GB2199071A

GB2199071A - Strengthening load bearing walls

Info

Publication number: GB2199071A
Application number: GB08729263A
Authority: GB
Inventors: Yiu Chin Eugene Yau; Kwok Ki Ma; Kai Yuen Wong
Original assignee: SHUI ON ENGINEERING SERVICES L
Current assignee: SHUI ON ENGINEERING SERVICES L
Priority date: 1986-12-15
Filing date: 1987-12-15
Publication date: 1988-06-29
Anticipated expiration: 2007-12-15
Also published as: HK11691A; GB8729263D0; GB8629899D0; GB2199071B

Abstract

A system for strengthening or reinforcing load-bearing walls is disclosed in which a framework normally of steel is assembled against the wall on suitable foundations, and up to 100% of loads borne by the wall are transferred to the framework. The system is applicable to any existing building, but has primary application in multi-storey blocks in which a design deficiency exists. Load transfer is effected by forcing a support component against the load itself adjacent the wall, and securing the support component with respect to the framework. The framework itself will normally comprise upwardly extending columns between which support components are disposed. <IMAGE>

Description

THE STRENGTHENING OF LOAD-BEARING WALLS This invention relates to the strengthening of load-bearing walls, and has a particular application in existing buildings where the primary structure requires reinforcement. This situation can develop where the use of a building is changed, there is a deficiency in the primary design; a building has been damaged or has deteriorated; or a building is to be extended by for example, the addition of further storeys. The term "load-bearing walls" includes structural walls subject to vertical andlor horizontal forces.

In Hong Kong and places of comparable population density, the use of tall blocks has become commonplace as a means pf providing business and personal premises. Blocks in excess of forty storeys have been built, and as building techniques develop more extensive consideration has been given to the manner in which such blocks can be reinforced as they get older, or to increase the loading that they can accommodate. There have additionally been examples recently of completed buildings being insufficient to accommodate design loads, with the result that even new buildings can be unsatisfactory. Either circumstance generates serious problems in that the available options are very limited. The primary option is demolition followed by rebuilding, and this is of course very expensive.Proposals for reinforcement of existing buildings have also been made, but generally these have required a building to be substantially wholly evacuated while the work takes place. This is of course highly disruptive, and poses the immense problem of relocating the occupants of the building while the work is taking place.

The present invention is a system and technique for strengthening load-bearing walls in existing buildings which involves the minimum disruption of their use, and also which minimizes the loss of usable space within the building.

In accordance with one aspect of the invention, a method of strengthening a load-bearing wall supported on a base structure comprising assembling a framework against the wall, mounting the framework relative to the base structure, and securing the framework to the wall and with respect to loads borne by the wall such that at least part of said loads are transferred to the framework.

In accordance with another aspect of the invention framework for strengthening a load-bearing wall on a base structure comprising at least two columns on each side of the wall, the two columns on one side being positioned in an adjacent area to the two columns on the opposite side, at least two primary cross-members each of which extends between two columns on the same side of the wall, and securing means to connect the columns to the wall.

The system is basically a framework which is mounted in the base structure supporting the wall, and secured thereto. Means are provided for transferring some or all the loads borne or to be borne by the wall to the framework. - The framework will normally itself be of steel, and comprise a plurality of vertical members which will occupy a minimum of space in the region bounded by the wall. In most cases, these members will be disposed spaced from each other on the same side of the wall and clamped thereagainst.

Typically, each member will be one of a pair, juxtaposed on opposite sides of the wall such that the wall is strengthened on both sides.

The primary loads that are borne by the type of walls with which this invention is most concerned will be floors extending laterally from the walls at space heights thereon. In order to transfer these loads to the framework, one or more support components are arranged beneath the respective floor, forced against the floor to take some or all of the load thereof, and then permanently secured to the framework.

Conveniently, this is accomplished by the mounting of jacks with respect to the framework, and jacking the support component or components into position prior to locking it with respect to the framework. In one system of the invention, such a support component may be a cross-member or beam extending between two vertical framework members on the same side of the wall which is eventually welded or otherwise secured to the vertical members. A bearing plate may be disposed between the support component and the floor to ensure that a point loads on the support component are avoided.

When adopted in a multi-storey block, the system of the invention has the particular advantage of being suitable for progressive assembly. The framework may be built upwards and through the first floor slab, and the transfer of the loads generated by the first floor slab be completed before work needs to be commenced on the second floor slab. Thus, as the work extends upwards through a building, at any one time as few as two adjacent levels in the building need be disrupted, although usually three will be involved.

Even then, the major work will be confined to the regions where the vertical members of the framework must pass through a floor slab, and disruption is further minimized.

Where the loading of one floor slab on the wall is to be transferred to the framework, it is of course important to complete this transfer accurately and effectively. The use of jacks as described above can be carefully controlled, and of course properly synchronized. A previous analysis of the building defects will determine exactly how much of a floor load is to be transferred, and computer analysis can be used to set the proper reinforcement that is to be accomplished at each level. It will be appreciated that in some buildings the degree of reinforcement required at different floors will vary. -The system of the invention is well suited to accommodate variations of this kind as the degree of transfer will not relate to the bulk of the framework at a particular level, only to the total load characteristics of the framework as it extends through the building.Thus, the amount of reinforcing framework at each level can be the same, although the actual degree of reinforcement may vary.

Of course, a full analysis of the basic design of the building must be made prior to the adoption of a system or method according to the invention, and it is essential of course that the basic foundations of the building are capable of taking the resulting total loads. Frameworks used in the system and method of the invention will in almost all circumstances be mounted on the basic foundations of the building in question and generally, these foundations will be sufficient. For example, many high rise buildings are supported on piles extending deep into the ground, and the pile cap at ground leVel or below constitutes a suitable base structure in which the framework can be mounted. In some circumstances though, it may be possible to use different foundations or base structures for the framework, although this could add substantially to the costs involved.

The benefits of the invention in terms of cost and minimal disruption to existing occupants of a building will be readily apparent from the above. It is estimated that the application of a system according to the invention to a sixteen-storey block could be completed within three months, and require the evacuation of as few as three floors at any one time.

In terms of cost, this is likely to be of the order of 25% or less of demolition and rebuilding. The amount of materials and equipment required will be relatively small, bearing in mind that those items used to effect the transfer of loads to the framework can be re-used on successive floors.

Once the system of the invention has been installed, floor portions which have had to be cut away can be readily restored, and the visible parts of the framework suitably disguised and made to blend in with the original walls. The loss to usable space will be a minimum, and the strength of the reinforced building can be at least as great as that required by the original design, or as a new design dictates.

A system embodying the invention will now be described by way of example and with reference to the accompanying diagrammatic drawings, wherein: Figure 1 is an isometric view showing the partial installation of a framework on a building wall; Figure 2 is an elevation showing the mounting of the framework in the building foundations, and against the load-bearing wall; Figures 3, 4 and 5 are elevations showing the transfer of a floor load to the framework; Figure 6 is asectional plan view of a floor of a building showing the installation of frameworks in accordance with the invention; Figure 7 is another isometric view showing a part of the completed installation; Figure 8 is a side cross-sectional view of another framework in accordance with the invention; Figure 9 is a side view of a portion of the framework of Figure 8 during jacking; and Figure 10 is an end view of a portion of the framework of Figure 8.

Referring first to Figures 1 and 2, a load-bearing wall 2 is shown reinforced by two posts, each in the form of an I-section steel column 4, the columns being juxtaposed on opposite sides of the wall 2, and clamped thereagainst by tie rods 6. Steel plates 8 are disposed between each column 4 and the wall 2 to spread the clamping load, and stiffeners 10 are included to reinforce the column at the clamping sections. The plates 8 can be accommodated within the existing rendering 12 on the wall 2 as shown in Figure 2.

Each column 4 is securely mounted in the foundation 14 on a boss 16 held by tie rods 18 which are themselves secured by epoxy grouting.

As shown, the columns 4 extends substantially vertically through holes which are cut in the floor slabs 20, and an I-section cross-member 22 extends laterally from the column beneath the floor to take a proportion of the load thereof. The securement of the cross-member 22 to the column 4 is described below.

The openings in the floor slabs 20 can be relatively small; typically they will be of the order of 400 millimetres square. Although 1-section columns and cross-members are illustrated, it will be understood that any suitable sections may be adopted. For example, channel section elements could be more suitable where a building is for domestic occupation and a less functional final appearance is desired.

The columns 4 will normally be arranged in pairs on each side of a wall 2, as illustrated in Figure 3. The cross-member 22 is of length slightly less than the spacing between the columns 4 such that it can be easily raised and lowered in substantially the same plane. As shown in Figure 3, the cross-member 22 is supported on jacks 24, which are themselves mounted on brackets 26. A bearing plate 28 is placed on top of the cross-member 22 to prevent point loading between the potentially uneven lower surface of the floor slab 20 and the cross-member 22.

Figure 3 shows the next stage in the transfer of the floor load to the framework. The hydraulic jacks 24 are extended to raise the cross-member 22 into engagement with the floor slab 20, and by further extending the jacks 24 the floor loading is transferred from the wall 2 to the jacks and thereby to the columns 4. This transfer will usually involve some physical movement of the floor slab 20, but this will be very small. During this stage, it is important to secure good contact between the cross-member 22 and the floor slab 20 and the bearing plate 28 can consist of a flexible strip, for example of synthetic rubber, with a layer of epoxy mortar between it and the floor slab.

Once the bearing plate 28 makes contact with the floor slab 20, it is recommended that this contact be maintained until the epoxy mortar has set; e.g., for around thirty minutes. Of course, the precise composition of the bearing plate 28 can be varied for different circumstances. Care should be taken though, to ensure that a good bearing contact is achieved between the cross-member 22 and the floor slab 20.

Once the requisite location of the cross-member 22 has been achieved and the desired floor load transferred, steel I-section stanchions 30 are disposed directly between the brackets 26 and the cross-member 22 to relieve the jacks 24. Folding wedges can be used at the top of the stanchions 30 to ensure their proper disposition. The ends of the cross-member 22 are then secured to the columns 4 by butt welds 32. The jacks 24 can be removed at this stage.

Figure 5 shows the final stage in the fixing of the cross-member 22. Still with the stanchions 30 in place, angle cleats 34 are fitted above and below the cross-member 22, and secured by fillet welds both to the columns 4 and the cross-member 22. Thereafter, the stanchions 30, and the brackets 26 can be removed.

The above procedure can be repeated at all the joints between floor slabs and their supporting walls at this level, and it is appropriate to synchronize the operation of the jacks 24 operating at the same level. Prior structural analysis of the building can determine the degree of load transfer required at each junction, and the requisite hydraulic pressure can be delivered to each jack in accordance with this analysis. A typical arrangement is shown in Figure 6 where hydraulic lines connect jacks just at either end and at both sides of each loading wall to the central pump 38. Such a synchronized jacking system can operate as many as two hundred jacks at the same time, and will minimize the differential movement between the various junctions, and therefore any damage to the load-bearing walls and floor slabs.Human error is also minimized as are the labour costs and the time required to complete the operation.

Once the load transfer is completed, and the cross-member 22 secured to the columns 4, and the jacks 24, stanchions 30, and support brackets 26 have been removed, the floor slab itself can be immediately restored in the region where the column 4 passes through it, and elsewhere as required, and once any new concrete has cured the space below the supported floor is ready for re-occupation. All the equipment can be moved to the floor above, and the procedure repeated.

As shown in Figure 7, only the columns 4 interfere substantially with the working space defined by the wall 2, although the cross-member 22 will normally also be visible. These remaining components of the framework can be easily disguised or made part of the decorations in a room, as desired.

It will be appreciated that during each stage only two storeys of a building need be evacuated or disrupted. Normally though, as one floor load is being transferred to the framework consisting of the columns 4 and cross-members 22, the columns 4 will be extended through the floor above, ready for the next load transfer step. Thus, each stage will usually involve the evacuation of three adjacent storeys in a building.

It will be appreciated from the above that as little as none, or as much as all of the loading on the wall 2 at a particular junction with a floor slab can be transferred to the columns 4. Depending upon the condition of the building in question and the nature of its original design, the invention thus enables the degree of transfer to be selected for each floor. If no load transfer is required, then the deployment of a cross-member 22 can be omitted. Generally though, cross-members will be included in any event to assist in retaining the integrity of the framework itself. On the other hand, it will be appreciated that by transferring floor loads at upper floors, stresses will be relieved in the load-bearing wall at lower levels, rendering it better able to accommodate the loads thereon.These factors will be essential parts of the structural analysis of the building that will usually be carried out before the present invention is applied thereto. Such analysis techniques for the analysis of building structures are well-known, and the details thereof need not be discussed herein.

While a simple rectangular framework structure is described herein, it will be appreciated that the framework may take different forms depending upon the nature of the building wall to be strengthened or reinforced. Thus, the framework may be extended to include any desired number of columns 4 on one side of the wall 2, with a separate cross-member 22 extending between adjacent columns or otherwise as required.

Where an even number of columns is used, cross-members may be disposed alternately for example. Further, the framework may be diagonally braced, although the rectangular framework described should normally be sufficient to reinforce the wall against lateral loads; i.e., wind forces, in the plane of the wall. It should be noted that the invention also contemplates adjacent vertical frameworks being linked by additional horizontal members at floor or ceiling level in circumstances where the building as a whole also requires reinforcement against lateral loads in the plane perpendicular to that of the framework.

In this regard Figure 8 illustrates a more complicated framework to that shown in Figure 1, although still using most of the same basic components.

In contrast, the framework of Figure 8 comprises a set of three columns 4 with primary cross-members 22 extending therebetween and secondary tie beam cross-members 23 extending at right angles to the cross-members 22 to other columns (not shown).

The columns 4 are securely mounted and load transferred indirectly via a base girder 25 and inclined strut 27 to the dowel bars 18 of pile caps 14.

In Figure 9, the jacking and supporting arrangement of primary cross-members 22 is shown, as well as the supporting arrangement of secondary tie beam cross-members 23. The erection process of cross-members 22 comprises securing the cross-member 22 between two angle irons 27 with friction grip bolts, filling the gap between cross-member 22 and floor 20 with monolith material, securing the bracket 26 to the column 4 and placing a jack 24 thereon, and operating the jack 24 to raise the cross-member 22 up and against the floor 20, all gaps therebetween being filled with the monolith material.In particular, the filling of the gap between the cross-member 22 and floor 20 comprises placing along each top edge of the cross-member a strip of material (e.g. foam rubber) to enclose the gap between the cross-member 22 and floor 20, and pouring liquid monolith material 28 (e.g. a plastic epoxy concrete mixture) into the opening 29 of the gap to fill the gap. To ensure that liquid monolith material is positioned along the whole length of the gap, means in the form of a strip of wire 31 is provided to move liquid further along the gap. The liquid monolith material is then allowed to set and strengthen for twenty-four hours before the jacking operation is started.

In Figure 9, the whole arrangement is secured together by means of friction grip bolts, which is in contrast to the arrangement in Figure 5 which is on the whole secured by welding. In particular, the bracket 26 is secured to the column 4 by bolts'33 with the help of a steel plate arrangement 37, and the cross-member 22 is secured to the column 4 via the angle irons 27 and two angle cheats 34 by bolts 35. At the start of the jacking operation, only the bolts 35 securing the angle irons 27 to the column are tightened, while the bolts 35 to secure the angle irons 27 to the cross-member 22 are inserted but not tightened. These particular bolts pass through vertical slots 39 provided either in the angle irons 27 or cross-member 22 to allow relative movement between the two members during jacking.After jacking is finished, the bolts to secure the angle irons 27 to the cross-member 22 are tightened, and the various other bolts 35 are inserted and tightened.

In Figure 10, the supporting arrangement for secondary tie beam cross-members 23 and an arrangement to interconnect the two columns 4 on opposite sides of the wall 2, are shown.

In particular, the cross-member 23 is secured to column 4 by means of a vertical steel plate 41 and a horizontal steel plate 43 all connected together by friction grip bolts 47 (see also Figure 9). The cross-members 23 are generally not load supporting, and are purely to interconnect the framework against one wall to framework against another wall.

The column interconnecting arrangement shown in Figure 10 which is provided just above each floor level comprises a short I-beam section 49 spanning the two columns 4 on opposite sides of the wall 2. The section 49 is secured to the columns 4 via a number of angle irons 51 by friction grip bolts 53, and passes through a hole 55 which has to be made in the wall prior to erection of the section 49. After the section 49 is erected, the hole 55 is filled with concrete material with the section 49 in situ, thereby providing bracing for framework on one side of a wall with framework on the other side of the wall, and additional securing of the columns to the wall over and above the lie rods 6.

The framework shown in Figure 8 is estimated to take around 25% of the load borne by the wall 2, which in consequence strengthens the wall considerably.

It is also estimated that the framework need only be installed in the lower levels of a building, since it is the load-bearing walls in these lower levels which bear the most load. For example, in a sixteen storey building, only the lower six levels require framework installed.

The embodiments of the invention illustrated use vertical columns 4. However, the columns may be inclined to the vertical in some applications; for example where some reinforcement against lateral loads is needed, but not so much as to justify extensive diagonal bracing of a basic rectangular framework.

Normally in such a variant, the inclined columns will converge upwards. The precise geometry of the framework most suitable for a particular building can be determined from the structural analysis of the building carried out prior to the strengthening system being designed, and the design will also of course take account of the labour and material costs involved in the implementation of suitable options. For example, in some circumstances where horizontal as well as vertical loads must be taken into account, a triangular framework structure may be more appropriate.

Buildings constructed primarily in concrete are particularly adapted to repair and reinforcement according to the present invention, and as many such buildings are now reaching an age where consideration of their structural integrity is becoming essential, the invention offers a viable alternative to demolition and rebuilding. By transferring loads on walls to a framework clamped or otherwise secured thereagainst the invention can provide sufficient reinforcement to enable a building to be restored to a usable state at a relatively low cost, with minimum disruption to the occupants, something which has heretofore not been a realistic prospect.

Claims

CL A I M S:

1. A method of strengthening a load-bearing wall supported on a base structure comprising assembling a framework against the wall, mounting the framework relative to the base structure, and securing the framework to the wall and with respect to loads borne by the wall such that at least part of said loads are transferred to the framework.

2. A method according to Claim 1 wherein the framework comprises at least two columns on opposite sides of the wall and clamped thereto.

3. A method according to Claim 2 wherein said two columns are juxtaposed.

4. A method according to any preceding Claim where in the framework comprises at least two columns spaced on the same side of the wall, and a cross-member extending therebetween, the cross-member receiving saidtransferred loads.

5. A method according to any of Claims 2 to 4 wherein the columns are vertical.

6. A method according to any preceding Claim wherein the wall is part of a building having elements extending laterally at spaced heights therefrom, the framework being extended past said elements, and up to 100% of the load of said elements being transferred to the framework.

7. A method according to Claim 6 wherein the framework includes columns extending upwardly through said lateral elements.

8. A method according to Claim 6 or Claim 7 wherein the lateral elements are floors extending from one or more load-bearing walls.

9. A method according to Claim 7 including the steps of supporting the floor on jacks mounted on the framework; extending the jacks to transfer said loads; and locking the framework to secure the transfer.

10. A method according to Claim 9 wherein the framework includes at least one support component between the jacks and the floor, the locking step comprising locking the support component to the framework.

11. A method according to Claim 4 and Claim 10 wherein the support component is the cross-member.

12. A method according to any of Claims 8 to 11 wherein the transfer of loads to the framework is carried out successively at ascending heights.

13. A method according to Claim 12 wherein the framework is progressively extended upwards past successive heights.

14. A method according to Claims 5 to 12 wherein the transfer of loads at one said height is synchronized at a plurality of locations on the framework at said height.

15. A method according to any preceding Claim wherein the framework is braced to resist lateral loads in the plane thereof.

16. A-method according to any preceding Claim wherein the framework comprises at least two columns on opposite sides of the wall, and a cross-member extending therebetween through the wall.

17. A method accordingly to Claim 16 wherein the cross-member extending between the two columns on opposite sides of the wall, is embedded in the wall.

18. A method according to any preceding Claim wherein the framework comprises at least two columns on different walls, and a cross-member extending therebetween.

19. A method according to any preceding Claim wherein framework is assembled only on the lower levels of a building.

20. Framework for strengthening a load-bearing wall on a base structure comprising at least two columns on each side of the wall, the two columns on one side being positioned in an adjacent area to the two columns on the opposite side, at least two primary cross-members each of which extends between two columns on the same side of the wall, and securing means to connect the columns to the wall.

21. Framework as claimed in Claim 20 wherein installment means are provided to raise the cross-members into position for connection with the columns.

22. Framework as claimed in either Claim 20 or 21 wherein secondary cross-members are provided which extend at right angles to the primary cross-members between two columns adjacent different walls.

23. Framework as claimed in any one of Claims 20 to 22 wherein a connecting member extends through the wall and between the two columns on opposite sides of the wall.

24. Framework as claimed in any one of Claims 20 to 23 wherein the columns extend several storeys in a building.

25. Framework as claimed in any one of Claims 20 to 24 wherein supporting and or mounting means is provided for the columns relative the base structure.