Design Against Floor Vibration
B-1 General
Floor with longer spans of lighter construction and less inherent damping are vulnerable to vibrations under normal human activity. Natural frequency of the floor system corresponding to the lowest mode of vibration, damping characteristics, are important characteristics in floor vibration. Open web steel joists (trusses) or steel beams on the concrete deck may experience walking vibration problem. Fatigue, overloading of floor systems and vibrations due to rhythmic activities such as aerobic or dance classes are not within the scope of this annex.
B-2 Annoyance Criteria
In the frequency range of 2 to 8 Hz in which people are most sensitive to vibration, the threshold level corresponds approximately to 0.5 percent g, where g is the acceleration due to gravity. Continuous vibration is generally more annoying than decaying vibration due to damping. Floor systems with the natural frequency
less than 8 Hz in the case of floors supporting rhythmic activity and 5 Hz in the case of floors supporting normal human activity should be avoided.
B-3 Floor Frequency
The fundamental natural frequency can be estimated by assuming full composite action, even in non-composite construction. This frequency, for a simply supported one way system is given by:
where
modulus of elasticity of steel, in MPa;
= transformed moment of inertia of the one way system (in terms of equivalent steel) assuming the concrete flange of width equal to the spacing of the beam to be effective, in mm ;
span length, in mm; and
PART 6 STRUCTURAL DESIGN — SECTION 6 STEEL
dead load of the one way joist, in
If the one way joist system is supported by a flexible beam running perpendicular with the natural frequency , the floor frequency may be reduced to , given by:
B-4 Damping
The percentage of critical damping may be assumed approximately as given below:
| Sl No. | System | Critical Damping Percent |
|---|---|---|
| (1) | (2) | (3) |
| i) | Fully composite construction | 2 |
| ii) | Bare steel beam and concrete deck | 3-4 |
| iii) | Floor with finishes, false ceiling, fire proofing, ducts, furniture | 6 |
| iv) | Partitions not located along a support or not spaced farther apart than 6 m and partitions oriented in orthogonal directions | Up to 12 |
B-5 Acceleration
The peak acceleration , from heel impact for floors of spans greater than 7 m and natural frequency , less than 10 Hz may be calculated as:
where
total weight of floors plus contents over the span length and equivalent floor width in N;
when overhang is only on one side of the beam);
= equivalent thickness of the slab, averaging concrete in slab and ribs; and
g = acceleration due to gravity.