PERFORMANCE STANDARDS FOR WALL TIES

Lawrie R. Baker *

1. ABSTRACT

The behaviour of various types of masoruy waIl systems is described and used to extract performance requirements for wall ties in the various systems. Quantitative performance specifications are then suggested for the waIl ties, using the results of standard tests on smaIl specimens representative of the waIl system to classify the tie. It is maintained that the classification allows masoruy codes to rationaIly specify the use of waIl ties in a range of practical constructions.

2. INTRODUCTION

Masonry connectors1 are important mechanical components used in masonry construction to attach the various building elements together. They include anchors­used to connect masoruy waIls at their intersections with other masoruy waIls and structural members, fasteners- used to connect attachments to the masoruy wall, and wall ties- used to connect wythes of masoruy together or a masoruy veneer to a backup wall. Tlús paper deals only with the latter type of connector and attempts to set out performance standards for them.

Clearly the performance standards for wall ties cannot be set without an understanding of their behaviour in practical masoruy constructions. Never-the-less the performance standards finally adopted must be specified independently of practical construction so that a particular waIl tie can be simply tested and classified by a standard. Suitable clauses and rules for the use of the classified wall tie in practice can then be incorporated in a standard for masoruy construction. It is important to clear1y distinguish between the specification of the waIl tie itself and the specification for its use in practice.

The behaviour of wall ties in various practical constructions is first considered, performance requirements then extracted and finally some performance standards suggested

* Professor and Head, School ofEngineering and Technology, Deakin University, Geelong, Australia.

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3. MASONRY WALL BEHAVIOUR

Potentially the wall ties between two wythes of masonry or between a masonry wythe and a backup wall can influence the resistance of the wall system to out-of-plane flexure, in-plane shear, and axial compression. The degree of influence on wall behaviour depends on the type of wall tie used. In this sense there are two extreme types of tie, namely the jZexible wall tie and the rigid wall tie.

Theoretically the flexible wall tie has infinite axial stiffuess (that is, in the direction of its length) but zero flexural stiffuess. Hence this tie forces both wythes to laterally deflect by equal amounts at the tie locations and transrnits lateral force between wythes, but cannot transmit shear between the wythes. These flexible wall ties generally increase the resistance of wall systems to flexure because of this load sharing between wythes. In general such ties do not increase the compressive strength of the wall system because there is no shear transfer between the wythes. There may be some increased compressive strength if both wythes carry vertical load as the axial stiffness ofthe wall ties may give increased lateral support to the wythe that tends to buckle the greater amount. If only one wythe is loaded however the unloaded wythe is unlikely to have sufficient lateral strength to increase the buckling resistance of the loaded leaf via the wall tie. Hence flexible wall ties increase the flexural strength of the wall system but not its compressive nor shear strengths.

This theoretical flexible tie is approximated in practice by the commonly used metal strip and wire ties which have substantial strength and stiffuess in the direction of their length but flexibility in bending and therefore cannot transrnit significant shear between the wythes. Another form of flexible tie incorporates sliding or rotating parts which provide axial stiffuess but permit free shearing displacements over a limited distance.

At the other extreme, the rigid tie has theoretically infinite stiffuess in resisting both axial and shearing movements. Hence this tie ensures the integral action of the wall system resulting in increased strengths in bending , shear, and compression. In practice this action is often obtained by using header bricks of brick webs between wythes . Only specially produced metal ties will achieve full composite action of a cavity wall but some of the normally produced heavy metal ties will achieve some degree of composite action. The behaviour of various wall systems using both flexible and rigid wall ties will now be considered.

3.1 Cavity Walls

Cavity walls consist of two wythes of masonry separated by an air space or cavity which inhibits the passage of water from the exterior wythe to the internai wythe. The two wythes may be similar or dissimilar, commonly consisting of an externai c1ay masonry exterior and a concrete masonry interior. Wall ties ensure that both wythes act together to resist lateral loads but the detailed behaviour is strongly dependent on whether flexible or rigid ties are used.

Flexible Ties When infinitely flexible ties are used the lateral load is shared between the wythes in proportion to their individual flexural stiffuesses and the flexural stresses in a wythe are computed using the section modulus of that wythe. For the wall to be

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structuraIly effective as a cavity waIl the strengths and stiffuesses of the two wythes must be of the same order. If one wythe, for example, has a stiffuess double that of the other wythe it wiIl resist 89% of the total load leaving the other wythe to resist only 11 % of the load. Little benefit in load carrying capacity is achieved by the combined action of the wythes in such a cavity wall.

Differential in-plane movements between the two wythes may result from environrnental factors such as long term moisture movements, temperature expansion, wetting and drying etc. Since the flexible ties cannot transfer shear between wythes these movements are free to take place provided adequate control joints are incorporated in the masonry and fitments such as windows and doors do not interfere with such movements. Differential in-plane movements can also occur when the waIl system acts as a shear waIl to resist wind or earthquake. Since the waIl ties cannot resist shear, each wythe must be capable of resisting any shear force applied to it. The shear movement of the wythes from this source wiIl be smaIl because of the large values of in-plane waIl stiffuess associated with masonry. Flexible waIl ties must therefore be capable of accommodating the differential in-plane movements between wythes that result from both environrnental factors and shear. For cavity waIls these relative movements are likely to be smaIl, say 10mm.

Rigid Ties When rigid ties are used the two wythes are said to be coupled and the waIl system acts compositely. Hence the full section properties of the combined waIl system are used in computing stresses from bending, shear and compression. On this basis there is a strong incentive to use rigid ties rather than flexible ties because they can increase the lateral load resisting capability of the cavity wall by a factor ranging from about 3 to 5. (Based on the ratio of composite section modulus to independent section modulus for common cavity walls).

Unfortunately, the rigidity of the ties which produces this enhancement also prevents the in-plane differential movement between wythes that tends to occur from the environrnental factors mentioned before. As a result internai stresses are induced in the waIl system in proportion to the suppressed movements. Unless these suppressed movements are very small it is likely that the induced tensile stresses wiIl lead to failure of the masonry. The use of rigid wall ties in cavity waIls is therefore severely limited in practice to cases where both wythes have very similar material properties (ie effectively constructed from the same masonry unit and mortar), are single storey buildings (to limit the accumulation of differential movement over the height of the wall), and do not have greatly differing environrnental conditions on the exterior and interior faces of the waIl system. Even in these instances the induced stresses concentrated at the points of anchorage of the rigid wall ties in the masonry (particularly near the top of the waIl) may cause rupturing of the masonry. This situation is alleviated somewhat if the wall ties are not infinitely rigid, but then the behaviour of the waIl is difficult to evaluate.

These considerations indicate that it is not desirable to specify rigid waIl ties for use in cavity walls and that in the special cases where composite action may be warranted the tying of the two wythes should be by masonry webs, as in the diaphragm waIl, thus eliminating flexural stress concentrations at waIl ties.

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3.2 Composite Walls

Here, a composite wall is defined as a masonry wall system comprised of two wythes separated by a mortar fiUed collar joint approximately 10mm wide, or by a wider grout filled cavity. The composite action of the two wythes is due to the shear strength generated in the collar joint from a combination of bond/friction at the interfaces and the shear resistance of wall ties across the collar joint. Friction at the interface is facilitated both by the pressing action of any applied lateral load and by tension induced in the wall ties as the interface tends to slip. The wall ties in this case must therefore have adequate tensile strength and shear strength. Since the ties are confined by the mortar in the collar joint or by the grout in the cavity their compressive and bending strengths are irrelevant and it is not necessary to distinguish the tie as flexible or rigid. Also such ties do not have to prevent the passage of moisture across the collar joint or grouted cavity.

3.3 Veneer Walls

There are two types of masonry veneer wall: one conslstmg of an ou ter wythe of masonry attached to a stiffer inner wythe of masonry or concrete wall, the other an outer wythe of masonry attached to a more flexible inner structural wall constructed of timber or steel studs. In neither case is the veneer expected to resist applied compressive loads. These types are distinguished according to the relative stiffuess of the inner wall as being either a rigid backup wall or a flexible backup wall . In either case the argument given in 3.1 against the use of rigid wall ties applies and only flexible ties need be considered.

Rigid Backup This situation is very similar to the cavity wall constructed with flexible ties as considered in 3.1. the difference is that the outer veneer is so flexible compared with the rigid backup that it resists insignificant lateral load andlor that it is so weak as to fail in flexure before ultimate resistance of the combined wall system is reached. In both cases the design philosophy used is to design the backup wall to resist the fulllateralload. Wall ties in this case are not required to have a high axial stiffness to facilitate load sharing between the wythes. The tie must simply have sufficient strength in tension and compression to transmit lateral load applied on the veneer to the backup wa11. As with the cavity wall the two wythes of masonry should be designed to resist in-plane shears independently.

F/exible Backup In this case the veneer is stiffer than the backup and hence resists the greater portion of the lateral load applied to the wall system (assurning axially stiff wall ties are used) . Usually the strength of the veneer in flexure is not great and it cracks, perhaps several times, before ultimate lateral load capacity of the wall system is reached. With each phase of cracking of the veneer more load applied on the veneer is progressively shed to the backup wall . Ultimately the share of load resisted by the veneer may be insignificant and design philosophy is to assume that ali lateral load is resisted by the backup alone. Again in this case the wall ties are not required to have a high axial stiffuess to facilitate load sharing between the veneer and the backup wall . They must simply be strong enough in both tension and compression to transmit load applied on the veneer to the backup.

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There is however a special problem that can occur due to racking action in areas of high wind or seismic activity. This has been pointed out by Lapish2 in relation to brick veneer on timber framed houses subjected to earthquakes in New Zealand. Horizontal seismic forces besides causing out-of-plane flexure in the walls normal to the forces also cause shear in the walls aligned with the forces . While the shear displacement at the top of the veneer may only be a few mm. it is reported that the less stiff backup timber stud walls may experience displacements in the order of 40mm. at the top plate leveI. Such differential in-plane movements cannot be accommodated by the commonly produced strip metal or wire tie. Hence damage often occurs by vertical cracking of the veneer at the comers of the house to accommodate the racking rotation of the backup wall. Also the wall ties in the racking wall may fail in bending or cause the veneer to fail in shear or a combination of both. The seismic shearing forces on the timber frame are worst when it supports a high roof constructed of heavy materiais such as tiles. The solution to the problem includes providing control joints in the masonry veneer at comers, ensuring that fitments such as door and window frames allow differential shear movement between the veneer and backup to occur, and by reducing the shear flexibility of the backup. As far as the wall ties are concemed they must be capable of accommodating the expected differential in-plane movement without causing excessive damage. In severe cases this may mean that special wall ties with sliding sections are necessary. Similar behaviour to that described above could be expected in low rise commercial buildings constructed in masonry veneer.

4 . PERFORMANCE REQUIREMENTS FOR W ALL TIES

From these brief descriptions of wall behaviour the range of performance requirements of wall ties can be extracted. Firstly, it is c1ear that flexible wall ties are desirable because of the many indeterminate factors associated with rigid wall tie behaviour. It would seem reasonable therefore that a wall tie standard should only give specific requirements of flexible wall ties leaving designers to cope with the complexities of rigid wall ties where required in special cases. Hence standard wall ties are not required to transmit shear across a cavity.

By far the greatest structural use ofwall ties is to transfer lateralload from one wythe of masonry to another or to a backup wall. Wall ties generally must therefore have appropriate strengths and stiffuesses in tension and compression however ties for composite walls do not have to resist compression. Since a veneer does not share in resisting lateralload, the ties for veneer walls may be generally less stiff and less strong than for cavity walls. While wall ties for composite walls do not have to accommodate any shear displacements between wythes, those for cavity walls must accommodate a medium amount, while in extreme cases the ties used in veneer construction with a flexible backup must accommodate a large amount of movement. Having regard for the poor performances recorded for some ties in resisting corrosion and the difficulty of replacing such ties it is prudent to insist on ali ties at least having a medium corrosion resistance but a high corrosion resistance in severe environmental locations. Ali ties that span an air cavity should resist water transfer across the cavity, although this requirement becomes less important in construction where insulation and an air barrier are used in the cavity. A summary of these minimum requirements for each type ofwall system is given in Table 1.

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Flexible Wall Tie - Minimum Requirements Performance Composite Walls Veneer Walls Requirement Cavity

Walls çollar W°l,lt\jd ~~h~up ~exkble joint aVlt ac up Axial Stiffness

Tension low • • • • medium • high •

Compression low • • medium • high •

Axial Strength

Tension low • • • medium • • • • • high • •

Compression low • • medium • • • high •

Shear low • • • Movement medium •

high • Water Transfer Resistance • • •

Corrosion Resistance medium • • • • • high • • •

Table 1. Mirumum Performance Requirements for Flexible Wall Ties

In addition to these, there are other requirements that all ties must possesso They must have adequate anchorage into the masonry or onto a backup wall to resist tensile and compressive forces in the ties and to allow the required shear movements to take place without disrupting the masonry or the tixings. They must have the appropriate degree of tire resistance for the application. They must also be capable of being safely installed which generally means that they have no sharp edges and/or are installed in two stages.

5. PERFORMANCE STANDARDS FOR WALL TIES

It remains to quantify the performance requirements in Table 1 and to specify standard tests so that a given wall tie can be c1assified according to the standard. Three types of tests are called for. Firstly, a test to measure the axial strength and stiffness in tension and compression. This test can be devised so that it also ensures the anchorages of the tie at both its ends are satisfactory and that the shear movement requirement (at least for the low case) is achieved. Secondly, a test to ensure that the tie has adequate resistance to water transfer. Thirdly, a test to establish the corrosion resistance or

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durability ofthe tie. Each ofthese tests will be considered in tum. In devising the tests the philosophy of using small specimens that reproduce practical conditions as far as possible has been adopted. This was the approach adopted for the Australian wall tie standard in 19843 but use of that standard over the past decade has indicated that some simplifications of practical construction incorporated in it did not give a good indication of actual performance. For example, it is not possible to correctly assess the water transfer resistance of a tie unless it is built into a specimen simulating the actual construction. Hence it is suggested that specimens used in the following three tests should ali consist of a tie installed as used in practice, one end anchored in a masonry couplet and the other end anchored into a masonry couplet, or fixed to a backup wall as appropriate. Examples are shown below.

(a) Cavity Wall and Composite Wall

(b) Veneer Wall stud backup

(c) Veneer Wall rigid backup

In each case the cavity distance should correspond to that for which the tie is designed. For the composite wall case the cavity need not be mortar or grout filled . Also the fixings specified by the manufacturer should be used in the attachment to the backup It should also be pointed out that although the actual values nominated in the following sections are largely based on the existing Australian Standard, they are by no means firrnly established. They are however achievable by the better quality ties manufactured in Australia which have performed satisfactorily.

5.1 Axial Strength and Stiffuess

To test axial strength and stiffuess the specimen is set up as follows • c1amp one brick of the couplet representing the outer wythe to the bottom platen ofthe tension/compression testing machine • c1amp one brick of the inner wythe, or stud, or section of backup to the top platen (if a metal stud is used it is c1amped to the platen along its top flange only) • slide the top c1amp without rotation and without inducing tensile force into the tie as follows:

cavity wall composite wall veneer wall

'vertically' lOmm

O IOmm

'horizontally' IOmm

O lOmm

Note: 'vertical' and 'horizontal' refer to the directions in the plane ofthe practical walL

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The specimen should now be tested in tension or compression in the lateralIy displaced position, recording the axial load - axial displacement graph until failure . At least five but preferably fiom ten to fifteen specimens should be tested in tension and compression (except for composite walIs where only tension tests are required) . Characteristic strengths and stiffuesses are then calculated using statistical methods.

Strength: The ultimate strength of an individual test should be taken as the load at an axial displacement of 1.5mm or the greatest load recorded if it occurs before a displacement of 1.5mm. Any fiee movement in adjustable ties should be included in the 1.5mm deflection limit. Such slack movement indicates poor tie design and should not be encouraged. This limit is imposed to prevent excessive rei ative deflections between wythes even under failure conditions. The strength of the ties can be classified as low, medium, or high if the characteristic strength of the sample for both tension and compression exceeds certain values, for example:

Strength Classification Characteristic Strength (minimum) low 0.3 kN medium 0.6 kN high 1.2 kN

Such a classification is useful for designers when specifying ties to be used but the actual characteristic strength should also be reported so that designers may take fulI advantage of the strength of a specific tie.

Stiffness: The stiffuess of an individual tie can be measured fiom the load -displacement graph ofthe test for that tie and having found the stiffuess for each tie the characteristic stiffuess of the sample can be calculated. The problem to be decided is which portion of the graph is to be used to measure the stiffuess and should it be a tangent or secant value. From the experience oftesting many types ofties over the past decade it is suggested that

• the measurement should commence fiom the origin of the graph. (ie. any slack in the tie or fixing should be included in the displacement. ) • a secant rather than a tangent value should be used as it is the stiffness over a range ofload that determines behaviour rather than the initial value. • the secant should be measured to a characteristic strength value as this is the strength range used in designo • two characteristic strength values should be used to determine two stiffnesses, firstly the calculated characteristic strength value and secondly, the appropriate strength classification value listed before.

The first stiffuess alIows the designer to use the actual measured characteristic stiffness ( and strength) of a specific walI tie, while the second stiffuess alIows classification of the tie as having low, medi um, or high stiffuess. For example:

Stiffuess Classification Characteristic Stiffness (minimum) low 0.5 kN/mm medi um 1.0 kN/mm high 2.0 kN/mm

Although it is possible to specify ties having various combinations of strength and stiffuess classifications it is preferable to standardise on a limited number such as:

Light duty - a tie having low strength and stiffuess classifications Medium duty- a tie having medium strength and stiffuess classifications Heavy duty- a tie having high strength and stiffuess classifications

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5.2 Water Transfer

The resistance of a wall tie to the transfer of water across its length can be established by testing only a small sample of say three ties . Further econollÚes can be made by installing the three ties in the one specimen described before. For this test the ties should be dipped in a cement slurry and the coating allowed to dry prior to installation in the specimen. This coating is to negate the effect of any oil or grease left on the tie from the manufacturing processo A special rig is required to set up the specimen for testing as follows:

• clamp the test specimen by the couplet representing the outer wythe such that the cavity surface is flush with a roughened vertical surface simulating the masonry wall above and below the couplet. (The roughened surface is to ensure an even flow of water later in the test and may be obtained by covering hardboard with a sheet of fine sand paper.)

c\amp the couplet, backup, or stud section at the other end of the tie such that the tie is in its normal installed orientation. • displace this inner clamp vertically downwards a distance of 10mm without allowing rotation of the couplet, backup, or stud section. • allow water to flow evenly down the vertical rough surface and the cavity face ofthe outer wythe couplet at 50 rnL/s per metre of distribution width for I IlÚnute.

If no water is transllÚtted across any of the three ties then the tie is classified as resistant to water transfer. If the tie has no specified top face then the test should be repeated with the ties installed in an inverse position.

Water

5.3 Durability

Usually one speaks of corrosion resistance of wall ties but to allow for non-metallic ties one should generally use the term durability. Ali wall tie codes presently specify the corrosion resistance of a tie in terms of the composition of the material from which it is made or by the nature and amount of protective coating applied . With worldwide concern4 for the corrosion of wall ties and the lack of authoritative guidance on the durability of specific wall ties it is essential that a standard durability test be devised . Following previous philosophy , the durability test should be done on wall ties installed in masonry specimens. This is essential as prelillÚnary testing has shown that corrosion within the mortar joint can be 14 times higher than within the air space of the cavity,5

and accelerated corrosion can occur at points on the tie where water is trapped. It is not the intention here to specify a standard test to classify the durability of wall ties as such a task is difficult though not impossible using our present state of knowledge.

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Such a test may involve: placing the specimen in a carbonising chamber for, say, three weeks exposing the specimen to accelerated enYÍronrnental conditions in aclimate

chamber over, say, a six week period. • maintaining a specified temperature (eg 30°C) and rei ative hurnidity (eg 90%) for three days prior to and over the six week period ofthe testo • spraying for a given time (eg lO rninutes) with a given concentration of salt water at the beginning of each week. • assessing the degradation ofthe tie and c1assifying its durability.

The c1assification cri teria of metal ties will be in terms of degree of corrosion whereas that of non-metallic ties would have to be in terms of embrittlement, delarnination etc.

5.4 Other Tests

Other tests may be desirable in special circumstances. In seisrnic and high wind areas a test is desirable to ensure that medi um and high shear displacements can be accommodated. The strength and stiffness tests described above are for ties requiring only a low degree of shear movement defined by:

Shear Movement Classification low medium high

Horizontal Displacement 10mm 20mm 40mm

While a flexible strip or wire tie can satisfy the low and possibly the medium c1assification, the high c1assification could only be achieved with ties having sliding or rotating parts. Fire resistance tests may also be necessary to give a finer c1assification than simply regarding metallic ties as fire resistant and others as non-resistant.

6. CONCLUSIONS

This paper presents an outline of the performance specifications in quantitative terms necessary to c1assify wall ties as being appropriate for use in various masonry wall systems. These specifications:

(a) are distinct from specifications goveming the use ofthe ties in construction. (b) are based on the action of masonry wall systems subjected to lateral loads. (c) require standard tests on representative small specimens, containing a wall tie.

I Drysdale R.G., Hamid AA, Baker LR., "Masoruy Structures. Behaviour and Design, Prentice Hall , New Jersey 1993. 2 Lapish E.B., "Aseismic Designs ofBrick Veneer and the New Zealand Building Codes", Pacific Conference on Earthquake Engineering,New Zealand, Nov. 91 . 3 Standards Association of Australia,AS 2699-1984; Wall Ties for Masoruy Construction, SAA, North Sydney, N.S.W., 1984. 4 Maurenbrecher A.,Brousseau R. ,"Review of Corrosion Resistance ofMetal Components in Masonry Cladding on Buildings," Repor! No. 640 National Research Council Canada, Ottawa, Feb. 1993 . 5 Cole I.S, Ganther W., Linardakis A , "Corrosion ofBuilding Materiais In Tropical Environrnents: Prediction and Testing",Eighth Asian-Pacific Corrosion Control Conference,Dec. 1993.

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