Incompatibility: Cavity Walls and Style

by Don Friedman on October 13, 2016

Let’s assume we’ve made it past the stiff-wall/flexible-frame trap. Like the hero of an adventure movie, we’ve designed our way past by swinging on vines over the abyss. We’ve still got a problem: how do we build our masonry walls?

Old building codes assumed bearing-wall structure and provided rules for the thickness of the walls. Here’s a page of the 1892 New York Building Code:


laws-relating-to-buildings-in-the-city-of-new-york-1892-unknown-dragged


There’s information about the minimum allowable thickness of the wall, but what I want to emphasize is the wall itself. It is a solid undifferentiated mass of brick.

From the good folks at Architekwiki, here’s a modern cavity-wall detail, showing how a standard masonry wall is built today:


b2010-b-masonrycavitywall


By my count there are eight separate elements making up the wall proper, with more to finish off the interior. This is no longer undifferentiated. This is now a system of some complexity.

So far, I’ll I’ve done is point out changes in construction technology over time, which are no surprise to anyone in the field and are a natural occurrence with any technology. But if we combine that with historicist architectural styles, the problems start. First, the single wythe of veneer in the modern system is tied back to the heavier concrete block, but that’s not enough for complete load transfer of the kind that happens in a solid wall. If the building is designed in a per-modernism architectural style – American Renaissance, for example – it will have projecting ornament. Structurally, water-tables and window hoods are cantilevers  that have to be restrained from rotating outwards. The single wythe of veneer is usually not capable of doing this, so we need to introduce more pieces of metal into the cavity as lateral structural supports for the projections. In a wall with a lot of sculptural relief, the amount of projection bracing may outweigh the ordinary veneer ties.

This kind of thing works – as matter of fact, we’ve worked on exactly this kind of wall design multiple times – but it’s a kludge. The architectural design developed for use with solid walls and needs a lot of hidden supports to work with cavity walls. There’s a basic mismatch between the architectural style of the wall and the constructive form of the wall. Unfortunately, telling people that their technology works better with a different architectural style is not a way to make friends.

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