Contents | 1. Introduction to structural design | 2. Loads | 3. Wood |
Introduction to steel |
Steel is subject to corrosion if not protected, and loss of strength and stiffness at high temperatures if not fireproofed (except that, as noted above, weathering and stainless steels resist corrosion). While these are extremely important material properties, the structural design of steel elements presupposes that these issues have been addressed within the architectural design process.
Steel has a distinct elastic region in which stresses are proportional to strains, and a plastic region that begins with the yielding of the material and continues until a so-called strain-hardening region is reached (Figure 4.1). The yield stress defines the limit of elastic behavior, and can be taken as 36 ksi for ASTM A36, or 50 ksi for what is becoming the de facto standard, at least for wide-flange (W) shapes: ASTM A992.
Within the plastic range, yielded material strains considerably under constant stress (the yield stress), but does not rupture. In fact, rupture only occurs at the end of the strain-hardening region, at an ultimate or failure stress (strength) much higher than the yield stress. Bending cold-formed steel to create structural shapes out of flat sheet steel stretches the material at the outer edges of these bends beyond both the elastic and plastic regions, and into the strain-hardening region. This actually increases the strength of these structural elements, even though the direction of stretching is perpendicular to the longitudinal axis of the element.
High-strength steels (with yield stresses up to 100 ksi) are available, but their utility is limited in the following two ways: First, the modulus of elasticity of steel does not increase as strength increases, but is virtually the same for all steel (29,000 ksi). Reducing the size of structural elements because they are stronger makes it more likely that problems with serviceability (i.e., deflections and vibrations) will surface since these effects are related, not to strength, but to the modulus of elasticity.
Second, increased strength is correlated with decreased ductility, and a greater susceptibility to fatigue failure. Therefore, where dynamic and cyclic loading is expected, high-strength steel is not recommended; where dead load dominates, and the load history of the structural element is expected to be relatively stable, high-strength steel may be appropriate, as long as the first criteria relating to stiffness (modulus of elasticity) is met. The most commonly used steels, along with their yield and ultimate stresses, are listed in Appendix Table A-4.1. Allowable stresses and available strengths are found in Appendix Table A-4.2.
Hot-rolled steel shapes contain residual stresses even before they are loaded. These are caused by the uneven cooling of the shapes after they are rolled at temperatures of about 2000° F. The exposed flanges and webs cool and contract sooner than the web-flange intersections; the contraction of these junction points is then inhibited by the adjacent areas which have already cooled, so they are forced into tension as they simultaneously compress the areas that cooled first. The typical pattern of residual stresses within a wide-flange cross section is shown in Figure 4.2. Residual stresses have an impact on the inelastic buckling of steel columns, since partial yielding of the cross section occurs at a lower compressive stress than would be the case if the residual compressive stresses "locked" into the column were not present.
Aside from standard rolled structural shapes, several other structural applications of steel should be noted:
Cold-formed steel is made by bending steel sheet (typically with 90° bends) into various cross-sectional shapes, used primarily as studs (closely-spaced vertical compression elements), joists (closely-spaced beams), or elements comprising lightweight trusses. Manufacturers provide tables for these products containing section properties and allowable loads, or stresses.
Hollow structural sections (HSS) are closed tubular steel shapes that can be formed and welded in various ways from flat sheets or plates; these shapes can be circular, square, or rectangular. Circular pipes are similar to round HSS, except that they are fabricated with a different grade of steel.
Open-web steel joists (OWSJ) are lightweight prefabricated trusses made from steel angles and rods. Spans of up to 144 feet are possible with "deep longspan" or DLH-series joists; regular "longspan" (LH series) joists span up to 96 feet, while ordinary H-series joists span up to 60 feet. These products are relatively flexible, subject to vibration, and are most often used to support roof structures in large one-story commercial or industrial buildings.
Space-frame (actually "space-truss") systems consist of linear elements and connecting nodes based on various geometries, most commonly tetrahedral or pyramid shaped.
Corrugated steel decks constitute the floor and roof system for almost all steel-framed buildings. For floor systems, they are often designed compositely with concrete fill, effectively creating a reinforced concrete floor system in which the reinforcement (and formwork) consists of the steel deck itself.
Cables and rods can be used as structural elements where the only expected stresses are tension, or where the element is prestressed into tension: the flexibility of these elements prevents them from sustaining any compressive or bending stresses. Applications include elements within trusses, bridges, and membrane structures.
© 2020 Jonathan Ochshorn; all rights reserved. This section first posted November 15, 2020; last updated November 15, 2020.