Buildings must be designed to perform in many aspects: resisting forces from gravity, wind, seismic, and other environmental sources. They also need to be serviceable and provide a functional work environment. One measure of serviceability is the structure’s vibrational characteristics. Vibrations can be sensed by building occupants, they can impact sensitive equipment, and they can effect sensitive activities.
In many buildings, such as office buildings, malls, theaters, etc., floor vibrations can be sensed by the occupants, and perhaps cause discomfort. Many types of equipment that work on a microscopic level, such as tools in wafer technology manufacturing facilities, requires minimal vibration for accurate operation. Medical equipment, such as imaging equipment, is negatively impacted by vibrations. Certain surgical procedures that work on a microscopic level also require a “quiet” structure.
Building vibrations can result from human or mechanical sources. Human activity, mainly walking, is the most common source of vibrations. We’ve all been in a building and felt floor vibrations when a person walks near us. But other sources, such as chillers, air handlers, and rotating machinery can excite a floor structure. In some cases, mechanical activity such as vehicular traffic or construction can transmit vibrations to a building.
Building structure vibration has been extensively researched, yet is somewhat in its infancy. As the computational ability of computers has increased, so has the ability of the Structural Engineer to analyze the vibrational characteristics of the floor system. The important parameters considered are the floor structure’s stiffness and mass, and the mass and frequency content of the source of excitation. Vibrations generally impact a structure adversely when the source of vibration and the structure have frequencies that are near each other or “in synch.”
Floor vibrations generally are manifested in building structures in the form of vertical movement of the floor plate. The vertical movement occurs with a certain acceleration and velocity. Structural Engineers can predict the levels of acceleration and velocity in the structure using modern analytical tools, and compare them to established benchmark criterion. ANSI provides velocity and acceleration criterion recommendations for different building uses. Many equipment manufacturers provide criterion as well. A qualified acoustic engineer with relevant experience can assist in establishing criteria.
While predicting floor vibration characteristics utilizes state-of-the-art analytical techniques, there are many assumptions in the input data and parameters that affect predicted results. Beam connections, symmetry of structural bays, concrete quality, partition layout and walking speed/direction are but a few of the factors that can drastically affect analysis results. Many in-situ measurements of floor vibrations have shown that current design methodology based on AISC Design Guide 9, the state-of-the-art guide for analysis and design yields somewhat conservative results.
At catena, our approach to the design of floors for vibration sensitive environments blends our experience with the application of the design guidelines with the in situ vibration measurements of floor structures that we have designed. We understand the balance between overly conservative analysis and under-design which can result in expensive retrofit measures. We use a bounding approach, looking at all variables in the situation at hand, as well as the intended use of the building. For instance, we would tend to “push” the design of a non critical structure, such as an office, more than a critical facility such as a laboratory or surgical suite.
Some structures require unique approaches. Recently, we provided the structural engineering for the expansion to the Shriners Hospital for Children in Portland, Oregon. The base level of the expansion spanned up to 90 feet over an existing parking structure. Full story steel trusses were utilized to support the four story hospital above, which housed patient rooms, laboratories, and surgical suites. The truss level housed many of the air handling units for the hospital.
The structure, use, and sources of excitation posed a unique analytical challenge for evaluating vibration response. Our team of engineers developed three dimensional models of the entire building system to evaluate transmission of walker induced vibrations and transmission of vibrations of mechanical systems into the building. The result of our analyses was specifying thicker concrete topping at the floor structures that correspond to the lower and upper chords of the supporting trusses. By doing so, we tuned the frequency of the truss levels to compliment that of the floor systems. Adding 1 inch of concrete topping at these levels was an economical and practical solution which helped the project meet tight budget and schedule constraints. Floor vibration characteristics were measured and found to be within established criteria for the hospital.
Architect: SRG Partnership, Inc.
Contractor: Andersen Construction Company