GB2138464A

GB2138464A - Lateral restraint strap for building construction

Info

Publication number: GB2138464A
Application number: GB08408030A
Authority: GB
Inventors: Harold John Andrews
Original assignee: PRESS BAT HOLDINGS Ltd
Current assignee: PRESS BAT HOLDINGS Ltd
Priority date: 1983-04-16
Filing date: 1984-03-28
Publication date: 1984-10-24
Also published as: GB8408030D0; GB2138464B

Abstract

The strap (19), of light weight metal strip or high tensile plastics such as polypropylene, for use in providing lateral restraint to a wall of a building (both in the case where joists run parallel to the wall and where they run perpendicular to the wall), has a disc (21) attached as a laminar bearing member to one end and has a 90 DEG twist (25) at a position spaced from the disc (21) by the thickness of the inner leaf (18) of the cavity wall. This portion of the strap can pass either through a space between adjacent blocks or through a coursing joint of the wall. Where joists (13,14), are parallel to the wall (18), the strap is hammered into the top surfaces and nailed, the free end being hammered down against the side of the further joist (14) and nailed at (27). Where joists are perpendicular to the wall, the strap passes through a coursing joint. The downturned end can give extra strength to the strap fixings. Damage to lightweight block work of the inner leaf (18) is minimised by the large bearing area of the disc (21). It is unnecessary to channel either the blockwork or the joists to use the strap (19) as was necessary with a prior art strap (10). <IMAGE>

Description

SPECIFICATION Lateral restraint strapfor building construction and methods of providing lateral restraintto a building structure This invention relates to restraint straps for building construction and to methods of providing lateral restraintto a building structure.

When the walls of a building are subjected to high winds, side loads are caused which could in severe cases exceed the strength ofthe wall and cause collapse. To prevent such collapse, the practice has arisen of adding restraining straps of metal to the building during construction so asto linktogetherthe uprightwalls and the horizontal "diaphragms" afforded by the floor andlor ceiling platforms. Internal walls are also sometimes linked to the horizontal diaphragms to provide restraint. Such straps are called "lateral restraint straps".

Other straps maybe used vertically to secure the roofstructuretothewallsofthe building butthe present invention is concerned primarily with lateral restraint straps.

When the Building Regulations covering the use of lateral restraint straps were first introduced, certain specific requirements were applied to such straps, which governed for example their size and shape, and the material ofwhich they were made. These requirements were later modified so that the present Building Regulations require only that the strength of lateral restraint straps should be sufficient to meet the design requirements.

Conventional lateral restraint straps are of 30mm.

wide, Smm.thickmild steel and are supplied pre-bent to an L-shape having a short limb of approximately 1 50mm. in length and a long limb of between 650mm.

and 1450mm. in length, depending on the intended use.

For lateral restraint,the short limb abuts the inner leaf masonry in the cavity of a cavity wall and the long limb,afterpassing in a horizontal planethroughthe masonry, is nailed to the joists.

Where the joists are parallel to the wall, the long limb lies in a horizontal plane, the tops of the joists are notchedto receive it and it is nailed in place to at least threejoists.

Where the joists are perpendicularto the wall, the strap is supplied with a preformed 90" twist so that, after it has passed through the wall in a horizontal plane, ittwists to a vertical plane. ltthen lies along an upright side face of a joist to which it is nailed.

This conventional strap has certain disadvantages which the present invention seeks to overcome.

Firstly, it requires a considerable amount of steel, which is used relatively ineffectively. The strength of the conventional strap is considerably greater than the design requirement. It is secured to the joists only by nailing and hence needs to be long enough to span at least three joists to give adequate fixing. Its thickness means that the tops of joists which it crosses must be notched so that it does not stand proud ofthe joists.

Secondly, the rigidity of the conventional strap means that it must be pre-bentto shape and hence the correct number of plain and twisted L-shaped straps needs to be estimated before ordering. Bending on site is not possible. No adjustment can be made for tolerances met in the building itself.

Thirdly, with the increasing use of relativelysoft lightweight blocks for building the inner leaf of a cavity wall, the bearing area abutting the masonry (typically 1 50mm. x 30mm.) may be insufficientto take the imposed load on the strap without causing crumbling ofthe blockwork. In particular, the loading is concentrated at the top edge ofthe block which is abutted by the bend ofthe conventional lateral restraint strap.

Finally, the need to place the strap in a horizontal plane in the masonry may mean that, where large blocks are used, these need to be channelled to allow the strap to be inserted at the correct level to suit the joists.

It is an object of the invention to overcome or reduce some or all of these disadvantages.

According to a first aspect ofthe invention, there is provided a lateral restraint strap comprising, in combination, an elongate strip and a bearing member adapted to be mounted on the elongate strip and to be located at a first end portion thereof.

The elongate strip may have location means at said first end portion to locate the bearing member. For example, the strip may be split lengthwise at said first end portion and the split parts may be bent in opposite senses to locate the bearing member.

The elongate strip may have a 90" twist preformed in the strip at a position near said first end portion, whereby, in use, the bearing member may abut one face of a wall and the twist may lie adjacent the other face of the wall, th rough which the strip passes.

Since the exact position ofthe bearing member along the strip is determined on site, small variations in wall thickness can readily be accommodated.

The bearing member may comprise a rigid laminar member. The laminar bearing member may have a slot adapted to receive the strip as a sliding fit. The slot maybewhollyenclosed bythematerialofthelaminar bearing member.

The invention also provides a method of providing lateral restraint to a building structure comprising an uprightwall and a plurality of joists lying generally parallel to the wall, the method comprising: taking a lateral restraint strap according to thefirst aspect ofthe invention set out above; disposing the lateral restraint strap with its bearing member abutting the face ofthe wall remote from the joists and with the elongate strip firstly passing through an uprightjoint between masonry elements of the wall and then twisting to lie in a generally horizontal plane on a plurality of joists; nailing the strip to the joists on which it lies;; and bending the end of the strip remote from the wall downwardly into abutment with an uprightface ofthefurthermostofsaid plurality ofjoists onto which the strip is nailed.

The strip may be hammered to indent it into the joiststowhich it is nailed, to afford a substantially flush surface.

Preferably, the strip overlies two joists only.

The invention also provides a method of providing lateral restraintto a building structure comprising an uprightwall and a joist lying generally perpendicular to the wall, the method comprising: taking a lateral restraint strap according to the first aspect ofthe invention set out above; disposing the restraint strap with its bearing mem berabuttingtheface of the wall remote from said joist and with theelongate stripfirstly passing through a generally horizontal coursing joint between superimposed courses of masonry elements ofthewall and then twisting to lie in a generally upright plane along a generally uprightface of said joist; and nailing the strip to said uprightface of the joist.

Lateral restraint straps embodying the invention will now be described in more detail, byway of example only, with reference to the accompanying drawings, in which: FIGURE lisa perspective view of part of a building structure showing, by way of comparison, the use of both a conventional lateral restraint strap and a lateral restraint strap embodying the invention in a building structure including joists parallel to a wall; FIGURE2is a side elevational view of a lateral restraint strap embodying the invention in use in a building structure including joists parallel to a wall; FIGURE3 is a perspective view of part of a building structure in which a joist lies perpendicu larto a wall, showing a lateral restraint strap embodying the invention in use.

Referring firstly to Figure 1, a conventionaltype of lateral restraint strap is shown at 10 and comprises a rigid mild steel element of L-shape, having a long limb 11 and a short limb 12 at right angles to the long limb.

The short limb 12 is about 150mm. long, while the length ofthe long limb 11 is selected according to the likely loading, and is equivalent to at least the span across three parallel joists. The conventional strap is provided with groups of nail holes along its entire length. It has a thickness of about 5mm.

The conventional strap 10 is secured to three joists, 13, 14, 15 by nailing through the nail holes 16 provided throughout its length. Since its effectiveness as a restraint depends on the security of th is connection to the joists and since onlythe nails provide the connection, it is necessary to fix to three joists. This determines the minimum length ofthestrap 10.

Atthewall, the strap 10 is shown channelled into the blockwork 18forming the inner leaf at 17, because the top ofthe large blocks does not coincide with the level ofthejoists 13,14, 15. This channelling is undesirable because itweakensthe block.

Where the blockwork is of soft, lightweight thermal insulation blocks, there is a risk of crumbling when a load is applied to the strap, even if the block is not channelled, becauseofthe relatively small bearing area ofthe short limb 12, and also because all the load is transmitted only to one block of the inner leaf 18.

Figure 1 also shows a lateral restraint strap of a type embodying the invention, indicated at 19. The strap 19 is in two parts, which are assembled together on site.

The first part is a strip of a material capable of carrying a substantial tensile load, for example a metal or suitable plastics material In the example shown, the strip 20 is of stainless steel. Another suitable material would be polypropylene. The second part ofthe lateral restraint strap 19 is a laminar bearing member in the form of a disc 21 which abuts againstthe masonry of the inner leaf 18 ofthe cavity wall. The disc 21 has a slot 22 in its centre, which can receive the strip 20 as a sliding fit. Some form of location means is provided at the end ofthe strip 20, to retain the disc 21 when it has been assembled to the strip. For example, when the strip is of metal such as stainless steel, a split may be made lengthwise in its end and the split portions 23 and 24turned in opposite directions to form location means.Where the strip 20 is of polypropylene, location means are moulded on in manufacture.

The strip 20 has a 90" twist 25 formed in it, nearthe end having the location means. The twist is so positioned that it lies adjacent the masonry ofthe inner leaf 18 on the side opposite to the disc21, but within thethickness normally allowedforfinishing the innerface of the wall with plaster, skirting boards and soon.

In a conventional strap, the relatively large crosssection has made it impossible to accomodate a twist in the strap within the normal finishing thickness.

Only when used belowfloorlevelwiththejoists perpendicularto the wall can such a strap be provided with a twist. By reducing the cross-section of the strip 20to match the design load more closely, the advantages of using a twisted strap can also be achieved in structures such as that shown in Figure 2.

It has been found that a strip of sufficient strength can be made in stainless steel with a thickness of only 2mm. and a width of 16mm. The small thickness meansthatthe joists need not be notched to receive the strip 20 which is merely hammered into the top surface to lie flush with the joists. Reference to Figure 1 will showthatthe conventional strap, having a thickness of about 5mm., has had to be sunk into notches in the joists to prevent itfrom lying proud of the joists. This notching weakens the joists and takes up extra time, particularly as several joists have to be notched for each strap.

The strap 19 is positioned so that the disc21 lies in abutmentwith the inner leaf ofthe wall 18. The strip 20 is disposed upright in a joint between adjacent masonry elements ofthe inner leaf 18. The twist lies adjacentthe innerface of the wall,the remainder 26 ofthe strip 20 then lying in a generally horizontal plane on top of the joists 13,14. The strip is nailed to these joists.

The restraint strap is secured to two joists only.

Extra resistance to imposed loads is given by bending down the innerfree end portion 27 ofthe strip against the face ofthe joist 1 4facing awayfrom the wall.

Wherethe strip is of metal,the relative thinness ofthe strip allows itto be bent on site using a hammer.

Tolerances in building construction have made it impossible to prebend the conventional strap, which has therefore had to be held in place only by nailing and hence has needed to span three joists at least. A spacer block 9 between the inner leaf masonry 18 and the joist 13 provides support againstwind loading tending to push the wall inwardlytowardsthejoists.

The strip can be nailed at 27 to the side face ofthe joist 14 if desired. Where it is made of a plastics material, the strip must be nailed to the side face.

The lateral restraint strap 19 may be supplied with the twist 25 preformed and with the split portions 23 and 24 lying in the plane ofthe strip. The disc will be supplied separately for easy transportto the site. On site, the split end portions 23 and 24 will be separated and the disc 21 slid onto the strip 20 from the other end. The slot 22 is of sufficient size to slide freely on the strip 20 and past the twist 25 up to the location means.

A method of using the lateral restraint strap 19 in a building construction where the joists lie parallel to the wall has already been described. In applications where the joists lie perpendicularto the wall, a slightly different method is used.

Referring to Figure 3, the inner leaf 18 of a cavity wall is shown as being made of courses of large blocks. The lateral restraint strap 19 is disposed so thatthe disc 21 lies against the outer face ofthe leaf 18 ofthewall, as before, but the portion ofthestrip 20 between the disc 21 and the twist 25 is passed in a horizontal plane through the coursing joint between adjacent superimposed courses of blockwork, and not vertically as before. At least one coursing joint is likely to occurwithin the depth of a joist 30.

The strip is positioned along the side face 29 ofthe joist 30 and nailed in place using the groups of holes 31. Itwill be appreciated thatthe end ofthestrip is allowed to remain straight along the face 29 of the joist.

In either mode of using the strap 19, it will be seen that no cutting of either joists or masonry is neces- sary. The large bearing area of the disc 21 spreads the loading imposed on the masonry. Furthermore, in either mode of use, the disc always bears against at least two, and sometimes more than two ofthe masonry elements of the wall, giving added security.

It will be appreciated thatthe strap uses a reduced amount of material, compared with a conventional strap, when made in metal, partly because its fixing is more secure than hitherto and enables its length to be reduced to span onlytwo instead of three joists. The thinner section and narrowerwidth of the strip 20 also give rise to economy in material.

Time and labour costs may also be reduced because the new lateral restraint strap 19 does not require channelling of either blockwork orjoists for fitmentin a building structure.

Claims

1. Alateral restraintstrapcomprising,incom- bination, an elongate strip and a bearing member adapted to be mounted on the elongate strip and to be located at a first end portion thereof.

2. A lateral restraintstrap according to claim 1 wherein the elongate strip has location means at said first end portion to locate the bearing member.

3. A lateral restraint strap according to claim 2 wherein the strip is split lengthwise at said first end portion and the split parts are bent in opposite senses to locate the bearing member.

4. Alateral restraintstrap according to any preceding claim wherein the elongate strip has a 90 twist pre-formed in the strip at a position near said first end portion whereby, in use, the bearing member may abut one face of a wall and the twist may lie adjacent the otherface of the wall through which the strip passes.

5. Alateral restraintstrap according to any preceding claim wherein the bearing member comprises a rigid laminar member.

6. A lateral restraint strap according to claim 5 wherein the laminar bearing member has a slot adaptedto receivethe strip as a sliding fit.

7. A lateral restraint strap according to claim 6 wherein the slot is wholly enclosed bythe material of the laminar bearing member.

8. A method of providing lateral restrainttoa building structure comprising an uprightwall and a plurality of joists lying generally parallel to the wall, the method comprising: taking a lateral restraint strap according to any one of claims 4to 7; disposing the lateral restraint strap with its bearing member abutting the face of the wall remote from the joists and with the elongate strip firstly passing through an upright joint between masonry elements ofthe wall and then twisting to lie in a generally horizontal plane on a pluralityofjoists; nailing the strip to the joists on which it lies; and bending the end of the strip remote from the wall downwardly into abutment with an uprightface ofthefurthermostofsaid plurality of joists onto which the strip is nailed.

9. A method according to claim 8 wherein the strip is hammered to indent it into the joists to which it is nailed, to afford a substantiallyflush surface.

10. A method according to claim 8 or claim 9 wherein the strip overlies two joists only.

11. Amethodofproviding lateral restrainttoa building structure comprising an uprightwall and a joist lying generally perpendicularto the wall, the method comprising: taking a lateral restraint strap according to any one ofclaims4to7 disposing the restraint strap with its bearing member abutting the face of the wall remotefrom said joist and with the elongate strip firstly passing through a generally horizontal coursing joint between superimposed courses of masonry elements ofthewall and then twisting to lie in a generally upright plane along a generally upright face of said joist; and nailing the strip to said uprightface ofthe joist.

12. Anovel lateral restraint strap substantiallyas hereinbefore described with reference to and as illustrated in the accompanying drawings.

13. A method of providing lateral restrainttoa building structure according to claim 8 and substantially as hereinbefore described with reference to Figures 1 and 2 ofthe accompanying drawings.

14. A method of providing lateral restrainttoa building structure according to claim 11 and substantial ly as hereinbefore described with reference to and as illustrated in Figure 3 of the accompanying drawings.