Eight things I learned by reading Section C8

I know, I know: a listicle. How 2011! But this is a site about heritage, after all… Here’s the pitch. I’ve been reading Section C8 Unreinforced Masonry Buildings from The Seismic Assessment of Existing Buildings, written by a team of researchers, academics and professionals and published at eq-assess.org.nz. (I mentioned it in the post on bed joint sliding shear.)

You might think it sounds tedious, but it’s not. It’s well-written, well-illustrated, and provides some really useful and clear categorisation of structures and the ways they can fail. I needed to read it: this is the work I want to do. Some of you might be in the same position. But even if you’re not interested in doing seismic retrofit, if you’re involved in any way with building works on heritage buildings made from URM, you should probably leaf through the first few sections.

It would be presumptuous and preposterous for me to write a “review” of Section C8. I don’t understand it well enough to do so. But I thought I’d share my own highlights, as a way of enticing others to have a look. We’ll see how it goes.


On a more serious note, this article includes images of damaged and collapsed buildings from Christchurch. The following is presented with sincere respect to the 185 people who died in the 2011 earthquake and to their families. Let’s work to try to stop something like that happening again. Kua hinga te tōtara i Te Waonui-a-Tāne. HT


Section through a cavity wall. Note also the change in thickness at the first floor, creating a ledge for the joist to sit on. From Section C8 Unreinforced Masonry Buildings, in The Seismic Assessment of Existing Buildings, at eq-assess.org.nz. Attributed to Holmes Consulting Group.

1.  Spot cavities with algebra

It was common practice to have a vertical cavity in brick walls. The cavity provided a barrier to moisture, and the outer wall could be made from higher-quality bricks that gave a finer appearance to the building. (The cavity is the black line running down the wall in the picture above.)

There are various complexities pertaining to the ties that were used to hold the layers of wall together across the cavity, how those ties have lasted, and how to assess the relative motion of the outer wythe (the veneer of good bricks) and the inner wythe(s). I won’t go into those here beyond noting that they exist. The question is, how do you know whether there’s a cavity, if you haven’t made a hole in the wall?

Algebra to the rescue! Well, multiplication, anyway. Brick are standard sizes. They’re usually 110mm thick (taking “thickness” as the dimension going into the wall). With some mortar in between wythes, that means that two wythes (layers) ≈ 230mm, three wythes ≈ 350mm, and four wythes ≈ 450mm. So if your wall thickness isn’t pretty close to one of those numbers—check for a cavity.

Insufficient connections between (floor and ceiling) diaphragms and walls leading to out-of-plane collapse. Section C8, as above, eq-assess-org.nz

2. Loose diaphragms fall out (sorry)

We’ve heard on site about floor joists sitting on ledges and maybe falling off when the ground shakes. We’ve also heard, on almost every site we’ve visited, about how tying the diaphragm into the walls at floor height, ceiling or both, can improve the structural performance of the building. What Section C8 makes clear is that the diaphragm can act to redistribute loads from out-of-plane to in-plane walls. If the diaphragm deforms too much, though, it won’t be able to support the walls effectively. In fact, too much diaphragm deformation can actually shove a face-loaded wall right off the edge of the building.

In-plane sliding on a damp-proof course. Section C8, as above, eq-assess.org.nz

3. Slip-‘n’-slide on a damp-proof course

For walls loaded in-plane (along their long axis), one of the possible failure modes is in-plane sliding. I wrote about one version of this in my post on bed joint shear testing. Section C8 points out that damp-proof courses can also be vulnerable to sliding.

To stop moisture wicking upwards through porous masonry, a layer of damp-proof non-porous material was commonly included in brick walls. Bitumen, slate, lead, or similar waterproofing was laid down in a continuous layer, usually not far above the foundations. (At a building I saw this summer, the DPC is granite, but that’s exceptional.)

Turns out, those damp-proof materials are softer than the masonry, or perhaps bond less well to the surrounding materials. Or maybe the change of material just provides a stress concentration. I don’t know! Still, I’d never seen or heard about DPCs as a site for sliding until I read C8.

Parapets tied back to a roof with raking braces — but are they sufficiently restrained in the vertical direction? Section C8, as above, eq-assess.org.nz. Attributed to Dmytro Dizhur.

4. Vertical tie-down for parapets

It’s a common sight around the traps to look up and see a steel brace holding back a parapet. Job done, right? Maybe not. Section C8 points out that such braces may not have enough capacity to deal with vertical displacement, especially when shaking of the roof plane is amplified by the brace and transmitted to the parapet. Quoth C8: “The danger of non-robust strengthening is that the parapet still fails, but collapses in larger, more dangerous pieces.” Not good. The parapet may need to be drilled and post-tensioned onto the top of the wall below.

As axial load increases, masonry walls gain strength from confinement. From Section C8, as above, eq-assess.org.nz. Attributed to Dunning Thornton.

5. Look out above

I suspect if you’d asked me whether masonry buildings suffered more damage at the top or the bottom in earthquakes, I’d’ve said the bottom. Makes sense, right? The walls crush and they turn over? Wrong. Generally speaking, the more axial load that is on the masonry walls, the stronger they are and the better they resist disintegration. Of course, this depends on the building form, the shaking, and other things, but the relationship between axial load and strength is useful to know.

Failure modes. Left, out-of-plane failure: instability of wall insufficiently tied in. Right, in-plane failure: spandrel failure, diagonal tension cracking, toe-crushing of piers. [Right, Section C8, as above, eq-assess.org. Attributed to Sharpe. Left, CCC Heritage, licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 New Zealand License]

6. We don’t talk about failure here

Section C8 enumerates the failure modes of URM buildings. I won’t write a list or a summary, but I will say that I found it really clarified my thinking about how buildings work to consider a finite list of failure modes. When I look at a structure now, I feel much better informed about how to break it into chunks and think about how each chunk might move and how it might fail. From that perspective, reading C8 was like taking your American mate to the cricket and telling him where to look.

Hierarchy of vulnerability, Section C8, as above, from eq-assess.org.nz

6b. And what to look for

An afterthought to the above. In assessing the structure, the C8 guidelines suggest working from left to right on this hierarchy chain, with the idea that the most vulnerable components of the building, those which are likely to fail first, are at the left. No good wasting time and money diagnosing a complex in-plane failure mode if the parapet’s not secured.

Ten steps. The assessment procedure, Section C8, as above, from eq-assess.org.nz

7. Ten simple steps…

I suppose I haven’t much to add to the image above. These are the guidelines which C8 provides to engineers as advice on how they should approach assessing a building. As with #6 above, for me this helped to understand how engineers divide the building into a set of observations and parameters which allow a model to be created—and don’t bother creating one that exceeds the complexity of the structure! I will be trying to hold these ideas in my head on our next site visit (St Paul’s Church on the 13th of March) and to think about how I’d approach the task of assessing the building. Thankfully we’ll have Peter Liu from EQ STRUC with us to show us how it’s really done!

A video from the Uminho Research Group on Historical and Masonry Structures. This is apparently a strengthened model. Still, watch the upper sections of the wall crack at the floor line and rock.

8. Walls rock

Walls under face load can be modelled as rocking. This means if the load is perpendicular to the wall, the wall can be “assumed to form hinge lines at the points where effective horizontal restraint is assumed to be applied… At mid height between these pivots… a third pivot point is assumed to form.”

When I read those words I recalled John O’Hagan talking about this at Hopetoun Alpha, but I think I understand it a little better now. To me, it feels different to think about a masonry wall in an earthquake as two rigid panels teetering one atop the other, as opposed to thinking about n bricks shuffling about independently, or as one rigid surface.

And this is the note on which I’ll leave this post. C8 offers quite a lot of guidance about how to make simplifying assumptions that allow analyses to be made, the rocking walls being one. It also offers suggestions for how to calculate important parameters if you can’t or haven’t tested them—things like tensile strength of the masonry. The impression I had, on reading these guidelines, was that the task of doing this kind of work myself someday seemed not impossible. Surely that’s the most sincere praise I can offer.