Confined Masonry Madness

How does a confined masonry wall fail out of plane? Earlier yesterday, Tim and I showed the construction supervisors in the field examples of good and bad quality work in houses originally built by another NGO (I'm leaving this NGO nameless), this NGO is also one the ones we are reevaluating their designs and construction. The houses all use a confined masonry wall design. I couldn't stop thinking why are we even bothering trying to increase the quality of the work when there is a design error, at least in my mind. There is no reinforcing in the masonry to prevent it from blowing out in an earthquake, but then at the same time, it's not the notorious "unreinforced masonry" it's confined, it's different.

Later that day Elizabeth, Tim and I had a huge discussion on the behavior of confined masonry that led us into the late hours of the night. I was so convinced this theory was right, that I wouldn't drop it. So what is confined masonry?

Confined masonry is where you have two reinforced concrete columns on either side of a masonry wall. On the top of the wall, you have a reinforced concrete beam, this beam will be connected to columns and continues running along the top of adjacent walls eventually going all the around the perimeter of the building which give its name, ring beam. On the bottom of the wall you'll have a plinth beam, which does the same as the ring beam but also ties the wall to the foundation.

In order for the masonry to "blow out" due to an acceleration of an earthquake, I think each brick would have to rotate about its own axis to some degree, theta, and translate a distance, delta, where the wall would start to arch out, until at some point where the wall would break apart. Think of a stone arch, each stone in it is rotated to some degree to make the arch shape. The same would happen, only horizontally in each row of bricks during ground acceleration. But in order for a rotation to happen, the wall must expand in length to allow room so the bricks can rotate. But in a confined masonry, the walls are tied together by the beams and columns on all fours side, not allowing the wall to expand. So in order for the wall to fail out of plane, the mortar in between the bricks, I think, must break apart from the induced in-plane compressive force, Pu. So if you have a well built wall, you can calculate the out of plane strength, by the stresses the brick applies on the mortar from trying to rotate.


Now, all of this is wrong if the mortar fails in shear, and the brick just slides out, with no rotation. But does is fail in shear? Or even yet, do the bricks arch out vertically and not horizontally as I explained earlier. I believe that would be dictated on the geometry of the wall. It'll span the shortest distance, the most ridge path. Remarkably no one has done testing on this (at least what we can find), so do we really know what we are talking about? Elizabeth can't find grants or funding to do a shake table test at a university back in the States (Berkeley, UCSD, Washington or Illinois). NSF won't fund it, due to it not having an application in the United States. And there are no shake tables at any of the universities in Indonesia, though there is one in Bangkok.

And to complicate things more, do the columns and beams act as ridge elements and adequately confine the masonry due to the poor quality of construction? How do you measure that without testing?

And to even further complicate things, Elizabeth did a recon trip to central Java right after the earthquake in May. Her findings were that in a confined masonry wall house, the failure point was the connection of the ring beams and the whole wall just fell over, not the masonry blowing out into rumble. So if the connections were strengthen, would the next failure point be the masonry blowing out? At what point does the masonry fail and connections don't? Every engineer is the States, or at least on the West Coast where there are earthquakes, would say "you have to reinforce that masonry!"

So what do we do in the meantime? Tell the NGO to demolish all the houses because it doesn't have reinforcing in the masonry and build it right and safer? Or can they leave it because it might have some out of plane strength. I'm not even talking about the poor quality of construction in many of the houses, which alone is merit for demolition. We're talking about a lot of houses, hundreds, maybe even a thousand, demolished when families are waiting in tents and temporally housing.