Influence of Back Support Conditions on the Apparent Mass of Seated Occupants under Horizontal Vibration

Abstract

The response characteristics of seated human subjects exposed to fore-aft (x-axis) and lateral (y-axis) vibration are investigated through measurements of dynamic interactions between the seated body and the seat pan, and the upper body and the seat backrest. The experiments involved: (i) three different back support conditions (no back support, and upper body supported against a vertical and an inclined backrest); (ii) three different seat pan heights (425, 390 and 350 mm); and three different magnitudes (0.25, 0.5 and 1.0 m/s2 rms acceleration) of band limited random excitations in the 0.5-10 Hz frequency range, applied independently along the fore-aft and lateral directions in an uncoupled manner. The body force responses, measured at the seat pan and the backrest along the direction of motion, are applied to characterize the total body apparent mass (APMS) reflected on the seat pan, and those of the upper body reflected on the backrest. Unlike the widely reported responses of seated occupants under vertical vibration, the responses to horizontal vibration show strong effect of excitation magnitude. The large displacements at lower frequencies cause considerable rotations of the upper body, and the knees and ankles, particularly when seated without a back support, which encouraged the occupants to continually shift larger portion of the body weight towards their feet. This together with the strong dependence on the excitation magnitude resulted in considerable inter-subject variability of the data. The addition of a back support causes stiffening of the body to limit the low frequency rocking motion of the upper body under x-axis motion, while considerable dynamic interactions with the backrest occur. The mean apparent mass (APMS) responses measured at the seat pan and the backrest suggest strong contributions due to the back support condition, and the direction and magnitude of horizontal vibration, while the role of seat height is important only in the vicinity of the resonant frequencies. In the absence of a back support, the seat pan responses predominate at a lower frequency (near 0.7 Hz) under both directions of motion, while two secondary peaks in the magnitude also occur at relatively higher frequencies. The addition of back support causes the seat pan response to converge mostly to a single primary peak, resulting in a single-degree-of-freedom like behavior, with peak occurring in the 2.7-5.4 Hz range under x-axis, and 0.9-2.1 Hz range under y-axis motions, depending upon the excitation magnitude and the back support condition. This can be attributed to the stiffening of the body in the presence of the constraints imposed by the backrest. A relaxed posture with an inclined backrest, however, causes a softening effect, when compared to an erect posture with a vertical backrest. The backrest, however, serves as another source of vibration to the seated occupant, which tends to cause considerably higher magnitude responses. The considerable magnitudes of the apparent mass response measured at the seat ba...

Apparent Mass and Seat-to-head Transmissibility Responses of Seated Occupants under Single and Dual Axis Horizontal Vibration

“…It has been shown that the proposed W d -weighting correlates reasonably well with the biodynamic responses of the body seated without a back support 4) . The biodynamic responses of the occupants seated with a back support and exposed to fore-aft WBV, however, differed significantly from those corresponding to sitting with a back support and exposed to lateral vibration 1,3,4,6,8) . The characterisation of biodynamic responses of the body seated assuming typical driving postures (back and hands supports) is thus essential for defining adequate frequency weightings for exposure assessments.…”

“…Furthermore, the studies of upper body interactions with the back support together with the cross-axis biodynamic responses are vital for enhancing the seated body responses to single and dual-axis horizontal vibration. Only a few studies, however, have considered the backrest as the second important driving point and obtained the APMS responses at the backrest under fore-aft vibration 3,8) . A single study has reported the upper body interactions in terms of cross-axis APMS along all the three axes under fore-aft vibration 8) .…”

“…Studies on horizontal biodynamics have mostly considered a sitting posture without a back support with only few exceptions [6][7][8] . The vast majority of the studies reporting the biodynamic responses of subjects seated with back support have primarily focused on the body interactions with the seatpan alone, although the backrest is known to serve as an important secondary driving-point 3,9) .…”

“…The majority of these studies focus on response analyses of seated body exposed to vertical vibration although a few have investigated the responses to foreaft (x) and lateral (y) vibration 1,3) . Furthermore, most of the studies have been limited to single-axis vibration and response measurements in the direction of the applied vibration.…”

“…The biodynamic responses of the seated occupants exposed to whole body vibration (WBV) have been widely investigated in terms of apparent mass (APMS) or driving-point mechanical impedance, seat-to-head vibration transmissibility (STHT) and absorbed power, under broad ranges of vibration and postural conditions [1][2][3][4][5][6] . The majority of these studies focus on response analyses of seated body exposed to vertical vibration although a few have investigated the responses to foreaft (x) and lateral (y) vibration 1,3) .…”