Unit 4: Designing Structural Steel
Structural steel design is an area of structural engineering. It started as a simple process, much like artwork, with designers using intuition and experience. Later it became more complex, as designers began using scientific methods and computations to check the safety of structures. Steel design seeks to find the optimum structure that will minimize cost, weight, construction time, labor, and cost of manufacturing the components, while maximizing efficiency in operations.
After completing this unit students will have an introductory grasp of:
- Structural steel design
- ASD and LRFD design philosophies
- Design rules of thumb and their great importance today
- Design of members, member shapes, column base plate design
Builders and designers began using metal as a structural material in 1777-1779 when cast iron was used to support a 100 ft. arch span on a bridge built in England. From 1780 to 1820 a number of cast iron bridges were constructed, mostly arch-shaped with the main girders consisting of individual cast iron pieces forming bars or trusses. Until 1840, cast iron was also the primary material used for chain links on suspension bridges. Wrought iron replaced cast iron around 1840, the earliest example being the Brittania Bridge in Wales, made from wrought iron plates and angles. While cast iron and wrought iron were becoming more popular in the construction industry, the process of rolling various shapes was developed. The rolling of rails began about 1820 and incorporated I-shapes by the 1870s. Three key developments led to the wide spread use of iron ore products in building materials: the Bessemer process (1855), a basic liner in the Bessemer converter (1870), and the open hearth furnace. Since 1890, steel has replaced wrought iron as the principal metallic building material. With a wide range of yield stresses and shapes, steel has become widely available for structural use.
Structural steel is about 98 percent iron alloy and available in a wide range of grades. These grades are grouped together based on properties such as strength, malleability, corrosion resistance, cost, and welding capability. The tallest structures today, commonly referred to as "skyscrapers," are built using structural steel due to its constructability, as well as its high strength-to-weight ratio. In comparison, concrete, being less dense than steel, has a much lower strength-to-weight ratio. This is because a structural concrete member, supporting the same load as a steel member, requires a much larger volume. Steel, being denser, does not require as much material to carry the load. This advantage becomes insignificant for low-rise buildings, which distribute much smaller loads, typically making concrete the economical choice (especially true for simple structures, such as parking garages). However, structural steel can be erected as soon as the materials are delivered on site, whereas concrete must be cured at least 1–2 weeks after pouring before construction can continue, making steel the schedule-friendly construction material. Steel is also inherently a noncombustible material, but when heated to temperatures seen in a fire scenario, the strength and stiffness of the material is significantly reduced. The International Building Code requires that steel be enveloped in sufficient fire-resistant materials, which can increase the overall cost of a steel structure building. Steel, when in contact with water, can corrode, creating a potentially dangerous structure. Measures must be taken in structural steel construction to prevent any lifetime corrosion. Fire resistant materials used to envelope steel are commonly water resistant. If no fire suppression is being used, the steel can be painted to provide water resistance. These are important material properties to keep in mind when designing with steel.
Structural steel design is an area of structural engineering. It started as a simple process, much like artwork, with designers using intuition and experience. Later it became more complex, as designers began using scientific methods and computations to check the safety of structures. Today, steel design requires a knowledge of methods and materials, structural analysis, and structural stability. Steel design seeks to find the optimum structure that will minimize cost, weight, construction time, labor, and cost of manufacturing the components, while maximizing efficiency in operations. The design procedure contains two main parts, functional design and structural design. Functional design focuses on adaptations to the site: working area, ventilation, transportation, lighting and aesthetics. Steel can be dangerous to erect, especially for tall structures. Therefore, in the construction planning process, it becomes very important for contractors to plan the worksite flow and notify workers of any hazards. Structural design is the selection and arrangement of structural members. These members must be checked for load-bearing stability, or safety. This process includes structural configuration, establishment of loads, member selection, structural analysis, evaluation of safety, and then a re-design of the member if needed.. Strength can also be reserved for occasional overloads and variations in member strength based on workmanship and installation. Design of structural steel follows the rules developed by AISC, ASD and LRFD standards.
Steel structures are made up of members, or elements, such as a beam, column, girder, or brace. A column is a structural member that carries its primary loads in compression or tension parallel to its axis. A girder is a large primary element used to carry point loads along its length, usually supporting a beam or column. A beam, opposite a column, carries its primary load in bending perpendicular to its axis. A brace is a structural element used to stiffen or support a portion of the structure or frame. An anchor bolt is an embedded bolt or threaded connection used to attach column bases and transfer loads to the foundation.
A column base plate is a thick plate at the bottom of a column through which anchor bolts mechanically connect the column and transfer forces to the foundation. You can see these members in Figure 1, i.e. with a floor slab (hidden) sitting on this frame acting as the primary load. Members also come in different shapes, as you can see in Figure 2. Standardized shapes satisfy the strength requirements of a member while minimizing the cost of the material. Many structural steel shapes are available: the American standard I-beam or S-shape, the wide flange or W-shape, the American standard channel or C-shape, HSS shape (hollow structural steel), L-angle bar, T-bar and the Z-bar.
For load-bearing beams and columns, the W-shape is most commonly used because its large cross-sectional area provides superior strength. Note the location of the web and flange on the I-beam pictured in Figure 3.
Philosophies of Design
Structural steel design follows ASD and LRFD standards. ASD, Allowable Stress Design, was the first to be developed by AISC, but it contained no variability between dead and live loads, and no probability of occurrence for snow, rain, and wind. These disadvantages and drawbacks of ASD led to the development of LRFD. Load and Resistance Factor Design, or LRFD, incorporates variability and probability through load factors in both resistance and effects of the load. Please refer to the most current AISC Steel Construction Manual for the latest ASD and LRFD load combinations. The required design strength, Ra, is determined from the largest load combination. Rn, or nominal design strength, is a given property of the steel member. If using ASD, you then divide Rn by the safety factor, Ω, and make sure the following equation is satisfied: Ra ≤ Rn / Ω. For example, the safety factor for elements in bending is 1.67. If using LRFD, you then multiply the nominal strength by the resistance factor, φ, and make sure the following equation is satisfied: Ra ≤ φ Rn. Essentially, you want the required design strength to be less than the actual design strength, which is the nominal strength multiplied by φ, or divided by Ω.
The resistance factor varies depending on the type of member: tension member (yielding state), tension member (fracture state), compression member, beam, or fastener. LRFD is used in most structural steel design applications because it incorporates probability and is more accurate than ASD.
Design of Members
When designing structural members it is important to first look at the architectural model and understand the overall concept and use of each particular member. Resistance to deformation is based on the material, length, and cross-sectional area of a member. Three strength limits need to be calculated and checked for safety: flexure, shear, and torsion. The serviceability limits, cracking and deflection, also must be calculated for confirmation of the structure’s stability. After loads are calculated and analysis is done, you can then decide if these members are adequate based upon these limits. The AISC Steel Construction Manual is a very useful resource when looking up material properties and completing strength calculations.
This unit provided an introduction to the history of structural steel in the construction industry. It covered the basics of design philosophy for structural steel and discussed the various shapes that can be utilized in structural steel construction. The unit also took a closer look at structural steel design by walking through the concept of designing a steel beam and coordinating a designed member for real world coordination problems. Structural steel design can be complex, as designers use scientific methods and computations to check the safety of structures.