By Steve Wright
In the simplest terms, the purpose of a building is to protect the occupants and contents, not only from the elements but also from the structural failure of the building. Structural failure directly impacts the safety of the occupants and contents, the converse of the building’s purpose. Therefore, structural loads are important to buildings because they directly impact safety.
Unfortunately, the dueling priorities of maximum safety and economy create a fine line to balance. Most of the time, building designs meet the challenge. Sometimes, they do not because cost savings are driving the project. Designers and architects are the first line of defense for safety and should have impenetrable arguments against cost-cutting and for a complete analysis of any changes to the structural drawings.
Design load failures
The following examples of design load failures (www.complex.com/style/2011/12/the-50-worst-architecture-fails) are extreme, but total structural failure can be caused by seemingly minimal errors as well as the stubbornness of a builder or owner.
Hotel New World, Singapore (1986) — The Lain Yak Building, also known as Hotel New World, collapsed in 1986. Upon investigation, it was discovered that, while the structural engineer calculated and designed for the building’s live load, he neglected entirely to consider the dead load. This error led to the rapid collapse of a six-story building housing a hotel, bank, and nightclub. The collapse killed 33 people and entrapped 17 others.
C.W. Post College Dome Auditorium, Brookville, N.Y. (1978) — The domed coliseum collapsed under a snow load during a blizzard. The dome was compliant with all existing building codes, but the engineer only considered uniform loads, thus creating a minimally designed structure that was unable to hold the type of unequal loads of snow and ice deposited by the storm. Fortunately, there were no people in the building at the time of the collapse.
Versaille Wedding Hall, Jerusalem, Israel (2001) — The Versailles Wedding Hall originally was designed to have one, three-story side and one, two-story side. However, the owners made a change in the plans late in the construction phase. They decided to add another floor to the two-story side, exposing the former roof to greater loads than it was designed to handle.
Although partitions were added to help support the extra weight, they were removed for reasons unknown. The floor began to sag, and the owners responded by adding grout and fill, adding more weight to the unstable structure. The floor failed underneath a wedding party that was in progress, killing 23 people and injuring 380 others.
LRFD versus ASD for safely calculating loads
Jim Collins, Ph.D., P.E., at MiTek USA, Inc., provided a comparison of load and resistance factor design (LRFD) with allowable stress (or strength) design (ASD) (www.uspconnectors.com/resources/icc-code-asd-vs-lrfd-design).
Allowable (Available) Stress Design:
- ASD says that the required strength must be equal to or less than the allowable (available) strength.
- The required strength is the applied load, determined by the building code or standard. Required strength represents actual loads that are expected to be applied to the structure.
- The allowable (available) strength reflects the force the keeps the material stress below a predefined maximum allowable stress.
- Allowable strength is the calculated nominal strength divided by a factor of safety (Ω) and represents the actual strength based on engineering principles and the appropriate design specification for the material in question.
Load Resistance Factor Design:
- LRFD also states that required strength must be less than or equal to available strength; however, there is no safety factor term as there is with ASD.
- The nominal strength is instead reduced by a resistance factor (ø), representing the specific material type under a specific load type. Resistance factor is often used for a specific type of failure mode or limit state.
- The resistance factor considers known variations in design assumptions and fabrication or installation factors that are known to have the potential to impact the available strength negatively.
- Also, the required strength is determined by increasing the applied loads based on how they are combined, which increases the expected applied loads and their effects that closely balances the LRFD available strength.
- The increased loads increase the expected applied loads and acknowledge uncertainties about dynamic, time-variable transient loads, using individual load factors to increase them when combined with other loads.
- Load factors also reflect the possibility of overload and probability of maximum transient peaks that may be different when acting in combination with other loads.
In Collins’s opinion, because LRFD uses load factors that increase the “required strength” on the left of the equation, this method of load determination contains a dynamic safety factor and results in a stronger structure for more highly dynamic loads than ASD.
Importance of continuous education in structural engineering
Understanding and designing for structural loads are critical to constructing a building that meets its primary purpose — safety. Not only must a building maintain a safe environment for the occupants, but it must keep the surrounding community safe from the release of toxic or explosive contents.
Essential facilities must withstand structural loads to continue to perform a service, such as protecting standby power generation equipment or government operations.
Local building codes are modified to include typical conditions within the geographic area and act as a reminder to designers of the typical loads expected. However, failures still occur.
According to M. Kevin Parfitt (www.mdpi.com/2075-5309/2/3/326/buildings-02-00326.pdf), interim department head of Architectural Engineering at Pennsylvania State University, following the building codes may not be enough. He feels that continuous education (as opposed to continuing education) is the answer to the historical loss of institutional memory.
Each designer increases his or her knowledge throughout a career, but that knowledge is not always passed along to others. Continuing education impacts a single person, but that education must be learned anew by the next designer through making similar mistakes.
Unfortunately, passing knowledge down to the next generation of designers is made difficult by the fear of exposing errors to the public and through the action of non-disclosure agreements.
Parfitt stresses that the industry must determine a method of sharing information, if only generically, through comprehensive failure dissemination and must develop educational repositories. Progress is being made in the form of the NIST Disaster and Failure Events Data Repository, but more work is needed. Individual diligence in self-education about failure causes and things that go wrong on a project is necessary. Experienced designers must mentor younger designers so they can learn from prior experiences and mistakes.
Structural loads are at the heart of structural safety calculations. A building that is designed without safety in mind is at risk of failing to fulfill its essential purpose.
Steve Wright works for Whirlwind Steel (www.whirlwindsteel.com), a manufacturer of pre-engineered metal building and steel building components.