Tuesday, August 08, 2006

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.


Scott said...

The FEMA documents (i.e FEMA 310)have criteria for how far unreinfoiced masonry can span between supports without reinforcing (Table 4-4). The thickness of the wall and quality of the mortar are the governing factors. In the States we would test the mortar strength per the procedures outlined in FEMA 310 but I am not sure what to do in Indonesia. Use a low estimate I suppose.

The concrete frames will also be moving in an earthquake. The ground will shake both east and west and then east again which will "whip" the top of the building more than the bottom. If the bond betwen the mortar and the conrete is poor, the entire brick wall could fall out of the frame in one piece. If the bond was good then one might expect the masonry to crack in the middle of the wall, bow and then collapse under its own weight. The low compressive stregnth of mortar means that local crushing of the mortar will not be difficult (as is shown in your sketch).

We have this type of construction in Seattle (confined masonry as you call it - we usually call it masonry infill). It was popular in the teens, twenties and thirties. So far it has performed OK, but it has never really been tested like in Indonesia. And I think the quality of mortar may be better.

If the span to thickness ratios meet the FEMA requirements we usally leave it alone. Or you could fasten full height metal studs to one side to prevent collapse. Or bond composite fibers to one or both sides (very expensive)

Scott Tinker

Nick said...

Good to see that you are
Here are my two cents on your question. As Scott said, we would typically reference FEMA and see if the walls meet the height to thickness ratios; if they do more on and if not strengthen the wall with strong backs (typically steel). In your case, where who knows that the mortar strength is, it may require a little more thought. However, I do think that the h/t ratios are a great place to start. From what I understand they were developed based in LA after the San Fernando Earthquake, I think, and were a mix of observations and testing. But they are based on the theory of maximum allowable tension in the brick's mortar and that the unreinforced bricks are only able to span a certain distance before cracking and failure. The entire system is very brittle and once the mortar cracks out of plane there is not added ductility or displacement. In-plane rocking of piers is an accepted behavior, but does not work so well out of plane.
The other good thing that you have going for you is the "reinforced" concrete columns, while lightly reinforced they will help the structure significantly.

For the typical one story home or building I would be very concerned with the roof and the connection of the ring beam to the roof joists. Because the brick walls and mortar are only good if they are well tied together at the roof.

hope this helps -

Juan Balas said...

Hi Forrest. Juan Balas Here. I read your blog with great interest because most of the home construction I did back in Argentina was actually using confined masonry. I know the INPRES (National Institute of Seismic Prevention - Argentina) Has a good sized lab and has done extensive testing in the matter. West side of Argentina is a high seismic risk area, due to the pacific plate/ Andean plate activity. I remember our best practice was building the plynth beam, then the masonry FIRST, alternating reinforcing rebar every three brick courses and then forming two open sides and pouring the columns once all the masonry was up. Irregular surfaces of brick would provide additional bond and a more tight system. Our details for column/ ring beams/ masonry connections were pretty tight, and of course, strictly enforced by the INPRES. I wish I could get those details. In any case, I will do some research and try to get them from a friend of mine.

I can see you are having fun. Take care! Juan

Juan Balas said...

I could get studies but in spanish.

Check this out