VERTICAL MOVEMENTS IN STONE MASONRY FACING WALLS.

Arnold W. Hendry

1. ABSTRACT.

Available information on factors influencing the vertical movements in a stone masonry wall forming the outer leaf of a cavity wall is summarised. Values are suggested for elastic, creep, moisture and thermal effects and used in an illustrative caleulation of movements in an eight storey wall. The results are diseussed in relation to differential movements between the masonry wall and the main strueture.

2. INTRODUCTION

Although stone masonry is seldom used for load bearing walls in modern buildings of any size, it has been used reeently for the outer leaf of eavity eonstruetion in a number of multi-storey buildings in Seotland, the appearance of whieh has been required to harmonise with that of old buildings in elose proximity. As an example, Fig.1 shows the Seandic Crown Hotel in Edinburgh whieh is a large panel eonerete strueture, ehosen for speed of eonstruction, loeated in a historie street of traditional buildings. The architeet required that the masonry facing wall should have the appearanee of solid masonry without being broken up by horizontal joints.

In the design of eavity walls it is neeessary to take into aecount differential movements between the outer leaf and the inner wall to avoid loosening of the ties or fixtures between them and to avoid problems at windows and eaves. Some eodes of praetice aehieve this by putting a limit on the uninterrupted height of the outer leaf. Thus the British Code, BS 5628 Part 1 [1], states that the outer leaf of an

Keywords: Masonry; Stone; Cavity walls; Movements

Professor Emeritus, Department of Civil Engineering, University of Edinburgh, Seotland, EH9 3JL.

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externai cavity wall should be supported at intervals of not more than three storeys or 9m (12m in a four storey building). When as in the example quoted above, it is desired to exceed these limits, the alternative is to estimate the relative movement between the inner wall and the outer leaf and to provide ties and details which will permit such movement to take place without resulting in damage to the construction.

Fig.1

The causes of movements in brickwork and have been discussed in some detail by

[2]. It is the purpose of this paper particular case of stone masonry for comparatively less published data.

3. MOISTURE MOVEMENTS

blockwork walls Fenton and Suter to consider the

which there is

Dimensional changes take place in masonry materiais with change in moisture contento These may be irreversible following manufacture - thus clay bricks show an expansion after manufacture whilst concrete and and calcium silicate products are characterised by shrinkage. There is very little information about irreversible shrinkage in natural stone although one reference [3J states that it does not existo Ali types of masonry, however, exhibit reversible expansion or shrinkage with change in moisture content and references 4 and 5 give values of 0.002 - 0.01% for

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for limestones and 0.01 - 0.07% for sandstones . For mortars the same sources quote a range of 0.02 - 0.10%. The extent of moisture movement of mortar depends on the cement content and for the relatively weak mortars likely to be used in natural stone masonry, the drying shrinkage over a period of six months may be of the order of 0.03% [5].

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g 0·OG5

~ 0 ·010 (U

~ '0·015 UJ'

50 100 150 ../

Oays f rOm Const ruct ion

Fig.2 Moisture movement in a sandstone masonry panel.

Measurements at Edinburgh University on a stone masonry test panel built of a carboniferous sandstone set in 1:2:9 cement:lime:sand mortar produced the result shown in Fig.2. The overall dimensions of the panel were 2 x 1.42 x 0.15m. with average joint thickness of 15.7mm. Measurements were taken on horizontal sections over which the proportion of mortar jointing was approximately 5%. Shrinkage appeared to stabilise at slightly over 0.01% some three months after building. At 160 days, the panel was sprayed on one side with water at relatively low pressure for about 15 minutes. This produced an immediate expansion beyond the initial dimension followed by drying out and shrinkage over a further period of 100 days.

It is not possible to draw general conclusions from such a limited experiment but considering the length of time required for the masonry to dry out even in the laboratory it would seem likely that, exposed to outdoor conditions with periodic wetting by rain, the variation in moisture content and the resulting dimensional changes would be a good deal smaller than suggested by the laboratory test figures for the component materiaIs. Thus it would appear thatthe values given in references 3 and 4 and in codes of practice may be regarded as conservative as a basis for estimating moisture movements in stone masonry walls.

4. ELASTIC AND CREEP MOVEMENTS

Estimation of these movements requires knowledge of both the elastic and creep properties of the masonry and again experimental data is scarce. The results of a number of short term compressive tests on rubble masonry piers are summarised in Table 1. The piers were built in either 1:2:9

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or 1:3:12 mortar and the stone strength lay in the range 35 to 187N/mm2. Each pier was five courses in height and the joint thickness was approximately 20mm. The relation between stone strength and masonry strength is shown in Fig.3. There is considerable scatter in the results the causes of which are not known in detail but which may include irregularity of stone shape and the difficulty of ensuring that the joints are completely filled. An approximate lower bound relationship between stone strength and masonry strength is indicated.

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100

Stone .Strength-N/mm 2

Fig.3 Compressive strength of stone masonry.

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The E values quoted were measured on prisms built of the same materiais as the piers of approximate dimensions 260x180x350mm (wxtxh). The results are equally scattered but generally increase with masonry strength. It is common practice [6] to estimate the elastic modulus of masonry by multiplying the compressive strength by an appropriate factor. It is not possible on the basis of the present results to suggest a defini tive value for such a factor but multiplying the compressive strength (fc) from Fig.3 by 300 gives the estimated value for E shown in the last column of Table 1. Correspondence with the measured results is very rough but in most cases gives a conservative figure of the right order.

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TABLE L

Results of Compressive Tests on Stone Masonry Specimens.

Stone Stone Mortar Masonry E E/fm Eest strength mixo strength(fm)(meas.) (fcx300)

N/mm2 C:L:S N/mm2 N/mm2 N/mm2

Limestone 31 1: 2: 9 4.88

Sandstone 35 1: 3: 12 4.09 1346 329 1200

1:3:12 4.36

39 1:2:9 5.36

49 1:2:9 5.96 4221 708 1530 1: 2: 9 6.38 2306 361 1530 1: 3: 12 7.14 2162 302 1530 1:3:12 7.07

65 1:2:9 10.22 3184 311 2100 11.16 3448 308 2100

83 1: 2: 9 10.98 2940 267 2700 1: 2: 9 10.14

87 1: 2: 9 11.80 2274 192 3000

Granite 131 1: 2: 9 12.32

Whinstone 168 1: 2: 9 9.86 2734 277 3000

Average dimensions of piers: 775 x 410 x 930mm (w x t x h) Number of courses : 5 Average stone size: 360 x 200 x 160mm Average joint thickness: 20mm

No test results are known for ashlar masonry but it has been suggested [7] that thin jointed masonry built with accurately dimensioned stones could be treated in the same way as concrete blockwork using appropriate code values for masonry strength. This should result in somewhat higher strengths than for rubble masonry but information is lacking as to the value of the elastic modulus of ashlar masonry.

No measurements of creep in stone masonry are available so that allowance for this effect must be arbitrary. Various formulae have been developed for the estimation of creep in clay brick [8] and concrete block [9] masonry but in the absence of experimental data there would appear to be little point in using anything more elaborate than the provision in BS.5628 Part 2 (6) which states that creep deformations may

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be assumed numerically to be between 1.5 and 3.0 times the elastic deformation, depending on the type of masonry.

5. MOVEMENT DUE TO TEMPERATURE CHANGE.

Approximate values for the coefficient of thermal expansion are given in reference 4. For limestones the range is 3 - 4 and for sandstones 7 - 16 x 10 per degree C. The value given for mortar is similar to that for sandstones. Calculation of the thermal movement of a wall is, however, rather complicated as this depends not only on the coefficient of expansion but also on other thermal properties such as absorptivity and capacity, incident solar radiation and the temperature at the time of construction. Fenton and Suter have developed a computer program for the calculation of wall temperatures to be used in estimating thermal movements [2]. This, however, requires the input of an amount of data which may not be available and approximate values for service temperature ranges have been published. Thus for the United Kingdom the Building Research Establishment has suggested for heavyweight walling of light colour a minimum of -20 C and a maximum of 50 C [4]. The base temperature proposed for calculations is 10 C.

6. EXAMPLE OF CALCULATION OF VERTICAL MOVEMENT.

To illustrate the approximate calculation of vertical movements in a 150mm thick rubble masonry outer leaf and to examine the relative importance of the various effects, we may consider a wall 24m in height having eight equa l storeys. The assumptions made will be stated under each heading.

6.1 Moisture movements.

Irreversible shrinkage of stone - nil, of mortar 0.07% Average height of stones 200mm. Thickness of mortar joints 15mm Proportion of mortar 7.5% Shrinkage of masonry 0.07xO.075 = 0.00525% 3

Shrinkage in height of wall 0.0000525 x 24xl0 = 1.26mm Reversible moisture movement +/- 0 . 04% Moisture movement over height of wall 0.0004x24x1~ =9.6mm Actual movement depends on moisture content at time of construction. Assuming initial 50% saturation at this stage, movement may be +/- 4.8mm.

6.2 Elastic and creep movements.

Unit weight of stone 2100kg/m3 (= 21.4kN/m3) Compressive strength of stone 70N/mm2 Compressive strength of masonry 7.0N/mm2 Elastic modulus 2100N/mm2 Creep deformation 1.5 x elastic deformation

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TABLE 2.

Elastic and Creep Deformations.

Storey Average Strain Compression stress in storey

kN/m2 10 mm

8 56.1 26.7 0.08

7 168.4 80.2 0.24

6 280.7 133.7 0.40

5 393.0 187.1 0.56

4 505.3 240.6 0.72

3 617.5 294.0 0.88

2 729.8 347.5 1. 04

1 842.2 401. O 1. 20

6.3 Thermal movement.

Assumed temperature at construction Minimum mean temperature of wall Maximum mean temperature of wall

, Cumulative

comp. mm

4.96

4.88

4.64

4.24

3.68

2.96

2.08

1. 20

100 C _200 C +50c

C

Creep comp.

mm

7.44

7.32

6.96

6.36

5.52

4.44

3.12

2.25

Range in service from 10 e C -30 degrees +40 degrees Coefficient of thermal expansion 10x10-b per degree C.

-b Overall contraction of wall 30x10x10 Overall expansion of wall 40x10x10-b

x24x103

~ x24x10 = 7.2mm =12.8mm

The maximum movement at the top of the wall due to the sum of these effects is shown in Table 3. It has been pointed out [10] that moisture and thermal movements will not be additive as increase in temperature will be accompanied by a reduction in moisture contento It is, however, not practicable to quantify this interaction and addition is conservative in the sense of representing a worst case.

7. DIFFERENTIAL MOVEMENTS.

The main purpose of this paper is to discuss vertical movement in stone masonry walls forming the outer leaf or cladding of a building the main structure of which is of a material having different properties and subject to different environmental conditions. In this discussion it is assumed that for aesthetic reasons the stone masonry wall supports its own weight from the foundation to the top of the building and is stabilised by ties to the structure

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TABLE 3.

Maximum movements at top of walls from construction.

Stone masonry outer wall

Irreversible movement - 1.3 Reversible moisture mvmt - 4.8 Elastic deformation - 5.0 Creep - 7.0 Thermal movement - 7.2

-25.3mm

Brickwork inner wall

+9.6

-0.8 -1.2 +4.8

+12.4mm

which are capable of transmitting tension or compression but not shear. The vertical movement of the masonry is assumed unrestrained and the ties must be capable of accommodating this movement without distress. What is important is of course the relative movement of the elements at each leveI of the building.

If the main structure is a steel frame, the only significant movement in this will be the result of temperature change from that assumed at construction to a maximum during service. In the UK this range would be 20 C [11] giving an expansion of 24x1~x18xlO-hx20 = 8.6mm in a 24m height. The differential movement between the stone masonry outer wall and the steel frame would thus be 33.9mm at the top of the building.

With a concrete blockwork inner wall built concurrently with the outer walls, the differential movements will be much smaller but if the main structure is load bearing clay brickwork this type of masonry is liable to develop moisture expansion so that differential movements may be significantly greater. The movement in an internaI clay brick wall have been estimated on a similar manner to that used for the outer stone masonry wall and are compared with those for this wall in Table 3. It has been assumed that the inner wall will be in a stable moisture condition and therefore no allowance for reversible moisture movement has been included. The total differential movement between the outer and inner walls in this case could therefore be as great as 38.1mm. However, if the walls are built at the same time, the differential movement due to elastic compression is reduced since the compression below each leveI will have taken place before the ties are positioned. Thus the relative wall tie movement due to elastic compression at the top of the wall will be zero and the vertical movement experienced by the ties at each storey leveI will be as shown in Table 4. The movement in a brickwork wall at each storey leveI is also shown, omitting the small creep effect, along with the total relative movement which the ties would have to accommodate .

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TABLE 4.

STONE MASONRY OUTER WALL - CLAY BRICKWORK INNER WALL: RELATIVE WALL TIE MOVEMENTS AT STOREY LEVELS.

Storey: 8 7 6 5 4 3 2 1

Shrinkage -1.3 -1.1 -1.0 -0.8 -0.6 -0.5 -0.3 O

Rev. moisture-4.8 -4.2 -4.0 -3.0 -2.4 -1.8 -1.2 -0.6 movement

Elastic comp. O -0.3 -0.4 -0.6 -0.8 -0.9 -1.1 -1.2

Creep -7.4 -6.7 -5.9 -5.2 -4 . 5 -3.8 -3.0 -2.3

Thermal -7 . 2 -6.2 -5.3 -4.3 -3.4 -2.4 -1.5 -0.5

Total -20.7 -18.6 -16.6 -14.9 -11. 7 -9.4 -7.1 -4.6

Total movement in brickwork inner wall: +14.4 +12.5 +10.5 +8.6 +6.7 +4.8 +2.8 +0.9

Mvmt. across wall ties: 35.1 31.1 27.1 23.3 18.4 14.2 9.9 5.3

Movements of this order across the cavity requires the use of special wall ties many varieties of which have been described by Grimm [12]. It will also be necessary to allow for differential movements across the cavity at window openings and at the roof leveI requiring careful detailing to preserve water exclusion as well as permitting free movement. The solution depends on the type of construction of the inner wall, windows and roof but discussion of this aspect is beyond the scope of this paper.

8. CONCLUSIONS

Considerable uncertainty exists concerning the essential data required to estimate the free movement of stone masonry outer walls arising from moisture, thermal, elastic and creep effects. However, the approximate calculations shown in this paper, using available data, give an indication of the likely magnitude of these movements.

Differential mo vements between this type of outer the main structure are likely to be appreciable if cladding to a multi-storey steel frame and even relation to a loadbearing brick inner leaf if liable to moisture expansion.

wall and used as a larger in this is

In the latter case relative movement across the wall ties may exceed 30mm in a 24m wall height, necessitating the use of dovetail slot or other special ties. With these there should be n o difficulty in accommodating movements of this order, provided that window and wall top details are appropriate.

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1.

2. Fenton G.A. and Suter G.

3. Fitzmaurice R.

4.

5.

6.

7. Hendry A. W.

8. Warren D. and Lenczner D.

9.

10.

11.

12. Grimm C. T .

REFERENCES.

British Standard Code of Practice for Use of Masonry. BS5628 Part I, London 1985. "Differential Movement Between Clay Brick Veneer and Concrete Block in Loadbearing Highrise Structures" Proc. 7th Int. Brick Masonry Conf., Melbourne, 1985. pp.305-16. "PrincipIes of Modern Building" Vol.1. H.M.S.O., London 1938. p.147. "Estimation of Thermal and Moisture Move­ments and Stresses: Part 2" Building Research Establishment Digest 228. 1979. "PrincipIes of Modern Building" Vol.1 3rd Ed. H.M.S . O. London 1959. p.192. British Standard Code of Practice for Use of Masonry. BS5628 Part2 1985. "Assessment of Stone Masonry Strength in Existing Structures" Proc. 5th Int. Conf. on Structural Faults and Repair Edinburgh 1993. pp.265 - 8. "A Creep-Time Function for Single Leaf Brickwork Walls" Int. J. of Masonry Construction Vol.2 No.1 1981. pp.13-19. Manuel de Calcul Effets Structureaux de Fluage et des Deformation Differees" Bulletin d'Information No.36,1980(cf. [2]. British Standard Code of Practice for Use of Masonry BS5628 Part 3 London 1985. "Brick Cladding to Steel Framed Buildings " Brick Development Assn. and British Steel Corporation Windsor 1986. "Masonry Veneer Anchors and Cavity Wall Ties" The Masonry Soe. Jour.Vol.12 No.1 1993.

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