Page 47 - Vol.06

100
Upper Bound

NJ= 2.7dB = 36%
10 Outdoor Footbridges ǎ NJ
Rhythmic Activities,
Peak Acceleration (% Gravity) 1 Dining and Dancing Log Velocity Average, x
Indoor Footbridges,
Shopping Malls,
Offices,
Residences

0.1
Lower Bound
ISO Baseline Curve
for RMS Acceleration
Log Stiffness
Measured floor vibration data and Gordon Model
0.01
1 4 8 40
Frequency, Hz
Figure4. Acceptance criteria for human comfort Figure5. Measured vibration data and Gordon model

frequencies can be estimated from the fundamental natural : axial shortening of column due to
frequency equation of a uniformly loaded, simply-supported, the weight supported
beam:
• Design For Walking Excitation
when we consider a simple supported beam When use equation (1) to calculate
peak acceleration, since the floor system
with uniform load, the midspan deflection of the beam is is actually a two dimension system
which consists of joist (or beam) and
, then the equation of natural frequency can
girder mode, so we should consider the
parameters and with (a) joist panel
be rewritten as For the combined mode, if both
mode, (b) girder panel mode and (c)
the beam or joist and girder are assumed simply supported, combined panel mode.
If the natural frequency of a floor
using Dunkerley relationship , the equation can be is greater than 9-10 Hz, significant
resonance with walking harmonics will
rewritten as
not occur, but walking vibration can
where still be annoying. Experience indicates
that a minimum stiffness of the floor to
: beam or joist deflection due to the weight supported
a concentrated load of 1 kN per mm (5.7
: girder deflection due to the weight supported
kips per inch) is required for office and
residential occupancies; the scanner used
for semiconductor fabrication requires
Constant Force Damping Ratio Acceleration Limit
100 kN per mm, that is the difference of
Pο ß аο/g × 100%
soft and hard floor. The parameters of
Offices, Residences, equation (1) is listed in Table 2
Churches 0.29 kN 0.02-0.05* 0.5%
Shopping Malls 0.29 kN 0.02 1.5%
Footbridges- Indoor 0.41 kN 0.01 1.5% Example 1
Footbridges- Outdoor 0.41 kN 0.01 5.0% An outdoor footbridge of span 12m
*0.02 for floors with few non-structural components (ceilings, ducts, partitions, etc.) as with pinned supports and the cross-
can occur in open work areas and churches
0.03 for floors with non-structural components and furnishings, but with only small section shown is to be evaluated for
demountable partitions, typical of many modular office areas walking vibration.
0.05 for full height partitions between floors.
Table 2. Recommended Values of Parameters and a 0 /g Limits

NEW FAB TECHNOLOGY JOURNAL APRIL 2012 47