ARCHITECTS and ENGINEERS

Reducing Structural Vibrations in Academic Lab Buildings

2655_002_00_ELACmath la 28_medium.jpgAcademic laboratory buildings are designed to meet industry standards for safety, often focusing on indoor air quality, ventilation, life-safety systems, lighting, and controlled entrances/exits. The structural integrity of academic lab buildings is additionally important, and structural engineers will design the building’s structural components with the strength and stiffness required to resist building code live load, snow, wind, and seismic forces.

Common sources of vibration, typically caused by humans and machinery, also require special design considerations in building areas planned for laboratories. These spaces contain sensitive instruments, such as microscopes, balances and lithography equipment. Unwanted floor vibrations that are seemingly unperceivable to humans can affect the research quality by disrupting equipment processes or measurement accuracy.

At HGA, our architecture and engineering teams have a successful history providing design, engineering and planning services for projects with these types of design challenges. More recently, our structural engineering practice has built on this expertise by developing a new set of powerful computational tools and design procedures that can solve even trickier vibration design problems for more complex structural systems of varying materials.

Traditional structural vibration design standards were developed decades ago and are heavily based on rudimentary analytical procedures with a list of stringent limitations and assumptions. Most of these standards only apply to basic steel and concrete structures of simple orthogonal forms, so when structural systems become complicated, engineers are usually left to design outside these limitations. As a consequence, an engineer will apply engineering judgment, which frequently results in the application of conservative “rules of thumb.”

In laboratory floor design, for example, industry standards are limited. Standards use simplified equations for designing floors where sensitive equipment may be present. An engineer is required to predict whether a person may walk in that given space either at a fast, moderate or slow walking speed. A fast walking speed may occur along corridor whereas a moderate or slow walking speed may occur in the laboratory itself. The fast walking speed usually results in the highest vibration response. So how would an engineer design the floor of a lab bay that is adjacent to a corridor bay exposed to a fast waking condition? The answer is usually by applying a conservative rule of thumb which assumes the fast walking occurs “in” the laboratory bay rather than the corridor. Some may even decide to ignore the design check, assuming excitation in one bay will not apply to the response in another bay. Unfortunately, the former judgement may lead to a stricter limitation on column spacing affecting lab planning and/or may lead to deeper floor structures—costing the owner additional money. The latter decision to ignore it may lead to floor vibrations that do not meet the required vibration criteria. The true behavior actually lies between these two extremes.

RESEARCHING COMPUTATIONAL TOOLS

To address these types of challenges, we developed a specialized work flow using a variety of custom-developed and commercially available software to estimate the vibrational response of complex floor structures. This research stemmed from our in-house Research Council Micro-Grant program, which investigates design solutions that can be applied to client needs.

Our goal was to:

  • Use comprehensive structural dynamics computations to develop methods for analyzing vibration response of structures that fall outside the scope of current industry standards.
  • Develop easy-to-use design tools for post-processing larger amounts of analytical data to streamline vibration analysis designs, especially during early design phases.
  • Integrate these computational tools as a real-time design component in all phases, developing robust designs throughout the entire design process.
  • Enhance our expertise to meet increasingly stringent owner-imposed vibrations criteria for sensitive equipment without impacting project cost and floor-to-floor heights.

The results of the Micro-Grant program work proved very successful. We rolled-out our new Specialized Vibration Analysis (SVA) tools and design procedures to other HGA structural engineers across other offices. Over the past two years, our engineers have successfully applied these design tools on a variety of projects and structural systems across all market sectors.

APPLYING COMPUTATIONAL TOOLS

One such project is the new Norman and Evangeline Hagfors Center for Science, Business, and Religion at Augsburg University in Minneapolis.

1826_029_00_Augsb Hagfors cg 013_medium (1).jpgThe 135,000-square-foot academic building integrates classroom, laboratory and research facilities for eight departments (biology, business, chemistry, computer science, math, physics, psychology, and religion) to promote cross-disciplinary learning. Current and future research lab space accounted for approximately 80,000 square feet of concrete pan and joist structure. In these areas, the floor structure was designed to meet a velocity vibration limit less than or equal to 2000 micro-inches per seconds, which is adequate in most instances for most optical microscopes up to 400x magnification. Working in conjunction with our architectural and design-build contractor teams, we rapidly prototyped several concrete floor designs throughout the pre-design phase. Each design studied the effects of varying column spacing, floor depths, formwork strategies, and laboratory planning options. The computation power of SVA allowed the design team to confidently react to fluctuating laboratory planning and respond to continuous feedback from our construction partner.

After this predesign effort, the structural depth, column-free lab layouts and nearly all of the steel pan formwork did not change. This pre-planning allowed the contractor to confidently procure steel pan formwork a month prior to the first constructing package, proactively solving a long lead-time concern in an already fast-track design environment. Our early structural engagement with the design-build team resulted in a structural design that is safe, meets required vibration criteria, fostered cost-effective reusable formwork, and aided column-free architectural lab planning without compromise.

Now let’s try to answer that question I proposed earlier regarding labs adjacent to corridors. This condition was present in many instances in the Hagfors Center laboratory areas. This type of planning was done to keep fast walking conditions out of the laboratory bays and near column lines. With SVA, we could better approximate the real response of a lab subjected to fast walking in an adjacent corridor bay. By comparison, if we would have made the conservative assumption that the lab bays contained fast walkers, floors designed with industry standard methods would have needed to be four inches deeper. This would have resulted in unnecessary added concrete volume, larger columns and footings, and taller floor-to-floor heights. For a five-story building, this would have resulted in significant structure and exterior enclosure construction costs. On the other hand, SVA showed that ignoring the fast walking along corridors would produce vibrations in the adjacent laboratories that exceeded the vibration criteria. Ignoring this condition may have negatively impacted University research activities.

TAKEAWAYS

Each lab building is different, and each lab building requires a thoughtful approach to reducing vibrations that may impede research activities. Predicting the vibrational response of a structure requires a certain amount of engineering judgment and appropriate assumption-making. HGA’s structural engineers have developed design tools and procedures that enhance our expertise to resolve these design challenges. Waiting to perform vibration analysis later in the design process leaves the project team exposed to risk that may result in costly changes and redesign. As such, we have continued to revise SVA to promote a standardized approach for analyzing vibration response, leading to earlier and more efficient structural designs.

Through an integrated team process involving architects, engineers and contractors, these tools provide direct benefits to owners planning labs:

  • With SVA, structural engineers can better understand how specific areas of floors respond to different walking speeds, allowing for more efficient designs.
  • Through rapid prototyping of different structural systems and configurations, engineers and project teams can collaboratively determine the optimal structural system based on multiple project-specific criteria.
  • Using computational tools, engineers can accurately select the right structural system, optimal structural depth and appropriate column layout early—with significantly less likelihood of design changes later in design process.

Topics: Education, Engineering

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