GB2163473A - Apparatus and method for strengthening structural members - Google Patents

Abstract

Two anchoring members 4 are secured to the member, e.g. joist 2, spaced along the length thereof, and a tensioned flexible strap 7 is looped around the two anchoring members 4. The anchoring members 4 are on the tension side of the neutral axis of joist 2 so that the tension in strap 7 adds to the tension in the joist to resist the bending moment. The apparatus may be used to support insulating material by providing straps running parallel to the joists. <IMAGE>

Description

SPECIFICATION Apparatus and method for strengthening structural members This invention relates to an apparatus and a method for increasing the strength of structural members, for example the joists in the ceiling or floor of a building. More particularly the invention relates to an apparatus and a method for increasing the strength of structural members to withstand bending moments caused by loads placed on the members.

The invention has numerous applications in building and architecture when it is required to reinforce structural members such as timber frames, joists, lintels, etc. In particular, it is especially useful for strengthening existing structural members when they are required to withstand a greater load than was originally anticipated when the building was erected. An example of such an application is described below.

Many existing buildings, particularly those with timber suspended floors, have been found to present excessive fire risks. It is desired to render such buildings fire resistant by providing them with fire retardant materials which remain intact when attacked by flames, and thus hinder the spread of fire for some time. A longer time is then available within which to bring the fire under control, and to evacuate people from the building. Insulating material may be installed in the spaces between the ceiling or floor joists, and one known material is foamed perlite which is first pumped into the spaces in foam or slurry form, and then allowed to set.Although such material is generally of very low density in comparison with other materials from which the building may be made, such as bricks and timber, it nevertheless imposes a high total load on the joists which reduces the available floor loading and may cause bending of the joists.

If the joists are strengthened according to the invention, prior to the fire retardant material being installed, they will be more resistant to this bending.

According to the invention, there is provided apparatus for increasing the resistance of a structural member to bending moments, comprising two anchoring members adapted to be secured to the structural member at spaced apart locations along the length thereof and each presenting a generally cylindrical portion extending transversely outwardly of the structural member, and at least one flexible elongate strap adapted to be formed into an elongate loop around the two cylindrical portions, and to be placed in tension, whereby the anchoring members may be positioned relative to a portion of the structural member which is placed in tension under bending moments such that tension in the strap(s) adds to the tension in the structural member to resist the bending moments.

The anchoring members may each comprise a cylindrical portion mounted at one axial end on a mounting plate which in turn may be screwed or otherwise fixed to the side of a beam or the like.

Preferably, however, the anchoring member consists of a rod or tube arranged to be fitted into a hole in the structural member. The hole and rod or tube may pass completely through the structural member and if desired a strap may be fitted to each side of the structural member. This arrangement is particularly suitable for strengthening rectangular-section structural timbers, such as joists.

In use the anchoring members would be positioned on the tension side of the neutral axis of the structural member. In the case of a cantilevered member this would be above the neutral axis and in the case of a joist supported at each end it would be below the neutral axis. The straps are preferably commercially-available clamping bands and they may be tensioned and joined into a loop by a conventional tensioning and jointing tool.

Another problem which arises when insulating materials are installed in floors is that of retaining the material in case of a fire. In a fire it can be expected that a ceiling will become weakened; it is necessary to prevent the material from falling through the ceiling or its fire retardant effect will be lost. It has been proposed to install generally Lshaped hangers along the length of the joists to support the insulating material, but this has the disadvantage that many floorboards have to be raised, thereby adding considerably to the time required for installation. Another proposal has been the use of tongues driven through the floor into the insulating material, but this may transmit forces from the floor, through the insulating material to the ceiling below, possibly resulting in cracking.Apparatus according to the invention overcomes these problems by providing one or more straps running parallel to the joists which may assist in supporting the insulating material but which may be installed very easily.

Thus, viewed from another aspect, the invention provides a method of increasing the fire resistance of a floor, comprising fixing an anchoring member between a pair of adjacent joists below the neutral axes and adjacent one end thereof, fixing an anchoring member between the pair of joists below the neutral axes and adjacent the other end thereof, forming at least one elongate flexible strap into an elongate loop around the anchoring members, tensioning the or each strap, and at least partially filling the space between the joists with a fire retardant material which sets around the strap(s).

As explained above this method is effective both to strengthen the joists against the added load of the fire retardant material and to support the material after it has set.

The anchoring members are preferably each formed as a pair of telescoping tubes fitted to screw-threaded studs which pass through holes in the joists. The fire retardant material is preferably foamed perlite, which is installed in a foamed state and sets on drying to a solid honeycomb material.

In order that the same sized studs may be used, the axially outer end of the outer telescoping tube is preferably provided with a cylindrical plug which is secured therein and serves to reduce the diameter of the end of the tube to the diameter of the narrower inner tube.

Viewed from another aspect, the invention provides an apparatus comprising a pair of telescoping tubular members and a pair of screw-threaded studs, the tubular members being provided with internal screw threads at their axially outer ends adapted to mate with the screw-threaded studs.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, wherein: Figure 1 is a sectional view through a floor and two supporting joists showing apparatus according to the invention; Figure 2 is a sectional view taken along the line ll-ll of Figure 1; and Figure 3 is a perspective view showing the apparatus in more detail.

Figure 1 shows a ceiling 1 suspended from a plurality of joists 2, two of which are shown in this Figure. The joists support floorboards indicated by reference numeral 3. A telescoping tubular anchoring member, indicated generally by the reference numeral 4, extends between each adjacent pair of joists 2. Each anchoring member 4 comprises a tubular portion 4b and a tubular portion 4a which slides within the portion 4b. The anchoring member is connected to the joists by means of threaded studs 5 which are positioned in suitable holes in the joists below the neutral axis of the joists. The holes in adjacent joists are substantially aligned and this arrangement serves to transmit forces directed longitudinally of the joists between the joists and anchoring members.

An anchoring member is shown in more detail in Figure 3. Each end of the member is tapped so that it may be screwed onto one of the studs provided in the joist. Since the portion 4b of the telescoping tube has a larger diameter than the portion 4a, a cylindrical plug 6 is placed in the end of the portion 4b. The inner diameter of the cylindrical plug 6 is the same as the inner diameter of the portion 4a.

The outer diameter of the plug 6 is equal to the inner diameter of the portion 4b so that it fits tightly within the end of the tube. It is secured in the position shown in Figure 3 so that it cannot slide further into the tube. The plug 6 is tapped so that this end of the tube may be screwed onto one of the studs provided in one of the joists. Where the studs 5 are of smaller diameter than the inner tube portion 4a, a plug of appropriate internal diameter may be provided in the end of each tube portion 4a and 4b.

A similar arrangement of anchoring members is provided at the other ends of the joists. A plurality of elongate flexible straps are formed into loops around the anchoring members. Reference numeral 8 in Figure 1 indicates the extent of the space within which the straps may be provided.

The number of straps required may be varied according to the particular application. Obviously, the greater the number of straps, the greater is their strengthening effect. An installer may be provided with a chart or table indicating the number of straps required for any particular span and joist depth.

This apparatus for strengthening floor joists may be installed in the following manner. Firstly a floorboard 3 close to the wall 9 is taken up in order to gain access to the joists, Holes in the joists are then drilled so that the studs may pass therethrough. The two portions 4a and 4b of the telescoping member 4 are pushed together in order that it may be fitted between two joists. One end of the telescoping member is then screwed onto one of the studs. The telescoping member is then extended so that it meets the opposite stud provided in a second adjacent joist. By simultaneously turning and extending the telescoping member the other end of the telescoping member is screwed to an opposite stud. This process is repeated between adjacent pairs of joists under the same floorboard.

Second telescoping members are similarly positioned close to the opposite end of the joist, i.e.

normally close to the wall at the opposite end of the room. Straps are then passed around the two anchoring members between each pair of joists by feeding them under the floorboards. The two ends of the strap are then pulled together and joined together using a suitable tool. This tool overlaps the two ends of the strap and makes a suitable joint at some point where they overlap such as X as shown in Figure 2.

Referring again to Figure 1, the depth of the joists is indicated by A. The distance B between the top of the joist and the centre of one of the studs is variable but for maximum strength is preferably about 0.8 x A. The distance C between the centre of the stud and the bottom edge of the joist should preferably be no less than 40 mm. (If the stud is placed too close to the edge of the joist, there will be very little material between the hole for the stud and the edge of the joist and therefore the joist may be weakened; and a certain safety margin is needed in case of fire burning the bottoms of the joists). Referring now to Figure 2, the distance D between the telescoping anchor and the wall of the building should preferably be a maximum of 450 mm.This might correspond to the second or third floorboard from the wall and the distance should be minimised so that a major proportion of the joist is strengthened. Referring to Figure 3, G represents the distance between the joists, which is typically about 375 mm. It is not desirable to have a large extent of the inner tube portion 4a exposed from the tube portion 4b, since the more of tube 4a which is exposed, the weaker the telescoping member becomes. The distance F by which the portion 4a extends out of the telescoping member 4 should preferably not exceed 75 mm.

The inner tube portion 4a may suitably be formed of steel tube of 32.7 mm outside diameter and 4 mm wall thickness and about 300 mm in length. The outer tube 4a may be about the same length but of 42.4 mm outside diameter and 3.2 mm wall thickness. The plug 6 and the end of the tube 4a may be tapped for 25 mm studs. Suitable straps 7 may be 19 mm x 0.6 mm metal clamping bands.

The embodiment of the invention described above is particularly useful for strengthening the joists of floors prior to filling the gap between the ceiling and the floorboards with a fire retardant material. The apparatus for strengthening the joists is simple to install since only a few floorboards have to be taken up. Thus, the joist-strengthening procedure adds only a little time to the time taken to install the fire retardant material.

The general principle of strengthening a structural member in this manner has many other applications in addition to that described above.

Claims (13)

1. Apparatus for increasing the resistance of a structural member to bending moments, comprising two anchoring members adapted to be secured to the structural member at spaced apart locations along the length thereof and each presenting a generally cylindrical portion extending transversely outwardly of the structural member, and at least one flexible elongate strap adapted to be formed into an elongate loop around the two cylindrical portions, and to be placed in tension, whereby the anchoring members may be positioned relative to a portion of the structural member which is placed in tension under bending moments such that tension in the strap(s) adds to the tension in the structural member to resist the bending moments.

2. Apparatus as claimed in claim 1 wherein the anchoring members each comprise a cylindrical portion mounted at one axial end on a mounting plate which in turn may be screwed or otherwise fixed to the structural member.

3. Apparatus as claimed in claim 1 wherein each anchoring member consists of a rod or tube arranged to be fitted into a hole in the structural member.

4. Apparatus as claimed in claim 3 wherein the hole and rod or tube pass completely through the structural member.

5. Apparatus as claimed in claim 4 wherein a strap is fitted to each side of the structural member.

6. A method of increasing the fire resistance of a floor, comprising fixing an anchoring member between a pair of adjacent joists below the neutral axes and adjacent one end thereof, fixing an anchoring member between the pair of joists below the neutral axes and adjacent the other end thereof, forming at least one elongate flexible strap into an elongate loop around the anchoring members, tensioning the or each strap, and at least partially filling the space between the joists with a fire retardant material which sets around the strap(s).

7. A method as claimed in claim 6 wherein the anchoring members are each formed as a pair of telescoping tubes fitted to screw-threaded studs which pass through holes in the joists.

8. A method as claimed in claim 6 wherein the fire retardant material is installed in a foamed state and sets on drying to a solid honeycomb material.

9. A method as claimed in claim 7, wherein the axially outer end of the outer telescoping tube is provided with a cylindrical plug which is secured therein and serves to reduce the diameter of the end of the tube to the diameter of the narrower inn tube.

10. An apparatus comprising a pair of telescoping tubular members and a pair of screw-threaded studs, the tubular members being provided with internal screw threads at their axially outer ends adapted to mate with the screw-threaded studs.

11. Apparatus for increasing the resistance of a structural member to bending moments substantially as herein described and with reference to the accompanying drawings.

12. A method of increasing the fire resistance of a floor substantially as herein described and with reference to the accompanying drawings.

13. Apparatus comprising a pair of telescoping tubular members and a pair or screw threaded studs substantially as herein described and with reference to the accompanying drawings.