Artistic, versatile, durable and resistant to fire, and sound, masonry construction has been used in architectural achievements such as the Egyptian Pyramids, the Colosseum in Rome, India’s Taj Mahal and the Great Wall of China.
In Australia, designers and engineers may soon be afforded greater flexibility when using the material amid revisions to the AS3700 Masonry Structures standard. These changes, which have been approved by the BD-004 standards committee and are expected to be adopted in NCC2019, removing barriers to masonry use. They result from extensive research conducted by the Concrete Masonry Association of Australia (CMAA) in collaboration with Queensland University of Technology.
According to Jake Ring, a civil engineer with the Concrete Masonry Association of Australia, Think Brick Australia and the Roofing Tile Association of Australia and member of the BD-004 committee, the biggest change involves the treatment of reinforced masonry under compression.
As things stand, Ring says there are several requirements in the standard in respect of reinforced masonry which are unduly costly and not realistic to comply with. As a result, engineers are often forced to treat masonry which is in fact supported by reinforcement as being unreinforced for the purposes of the standard.
The revised standard, however, has amended these requirements. This will enable walls which are in fact reinforced to be treated as such from a compressive strength viewpoint.
As an example, in order to use the reinforced masonry equations contained in the standard, it is necessary to laterally restrain the steel using horizontal ties. In theory, this would prevent steel from buckling when the wall is compressed. Due to the need for bricklayers to do this over an entire job, however, along with the difficulty in making grout fill all voids completely as a result of the amount of steel in the voids, this was considered to be both unworkable and unduly costly.
Furthermore, the research found that these restraints were of little benefit. This was largely because lateral restraint was already being provided by the existence of grout surrounding the vertical steel.
Accordingly, lateral restraints no longer required in the revised standard – albeit with new provisions requiring a certain volume of grout surrounding the steel and the steel remaining vertical (not off-kilter) during the construction process.
The ability to treat reinforced masonry as such matters. When masonry is unreinforced, engineers are required to apply a ‘capacity reduction factor’ (CRF) of 0.5. Essentially, this represents a safety factor which engineers need to apply when calculating the compressive strength of the structure. Where a CRF of 0.5 is applied, the strength of the structure is determined by multiplying the sum of the materials’ strength by 0.5. This effectively creates a safety margin of 50 percent.
Where masonry is reinforced, by contrast, engineers can apply a less punitive CRF of 0.75. This reduces the safety margin to 25 per cent.
Second, when masonry is deemed to be unreinforced, engineers are required to disregard any compressive strength benefits provided by the reinforcement.
Courtesy of these factors, where masonry is considered to be unreinforced, the amount of material required to satisfy the requirements of the standard from a compressive strength viewpoint is greater compared with what would be the case where masonry is considered to be reinforced.
Given AS3700’s status as a primary referenced standard under the National Construction Code (which means that adherence to the standard is compulsory), this imposes a considerable disadvantage on use of masonry as a design option.
By contrast, Ring says the new ability to treat masonry which is in fact reinforced as such from a compressive strength viewpoint will enable capacity gains of around 45 per cent.
The bottom line, he says, is that this will help masonry to compete with other materials from a compressive strength viewpoint. This will have cost benefits in terms of reducing the amount of steel or grout or block used and/or reducing the thickness of the wall. It will also provide designers and clients with greater options. These include maximising interior building spaces by using thinner walls and/or adding extra floors beyond what was achievable under the standard as it currently stands.
“In essence, from a strength perspective, engineers can design reinforced masonry walls (using the revised standard) as reinforced masonry walls,” Ring said. “From a compressive strength perspective, it makes it (masonry) a lot more competitive with the other materials which are on the market.
“What we are trying to do by communicating this (the revised standard) to engineers – it’s like saying ‘hey, masonry is back on the table as a material choice.’”
Beyond this, changes have been made in other areas.
First, a current requirement which limits the strength of the grout allowed to be used to 1.3 times the strength of the masonry block being used has been removed. This, Ring says was unnecessary as testing conducted by QUT indicated that you could use any strength of grout without negatively impacting upon the walls or wall strength. Removing this requirement, he says, will enable the design of higher strength walls.
In addition, new provisions have been added for stack bonded masonry – where bricks or blocks are layered in a vertical arrangement and do not overlap as per traditional bricklaying arrangements. Whilst such arrangements are often desired by architects for aesthetic reasons, they have previously provided difficulty for engineers as they did not comply with a number of the lateral loading provisions of the standard. As a result, engineers asked to deliver upon such arrangements were likely to provide designs which were highly conservative in nature and thus costly.
The new provisions overcome this, however, by enabling stack bonded masonry on the condition that reinforcing is provided at different points in the wall. This, as expected, will provide certainty for engineers in terms of a mechanism by which such alignments can be delivered which comply with the standard. In turn, this is expected to free up architects by enabling them to use this arrangement without being served with an overly conservative technical design from the structural engineer.
Professor Manicka Dhanasekar, a professor of civil structural engineering at Queensland University of Technology and member of the BD004 committee, said masonry has been saddled with a ‘double whammy’ from a compressive strength viewpoint under the current standard in terms of the aforementioned requirement to both apply the more penal capacity reduction factor and to disregard the steel entirely from compressive strength calculations.
After being approached by CMAA to look into this, Dhanasekar found that the current requirements date back to the 2001 version of the standard and were based on old literature. Upon conducting a review, he discovered that there had indeed been concerns about the testing upon which the current requirements were based. Further, Australia’s current standard is more restrictive compared with those which apply in Canada and the US – both of which enjoy high standing internationally.
Dhanasekar stresses that the objective of the revised standard remains conservative – just not more so than necessary.
“Fundamentally, the problem we have had and what prompted us to think of changing the current situation was that there was a double whammy in effect against the design of reinforced masonry under compression in the current standard – the 2011 version,” Dhanasekar said. “We thought the masonry could take a lot more load than what we have provided (in the standard).
“We have to make the standard conservative. The question is how conservative?
“We want to be conservative – we never want not to be conservative with the standard. How conservative was the issue. If it is so conservative that it cannot be used, that is not a great way forward.”
With all of the changes, Dhanasekar says designers will be freed up in their use of masonry.
“Say for example, if a designer wants to put walls in a multi-storey building for the lift core, they currently have only one option of concrete walls because masonry is not competitive under the current version,” he said.
“What we believe they can do now is that they can choose the material they want, they can have higher strength grout, they can make use of the strength of the steel – at least partly and then they can also have a higher capacity reduction factor.”
“With all these trade-offs they have got, they can navigate the design much better. It’s like a pilot who has access to the controls rather than autopilot.”
Australia’s standard for masonry structures is changing.
Once the changes are in place, the material will be more cost effective and designers will have greater choice in how it is used.