchor. In
FIG. 1
, it can be seen that a fixture
10
is provided on a base
12
having a channel
14
suitable for receiving a tendon
16
in a desired position. An anchor
18
is connected to the tendon
16
and resides against a wall
20
of the fixture
10
. In this arrangement, the wide end of the cavity of the anchor
18
faces outwardly. A compression mechanism
22
has a plunger
24
at one end. The compression mechanism
22
includes a hydraulic or pneumatic system
26
for the purpose of applying strong pressures to the plunger
24
. The plunger
24
includes an indentation
28
at the end
30
so as to allow the end of the tendon
16
to be inserted therein. When suitable hydraulic pressure is applied to the plunger
24
, the plunger
24
will move toward the anchor
18
so as to apply compressive forces onto the end of the tendon
16
for the purpose of establishing a strong interference-fit relationship between the tendon, the wedges and the wall of the cavity of the anchor
18
.
FIG. 2
is a more detailed view of the prior art system of
FIG. 1
showing the formation of the dead-end anchorage. In particular, in
FIG. 2
, there is shown the anchor
18
as having a steel anchor body
32
with a polymeric encapsulation
34
extending therearound. The anchor body
32
includes an interior cavity
36
which tapers inwardly from end
38
toward end
40
. Wedges
42
are positioned in the cavity
36
between the exterior of the tendon
16
and the inner wall
44
of the cavity
36
. The plunger
24
is shown as having indentation
28
at the end
30
. The plunger
24
will move toward the end
38
of the anchor
18
so as to force the tendon
16
and the wedges
42
into the cavity
36
.
In the normal process of using the system of
FIGS. 1 and 2
, the anchor
18
is initially installed within its desired position in the fixture
12
so that
Harrison & Egbert
Horton Yvonne M.
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