2.6 Section Types and Their Properties
Steel design extensively uses section properties. The most common are area A, plastic section modulusZ, elastic section modulusS, moment of inertiaI, and radius of gyrationr. A and Z relate to strength, while I and r relate to stiffness and stability, respectively. J and C are torsion properties—most commonly used for closed shapes. For simple shapes, Table 2.12 provides equations for properties of round and rectangular shapes. For more complex shapes, use the tables in Appendix 1 or the AISC Steel Construction Manual.
The AISC Steel Construction Manual7 provides dimensions and section properties for a multitude of structural shapes. Appendix A1 provides a selection of these shapes for use with this book. The tables typically have the heaviest, several intermediate, and the lightest sections in each size group.
- Wide Flange (W)—Used for beams, columns, and braces—Table A1.1
- Standard (S)—Used for crane rails
- Pile (HP)—Used for driven piles
- Channel (C)—Used for light beams and equipment—Table A1.2
- Angle (L)—Used for bracing and connections—Table A1.3
- Hollow Structural Sections (HSS)—Used for columns, beams, braces—Table A1.4
- Pipe (P)—Used for columns and bracing—Table A1.5.
2.6.1 Wide Flange Sections
Wide flange sections are the most common shape for moderate to heavy loads. They are denoted with a W, followed by their nominal depth and weight per foot. A W18 × 40 (W460 × 60) is exactly 17.9 in (454.7 mm) deep and 40 lb/ft (60 kg/m). Note that for heavy sections in the series, the nominal depth can be off by as much as 6 in (150 mm), which needs to be considered when laying out floor space.
Wide flange sections are grouped by flange width in the steel manual. This allows us to select shapes that are more suited as beams, columns, or
Table 2.12 Section property equations for round and rectangular shapes
| Round | Rectangle | |
 |
 |
|
| Area | ||
| A = πr2 | A = bh | |
| Moment of Inertia | ||
| I = πr4/4 | Ix = bh3/12 | Iy = hb3/12 |
| Radius of Gyration | ||
| rz = r/2 | ||
| Plastic Section Modulus | ||
| Z = 8r3/6 | Zx = bh2/4 | Zx = bh2/4 |
| Section Modulus | ||
| S = πr3/4 | Sx = bh2/6 | Sy = hb2/6 |
| Q = 2r3/3 | Qx = bh2/8 | Qy = bh2/8 |
| at center | ||
| Polar Moment of Inertia | ||
| J = πr4/2 | ||
| for any shape | ||
in between. Looking at the W12 (W310) series table in Appendix A1.1, we see five groupings of shapes. The bottom two groups (W12 × 14 through W12 × 35, or W310 × 21.0 through W310 × 52 in metric) are most suited for beams. The top groups (W12 × 65 to W12 × 336, (W310 × 97 to W310 × 500)) are most suited for columns. The middle two (W12 × 40 to W12 × 58 (W310 × 60 to W310 × 86)) work well for light columns, beams with axial loads, and beams without lateral bracing along their length.
The W12 (W310) tables in Appendix 1.1 are typical of the organization in the steel manual. For sections in each group, the flanges will line up when spliced, an important consideration when joining columns. The other shapes in Appendix 1 have been grouped together for space reasons.
2.6.2 Channels
Channels are used for light beams, often on platforms and stairs. They make poor columns, given their narrowness in the weak direction. There are two types of channels, C and MC. Channels are designated by C followed by their actual depth by weight per foot. A C8 × 11.5 (C200 × 17.1) is exactly 8 in (203 mm) deep, and 11.5 lb/ft (17.1 kg/m). They are grouped in the steel manual by depth.
2.6.3 Angles
Angles are commonly used as light braces and connection material. They are denoted by L and followed by the actual long and short leg dimensions, followed up by the leg thickness. An L3 × 2 × 1/4 (L76 × 51 × 6.4) is 3 in (76 mm) tall, 2 in (51 mm) wide, and 1/4 in (6.4 mm) thick. They are grouped in the steel manual based on leg size.
2.6.4 Hollow Structural Sections
Hollow Structural Sections are rectangular or round in shape. They are common as columns in shorter buildings, braces, beams, and truss members. They have substantially better torsional properties than their counterparts. These shapes are designated by HSS, followed their physical dimensions, and wall thickness. A rectangular HSS12 × 6 × 1/2 (HSS304.8 × 152.4 × 12.7) is 12 in (304.8 mm) deep, 6 in (152.4 mm) wide, and has a 1/2 in (12.7 mm) wall. A round HSS12.750 × 0.500 (HSS323.9 × 12.7) has a 12.75 in (323.9 mm) outside diameter and 1/2 in (12.7 mm) wall thickness. They are grouped by size.
2.6.5 Pipe
Pipes are used for light columns and braces. They are designated by nominal inside diameter—different from the actual. They come in three weights, standard (STD), extra strong (X), and double-extra strong (XX). A pipe designated as Pipe 6 X-strong (Pipe 152 X-strong) has a nominal 6 in (152 mm) inside diameter, and 5.76 in (146 mm) actual inside diameter, and 6.63 in (168 mm) actual outside diameter. They are grouped by diameter in the steel manual.
2.7 Construction
Steel construction progresses as follows:
- A fabricator takes the engineering drawings and prepares erection and shop drawings. The erection drawings show each piece of steel and where it is located, and any field details. The shop drawings show every hole, plate, and weld on each piece of steel.
- The fabricator makes each piece of steel and inspects their work. AISC approved shops have internal quality control, unapproved shops have external inspectors.
- Iron workers erect the steel, with two bolts in each connection initially and temporary bracing.
- Workers rack and plumb the structure, then final bolting and welding occurs, along with connections to lateral systems like shear walls.
- Workers lay down the corrugated metal deck and weld headed studs to the composite beams.
- Pump trucks place concrete over the metal deck to form the slab.
An important consideration for steel erection is whether to field weld or bolt most joints. Field welding can result in smaller joints, especially in seismic moment connections. However, wind can affect the shielding gas around the electrode and reduce quality. Additionally, in remote areas, field welders may be hard to find. Field bolting connections can be faster and reduce field welding, though the joints can become rather large in seismic connections. There is not one right answer, just possibilities.
2.8 Quality Control
Quality control of steel construction is critical to its successful performance. Variability in quality may come from steel rolling, fabrication, and erection. Ensure safeguards are in place that the things that matter are inspected. Table 2.13 summarizes key inspection activities, which are
Table 2.13 Summary of steel inspection requirements
| Inspection Type | Periodic | Continuous | Requirement |
| Submittals | Shop certifications | ||
| Erection and shop drawings | |||
| Mill test reports (MTR) | |||
| Welder qualifications | |||
| Inspector qualifications | |||
| Bolt storage and installation procedures | |||
| Weld Inspection | Complete and partial penetration welds, except flare-bevel welds | ||
| Multi-pass fillet welds | |||
| Single pass fillet welds greater than 5/16 in (8 mm) | |||
| Single pass fillet welds equal to or smaller than 5/16 in (8 mm) | |||
| Floor and roof deck puddle welds | |||
| Welding for stairs and railings | |||
| Weld Testing | Visually inspect all welds before releasing pieces for installation. | ||
| Test all complete joint penetration welds by radiographic or ultrasonic testing. | |||
| Ultrasonically test base metal thicker than 1 1/2 in (38 mm) when subject to through thickness weld strains. | |||
| Magnetic particle test beam-column CJP welds | |||
| Snug Tight Bolt Inspection | Fastener grades are installed where indicated in drawings. | ||
| Storage and cleanliness of high strength fasteners. | |||
| Faying surfaces are in firm contact for snug tight bolts. | |||
| Washers are installed as required. | |||
| PreTensioned and Slip Critical Bolt Inspection | Confirm fastener assembly and wrench systems are suitable for pretensioned and slip critical installations. | ||
| Test fastener combinations and calibrate wrench systems | |||
| Fasteners are in snug-tight condition prior to pretensioning. | |||
| Witness pretensioned and slip critical fastener installations and confirm conformance. | |||
| Verify surface condition of faying surfaces attached with pretensioned fasteners. |
Source: Ingenium Design
exhaustively detailed in the codes. Additionally, Chapter 10 in Special Structural Topics8 in this series provides an in-depth look at steel quality control.
2.8.1 Field Observations
We frequently visit the construction site to observe progress and handle challenges that arise. An engineer or architect should not direct the work. Rather, according to the contract, he or she should communicate their finding in writing to the contractor, architect, owner, and jurisdiction. Things to look for while in the field include:
- Member size and orientation. If the erection and shop drawings are correct, it is likely this won’t be a problem. But it’s always worth looking.
- Field holes. Confirm torch burning of holes has not taken place.
- Welding. Observe the welds and see if they are smooth and have a fluid V pattern, as shown in Figure 2.15. If they are bumpy and discontinuous, there is a problem.
Figure 2.15 (a) Good quality, and (b) poor quality single-pass fillet
- Bolts. Confirm all holes are filled with bolts. Observe the bolt head markings and confirm the right grade is used. Spot check the bolt sizes and make sure they follow the plans.
- Bolt Pretensioning. For pretensioned and slip critical joints confirm the bolts have been properly tensioned. This might be through direct tension indicator washers or field inspection reports.
- Base plate grouting. Confirm base plates are grouted and it stops at the bottom of the baseplate. If it goes up the sides, it will crack.
2.9 Where We Go From Here
From here we will dive into tension, bending, shear and compression member design. We then get into lateral design, and end with connections. By the time you are finished studying this book, you will be familiar with what structural engineers use 80% of the time in steel design.
Notes
1. Teran Mitchell. “Structural Materials.” In Introduction to Structures, edited by Paul W. McMullin and Jonathan S. Price. (New York: Routledge, 2016).
2. AISC, Specification for Structural Steel Buildings, AISC 360 (Chicago: American Institute of Steel Construction, 2016).
3. AISC, Code of Standard Practice for Steel Buildings and Bridges, AISC 303 (Chicago: American Institute of Steel Construction, 2016).
4. RSSC, Specification for Structural Joints Using High-Strength Joints (Chicago: Research Council on Structural Connections, 2014).
5. AISC, Seismic Provisions for Structural Steel Buildings, AISC 314 (Chicago: Research Council on Structural Connections, 2016).
6. AISC, Steel Construction Manual, 14th edition (Chicago: American Institute of Steel Construction, 2011).
7. AISC, Steel Construction Manual.
8. William Komlos. “Quality and Inspection.” In Special Structural Topics, edited by Paul W. McMullin and Jonathan S. Price. (New York: Routledge, 2017).