Static structures (e.g. – buildings) – Processes
Reexamination Certificate
1999-09-08
2001-06-05
Friedman, Carl D. (Department: 3635)
Static structures (e.g., buildings)
Processes
C029S452000, C052S223100, C052S749100, C081S027000, C081S463000, C227S147000
Reexamination Certificate
active
06240699
ABSTRACT:
TECHNICAL FIELD
The present invention relates in general to post tensioning concrete construction and, in particular to the anchoring of monostrand cables extending through a concrete structure.
BACKGROUND INFORMATION
Concrete is strong in compression, but relatively weak in tension. In most structures, such as buildings or bridges, forces such as wind, gravity, and earthquakes subject the structure, and therefore, the concrete to compressive and tensile stresses. Steel reinforcement is typically used to resist the tensile forces within a concrete structural member. The steel reinforcement is placed in the form before the concrete is poured within the tension zone. The steel reinforcement is designed to work with the concrete to resist tension forces and to control concrete cracking. For additional efficiency and economy, steel reinforcement, such as prestressing steel, can be stressed before the building forces are applied to the concrete member in the opposite direction of the to-be-applied force. Such stressing counteracts the applied force and allows the use of less steel reinforcing.
There are two basic methods of stressing concrete before the concrete is subjected to design forces (i.e., prestressing): post-tensioning and pre-tensioning. Pre-tensioned prestressed concrete is usually fabricated at a plant remote from the final construction site. In that case, the steel tendons are stressed before the concrete is placed. With post-tensioned prestressed concrete, steel tendons are stressed after the concrete has been placed and gained sufficient strength at the construction site. The present invention is primarily concerned with post-tensioned prestressed concrete.
With post-tensioned prestressed concrete, after the concrete has hardened, a hydraulic jack is used to pull strands of steel or tendons encased in the concrete. The prestressing steel strands are separated from the concrete by polyethylene sheathing. This sheathing allows the encased steel move with respect to the concrete. Thus, the steel can be stressed without frictional resistance from the concrete which effectively eliminates tensile stresses in the concrete due to the prestressing. The tension force in the steel is then permanently transferred to the concrete as a compressive force through anchoring devices at the end of the concrete member.
In post-tensioned concrete, the concrete is simply poured over the unstressed steel strands or tendons. Initially, therefore, both concrete and steel have no stress. As the concrete cures or hardens, it gains its compressive strength. After about 24 hours, it usually has reached about 75 percent of its full design strength. It is at this point when the steel is stressed. The steel is then stressed to high stresses, (e.g., 216,000 psi) but within its elastic limit. While stressed, the steel is then permanently attached to anchors at the ends of the concrete beam. Thus, the steel is in tension (i.e, pulling on the anchors) while the concrete remains in compression because both anchors are pushing on the concrete. The result is a considerable reserve of compressive stress at the bottom of the beam that in turn can be used to counteract the tensile stresses from an applied load.
Before the concrete is poured, anchorages are set and attached to the form. Once the concrete has hardened, the anchorages are embedded into the concrete. Typically, the anchorages are not flush with the concrete edge. Pocket formers are used to separate the anchorages from the edge of the concrete. Thus, after the concrete has hardened, these pocket formers leave anchorage cavities in the concrete. Anchorage cavities have tapered wedge-receiving seats and passages through which the tensioning cables extend.
FIG. 1
is a cross-section of an end of a concrete member
100
through anchorage
105
and anchorage cavity
102
. Steel strand
104
extends from polyethylene sheathing
106
through the center of anchorage
105
and anchorage cavity
102
.
A stressing ram (not shown) is used to pull the tendon
104
to the correct pressure and elongation. Then, anchoring wedges
108
a
and
108
b
are driven between tendon
104
and the surrounding anchorage
105
. The wedges grip tendon
104
as the jack releases it. Thus, the wedges
108
a
and
108
b
keep tendon
104
from slipping back through anchorage
105
into the concrete. Once the all of the tendons have been properly stressed, the ends are cut, the pockets are grouted, and the beam ends are encapsulated to protect the tendons from corrosion.
Wedges
108
a
and
108
b
are pieces of tapered metal with teeth that bite into the prestressing tendon during transfer of the prestressing force. The teeth are beveled at the front end to ensure gradual development of the tendon force over the length of the wedge. Two piece wedges are normally used for monostrand tendons. Proper installation of the wedges into the concrete is critical. The wedges must be evenly spaced and exactly fitted into anchorage
105
. If the wedges are unevenly spaced or not properly fitted, the wedges could slip and ruin the entire prestressing operation.
It has been found that it is difficult to properly align, install and seat the wedges. The wedges must be installed into the back surface
103
of anchorage cavity
102
which has limited access. Furthermore, both wedges need to be equally spaced and driven into anchorage
105
by an equal amount. Currently, a small hand held device is used to install the wedges in the anchorage cavity. This small hand held device consists of a bent longitudinal bar. The user positions one wedge into the space between anchorage
105
and protruding tendon
104
. Then with the other hand, slides the hand held device along tendon
104
with enough force to partially anchor the wedge into anchorage
105
. The user then repeats the procedure for the other wedge on the opposite side of the strand. The tool only allows for partial installation of one wedge at a time. Thus, the user must alternate seating the wedges on both sides of the cable constantly to ensure that the wedges are evenly aligned as they enter their respective wedge seats. This process is time consuming, inefficient and difficult due to the limited space available in the anchorage cavity.
What is needed, therefore, is an efficient and cost-effective device to place the anchorage wedges into the anchorages prior to tensioning the monostrand members.
SUMMARY OF THE INVENTION
The previously mentioned needs are fulfilled with the present invention. Accordingly, there is provided, in a first form, a hand-operated tool with a u-shaped bar with a handle, sliding hammer, a concentric stop and magnetic tip. The cross section shape of the tool is u-shaped to allow the tool to straddle and slide along the tendon. The slide hammer action in the handle of the tool allows the user to apply force to firmly drive the wedges into the respective anchor-wedge seats. The concentric stop is a means transferring the energy of the hammer's momentum to the magnetic tool head. The magnetic tip is designed to hold the wedges around the strand until they can be seated into the concrete anchorage. Thus, the installer can simultaneously set two wedges evenly and correctly in the cavity. After the wedges have been partially seated, the magnetic connection is easily broken as the tool is retracted.
The present invention allows for two or more wedges to be simultaneously installed, as opposed to a single wedge. Furthermore, both wedges can be aligned simultaneously on the magnetic head of the tool, as opposed to working within the confined space of the anchorage cavity. Finally, the seating of the wedges can occur simultaneously, and because the tool places a nearly equal force on both wedges, there is no need for repeated attempts of alignment and seating.
These and other features, and advantages, will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. It is important to note the drawings are not intended to represent the only form of the inven
Granger Dale Andrew
Scanlon Wayne Alan
Friedman Carl D.
Murphy, Esq. James J.
Wilkens Kevin D.
Winstead Sechrest & Minick
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