Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition

Chapter 1 – Introduction to structural design: Material properties

Wood, steel and concrete are actually extraordinarily complex materials. Of the three, wood was used first as a structural material, and some of the otherwise inscrutable vocabulary of structural analysis derives from this fact: the notion of an "outer fiber" of a cross section, or even the concept of "horizontal shear" are rooted in the particular material structure of wood.

Only certain material properties are of interest to us here — specifically, those that have some bearing on the structural behavior of the elements under consideration. The most obvious, and important, structural properties are those relating force to deformation, or stress to strain. Knowing how a material sample contracts or elongates as it is stressed up to failure provides a crucial model for its performance in an actual structure. Not only is its ultimate stress (or strength) indicated, but also a measure of its resistance to strain (modulus of elasticity), its linear (and presumably elastic) and/or nonlinear (plastic) behavior, and its ability to absorb energy without fracturing (ductility).

Ductility is important in a structural member because it allows concentrations of high stress to be absorbed and redistributed without causing sudden, catastrophic failure. Ductile failures are preferred to brittle failures, since the large strains possible with ductile materials give warning of collapse in advance of the actual failure. Glass, a non-ductile (i.e., brittle) material, is generally unsuitable for use as a structural element, in spite of its high strength, because it is unable to absorb large amounts of energy, and could fail catastrophically as a result of local stress concentrations.

A linear relationship between stress and strain is an indicator of elastic behavior — the return of a material to its original shape after being stressed and then unstressed. Most structural materials are expected to behave elastically under normal "service" loads; but plastic behavior, characterized by permanent deformations, needs to be considered when ultimate, or failure, loads are being computed. Typical stress-strain curves for wood, steel and concrete are shown in Figure 1.48. The modulus of elasticity, E, is the slope of the curve — i.e., the change in stress, σ, divided by the change in strain, ε. For linear materials:

E = σ/ε

(1.6)

The most striking aspect of these stress-strain curves shown in Figure 1.48 is the incredibly high strength and modulus of elasticity (indicated by the slope of the curve) of steel relative to concrete and wood. Of equal importance is the information about the strength and ductility of the three materials in tension versus compression. For example, structural carbon steel, along with its high strength and modulus of elasticity, can be strained to a value 60 times greater than shown in Figure 1.48 in both tension and compression, indicating a high degree of ductility. Concrete, on the other hand, has very little strength in tension, and fails in a brittle (nonductile) manner in both tension and compression. Wood has high tensile strength compared to concrete, but also fails in a brittle manner when stressed in tension; in compression, however, wood shows ductile behavior.

Figure 1.48: Stress-strain curves for structural materials

Aside from this stress-strain data, material properties can also be affected by environmental conditions, manufacturing processes, or the way in which loads are applied. These material-dependent responses are discussed in the chapters that follow.

Sustainability

Sustainability is a notoriously inadequate term, as its use in both casual speech as well as in green building guidelines has no consistent relationship to the ongoing maintenance (or rather, degradation) of human life and natural resources on planet Earth. Nevertheless, facts relating to at least one aspect of global environmental welfare — the production of greenhouse (global warming) gases — can be established for wood, steel, and concrete.

A tree, as is well known, extracts CO₂ from the atmosphere as part of the photosynthesis process; carbon — formerly in the atmosphere — is sequestered in the material of the tree itself until the wood is left to decay, at which time it releases the carbon back into the atmosphere in the form of CO₂. To the extent that trees are "farmed" on plantations, i.e., planted and harvested like any other agricultural crop, there is neither a net loss nor gain in greenhouse gases from the material itself. There are, however, greenhouse gases emitted from the cutting, transportation, and especially the drying of wood in kilns. Other greenhouse gases, e.g., formaldehyde, are associated with the glues used in various engineered wood products such as plywood. It is difficult, however, to find data about the overall impact of forest products on global warming.

Information is more readily available for steel and concrete, as both of these materials leave a significant mark on greenhouse gas emissions. The manufacture of steel releases CO₂ at an average rate of 1.8 tonnes per tonne of steel produced (where 1 metric ton, or tonne, equals 1000 kilograms, which in turn equals 2205 lb or about 1.1 ton). This corresponds to about 6.7% of overall global CO₂ emissions generated by humans (and results from about 1.6 billion tonnes of steel being produced annually). Much of this CO₂ is due to the persistence of less-efficient steel production technology in many parts of the world: old-fashioned and inefficient open hearth furnaces are still in use, for example, in Russia and Ukraine, while other inefficient technologies have been developed or persist because they exploit regionally available materials or expertise. In the U.S., steel is produced in electric arc furnaces (having the least global warming impact) or basic oxygen furnaces (with the next least global warming impact).

Concrete's large contribution to global warming comes about for two reasons. First, more concrete is consumed than any other material on earth, discounting water. Second, it turns out that heating limestone to obtain calcium — a necessary ingredient in portland cement — releases quite a bit of CO₂, as does the burning of fossil fuels to create the heat needed to drive this process of calcination. While "only" 1 tonne of CO₂ is produced for every tonne of cement (compared with 1.8 tonnes of CO₂ for every tonne of steel), the 2.35 billion tonnes of concrete produced annually result in about 5% of the world's human-generated CO₂. These statistics are, of course, subject to change, and reflect approximate values from the years 2010–2014.