The undergraduate level reader will appreciate that vibration of structures will be a part of their career.
Vibration issues for structures have been with us since the beginning of time and will be with us forever. The subject of vibration is not typically part of an engineer’s college education but is a subject worthy of attention and study if one wishes to avoid an embarrassing situation. Noteworthy problems resulting from vibration, i.e., the resonant response of structural elements to load input from outside sources, have certainly been newsworthy, starting with the Angers France Bridge collapse in 1850, to the Tacoma Narrows Bridge in 1940, to the Millennium Pedestrian Bridge that spans London’s River Thames that reopened in 2002 after tuned mass dampers were installed. Unwanted vibrations can develop in structures that are not structurally sound, but more often, a structure that is structurally sound is not psychologically sound to human users.
For hundreds of years, structures avoided vibration issues as a natural result of the construction used. You may want to consider that to be the “mass” solution. In other words, in the past, structural systems were more massive, spans were shorter, and supporting elements were thicker. As an example, just compare normally reinforced concrete rib or waffle slab floor construction to the lighter post-tensioned concrete floor construction frequently used today. Architects and engineers did not foresee that 8-inch thick post-tensioned slabs, supported by columns spaced at 20-foot to 30-foot intervals, while structurally sound, would have a “trampoline” feeling that vibration sensitive users imagined to have structural problems. Science and engineering readily dampened this vibration perception issue by providing lightweight, non-structural floor-to-ceiling partitions, realigning travel aisles and display cases, and, in extreme cases, installing mass vibration dampers tuned to counter the natural floor vibration frequency.
Architects and engineers often use prescribed methods to ensure precise, safe, and the efficient building designs. The vibration of structural systems due to human activity is a serviceability consideration handled primarily by structural engineers. Serviceability aims to meet performance criteria while maintaining comfort, usability, and aesthetics. When assessing serviceability, engineers evaluate deflection limits, durability, stability, cracking, and vibration.
The American Institute of Steel Construction’s AISC Design Guide 11 – Floor Vibrations Due to Human Activity (2nd Edition)
This an extensive resource focusing on floor and stair vibration design in buildings. The guide provides practical advice for structural engineers to assess and control vibrations caused by occupants’ movement.
The natural frequency of a building or component represents its inherent vibration when subjected to external forces. Resonance occurs when dynamic forces from human activity coincide with the structure’s natural frequency, causing amplified vibrations. e AISC design guide emphasizes analyzing resonance and frequency effects to minimize excessive vibrations. By following these guidelines, engineers ensure that the building can handle dynamic loads from human activity without compromising performance.
Humans are very sensitive to vibrations, causing discomfort, agitation, and uncertainty on the building’s overall functionality. According to the AISC Facts for Steel Buildings No. 5 – Vibration, humans can perceive motions of about 0.005 times the acceleration of gravity (0.5%g). Not only could vibrations be annoying and inconvenient, but the overall structural integrity of the building could be perceived as compromised, potentially leading to safety concerns for its occupants and long-term durability issues. Even if the vibrations do not pose any structural risks, the mere sensation of movement can lead to a sense of insecurity and disrupt the perception of stability in the building. Human perception of vibrations is highly sensitive, and even subtle motions can cause discomfort and uncertainty about the building’s overall functionality. In the context of building design, it is essential for the engineer and architect to recognize that human perception is beyond only assessing structural risks.
Besides thinner structural systems with longer spans lacking damping from partitions, the vibration of basic scissor stairs, and more specifically, the monumental stairs that appear on almost every project, present significant challenges to structural engineers. Architects look for a “sleek” design, often utilizing glass railings and treads to achieve the effect. Here again, this beauty, which reduces the mass that contributes to stiffness, may lead to perceptible vibration issues. e vibration in staircases is influenced by various factors, such as the stair structure’s design, material properties, and load distribution. A well-designed staircase should consider all these aspects to minimize vibrations and ensure its structural stability and performance. The AISC Steel Design Guide 11 provides valuable guidelines and recommended practices for designers to assess and control vibrations effectively. Following these prescribed methods is essential to achieve a staircase that not only can withstand the expected loads but also remains comfortable and functional for its intended use. By mitigating excessive vibrations, the overall perception of the structural integrity becomes more favorable. Please see the attached Figure No. 1 from Design Guide. While it may appear intimidating, all the elements of the equation should be familiar. AISC has taken the complex task of determining how they are combined to result in a usable vibration equation.
Architects hold the responsibility of envisioning the building’s overall design and functionality, carefully considering factors like traffic flow and seamless integration with the architectural concept. On the other hand, structural engineers bear the primary responsibility of ensuring the building’s structural integrity and serviceability. Engineers can meticulously calculate loads, stresses, and forces that the stairs must withstand, ensuring compliance with local building codes and safety regulations, but without a focus on vibrations, all is for naught.