Strength Design Method Gains Foothold in Structural Engineering
Evolution has a place in structural design, as in any field. Better materials come along. New techniques emerge. Technology changes everything. These and other circumstances routinely engender changes in the engineering of safe and functional structures.
An important evolutionary moment came more than 50 years ago when engineers began experimenting with a different way to determine what was needed to keep a building upright. They developed a method-called strength design-that emphasizes the strength of materials rather than the stress produced by loads. Their calculations use variable overload factors and resistance factors to establish structural specifications.
Strength design, first adopted in concrete construction, has stood its own test of time. Slow but sure, the method has come into wider use as an alternative to allowable stress design, an industry standard for generations of structural engineers.
Where is strength design half a century later? And what kind of foothold does it have in engineering design and construction?
Explicit Factor of Safety
The gradual rather than speedy acceptance of one method over another is understandable, say Fattah Shaikh, professor of engineering and chair of the Department of Civil Engineering and Mechanics in the College of Engineering and Applied Sciences at the University of Wisconsin-Milwaukee, and structural engineer Tom Whittow. They have been part of that evolution, in different construction materials and in the education of new engineers.
Strength design and allowable stress design both use combinations of dead and live loads to engineer a safe structure. Strength design additionally uses varying resistance factors for individual structural elements to calculate the factor of safety precisely, accounting for variations in material properties, design procedures, and construction quality.
Material values are not perfectly repeatable. Engineers assume, for example, that one steel beam always has a slightly different resistance from the next. That's where strength design principles come in, says Tom Whittow, vice president and partner in Computerized Structural Design, a Milwaukee engineering firm. Whittow's practice area is industrial structures and steel construction.
Although he is a strong proponent of traditional stress design, Whittow uses the newer method when appropriate. Allowable stress design held sway in steel until the American Institute of Steel Construction (AISC) began updating codes to include strength design more than 15 years ago. In the steel lexicon, strength design is known as load resistance factor design, or LRFD.
"The advantage of LRFD gets back to the meaning of 'factor of safety' in the first place," Whittow explains. "You can adjust that factor of safety by knowing the materials to be used, the failure type, and the mix of dead and live loads."
Strength design calculations allow the engineer to adjust for bending, axial compression, shear, and other load impacts at a level greater than what may in fact be needed. The engineer is designing for failure, thereby creating a greater factor of safety.
Shaikh says the ability to use material values in absolute terms is a serious advantage of strength design. He is an expert in reinforced concrete construction, the sector where strength design was first applied.
"When I use the known strength of materials as my guide, the factor of safety is very explicit and determinate. As an engineer, I know I have total control because I can assign appropriate load factors to service loads based on their characteristics with respect to type, probability of overloads, and other particulars," Shaikh says.
He adds that because the method considers factored loads and resistance factors together, engineers can be more precise in individual combinations, targeting strength where needed without overbuilding.
The strength design method also requires more detailed calculations with numbers specific to each factor of design and construction. Those specific numbers give the design engineer more flexibility in making changes or choosing materials. There is the chance of cost saving, too, with more economical use of materials. Finally, strength design offers a better assessment of structural behavior that could improve safety.
Growth of an Alternative
Strength design formally came on the scene in the mid-1950s when the American Concrete Institute added code language for its use. There were no prescriptive procedures at first, says Shaikh, but continued research in the method and application by newly minted engineers brought the method into the mainstream. In the latest code updates for reinforced concrete, the strength method predominates to the complete exclusion of stress design.
Shaikh estimates that 95 percent of practicing engineers who work in concrete now follow the methodology of strength design.
Meanwhile, in academia, engineering students have been learning the principles of strength design for years. Shaikh believes strength design cannot be the limit of what new engineers learn. They must be conversant in strength and stress design even in the emerging "strain" design method. He claims that understanding and being able to work in all methods is the measure of the complete engineer.
Whittow agrees. Nevertheless, he notes the current emphasis on strength design in engineering schools will influence the rewriting of codes to conform to the method in all materials. "When the time comes and those engineers trained in strength design are making the decisions in our firms, that method will become the primary method."
The argument for a broader choice of methods, Whittow says, rests with being able to make refinements in structural design when there is a notable difference in the characteristics of structural elements.
In concrete construction, dead loads or the weight of the building and other fixed elements tend to be more substantial than live loads or the activity, equipment, and people in the structure. That makes strength design a natural choice in concrete, Whittow says, because precise calculation of dead loads in the strength of structural elements means measurable savings from not overbuilding.
Steel is not quite the same, he notes, because live loads often are greater than dead loads. Steel construction is common in industrial operations where heavy machinery and materials constitute a more substantial live load. The allowable stress of those loads is a much larger portion so there is less advantage in using the strength design method.
Nonetheless, AISC introduced LRFD in its 1986 specifications. Three revisions later, the 1999 specifications established the method as "state-of-the-art" for steel buildings. All new research and investigations in steel construction now follow LRFD.
Strength design has been available in wood construction for only about five years and in masonry, a shorter time still. Shaikh says training programs using strength design in timber, wood, and masonry are just starting to appear. He reasons acceptance of the method in these materials should happen sooner and shorten the time it takes to apply it in the building process.
Architects and Engineers: Speaking the Same Language
What should architects know about strength design and why does it matter? In the construction equation, architects design, engineers make sure what architects design stays standing. The two disciplines concentrate in different areas and speak their own distinct languages.
But Whittow and Shaikh say the building process is more clear-cut when architects have a reasonable grasp of engineering design methods.
"Architects think about the structure as a whole when they create a design," Whittow adds. "When they do, we can choose LRFD or allowable stress calculations to better accommodate the structure into the building they've designed."
He notes that architects who take courses on structural design acquire a working knowledge of engineering that serves them well. Just understanding the difference between live and dead loads can affect how a building is proportioned.
In Shaikh's view, the more architects comprehend of stability bracing, materials properties, loading conditions, and other aspects of engineering design, the better. "It is absolutely useful for them to be aware of such things to take advantage of the flexibility and precision inherent in strength design."
Building on Strength Design
Advances in computer technology have helped acceptance of strength design as a way to measure the factor of safety in building construction.
Shaikh acknowledges that computers have had a big impact on all aspects of engineering and design. As analysis software improves, however, it becomes an essential and reliable tool. Multiple calculations of different load factors are done quickly and precisely, making the most of the method's flexibility.
Even as the advantages of strength design drive its expanded use and engineers educated in the method take the lead in projects, Whittow and Shaikh conclude there must be more. Engineers cannot rely simply on known values and investigations to establish structural integrity. "There is a need to augment any design method with other considerations," Shaikh asserts. "An engineer's own incisive judgment about the potential of the structure must also be brought to bear."
Tom Whittow and Fattah Shaikh are among several distinguished experts who participate in courses on strength design and other engineering topics offered by the Department of Engineering Professional Development at the University of Wisconsin-Madison. Learn more by visiting our Courses. Or call 800-462-0876.
Written by Mary Maher
This article is based upon work supported by the University of Wisconsin–Madison Department of Engineering Professional Development. It is for general information and distribution. It is not intended to provide specific solutions or advice for specific circumstances, which should be sought from appropriate professionals.