The CSA standard for masonry infill walls is being updated this year and likely, based in part, on the results ongoing research at Dalhousie University.
Dr. Yi Liu, associate professor, Department of Civil and Resource Engineering, has been working for two years on the properties of infill masonry walls with specific attention to masonry block.
Infills are panel walls between bearing columns and structures but little science has been applied as to what optimum levels of stiffness should be and how that stiffness might be enhanced for wind resistance and earthquakes, she said.
CSA Design of Masonry Structures S304.1-04 (R2010) which dates to 2004 and was reaffirmed in 2010, is due for a refresh this summer and is to be considered for the National Building Code of Canada. CSA says it has just completed public review and ballot is anticipated in late winter 2014 so nothing is final.
However, Dr. Liu says her work has provided some calculations for stiffness and hopes to contribute more.
The experiments looked at the behaviour and capacity of concrete masonry infills bounded by steel frames during in-plane and out-of plane behaviour and the capacity of masonry infill walls. She hopes to summarize current and previous research as she progresses.
She tested 10 concrete masonry infilled steel frame specimens as well as one bare frame specimen under an in-plane lateral loading, applied at the frame top beam level.
Components investigated the aspect ratio, extent of grouting, opening of the infill and frame-to-infill stiffness ratio as reflected by varying the orientation of the frame column.
The goal is to find the effect of each parameter on the failure mode, ultimate load and stiffness of specimens and the veracity of the equations used in the design of masonry in current practice.
In a paper published in the Canadian Journal of Civil Engineering, she said current Canadian design guidelines “provide a markedly conservative design of masonry infills whereas the American design standard gives an improved estimate of infill stiffness and strength when compared with test results.”
Part of the curiosity is that masonry infills are more or less dead weight which is inefficient.
“The common practice is that the masonry infill is actually isolated from the structure so it’s only sort of a cladding which serves no structural purpose,” she said.
“It’s the frame itself which provides the resistance. The problem is the masonry infill actually has pretty big or large inherent stiffness. If we can figure out a way to incorporate the stiffness into the structure we can probably come out with a more economical design.”
It could also prove a new market for masonry blocks and bricks, she added, noting Canada has a strong masonry industry but in eastern Canada, for example, masonry bricks and blocks are mainly used as cladding not as weight bearing structures.
She’s already been able to achieve the first goal of making initial recommendations on design. Ultimately, she wants to establish a set of unified equations and design charts to better set standards and give engineers more options in integrating the panels into the load bearing infrastructure of a building.
These types of infill walls with bearing structures have been used since ancient times, she said, but there’s been very little science applied to how it can be done better.
“There’s no real sound and verifiable data and science on how the system actually works and how it might be affected in earthquake zones for example,” she said. “So we’re not incorporating it into the frame.”
Her work has involved building scale prototype masonry block infill panels with steel frames and testing them with different loading using sensors to measure the impacts at different points.
The initial phase measured lateral loading and cracking and the second phase, which she is now in, is creating numeric values for a model which they will build to predict and then verify results in further lab tests of other prototype panels.
Eventually, once the computer modeling is proved effective, Dr. Liu, hopes to have a viable tool to explore further.
The work takes time, she said, because it involves the construction of a test panel, recruitment of competent students to assist, the testing, and while there is some software available most often she ends up writing her own code.
“The commercial software available is limited,” she said. “It is a process not something you can do overnight.”
Earthquake modeling is on her wish list but she will need to partner with another university where they have an earthquake table.
“You need 3D shaking for earthquake simulation,” she said.
“Up and down and side to side. We don’t have a table for that so we will have to partner with someone who does, perhaps, the (University of) Calgary, McMaster or Sherbrooke.
Despite the early progress, she said, there’s still some ways to go.
“To get a sound and waterproof standard we still need to do more,” she said.
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