DESIGN BASED ON LOADSHARING WITHIN MASONRY STRUCTURES: A REVIEW OF EXPERIMENTAL DATA

S K Arara. BSc (Eng), M Se.

I. ABSTRACT

Masonry buildings as built have a much better peIfoffi1anee under load in pracrice than rhat predicted by design rules which are usuaJly based on behaviour of individual elements in isolation . This is because real strctures behave as an 'iter­connected system ' which always has important inter-actions and degrees of loadsharing within ir. Engineers have therefore always appreciated that designs could be made much more economic if sueh inter-actions could be appropriately quantified and isolated for adoprion in design codes of practice. Research in the pasr in this area has been piecemeal and directed more to specific aspects than tackling the problem overaIl in a co-ordinated way. There is presently growing feeling in the UK indusrry and profession, particularly since the establishment of rhe BRE's Large Building Test Facility , that a change in emphasis is now required so as to quantify important inter­actions and whole building behaviour to base design on. This paper therefore reviews the published experimental data in this field in the UK so as to draw sound conclusions from the work to date. The pape r also outhnes areas for further research work and praposes that this should involve borh physical tests on walllfloor assemblages, as weJl as computational techniques.

2. INTRODUCTION

Masonry buildings generally have a much better peIfonnance under load in practice than that anticipated ar the design srage. This is because the design in accordance with current practice is usuaUy based on design of individual elements in isolation. largely as a series of simplified two dimensional (2-D) entities, eg waJIs, floors. TOof,

Keywords: Three dimensional; Masonry; Structures; Inter-action: Loadsharing

Crown Copyright (Building Research estabhshment, Garston. Watford, Herts.)

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partltlOnS etc.. Elemental design is necessarily based on simplifying assumptions conceming structural behaviour of the elemems. design loads and boundary conditions. This tends to make designs over conservative, less efficient and less economic than a design which would be based on treating the building structure as a three dimensional (3-D) entity. This is because buildings as built have interconnected elemems and therefore are bound to respond to loads as an 'inter-connected system' which always has a number of significam inter-actions and degrees of loadsharing within it providing additional robustness. Engineers have always appreciated that designs could be made much more efficient and economic if such loadsharing could be quantified and taken into account in designo There is presently a growing feeling among the UK industry and the profession that a change in emphasis is required and that the current design practice could be improved by focusing research on the fuH building behaviour and deterrnination of importam inter-actions within the building structure.

A number of organisations have carried out research in this area in the pas!. mainly in terrns of specific local inter-actions, eg wall/floor interaction, but occasionally whole building behaviour also. However overall the effort has been a disjointed one. The problem should be dealt with in a more substantial and a co-ordinated manner, with clear aims and objectives which can be defined as follows:

a) to enable the development of design ca!culation procedures which would take advantage of loadsharing that occurs within structures, more than is the case to date;

b) to provide a sound basis for the future development of masonry codes of practice, national and imemational.

As a preamble to such endeavours , it is however important to leam and draw sound conclusions from the said past work. Effort expanded on a review of the past experimental data in the field would help focus on the problem and idemify some of the importam inter-actions. This paper describes such a review and the main findings faH under the following categories: walI to floor interaction; wall to wall vertical loadsharing; laterallbuttressing performance of waJls; racking shear; and structural response to accidental damage. The review covers experimental data published in the UK and is not claimed to be exhaus!Íve.

lt is clear that any research programme for new work would need to be large and comprehensive and carried oU! over the long termo Taking firrn decisions about it right at the beginning of a long tem1 endeavour could not be credible and therefore would not be advisable. Broad areas however can be identified and the paper indicates these in the light of this review and industry discussions. Subject to further discussions and decisions, these areas of research would inevitably involve testing of a large number of 3-D element assemblages, conceivably comprised of a few elements to perhaps whole structures. However since an approach involving physical testing alone would clearly be prohibitively expensive. the scope of computational techniques such as suitable finite element modelling, must also be

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investigated and taken on board.

The programme of work would be large and would therefore necessarily involve a number of test houses. including universities. Co-ordination and management of such a large programme may therefore require a dedicated committee of professionals.

3. REVIEW OF THE EXISTING RESEARCH DATA

3.1 WalIlfloor interaction

Among the papers reviewed. a majority of six falI in this category. One common strand among these was to detennine the amount of the fixity which would be available at a wall/tloor junctíon, with a view to sorting out the effects it may have on the following:

(i) the effective height of wall, with respect to storey height H, to consider in the design of a wall element;

(ii) the resultant eccentricity of verticalload. in relation to walI thickness t, sustained by the wall element.

3.1.1 The paper by Colville and Hendry I describes a series of vertical load tests perfonned on a fulI scale two-storey single bay single leaf loadbearing brick masonry structure with concrete tloors. Magnitudes of the wall precompression and fioor loads were varied in the tests and the measurements made in respect of the detlection and rotation of the tloor slabs. The main finding was that over 75% fixity and a significant amount of restraining moment could be developed at the lower wall/fioor joint, even at a low precompression stress of around 0.30 N/mm'. Full joint fixity would only occur at an estimated 1.4N/mm' pre-compression.

3.1.2 Sinha and Hendry 2 carried out tests on a fuH scale two bay three storey single leaf brick structure, with concrete tloors tied back to one side to a strong abutment. They found that for a udl of upto 2.5KN/m~ on fioors. the structure could be idealised as a frame for the calculation of effectíve eccentricity of vertical load on waUs and the effective height of walIs , both of which varied depending upon the situation of the waU in the structure. The tests gave the effective eccentricity of vertical load of between O and 1.89t. A figure of 0.03t was also obtained for accidental eccentricity for walls . The effective height varied between 0.5H and 1.0H. with confinnation that ir could be taken as O.75H for grond f]oor walls, particularl)' when ali fIoors are loaded.

3.1.3 Sinha et aP tested a full scale single bay five storey brick cavity wall structure, with concrete tloors unifannly laaded to 2.0KN/m2 and tied back on one side to a strong abutment. The outer leaf of the externaI wall was unsupported for a maximum of three store)'s and the fioors supported on one leaf, or two leaves. Once again , for working load range . the structure could be idealised as a frame. In alI 86% of the

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fioor load was taken on the two inner leaves and the remaining 14% going into the two outer leaves. When fioors bear on one leaf, most momem is taken by the inner leaf causing it to crack at the junction, but the momem is equally shared between the two leaves away from the junction. Factors affecting fioor fixity at its junction with the wall are stiffness of the wall and the fioor and the average eompressive stress in the wall which in turn affeets the point at whieh tensile eracking will oeeur in the wall. For ali. the fioors loaded, the effective height can be taken as 0.75H for the ground fioor walls and around 0.5H away from the ground floor. Varying between 0.05t and 1.7t, no defini te values were obtained for the effective eeeentricity for waUs resulting from the floor loading. They should in praetiee therefore be calculated using the frame idealisation.

3.1.4 The above work appears to have been the basis to the proposals included m Hendry's paper 4 in regard to the eaIculation of the joim fixity leveis and the effective eceentrieity of vertical load on waUs. The paper gives practical examples of a cavity wall and a solid wall in a seven storey structure with concrete tloors and suggests that the effective eccentricity values in praetiee would be quite small , with a maximum of around 0.25t at the penultimate fioor.

3.1.5 The most recem work reviewed was carried out by Stokle and 8eU5 which confirms the development of elastic frame action at a joint between single leaf single storey wall and a concrete fioor. This happens even for wall compressions lower than 0.3 N/mm2 for the practical range of floor loads. The work suggests the use of an 'edge pivot model' as a tool for predicting the limit of elastic frame behaviour and the subsequent degree of interaction between the slab and the wall which occurs beyond this limit. The work is being extended imo testing of specimens in a two storey configuration. which would also include cavity walls and timber floors , and also the effect of creep in waIls on the effective eccentricity of vertical load.

3.1.6 Tests incorporating a single leaf brick wall/timber fioor joint, with the fioor supported on joist hangers are described in a paper by West et a16. For vertical load applied on the timber tloor. the tests gave the effective height for the ground tloor wall as I.OH and an effective eccentricity of vertical load at the joist hanger of around 90mm, ie 40mm from the face of the wall.

3.2 Verticalloadsharing between walls

3.2.1 Four verticalload tests were carried out on the brick/block externai cavity walls ai the front and rear of a two storey masonry house 10 years ago 7. The waUs concerned had large openings in both the storeys and were bonded to a brickfblock gable cavity wall at one end and connected to a solid brick party wall at the other. Tests 1 and 2 were carried out with the load applied on the inner leaf at the eaves levei only of the rear wall. Here the floor timber joists provided, although strapped, ran parallel to the wall. In tests 3 and 4, the load was applied at both the eaves leveI and the tloor levei of the front wall. The tloor timber joists in this case spanned perpendicular to the wall supported on joist hangers. In ali cases of the vertical loading, the fioor acted as an effective prop dividing the wall into two storeys

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limiting its slenderness. Once again the effective height for the ground floor waUs was shown to be between 0.7H to 0.75H. The tests also showed a substantial degree of loadsharing by the flanking gable wall and the party wall particularly in the ground floor storey. This relieved the central pier of some of the load, but limited the ultimate performance of the walls by generating shear cracks at the junction of the flank walls and retums.

3.3 Lateral/buttressing performance of walls

3.3.1 The whole of the 9.0m long cavity gable wall in the two storey house test described above was subjected to laterally applied udl 8. The front and rear wall s acted as effective buttresses with the loaded wall taking at least three times the required lateral resistance . The tloor appeared to restrain the gable wall and transferred load into the internaI block partition wall provided at the ground tloor. The overaIl concJusion was that the house proved to be extremely strong and lhe interactions developed enabled the structure to work compositely.

3.3.2 The triangular gable part of the gable wall as above was also subjected to a wind suction test 9 separately. The main concJusion here was that the gable part was able to cantilever effectively and sustain the suction load but only in conjunction with effective tying being achieved to the trussed rafter roof. The plasterboard ceiling appeared to play a significam part in stiffening up the TOof structure together.

3.3.3 In experiments in connection with a two storey high SLIM (Single Leaf Insulated Masonry) house lO, lateral load tests were carried out on the rig of an externaI wall of the house built in the laboratory. and the same wall in-situ in the actual house. The walls were 9.0m long by 5.0m high and the tests were destructive in the former case and non-destructive in the latter case. The laboratary wall simulated a 3.0 m wide section of the actual house, complete with built-in timber floor joists and two 1.1 m long stiffening walls .

The main concJusion from the work was that despite large openings , the failure pressure achieved in the laboratory wall gave a partia I safety factor for material strength of around 5.3, and the house itself appeared to be substantially stiffer in its response to the lateralload than was the rig built in the laboratory.

3.4 Racking shear

3.4.1 Racking load tests were carried out on single leaf 1.0m to 4.5m long brick and block buttressing wall s with and without a return at one end and subjected to vertical pre-compression to simulate vertical load from storeys above". The work cJearly demonstrated the substantial comribution that even a smaU retum could make in taking the racking load. The waUs with return, particularly the ones of 1.0m length , gave upto eight times the average shear strength obtained hom plain waUs.

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3.5 Structural response to accidental damage

3.5.1 The early post-Ronan Point work on loadbearing masonry structures subjected to gas explosions is described in a paper by Astbury et ai 12. An important conclusion reached was that masonry structures are generally competent to resist accidental loads such as those due to gas explosions and vehicle impact, etc .. Any damage caused ís contained locally without leading to progressive collapse type failure. The important factors for this behaviour to occur, which are usually satisfied in masonry buildings, are: the restraint available to walls provided by the superimposed load; and the degree of inter-connection within the structure, as is the case with cellular and cross wall/spine wall stiffened building fortOs. It is for these reasons that most masonry buildings are easily amenable to design against accidental damage in accordance with the BS 5628 Part 11 3. The provision of horizontal and vertical tie steel may only occasiona\ly be necessary in certain strategic positions in the top few storeys in multistorey buildings.

3.5.2 More recent work involving the two storey house described above (cf 3.2.1) is described in a paper by Templeton et al l4 . Two tests were carried out involving the front and rear loadbearing walls where in each case the central pier was suddenly removed . Despite the fact the walls were already cracked after the positive pressure tests on the gable wall (cf 3.3.1), in each case the wall above was able to span over by a combination of arching action, cantilevering lintels, fIoor and wall tie aclÍon.

4. CONCLUSIONS FROM THE WORK SO FAR

a) The main conc\usion from the work covered under 3.1 above is that masonry wall/tloor assemblies, particulary with concrete fIoors, effectively behave as a structural frame under verticalloads. The effective eccentricity of vertical load at any junction can therefore be obtained using basic frame analysis. The analysis may also be used for obtaining the effective height of various wall elements, although it can safely be taken as O.75H for ground tloor waUs.

b) The work of the type covered under 3.1 should be extended on the Iines being carried out at the Universiy of Manchester Institute of Science and Technology (UMIST) by Dr Phipps and Dr Bel\. This work breaks new ground by involving both concrete and timber tloors, with single leaf and cavity walls put together as test specimen in a two storey configuration simulating both externai and internaI wall/tloor joints. The work is also aimed at taking a look at the effect of creep on the walI elements under sustained load.

c) The verticalloadsharing has c\early been indicated taking place between perpendicularly placed loadbearing walls, but a great deal of work would be required in order that such loadsharing may be quantified for design purposes.

d) Masonry house structures are generally very efficient and effective in resisting positive lateral wind pressures because of the composite action developing

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within the structure. However the same cannot be said about with negative wind pressures applied to isolated positions, eg triangular gable part, and more work may need to be done in this area.

e) Another important lesson to leam is that a part rig of a house tested under lateral loads may not do justice to the full composite action that is Iikely to develop if the structure as a whole is tested.

f) Racking shear tests demonstrate the importance of even a small retum being added to the wall under load , but this work should be extended to test multistorey wall configurations.

g) Masonry structures generally behave with a great deal af integrity and composit action in resisting accidentalloads, gas explosions and vehicle impact. Any damage caused is usually contained locally and is unlikely to lead to progressive collapse type of failure.

5. OUTLINE AREAS FOR FUTURE RESEARCH

The following broad areas are indicated for discussion purposes.

a) Multistorey masonry assemblies with a range of materiaIs and construction aimed at development and determination of the frame action. Twa storey assemblies are being tested at UMIST (cf 4a».

b) Effect of bearing of hard (concrete) flaors on loadbearing walls.

c) Verticalloadsharing between perpendicularly placed loadbearing walls, including the effect of floar diaphragms and stiffening walls.

d) Diaphragm action of floors.

e) Lateral stregth of non-loadbearing partitions (eg timber studding) and their contribution to the composite action developed in masonry structures.

f) Racking strength of multistorey walls.

g) Development of by validation through physical experiments a suitable finite element modelling technique for application to determine structural behaviour of 3-D masonry structures.

h) Work aimed at Eurocode 6 Simple Rules parto

i) Resistance of masonry structures to local fire loads.

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6. REFERENCES

1. Colvelle, 1 & Hendry, A W, Tests of a loadbearing masonry strueture, British Ceramie Soeiety Proeeedings, No. 27, pp 77-84,1978.

2. Sinha, B P & Hendry, A W, An investigation into the behaviour of a briek eross-wall strueture, British Ceram. Soe. Proeeedings, No. 27, pp 67-76,1978.

3. Sinha, B P, Maurenbreeher, A H P & Hendry, A W, An investigation into the behaviour of a five storey eavity wall structure, British Ceram. Soe. Proeeedings, No. 24, September 1975.

4. Hendry, A \\T, The ealeulation of eeeentricities in loadbearing walls, Engineering file note No. 3, BOA, Windsor, 6pp ,1988.

5. Stokle, J O & BeU, A l, Frame aetions in masonry struerures: an experimental investigation, Masonry lnternational, Vo. 1. No. 3, 71-108,1987.

6. West, H W H, Dodgkinson, H R & Haseltine, B A, The effeet of floor and wind loads applied separateIy or simultaneously to a two storey height waLI, TN 252, BCRL, lan. 1976.

7. Edgell, G J & de Vekey, R C , The robustness of the domestie house, Par 1, Compressive loading test on walls, TN 350, BCRL. May 1983.

8. Templeton, W. Edgell, G J & de Vekey, R C. The robustness of the domestie house, Part 3, Positive wind pressure tes! on gable wall, TN 370, BCRL, February 1986.

9. Edgell, G J & de Vekey, R C, The robustness of the domestic house, Part 2, Wind suction tests on gable waUs, Proeeedings of the 7th International Brick Masonry Conference, V 2, pp 949-58, 17-20 February 1985.

10. Hodgkinson, H R & Haseltine, B A, The structural testing of a SLlM house, TN 351, BCRL. June 1983.

1 I. Cavanagh, C J, EdgeJl, G J & de Vekey, R C. The racking strength of lightly loaded partition walls, TN 349, BCRL, May 1983.

12. Astbury, N F, West, H W H, Hodgkinson, H R, Cubbage, P A & Clare, R, Gas explosions on Ioadbearing masonry struerures, SP No. 68, BCRL, 1970.

13. BSI, BS 5628 Part 1, Code of practice for use of masonry, Struerural use of unreinforeed masonry. 1985.

14 TempLeton, W, Edgell. G J & de Vekey, R C. The robustness of the domestic house. Part 4, Accidental damage, TN 371, February 1986.

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