GB2302896A - Arch reinforcement - Google Patents

Abstract

A method for reinforcing an arch or portion thereof includes the step of forming within or on the surface of the structure a framework of elongate rods (3,4) of a high tensile strength. The framework may be bonded to a surface of the structure or may extend within the structure and may be two or three dimensional. The method is particularly applicable to masonry arches.

Description

TITLE: Bridge Reinforcement DESCRIPTION Technical Field The invention relates to a process for repairing or reinforcing an arch, particularly of masonry or concrete construction in a bridge, chimney, viaduct or vault.

Background Art Masonry and like materials are generally strong in compression but very weak in tension and are therefore used in structures where there is substantially no tensile loading. For example, masonry is appropriate for the construction of traditionally shaped buildings where the forces are essentially vertical and compressive. It is necessary however to provide beams in the form of lintels over doors, windows and other openings.

Masonry is also an acceptable medium for the construction of arched structures such as arch bridges which ideally should be in a state of compression throughout. However, the forces in the masonry are much more complex than those in a building, and have both horizontal and vertical components.

If the forces within masonry structures are purely compressive and do not exceed the design strength of the material such structures are stable and are not liable to cracking or other deterioration. However, thermal or moisture movements, foundation movements and subsidence or physical changes in the material can cause a loss of support in some areas resulting in tensile forces within the masonry. As the masonry is unable to withstand these tensile forces, cracks will form above the unsupported areas.

There are various ways of peventing or repairing cracked masonry. Ties may be inserted to withstand the tensile forces. Alternatively, a beam may be created to span an area of poor support, such as an area of subsidence.

Patent Specification GB 2259120 discloses a method of creating a beam within a masonry structure by cutting a groove in a surface of the structure, injecting a grouting material into the groove, embedding a rod along - the groove in the grouting material, injecting further grouting material into the groove, embedding a second rod along the groove in the grouting material and injecting further grouting material into the groove. Upper brick courses are first reinforced to provide support above the groove.

Unlike the original masonry, this beam can provide resistance to bending moments and can bridge an area of poor support, eliminating harmful tensile forces within the masonry.

Provided that accurate reinforced masonry design calculations are carried out to determine the correct locations and strengths of the beams, the above approach can be very effective in structures where the compressive forces are essentially vertical. With an arch, however, the situation is much more complex because the compressive forces have both horizontal and vertical components and often have imposed lateral components.

Creating a beam within the masonry would tend to upset the balance of forces in the arch and, even if it solved a problem in a localised area, would be likely to cause difficulties in other areas of the arch.

cch Minortbridge repairs can be effected using ties, which link two areas of masonry and provide the tensile forces which the masonry cannot.

Major repairs to arch bridges are carried out as follows.

All bridge users and services such as cable, gas and water are diverted, and a reinforced concrete slab with edge beams is cast over the bridge to take the weight off the arch. The bridge is thus made into a simply supported beam spanning between supporting abutments. To provide adequate support it is sometimes necessary to strengthen existing abutments and/or to underpin using piles or a similar method to found into adequate soil substratum.

This method of repair is extremely disruptive and expensive and ruins the character of the bridge. Many masonry bridges are of architectural interest/listed and there is an increasing desire to maintain them in as close te their original state as possible.

The Invention The invention provides a method for reinforcing a structure being an arch or a portion thereof which includes the step of forming within or on a surface of the structure a framework in at least two dimensions of elongate rods of a high tensile strength, the rods being bonded to the structure such that substantially no relative movement may occur therebetween.

The method of the invention is particularly useful for masonry arches but may be applied to an arch of any material or materials which are originally or which have become relatively weak in tension. Porous materials may be improved by the injection of cement or another consolidating material, to improve their compressive strength and provide security for the rods.

The rods may be made of steel, plastic, carbon fibre or any other material or mixture of materials with sufficient tensile strength and preferably also good resistance to creep. The tensile strength of the rods should be at least about 460 N/mm for high tensile stainless steel, and may be more or less than this for other materials. The necessary strengths can be calculated using known techniques and/or referring to British Standards. The original structure withstands compressive forces and holds the framework of rods in place, preventing the rods from buckling when they are in compression. The rods withstand the tensile forces that the masonry structure cannot resist, and transfer forces away from weakened areas to adjacent areas of compression.Additionally, the compressive strength of the rods is significant if the structure is loaded excessively in any particular area, since it increases the compressive resistance of the core.

The rods may be curved (for example they may follow the arch of the bridge) or straight. They preferably cross one another at an angle of about 90 , but this angle may be significantly smaller, depending on the dimensions and shape of the bridge. The only limitation is that the rods should not all be parallel.

If the framework is a two-dimensional mesh framework the rods may be attached to a surface of the structure or may be embedded within the structure. If the structure is an arch bridge, such a two dimensional framework would preferably be fitted on or near the underside of the bridge as this is an area likely to suffer tensile forces if the bridge is unbalanced or not properly supported, and compressive forces if the bridge is overloaded.

The framework may extend in three dimensions with the rods preferably being positioned throughout the structure. The rods may be quite evenly positioned, or they may be concentrated in stress points. A computer analysis can be carried out to determine the best locations for the rods.

To insert the rods into the structure, slots may be cut into the structure or bores drilled into the structure and the rods inserted in the slots or bores and retained if necessary by mortar or resin. Clamps or propping or other forms of temporary or permanent fixings may be used to hold the rods in position, and if necessary support the structure while the fixing mortar or resin hardens.

It is important that the rods be held firmly in the mortar or resin, i.e. that the bond is good, so that the rods are able to transfer tensile forces to the structure. Shrink free mortars are therefore preferable.

To ensure that the mortar spreads around the entire bar to hold it in place, twisted or helical bars may be used.

The rods may be inserted singly or in multiples. They may be preformed or formed by inserting suitable material into the slot or bore to form the reinforcement. For example carbon fibre can be inserted into a slot in liquid form so that it cures into a rod within the slot.

The cured rod may bond directly to the masonry or a fixing mortar or resin may also be necessary The method may be used on an arch that has already suffered damage or may be used as a preventative measure to upgrade a structure subject to increased loading.

The reinforcement framework is relatively quick and easy to instal and is far less disruptive than traditional methods of undertaking major repairs. It is not necessary to divert services and traffic may flow under or move over the bridge while installation takes place.

Furthermore, the appearance of the bridge may be left unchanged since the reinforcements are either invisible, if within the structure or can be disguised if on the surface.

The Drawings Figure 1 is a schematic side view of an unreinforced masonry arch bridge with a partially collapsed foundation; Figure 2 is a schematic perspective view of an arch bridge which has undergone reinforcement by a method according to the invention; Figure 3 is a schematic perspective view of the bridge of Figure 2 but showing a partially collapsed foundation; Figure 4 is a schematic cross-sectional view of rods positioned as they would be in a bridge reinforced by a method according to the invention; Figure 5 shows the way in which consecutive layers of rods may be staggered; Figure 6 is a schematic cross-sectioal view similar to Figure 4 but showing additional radial rods.

Figure 7 is a schematic cross-sectional view showing an alternative way in which rods may be positioned.

Figure 8 is a schematic cross-sectional view showing an alternative way in which rods may be positioned.

Description with reference to the drawings Referring to Figure 1, an arch bridge 1 is of masonry construction. When the bridge 1 is properly supported and is not unevenly or excessively loaded, it is in compression throughout and the stresses in the bridge can be borne quite comfortably by the masonry. However, an area 2 of a foundation of the arch bridge 1 shown in the Figure has collapsed, leaving the bridge poorly supported in this area. This uneven support disrupts the balance of forces in the bridge and results in tensile forces around the poorly supported area. Since masonry is very weak in tension, cracks appear in the masonry and the bridge is weakened.

Figure 2 is a schematic perspective view of an arch bridge which has been reinforced by a method according to the invention. A two dimensional framework of elongated rods has been created on the underside of the bridge. The framework consists of a series of equally spaced parallel longitudinal rods 3 and a series of equally spaced parallel transverse rods 4. The rods are attached to the masonry structure of the bridge so that no relative movement therebetween can occur, and the masonry structure can be regarded as being split into "cells" bounded by these rods, all of which are part of the homogenous mass.

Figure 3 shows the bridge of Figure 2 but with a poorly supported area 5 at its foundations. A corner 6 of the bridge is essentially no longer supported by these foundations. However, just above this unsupported area is a transverse rod 7. This rod transforms the area into which it is embedded into a beam, which can withstand bending. Therefore, when an extra net downward force is applied to this beam above the unsupported area, the beam resists the bending moment induced in it by this force and hence transfers the force along the length of the beam to areaS oF well supported masonry.

Additionally the longitudinal rods 3 extending up away from the unsupported area are put in tension due to the loss of support of the masonry at ground level. These bars are firmly attached to the masonry and each masonry cell above the unsupported area thus experiences a downward force. However each masonry cell is located above a transverse rod 4, acting as a beam. Each beam resists the bending moment caused by the downward force at its end and spreads the force along its length as discussed previously. Thus, it can be seen that the framework of rods turns a damaging localised force in an unsupported area into a force distributed over a large part of the structure where the support is still good.

For clarity, Figures 2 and 3 show a two dimensional reinforcement framework on the underside of the bridge.

However, the framework may extend throughout the whole arch, and there may be several layers of reinforcement as shown in Figures 2 and 3. Figure 4 is a schematic cross section of a bridge reinforced with more than one layer of longitudinal 3 and transverse 4 bars. Figure 5 shows that the layers of longitudinal bars 3 are staggered in relation to each other.

In addition to longitudinal and transverse bars, the framework may also include radially aligned bars 8 as shown in Figure 6.

To reinforce a bridge using a three dimensional framework according to the invention, the following method may be used. Stage 1. Deep grooves are first cut into the structure for the longitudinal rods, or conversely the transverse rods. These rods are inserted, in shrink free mortar which is then allowed to cure. The bars may be clamped or propped in place while the mortar hardens.

Stage 2. Slightly shallower grooves are then cut for the next layer of rods which will pass across the rods which have been inserted first, so that they lie substantially orthogonally to the longitudinal rods.

These rods are inserted in shrink free mortar. Stage 3.

Once this mortar has cured, further shallower grooves may be cut for the next layer of rods and so on. The spacing of consecutive layers of longitudinal and/or transverse rods may be staggered. Radial rods can be inserted where appropriate and the timing of insertion is not critical. However, if radial rods are to be used, the staggering of the longitudinal and tranverse rods should be controlled so that there is a clear path through the structure for the radial rods. In general rods may either be inserted laterally into slots or lengthwise into drilled bores.

It is not necessary to follow a layer of longitudinal rods with a layer of transverse rods or vice versa. Rods and/or layers may be inserted in many different ways, such as those shown in Figures 7 and 8.

Monitoring devices may be attached to the structure to gauge the stesses and strains and monitor the performance of the structure. Thus, the performance of the reinforced structure can be analysed to check, for example, how it performs under increased load.

Claims (10)

CLAIMS:

1. A method for reinforcing a structure being an arch or a portion thereof which includes the step of forming within or on a surface of the structure a framework in at least two dimensions of elongate rods of a high tensile strength, the rods being bonded to the structure such that substantially no relative movement may occur therebetween.

2. A method according to claim 1 wherein the framework includes a first set of substantially parallel rods and a second set of substantially parallel rods, the first set of rods being oriented approximately orthogonally to the first set.

3. A method according to claim 2 wherein the rods are bonded to a surface of the structure.

4. A method according to claim 2 wherein the framework further includes a third set of substantially parallel rods oriented approxiately orthogonally to the first set and the second set.

5. A method according to claim 4 wherein the framework extends throughout substantially the whole of the structure.

6. A method according to any of claims 2 to 5 wherein the rods in each set are substantially evenly spaced.

7. A method according to any preceding claim wherein the rods are helical or twisted stainless steel.

8. A method according to any preceding claim wherein the rods are bonded to the masonry with shrink free mortar.

9. A method according to claim 8 wherein the rods are clamped against the masonry while the mortar sets.

10. A method substantially as described herewith with reference to the drawings.