Where is the most stress?

In designing and assembling a masonry building, the engineering work can provide helpful insight which is simple and powerful.  

For example, if we consider the masonry building I'm currently completing, it's insightful to ask: where is the highest stress in the building? Where is the highest compressive force, squeezing together?  Where is the highest tensile force, pulling apart?

The engineering for this building was done by Cheng-Ning Jong, PE.  He has some familiarity with my company's masonry system, since he helped compose, file and prosecute all of our patents.  We've worked together for several years and have a good rapport, a comfortable back-and-forth as we discuss, develop and fully articulate ideas.  

Mr. Jong's most critical role, in my opinion, is the detailing of the reinforcement and the size of the concrete footer from which the stem wall is laid.  A 'footer' is the base of the building, typically located in an excavated trench.  Here's a picture of the footer, with the first few block being arranged for the stem wall:

A stem wall is the bottom section of all the vertical walls buried below the ground, sitting on the footer.  Here is a completed stem wall for a room:

This building has arched masonry roofs, domes, half-domes, flying buttresses, arches meeting at intersections; there is a lot of structural configuration, rebar, weight, stress and so on within the building structure. Here are some architectural drawings, showing some of this detail.  Our architect for this building was Robert Ferry, AIA, RDP.

So if we consider this entire structure, where is the most stress?  Where is the highest compression?  Where is the highest tension?

The highest compression occurs at the bottom of the stem wall, where the stem wall meets the footer, on the outside of the building.  Why?  The entire weight of the building sits on this point.  In addition, the vertical wall acts as a giant lever, translating any thrusting force from the masonry roof and increasing this force by the length of the lever, or wall height to this location. This location, at the corner of the stem wall and the footer, wants to act as a hinge on which the lever of the vertical wall acts.  The highest tension occurs at the bottom of the stem wall, where the stem wall meets the footer, on the inside of the building.  The same lever action of the wall wants to pull up from the hinging on the outside, a mere 8 inches away: the wall thickness.

It's useful to note these areas of high stress.  It makes one pay closer attention to the detailing of rebar, rebar placement, connections, centering, etc., when you are consciously aware that the building you're making will have these high stress locations.  Build accordingly, get it right.

Harder, stronger, stiffer, tougher.

Today I’m taking a quick look at material properties and what exactly they mean in terms of the masonry system I’ve been describing.


Strength is defined as the ability of a material to withstand applied stress without suffering failure. As stated before on this blog, ceramics, concrete and glass have a high compressive strength.

Stiffness is defined as a measure of resistance of a material to deformation under an applied stress. Concrete and ceramics generally have a high stiffness. Stiffness is characterized by Young’s Modulus.

Hardness is defined as the measure of how resistant a material is to permanent shape change under an applied stress.

Toughness is defined as how resistant a material is to crack propagation. It is the resistance to fracture under applied stress; it is the amount of energy per unit volume that a material can absorb before rupturing.

Concrete and ceramics have a high compressive strength, a high stiffness and hardness. Concrete and ceramics are generally not very tough: they will tend to crack because of their somewhat brittle nature. The larger the item, the greater tendency to suffer cracking from applied stress. As the size increases, this tendency toward cracking increases exponentially.

The masonry system I’ve been describing is strong, stiff, hard and tough. This system gains its toughness by virtue of the fact that it is composed of relatively small unit shapes. In this sense, it is “pre-fractured” and is held together by the interlocking features. That is, a large structure is less likely to break if it is already “broken” into component parts.

The component parts in this case are masonry units which are free to move (strain) under an applied force (stress). Strain relieves stress. This results in a strong, hard and tough structure.