Design Guide for Masonry Reinforced by Bond Beams to Resist

Lateral Load

Design Guide for Masonry Reinforced byBond Beams to Resist Lateral Load

Introduction

This Design Guide has been developed from an extensive series of tests on full size walls,generally 8m x 5m (length x height) and reinforced at intervals up their height. The walls were made from concrete blockwork and, in the case of plain walls, were reinforced by bond beams located approximately one third and two thirds of the wall height. The bond beam course was atrough type concrete masonry unit containing two 16mm reinforcing bars, placed horizontally oneabove the other in the ‘trough’. The bars fitted into metal cleats fixed to columns at their ends and were concreted into the trough. At specified intervals vertical shear transfer rods connected the bond beam to the courses above and below it. For walls with window or door openings, the bond beams were installed at window head and sill level or at door head height respectively. Althoughthe testing programme was carried out using cleats and transfer rods of proprietry design the remainder of the components are readily available and comply with relevant standards and certifications. The Guide has therefore been produced in a relatively general way.

The experimental work was carried out at CERAM and sponsored by Wembley Innovation Ltd.

The production of this Design Guide has been steered by a working group consisting of:

Andrew Best Buro Happold Geoff Edgell CERAM Neil Tutt Jenkins and Potter Hamish Corbett Wembley Innovation Ltd

A full report of the experimental work is available from Wembley Innovation Ltd.

Foreword

This Design Guide provides guidance and recommendations for the design of masonry walls incorporating bond beams used horizontally to strengthen walls against lateral loading. It shouldnot be quoted as a specification and particular care should be taken to ensure that claims of compliance are not misleading.

This guidance was prepared on the assumption that the execution of its recommendations is entrusted to appropriately qualified and competent people.

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1. Scope

Recommendations are provided for the structural design of 140mm thick single leafconcrete masonry walls reinforced at determined intervals with horizontal bond beams, toresist lateral loads.

Note: Walls designed using this Guide may not always be adequate to satisfy other design requirements e.g.resistance to fire, thermal insulation, sound insulation. Reference should be made to BS 5628-3 for guidance.

It has been assumed in preparing this guidance that the design of masonry is entrusted tochartered structural or civil engineers or other appropriately qualified persons, for whose guidance it has been prepared, and that the execution of the work is carried out under thedirection of appropriately qualified supervisors.

2. References

The following documents should be referred to for the application of this Guide. For datedreferences, only the edition cited applies. For undated references, the latest edition of thereferenced document (including any amendments) applies.

BS 5628-1: 2005 Code of practice for the use of masonry – Part 1: Structural use of unreinforced masonry.

BS 5628-2: 2005 Code of practice for the use of masonry – Part 2: Structural use of reinforced and prestressed masonry.

BS 5628-3: 2005 Code of practice for the use of masonry – Part 3: Materials and components, design and workmanship

BS 8215: 1998 Code of practice for the design and installation of damp-proof courses in masonry construction

BS EN 771-3 Specification for masonry Units – Aggregate concrete masonry units

BS EN 998-2 Specification for mortar for masonry – Part 2: Masonry mortar

BS EN 1052-2 Methods of test for masonry – Part 2: Determination for flexural strength

DD 86-1 Damp-proof courses – Part 1: Method of test for flexural bondstrength and short term shear strength

BS 5838-1 Specification for dry packaged cementitious mixes. Prepacked concrete mixes.

BS 4449 Specification for carbon steel bars for the reinforcement of concrete

BS EN 10111 Continuous hot rolled low carbon steel sheet and strip for coldforming: Technical delivery conditions.

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3. Definitions

The definitions given in BS 5628-1 and BS 5628-2 apply together with those below.

3.1 Bond beam A course of trough shaped units mortared together in a wall. Reinforcing bars areplaced in the void which is then concreted.

3.2 Shear transfer rodsA flat steel section bent through 90o to form a leg and foot. The rod is placed withthe foot beneath the course below a bond beam with the leg in a cross joint. Therod passes through a hole in the base of the bond beam unit, the concrete infill andinto the mortar in the cross joint in the course above the bond beam.

4. Symbols

For the purpose of this Guide the following symbols apply

kxf characteristic flexural strength of masonry

dg design vertical load per unit area

mJ partial safety factor for material

fJ partial safety factor for load

5. Alternative materials and methods of construction

Where materials and methods are used that are not referred to in this Design Guide, theiruse is not discouraged However they are beyond the scope of this guidance, which is based upon that of the experimental programme. Characteristic flexural strengths formasonry outside the scope of this guidance may be determined in accordance with BS EN 1052-2.

Section 2: Materials, components and workmanship

6. General

The materials, components and workmanship used in the construction of masonry laterally loaded wall panels should conform to the appropriate clause in BS 5628-3.

7. Masonry Units

440mm x 140mm x 215mm aggregate blocks should conform to the relevant clauses of BS EN 771-3: Aggregate Concrete Masonry Units and have a mean compressive strength of at least 7 N/mm2.

8. Laying Masonry Units

Aggregate blocks should be laid on a full bed of mortar. This includes units used to form the bond beam and those in the course above. Raked joints should not be used.

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9. Rate of Laying

The maximum height of blockwork that should normally be built in a day is 1.5m.

10. Forming of chases or holes

Chasing of completed walls or the formation of holes should be carried out only whenapproved by the designer and then be in accordance with the recommendations of BS 5628-3.

11. Damp-proof courses

The provisions of BS 5628-1 should be followed.

12. Reinforcing Steel

Reinforcing steel should be 2 no. T16 bars in each bond beam conforming to the requirements of ribbed weldable steel reinforcing bars BS 4449.

13. Shear transfer rods

Shear transfer rods should b 40mm X 4mm mild steel to BS EN 10111. The rods should consist of an ‘L’ shape with a vertical leg of 610mm and a horizontal foot of 60mm in length.

14. Cement

The provisions of BS 5628-1 should be followed.

15. Aggregate for Mortar

Aggregate for mortar should follow the recommendations of BS 5628-3, Clause 4.3.

16. Masonry Mortars

The provisions of BS 5628-1 should be followed. Mortar of compressive strength class M4, mortar designation(iii), should be used. .

Note: In the experimental work a ready to use, retarded mortar 1:1:6 cement:lime:sand was used.

17. Concrete Infill

C40 pre-mixed 10mm aggregate (bagged) concrete to BS 5838-1 is recommended, however quality controlled ready mixed concrete can also be used.

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18. Head restraint anchors and frame cramps

Proprietary frame cramps 19mm X 2mm in cross section and projecting 175mm from theframe have proven to be suitable when used with a debonding sleeve.

Proprietary internal head restraints that are bedded in the bed joint beneath the top courseand which are suitable for a maximum gap above the top course of 25mm and permit bothvertical and in plane restraint have proven to be suitable.

Section 3: Design objectives and general recommendations

19. General

The design of concrete blockwork walls to resist lateral loads follows the guidance given inBS 5628-1 and BS 5628-2. In the case of walls containing bond beams the principle is todivide the walls into sub-panels using the bond beams and any vertical supports e.g. windposts, local vertical reinforcement. Each sub-panel is then designed according to BS 5628-1 using the relevant flexural strengths, support conditions and height/length ratio. The lateral loads from the masonry sub-panels are then applied to the bond beams, which are checked to see that their maximum design moments are not exceeded. When the maximum design moments are based upon the partial factors recommended in this guide,the serviceability limit states of cracking and deflection will be satisfied.

20. Division into sub-panels

The bond beam may be taken as consisting of the reinforced course acting together with the courses above and below it i.e. it is three courses deep. The sub-panel is then taken as receiving simple support at one course above or one course below the reinforced course.Alternatively if the designer carries out a more detailed analysis and can justify continuityacross all three courses then enhanced support can be assumed, in this case the support is considered to act at the mid height of the bond beam.

If there is sufficient precompression due to self weight of the masonry above thencontinuous support at the dpc at the base of the wall may be assumed. Alternatively flexural tension should only be relied at the damp proof course if it has been justified bytests (see DD 86-1). If the damp proof course is provided by damp-proof course brickscontinuous support may be assumed. If flexural tension cannot be relied upon at the damp-proof course then simple support should be assumed.

The designer will need to consider whether the head restraint can provide continuoussupport and if not should assume simple support.

Note: In the experimental work simple support was generally achieved. Where attempts were made to providemoment restraint, cracking tended to occur prematurely along the bed joint at the base of the top course.

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21. Limiting dimensions

The limiting dimensions of the sub panels should be in accordance with BS 5628-1 and of the whole wall in accordance with BS 5628-2

21.1 Limiting dimensions of sub-panels

For a wall thickness of 140mm the limiting dimensions of the sub- panels are:

a) Panel Supported on three edges; i) two or more sides continuous = height x length equal to 29.5m2 or

lessii) all other cases = height x length equal to 26.5m2 or less

b) Panel supported on four edges;i) three or more sides continuous = height x length equal to 44m2 or

less ii) all other cases = height x length equal to 40m2 or less

c) Panel simply supported at top and bottom. Height equal to 5.5m

In cases a) and b) no dimension should exceed 7m. In case c) there is no restriction onpanel length.

Note: Generally the sub-panels are assumed to span vertically between bond beams.

22. Compressive strength of blockwork

When using the materials specified for this form of construction there is no need to check against compression failure.

Note: Tests show that bond beams fail in flexure after extensive cracking and have effectively failed byexcessive deflection before fairly slow and localised compression failure occurs.

23. Characteristic flexural strength of concrete masonry

The characteristic flexural strength of masonry for use in design ( ) may be determined by tests according to BS EN 1052-2.

kxf

Alternatively the value may be determined from Table 1 below which is derived from Table 3 of BS 5628-1.

Table 1: Characteristic flexural strength of masonry, , N/mmkxf2

Mortar Strength Class/Designation

Plane of failure parallel to bedjoints

Plane of failure normal to bedjoints

M4/(iii) 0.22 0.52

Note: Test to determine the compressive strength of concrete blocks should be in accordance with BS EN 772-1

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24. Partial safety factors

It is assumed that the recommendations in clause 11 of BS 5628-2 for the special category of construction control for the reinforced elements will be observed and that this control extends to the unreinforced sub-panels. In these circumstances it is recommended that thevalue for flexure in BS 5628-1 for the special category of construction control i.e. 2.5 may be used. If this level of control cannot be achieved than a value of 3.0 should be used.

Although the recommendations for clause 11 of BS 5628-2 are assumed to apply, thepartial safety factor for materials for the design of the reinforced elements is recommendedto be 3.0. If this recommendation is followed the serviceability limits states of deflectionand cracking will be met.

25. Design of the sub-panels

The design procedure for the sub-panels should follow the provisions of clause 32.4 of BS 5628-1, although it should be noted that sub panels are normally designed as spanningone way vertically. In particular it should be noted that where a subpanel has a height:length ration of less than 0.3 it should be designed as spanning vertically. Where a panelhas a height: length ratio of greater than 1.75 it should be designed as spanninghorizontally.

The vertical load in the sub-panel acts so as to increase the flexural strength normal to thebed joint and the strength may be modified to + kxf mJ dg where is the design stressdue to vertical load (including self weight) normal to the bed joint.

dg

26. Design of Bond Beams

The maximum allowable bending moment for the bond beam has been derived throughtesting, and is 60kN-m. The basis of this assertion is given in Appendix A. This should be modified by the material safety factor (3.0) to give a maximum design bending moment for the bond beam in the ultimate limit state of 20kN-m.

The bending moment applied to the bond beam should be calculated assuming that thebond beam is loaded by the characteristic wind loads on the adjacent masonry sub panels,modified by the appropriate partial safety factor for loads of 1.2 and the bond beam is considered to be simply supported.

As shown in Appendix A when the recommended partial safety factors are used there is noneed for a further check against cracking or excessive deflection.

27. Detailing

The detailing of the restraints, bar placing etc. shall be such that the design strength of thewall can be achieved.

27.1 Head restraints and frame cramps In general manufacturers recommendations should be followed. Frame crampsshould be used to restrain the sides of wall and spaced vertically at maximum 450mm centres. Head restraints should be placed no more than 900mm aparthorizontally at the head of the wall.

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27.2 Shear transfer rods Shear transfer rods shall be placed at 900mm centres. These are placed in the cross joints of the courses above and below the bond beam. A small break out isneeded in the base of the bond beam to enable location.

27.3 Location of reinforcement The location of the cleats and any intermediate supports e.g. at the transfer rods, should be such as to ensure sufficient clearance from the side of the bond beamblock for adequate cover and compaction of concrete infill. In a 140mm wide and215mm high bond beam, two T16 bars should be placed vertically above one another with equal spacings between the base of the unit and the first bar, the twobars and the second bar and the top of the unit, as shown in Figure 1. The provisions of BS 5628-2 regarding cover, bar spacing etc, should be followed.

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4 5 6

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WI Beam System 1 Primary structural column 2 End Cleat3 De-Bonding Sleeve4 “U” Block 5 Transfer Rod 6 C40 Concrete Infill7 T16 Reinforcing Steel8 Blockwork Wall

Figure 1.Examples of the bond beam system, together with140mm “U” block

dimensions and reinforcement spacing.

28. Workmanship

Work should generally conform with BS 5628–3.

Concrete infill should be in accordance with 17. Special care should be given to the workability of concrete and the height of pour to ensure complete filling without spillage onthe face of the units.

WI 140mm U-BLOCK NOTE: NOT TO SCALE

215mm

30m 30m80mm

150mm

76mm

56mm

64mm

95mm

T16

T16

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Reinforcement should be in accordance with 12 and fixed as shown on the detail drawings. Care should be taken to ensure that the specified cover to the reinforcement is maintained,e.g. by using spacers. Where spacers are used they should be of such a type and thereinforcement so positioned that compaction of the infill concrete is not prevented.

Reinforcement should be free from mud, oil, paint, retarders, loose rust, loose mill scale, snow, ice, grease or any other substance that may adversely affect the steel of concretechemically, or reduce the bond. Normal handing prior to embedment is usually sufficient forthe removal of loose rust and scale from reinforcement.

Appendix A: Bond Beam Test Data

During the experimental work of Phase (V) two low height (1.1m) walls containing a single bondbeam with pinned end supports were built on a slip plane and loaded with a uniformly distributedload over their face area until failure occurred. The results of these two tests are given in Table A1.

Wall no. Load at first crack (kPa) Maximum Load (kPa)

2 2.4 8.54

3 2.6 7.05

Table A1. Experimental Bond Beam Results with Pinned End Cleat Connection

From these results the characteristic maximum load of the bond beam had been taken to be 7.05kN/m2 (for the present). The wall in this case spans 7.9m and assuming the ends are simply supported the maximum allowable bending moment resistance may be taken to be 60kN-m.

Applying the partial safety factors, mJ = 3.0 and fJ = 1.2 the design maximum load is derived as 1.96kN/m2 which is lower than either of the loads in the tests of 2.4kN/m2 and 2.6kN/m2 to cause cracking.

From the load vs deflection curves in the Phase (V) report the deflection at 1.96kN/m2 is 12mm(wall 3) which is Span/664. The general criterion for deflection is that the final deflection will notexceed span/250, including long term effects. In order to avoid damage to finishes it isrecommended that deflections, after they are constructed do not exceed span/500 (BS5628-2, cl 7.2.2.2.1). It is concluded that the deflections recorded in the tests are small enough that the serviceability limit state of deflection will be satisfied.

Phase (VI) of the work included two further low height (1.1m) walls in this case also with a single central bond beam. The difference between this and Phase (V) is that the cleats were firmlymounted onto the columns and in Phase (V) they had been set from the column face by bolted connections. The span in this case was 8.1m. The results were as follows:

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Wall No. Load at First Crack (KPa) Maximum Load (KPa)

1 4.0 9.0

2 4.2 11.6

Table A2. Experimental Bond Beam Results with Fixed End Cleat Connection

In this case the greater fixity had clearly an effect in delaying cracking and allowing greater loads tobe achieved. Using the same partial factors as above would give estimates of the design loads as2.5kN/m2 and 3.2kN/m2 which are again comfortably below the cracking loads. The deflections at these loads were span/582 and span/2862. At this stage it is considered prudent to use 60kN-m as the maximum allowable bending moment for the beam as although 9kN/m2 and 11.6kN/m2 were the recorded maximum loads at loads in excess of approximately 7kN/m2 there was a change inload:deflection behaviour i.e. a reduced rate of deflection increase with increased load. It is assumed that the wall was starting to deflect plastically in theses cases, and hence the higher load capacities have been disregarded.

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