Monday, January 16, 2012

The curious case of masonry building code for triangular block

Building code requirements can present a unique quandary to the introduction of fundamentally innovative techniques and methods in masonry construction.

A discussion of building code and innovative masonry construction begins by describing the alphabet soup of government and industry organizations involved in developing and commenting on this code.  Here are some of the major players in developing building code for masonry construction, both in the US and internationally:

ASTM (ASTMI): American Society for Testing and Materials (International)

TMS:  The Masonry Society

NCMA:  National Concrete Masonry Association

ACI:  American Concrete Institute

The requirements for concrete masonry units (block) are described in “ASTM C90 - 11a Standard Specificationfor Loadbearing Concrete Masonry Units.”  This document has evolved over the years, and is changed as industry and uses change.  It is written by committee members composed of ACI members.  This document is commented in a document produced by NCMA simply titled “NCMA Commentary Discussions to ASTM C90.”

The first difficulty lies in finding a proper classification for the block system I have developed.  Is it loadbearing or non-loadbearing?  According to ASTM et al, if a structure does not support anything other than its own weight, it is non-loadbearing.  Since my block system supports only itself, it is technically a non-loadbearing structure.  This is of course untrue, since “only itself” includes a dome or arched roof, which means that it is loadbearing.

Another difficulty lays in the fact that concrete masonry units have become standardized to such an extent that there is no allowance for a triangular-shaped masonry unit.  All masonry units are assumed to be rectangular, and are even illustrated as such within the code specification.

This rectangular bias, or prejudice, or way of assessing masonry creates a number of difficulties relative to triangular block.  Unit strength, cracking, density, appearance, water resistance (absorption), and thermal & acoustic insulation are all affected by whether or not the masonry unit is rectangular or triangular.  Fortunately, triangular block which are used to assemble into a sphere, or dome or arch are stronger, less crack prone, more water resistant; and may be made more or less dense depending on wall thickness – thus effecting insulation properties also.

Ultimately, the code is written in such a way to allow for the designer and builder to exercise their own expertise, experience, and professional judgment in their projects.  It remains a question whether or not the existing code could be modified to describe triangular block used to build spheres, domes, arches, cylinders, etc., or if a whole new section would have to be added.  I am reminded of how much I love to sit on committees. 


  1. where i live, in the south west united states theres not much code enforcement. if i wanted to experiment with triangular adobe blocks do you think that it would be possible to make forms for triangular bricks? im talking solid mud bricks. also do you think they would be too heavy? its a dome... how would you build the interior formwork until the dome is complete? i dont have much experience in this, but would love to experiment with some small structures.

  2. I think the key to your idea would be the compressive strength on your adobe. If they crumble easily, you might consider adding a cement to your mix. Also, you should weather proof the outer surface (water must shed, not dissolving the adobe). The wall thickness (from inside to outside) is a critical element in masonry domes. You can obtain a thick wall while reducing weight by forming a cavity, or depression, or hollow core in the block. Wall thickness depends on the strength of your adobe and the diameter of your dome. For more on this, you might want to look at some of my other entries:

    It is certainly possible to make your molds from plywood. This was how I did my first blocks. I hinged them together at the corners, and they simply "unfolded" to release the cast block.

    In terms of interior formwork, there are several ways to go. You can build a wooden form from plywood, or timbers, using basic geodesic pattern. You could also use bent saplings to describe a geodesic. I think the best solution is to use an inflatable bladder which creates the form. Block are laid against the form and -once the dome is complete- the bladder is deflated and removed. These require very low pressure (as low as 5 psi). I talked about this in a couple of entries:

    When assembling a dome from masonry, as soon as you complete any given horizontal row, it becomes self-supporting when that horizontal row is complete.

    I hope I've answered your questions? Please let me know if I can answer more fully.

    I wish you the best of luck! Masonry domes are wonderful structures. Properly done, they can literally last for thousands of years.

  3. I should also mention, take a look at Musgum 'domes' made in Africa. this catenary configuration would need less internal scaffolding, and they look pretty cool. See it here, toward the end of the entry:

  4. Hi, i'm looking into a design of triangular/pyramid interlocking bricks, wondered if you had some information that may be helpful to me or some comments? i'm studying architecture and my email is, many thanks