Patent Application
20050028477
The present invention concerns strengthening of structures and more specifically strengthening of structures by means of additional prestressing method.
Additional prestressing is a known method for strengthening or repairing concrete or masonry structures and has formed the subject of standard NFP95-104 (“Repair and strengthening of concrete and masonry structures”, published in December 1992 by the French standards association (AFNOR)).
It involves generating prestressing forces by putting reinforcing members into tension, for example prestressing cables, external to the structure to be strengthened. The reinforcing members transmit forces to the structure through anchorages bearing on reinforced concrete or metal parts, called bosses, offset from structural components such as existing bracings, beams or walls.
As specified in section 5.2 of standard NF P 95 104, these anchorages are generally fixed by pinning using prestressing bars.
Concrete bosses are large units, which can either be cast in place or precast. However, boss installation proves delicate in either case, especially because of the difficulty in accessing structural components on which bosses must bear. Openings or windows must sometimes be created in the structure to allow bosses to be installed.
Metal bosses are shop fabricated for adaptation to each structure, which ensures their superior fabrication quality compared with concrete bosses. To allow such bosses to have a surface with a sufficiently high friction coefficient bearing on a concrete structural component, this bearing surface must be serrated by machining it or by welding steel wires to it, leading to high construction cost.
According to the method recommended by standard NF 5 P 95-104, reinforcing members pass through the bosses in which they are anchored. Such an anchorage is always made inside the boss on the side opposite to the reinforcing member regular section. It thereby compresses the boss when the reinforcing members are tensioned.
Moreover, bosses foreseen by the standard are positioned at the ends of the structure to be strengthened, such that the additional prestress is applied over the longest possible distance. They are therefore often placed near to obstructions, such as structural walls, columns or bracings. This arrangement makes it difficult to place the reinforcing-member tensioning jack.
For this reason, it is generally necessary to displace the boss away from the obstruction to ensure enough clearance to position the jack, which effectively limits prestressing to only a subsidiary part of the structure. Alternatively, the boss may be placed near the obstruction on condition, however, that an opening is made in this obstruction to allow the jack to be positioned. Applying the resulting prestress is better than in the former case, but this solution requires concrete break-out or drilling work to be performed.
one object of the present invention is to overcome the drawbacks of the current methods recalled above by allowing prestress to be applied to a major section of the structural component to be strengthened, whilst avoiding implementation of work involving breaking out part of the structure for strengthening purposes.
Another object of the invention is to have an anchorage well suited to the structure to be strengthened, without requiring additional adaptation operations such as machining of an anchorage block surface bearing on a member of the structure to be strengthened.
The invention therefore proposes an anchorage unit for anchoring at least one prestressing member, capable of being tensioned, onto a structural component to be strengthened or repaired by additional prestressing, including means of fixing or attaching onto the said structural component and means of anchoring the prestressing member located adjacent to a first side of the anchorage unit oriented towards a regular section of the prestressing member. According to the invention, the fixing means are located adjacent to a second side, opposite the first side, of the anchorage unit, which is so provided that it may be put into traction when the prestressing member is tensioned.
When part of the structure, whose easily accessible, must be strengthened prestressing, this compressive stress considered area essentially adjacent to the adjacent to the stress is applied to the structure right up to the fixing means, in other words almost as far as the second side of the anchorage unit. This second side may be located near to an obstruction that would hamper implementation of conventional anchorages.
Moreover, the invention proposes a method for strengthening or repairing a structure by additional prestressing comprising implementation of such an anchorage unit. This method includes the following steps:
Reinforcing member used for prestressing are conventionally metallic. In particular, they may feature strands 5 capable of being tensioned and words between anchorage zones. Strands are generally protected from corrosion in their regular section, for 10 example by a high-density polyethylene (HDPE) sheath 4 enclosing the strands and injected with cement grout or a non-adhesive soft material such as grease or wax.
The anchorage unit 6 is integral and compact. It is favourably made of cast iron. At one of its ends (on 15 the right in
Furthermore, in an advantageous embodiment, the cast iron anchorage unit 6 may be cast to feature orifices that permit sealing of prestressing cable sheath 4 connections with the anchorage unit 6. A connection collar 9 and seals may then be inserted into these anchorage unit 6 entry orifices to ensure such a seal.
Moreover, the anchorage unit 6 has means of fixing onto the structural component to be strengthened or repaired, which may be an industrial building floor, for example.
According to the invention, the anchorage unit 6 section opposite the strand 5 anchorage, in other words the anchorage unit 6 section furthest away from the prestressing cable regular section, is fabricated such that it may be fixed onto the structural component to be strengthened. In
As represented in
The combination so formed therefore allows the resultant of the pinning and tensioning forces to be transmitted to the structure to be strengthened.
In an advantageous embodiment, the anchorage unit 6 fixed to the concrete floor slab 12 has a bearing surface featuring cast serrations 20 in contact with the structural component. Such casting of the iron enables effective bonding of the anchorage unit 6 with the concrete to be obtained without having to perform expensive additional operations, for example machining of the anchorage unit 6.
The arrangement foreseen by the invention is particularly advantageous because it allows the strand 5 to be tensioned without having to displace the anchorage away from the wall or obstruction 13, which backs onto the anchorage unit 6. The end of the strand 5 is effectively accessible at the housing 8 of the anchorage unit 6. The housing 8 is located at the distance from the anchorage unit fixing zone (illustrated by the orifices 7). A tensioning jack fitted with a curved tip can easily be installed in the available space beneath the anchorage unit 6. Resorting to breaking out or drilling the wall 13 to tension the strand 5 may therefore be avoided, unlike the common practice with conventional bosses.
Moreover, such an arrangement permits sufficient excess length of cable for retensioning or, on the contrary, relaxing the strand 5 after initial tensioning and trimming of the strand 5.
A protective cap or sleeve 11 can be advantageously installed to protect the end of the strand 5 after it leaves the housing 8, as illustrated in
When prestressing forces to be high, the prestressing bars units onto the structure may all the cable force friction because the friction coefficient of serrated cast iron on concrete (of the order of 0.6) requires a very high pinning force. In this case, it may be advantageous to transfer all forces between the anchorage unit and the structure by combining friction and direct bearing. Direct bearing is obtained, for example, by means of a recess 17 provided in the form of a blind hole in the anchorage unit face intended to be applied against the structure (
While a preferred embodiment has been disclosed, the invention is to be limited only by the following claims and equivalents. The embodiments disclosed are examples of the invention and the invention is not limited to the specific embodiments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 03 09225 | Jul 2003 | FR | national |
