What you need to know when designing with engineered wood, 2: Comfort and performance

After safety, comfort is the next most important consideration for people in buildings. When it comes to floor systems, there are two key “comfort” factors: Sound Transmission and Floor System Vibration.

The most frustrating thing about these two factors is that, even though they can be measured quantitatively, people’s reactions to them are totally subjective.

If you are interested in long-term customer satisfaction however, you should keep in mind peoples’ preferences in regard to sound and vibration, and (if possible within budget) always design for the highest possible performance.

Sound Transmission

Sound transmission is measured in two categories: Sound Transmission Class (STC), which measures the intensity of sound sounds such as music or conversation traveling through a floor/ceiling assembly, and Impact Insulation Class (IIC), which measures the transmission of impact sounds such as footfalls or dropped loads.

Floor/ceiling assemblies are usually tested by independent third-party testing agencies that provide certification for sound performance. Product manufacturers then publish performance characteristics as a guide for design professionals.

Some code bodies have set minimum requirements (expressed in numbers such as 50) for STC and IIC performance ratings of floor assemblies. These minimums are believed to satisfy the majority of people using a building (I’ve heard otherwise, but this is normal as people’s reaction to sound transmission is more subjective than anything else). A rule of thumb here: don’t settle for the minimum rating.

Vibration

Vibration is a side-to-side as well as up and down oscillatory movement of the floor system, initiated by footsteps, dancing, dropped items, etc. More than sound transmission, vibration has the potential to become alarming for occupants, because people perceive it to be in relation with the structural integrity of the building. In that sense it becomes a very important design consideration, as extra design measures may be needed to satisfy end users of a building. The U.S. model building codes don’t have vibration performance requirements, so you might find it helpful to look at Canadian building codes for guidance.

Designing for acceptable vibration performance requires consideration of three elements:

  • Frequency Content of the vibration
  • Amplitude of the vibration
  • The effects of vibration Damping

Frequency content is the cycle time of the vibration. This is measured as hertz.

Important to note: research finds that people feel more comfortable at higher hertz rates than at lower rates.

Vibration amplitude refers to the extent or magnitude of floor vibration. Picture a static horizontal line: Amplitude would be how far above and below the line the vibration cycle peaks fall. The stiffness of the floor (measured by deflection) is related to amplitude. Not surprisingly, high amplitude vibrations are more alarming to people than lower ones.

Using deeper framing members and bridging between joists can reduce amplitude. Continuous bridging perpendicular to the bottom flange of the joists (i.e. using strongbacks) is however, the most effective.

Damping is the effect of vibration amplitude being lessened, and duration of the vibrations being shortened. Things that can provide damping effects are the above- mentioned bridging but also, the addition all other framing and finishing materials and the application of loads as well as interior partition walls.

Aim for higher performance and comfort

Finally, it is important to recognize that every suspended floor system, regardless of makeup or depth or span, vibrates and transmits sound. For the sake of long-term customer satisfaction and comfort, though, it is best to aim for higher performance.

If you need any additional information on this subject let us know, we’ll be happy to help.

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