The Development of Masonry Reinforced by Bond Beams and Bond Columns to Resist Lateral Load

By AZoBuild

Table of Contents

Background

A series of tests on small beams, full size and low size walls resulted in the publication of a ‘Design Guide for Masonry Reinforced by Bond Beams to Resist Lateral Load’. The tests showed that it is possible to subdivide large walls into smaller panels using bond beams, the lateral load resistance was significant and could be compared to walls subdivided by wind posts. The system has been further developed to include using reinforced hollow blockwork columns, which allows the subdivision of walls by both vertical and horizontal reinforced elements. The column tests and developing of an extended and revised design guide are described in this paper. A major system application is at the Aquatic Centre for the 2012 Olympic Games in London. The internal blockwork walls in this iconic structure are up to 7m high and must accommodate a number of openings for services. This is done in an efficient and elegant way by the system.

Introduction

Ceram was approached in 2006 by a major masonry contractor wind posts. The system essentially comprised a horizontally reinforced blockwork course (a bond beam), at intervals up the height of the wall, so as to subdivide the wall into a number of smaller vertically spanning panels as shown in Figure 1. A series of laboratory tests were commissioned to study the structural performance of the system.

Figure 1. Wall containing one bond beam in concrete frame

Background of the Experiment

The procedure followed is detailed below:

• Firstly four walls were tested. These were each of 8 m L, 5m W and 140mm t, solid aggregate concrete blockwork.
• At around 1/3rd and 2/3rd of the wall height, ball beams were introduced.
• Two high-yield reinforcing bars each containing 16 mm diameter high-yield reinforcing bars arranged one atop the other at the wall midpoint in a range of trough-shaped units that were concreted subsequently with 40N/mm² concrete.
• The walls were built within a steel frame and the reinforcing bars fitted into simple cleats fitted to the columns.
• The bond beam was attached to the course below and above with shear transfer rods.
• Figure 2 shows an early test wall and a typical loading arrangement in Figure 3.
• Two further walls comprising wind posts at the wall centre line were tested for comparison purposes.
• One wind post was a 100 mm box section and the other, an 80 mm section, were placed in a vertical hollow block work void that is within the wall thickness.

The results obtained initially were very encouraging, in that the walls with the bond beams gave similar results to those with the wind posts. For example, inclusion of bed joint reinforcement concrete and tight tie spacing implied that the designs were conservative. A second similar phase was performed where these conservative measures were not followed.

Similarly, the results were encouraging. One of the walls showed extension to a 12 m horizontal span. There is no proof of cracking because of shrinkage.

Figure 2. Early test wall

Figure 3. Typical loading arrangement

One major observation of these initial work phases were that when there was a failure of the walls containing the wind posts failed, this happened by cracking. Totally, around 18 wall panels were tested, which were supplemented by 18 smaller beam tests.

The sub-panel design approach between the bond beams is straightforward and follows the principles of BS 5628-1 or EN 1996-1 and PD 6697. As there was no sign of sudden compression failure in any of the tests, and consequently a series of tests on low height walls were performed to determine some limiting bending moments that could be used for design. From the four tests performed the least result was used to specify an ultimate maximum bending moment for use in design in a way that when taken with the suggested partial safety factors, ensured that no cracking occurred and deflections were limited, and hence a serviceability check was not required.

Initial Application

The initial significant bond beam system application was in a huge data centre constructed in South East England. The centre was built as a strong two storey steel and concrete bunker in a huge steel framed building. The inside was divided into small rooms so that any explosion or fire does not spread.

Long runs of 140 mm thick block walls and design lateral load of 0.5kN/m² was the solution used for wall construction. The typical height of these walls was 6 m and the original design featured bed joint reinforcement in every course plus 200 x 200 square hollow section wind posts at maximum 4.5 metre centres. The total amount of blockwork used was 11,000m².

This Data Centre was an ideal use for the bond beam system, as it highlighted long, uncomplicated runs of high walls, loaded by a moderately high lateral load. The bond beam system was very successfully used on the Data Centre, but that project also pointed out the shortcomings of the system. While the bond beams enabled a much wider spacing of wind posts, it was not possible to entirely omit them. The next step in the development of the product was to investigate a vertical version of the bond beam.

Column Tests

Column tests were performed on what were essentially locally reinforced hollow blockwork. The column length was 0.89 m and either 3m or 5m high, in both 140mm blockwork and 190mm blockwork. Two block sections were used with two formed voids, separated by a central web and one where the central web was removed. Two vertical steel bars were used in each case and the system was eventually designed about the single voided block.

In the case of the 140mm blocks, only one of the four columns failed by the section failing, in the remaining three, the air bags in the loading system failed prematurely. Hence the design bending moments were fixed based upon these columns, and a check at working loads, showed that the deflection in all cases was acceptably low.

In 190mm columns, all the failures were by the bursting of air bags and although the actual failure moments were extremely high, they did not really provide a sound basis for fixing a limiting ultimate moment.

The completed system comprises both beams and columns, shear transfer rods, cleats to fix reinforcing bars to building columns, and to the vertical bars in the columns. All of the details along with the limiting moments to be used for both beams and columns are available in a revised design guide.

Application at The 2012 Aquatics Centre

The first application for the completed system was at the London 2012 Aquatic Centre. This is an architecturally impressive building built for the Olympic Games, and it features a large lower level containing large areas of plant rooms, changing areas and various other back-of-house functions as shown in Figure 4.

Figure 4. 2012 aquatics centre

The lower level is divided into a number of functions by 9,500m² of 140mm thick blockwork walls. The design of the walls was made complex by a number of factors;

• The lower level is a cavernous space and most of the walls were of height 6 – 7 m
• The designers had specified a lateral design load of 0.5kN/m², with higher loads at balustrade level along escape corridors
• Many of the walls were not full height and hence, had no restraint at their head
• There were large amounts of large services distributed at high level in the spaces, forming a number of penetrations through the full height walls.

The bond beam and column system was a suitable solution for stiffening and restraining these walls. The bond columns were able to span the full 6.5m from floor to soffit level, and the beams and columns could be positioned to avoid the ductwork and cabling runs. Bond beams can be located at the head of partial height walls to provide restraint, and also at balustrade height along escape corridors to resist the unusually high loads in these areas.

Figure 5. Completed wall showing openings for ducting

Figure 6. Complex services at wall head

Potential Future Developments

The Aquatics Centre Application showed how the system worked though it showed the disadvantages of the system with respect to movement joint spacing and blockwork shrinkage. Introduction of bed joint reinforcement overcame this problem, further testing would enable the investigation of the effect of bond beams on minimising shrinkage, allowing the reduction or omission of bed joint reinforcement on future projects.

Conclusions

It is possible to subdivide large blockwork walls without extensively using wind posts. The combination of the design of the subpanels using Code Guidance and a ‘design by test’ approach for the bond beams and columns has enabled an ultimate limit state design approach to be developed, without requiring serviceability checks. Throughout the test programme, progressive improvements were made to the components of the system.

The system has been successfully used on the complex 2012 Aquatics Centre project. The masonry contractor has reported that the block walls were erected in a quicker and more economic manner than using comparable traditional systems. This project demonstrated the value of the system for applications with long or high walls with significant lateral loads.

References

1. British Standards Institution Code of Practice for the use of Masonry Part 1: Structural use of Unreinforced Masonry BS 5628-1: 2005. Part 2: Structural use of reinforced and Prestressed Masonry BS 5628-2: 2005. Part 3: Materials and Components, design and workmanship BS 5628-3: 2005.
2. British Standards Institution Eurocode 6 - Design of Masonry Structures Part 1.1 General Rules for Reinforced and Unreinforced Masonry Structures BS EN 1996-1-1 2005.
3. British Standards Institution Draft for Development. Damp Proof Courses Part 1: Methods of Test for Flexural Bond Strength and Short Term Shear DD 86- Part 1.
4. Ceram Design Guide for Masonry Reinforced by Bond Beams and Bond Columns to Resist Lateral Load, 2010.
5. Corbett, H and Edgell, G J
   Design Guide for Masonry Containing Bond Beams Proc. 11th Canadian Masonry Symposium, Toronto, 2009.
6. Edgell, G J and Clear, L
   Comparative Tests on Aggregate Concrete Blockwork Walls Containing Wind Posts and Bond Beams. Proc. 14th Int. Brick and Block Masonry Conference, Sydney, Feb. 2007.

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