Comparison of 3 techniques for tying rebar for freeway

Comparison of three techniques for
tying rebar on freeway bridge decks
Jim Albers, Steve Hudock,
Yong-Ku Kong, Brian Lowe, Sunil
Sudhakaran, Edward Krieg
National Institute for Occupational Safety & Health
Organizational Science & Human Factors Branch,
Division of Applied Research & Technology
Cincinnati, OH
Background
Contractor requests NIOSH assistance
Construction reinforcing concrete
contractor requests that NIOSH
n
n
Evaluate ironworkers’ exposures to WMSD
risk factors during rebar tying
Investigate effectiveness of a portable
battery powered rebar tying tool for
reducing exposures to WMSD risk factors
Background
Rebar tying on freeway bridge
Contractors’ employees install and tie
rebar together to reinforce the concrete
deck of new elevated freeway bridge
n
Estimated 2.2 million ties for length of
bridge
Rebar is tied together using an
automatic tier and traditional pliers
n
Automatic tier used by contractor for ~1.5
years
Rebar tying techniques used before
NIOSH evaluation
Manual pliers
& wire tying
Battery powered
rebar tying tool
NIOSH introduced adjustable
extension for powered rebar tier
Battery powered rebar tying
tool + adjustable extension
Study objectives
Evaluate ironworkers’ exposures to risk
factors for developing upper extremity
and low back musculoskeletal disorders
Determine the effectiveness of using
power tiers to reduce exposures to risk
factors for upper extremity and low back
musculoskeletal disorders
Methods
Field study/Quasi-experimental design
Repeated measures-randomized ordered study
n
Eight ironworkers tie rebar using 3 randomly
assigned tying techniques
w Independent variables: 3 tying techniques (pliers, power tier, and power
tier + extension)
w Dependent variables: trunk position and hand-wrist motion
Data collected
n
n
n
n
Workers’ demographic & work history
Dynamic wrist motion (dominant hand) using
biaxial goniometers (Biometrics, Gwent, UK)
Video-tape for observational analysis of trunk
posture (MVTA, 2001)
10 point “perceived exertion” questionnaire (Borg,
1982)
Methods
Goniometric data
Flexible goniometers
used to measure wrist
position and motion in
the flexion-extension
(F/E), ulnar-radial
(U/R), and pronationsupination (P/S)
planes of wrist motion
Statistics calculated
for wrist position,
velocity, &
acceleration
Methods
Low back & economic data
Observational data
n
Computer assisted timed activity analysis
using the Multimedia Video Task Analysis
system [UW-Madison, 2001]
Economic data
n
n
Productivity affect
Payback period
Evaluation criteria
Rapid wrist movement in three planes
predicts risk of upper extremity
musculoskeletal disorders [Marras &
Schoenmarklin, 1993]
Trunk posture compared to published data
(Seidler et al, 2001)
Biomechanical data
n
Spinal disc compressive force estimates
(3DSSPP, 2001)
Ironworkers’ responses to a 10 point
“perceived exertion” survey (Borg, 1982)
Results (N=8)
Anthropometrics: Mean (SD)
Age 37 yrs (6 yrs)
Height 5 ft. 9 in. (2.2 in)
Wt 184 lbs (18.5 lbs)
Ethnicity
n
75% (N=6) Hispanic-Central American
Work history (Mean/SD)
n
n
Tying rebar: 10 yrs 4 mos (8 yrs)
Job site: 1 yr 4 mos (1 yr)
Results
angular degrees/second
Wrist velocity (deg/s)
120
100
80
Mean
60
Std Dev
40
20
0
Pliers
PT
PT+E
Flexion/Extension
Pliers
PT
PT+E
Ulnar/Radial
Pliers
PT
PT+E
Pronation/Supination
Tool Type by Wrist Plane
Mean High UE MSD Risk*
Mean Low UE MSD Risk*
*Marras & Schoenmarklin, 1993
P= Pliers; PT= Automatic tier; PT+E= Automatic tier + extension
Results
angular degrees/second
2
Wrist acceleration (deg/s2)
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Mean
Std Dev
Pliers
PT
PT+E Pliers
Flexion/Extension
PT
PT+E Pliers
Ulnar/Radial
PT
PT+E
Pronation/Supination
Tool Type by Wrist Plane
Mean High UE MSD Risk*
Mean Low UE MSD Risk*
*Marras & Schoenmarklin, 1993
PT= Power tier; PT+E= Power tier + extension
Results
Trunk posture analysis
Time in extreme (>90°) forward flexion
n
n
Tying with pliers - ~100%
Tying with power tier (PT) - ~100%
w Free hand/arm used to support trunk 93% time
n
Tying with power tier + extension (PT+E) – 0%
w 83% time in neutral (0° to -15°) trunk posture
Significance
Pliers vs PT
Pliers vs PT+E
PT vs PT+E
NS
p<0.0001)
p<0.0001)
Results
Tying productivity
Ties completed in 2.5 minutes (n=8)
n
n
n
Pliers - 42 ties
PT - 84 ties
PT+E - 52 ties
Significance
Pliers vs PT
Pliers vs PT+E
PT vs PT+E
p<0.0001
p<0.001
p<0.0001
Results
Perceived effort1 survey responses (N=5)
Mean perceived effort (0-10) during tying
Location
Pliers
Power tier
Power tier
+ extension
Hand-wrist
4.8
2.8
5.0
Low back
5.8
2.8
1.2
Hand-Wrist
p<0.05
NS
p<0.05
Low-Back
p<0.0001
p<0.0001
p<0.01
1 Borg, 1982
Significance
Pliers vs PT
Pliers vs PT+E
PT vs PT+E
Economic analysis
Assumptions of payback analysis
Productive hrs/year = 1440 hrs (180 days)
Hourly wage = $32.20
n
VA Davis-Bacon prevailing wage - $22.45 + $9.75 (benefits)
200% productivity increase with power tier
Tool Costs (assume annual replacement)
n
n
n
Pliers - $20
Power tier $3070 (RB392 w/ 2 extra batteries)
Tier battery charger - $3,000/year (generator + gasoline)
Wire Cost
n
n
Pliers - $0.005/tie
RB392 - $0.017/tie
Economic analysis
Power tier payback period
Tiers
1
2
3
4
a
Costa
Benefitb
$9,770.00 $38,866.00
$19,540.00 $84,284.00
$29,310.00 $127,902.00
$39,080.00 $171,520.00
Payback
Periodc
0.25 yr
0.23 yr
0.23 yr
0.23 yr
Cost includes auto tier, 2 extra batteries & battery charging system.
b Financial benefit due to productivity increase and reduction of labor
input.
c Proportion of year (180 days) to recover cost of using auto tier.
Discussion
Study only evaluated ironworkers exposures to
WMSD risk factors during rebar tying
Workers lacked experience using the extension for
the power tier
n
Inexperience may explain lower productivity using tier and
high “perceived effort” rating
w Similar study found auto tier w/ extension twice as fast as
pliers during 2-3 hr working period (Vi, 2004)
Low “perceived effort” scores for low-back during PT
tying suggests use of free arm/hand to support trunk
reduces biomechanical loading
Field studies present challenges not encountered in
laboratory
n
e.g., production constraints, scheduling, environment,
instrumentation care, etc.
Conclusions
For tying rebar at ground level
n
n
n
n
Using a pliers exposes ironworkers to a high level of risk for
upper extremity and low back musculoskeletal disorders
(UEMSD)
Using a power tier (PT and PT+E) reduces rapid & repetitive
wrist motion associated with UEMSDs
Using a PT+E eliminates extreme forward flexion
Using a PT may reduce back loading compared to pliers tying
Ironworkers inexperience using the PT+E likely
affected their responses to the “perceived exertion”
survey and their tying productivity using the PT+E
Automatic rebar tier cost-effective ergonomic
intervention
References
3DSSPP [2001] 3D Static Strength Prediction Program™ Version
4.3. Ann Arbor, MI: University of Michigan Center for Ergonomics.
Borg, G [1982] A category scale with ratio properties and
interindividual comparisons. In Geissler, H.G. and Petzold, P.
(Eds.), Psychophysical judgment and the process of perception.
Berlin: VEB Deutscher Verlag der Wissenschafter.
Marras WS, Schoenmarklin RW [1993] Wrist motions in industry.
Ergonomics 36 (4), 342-351.
Seidler et al [2001] The role of cumulative physical work load in
lumbar spine disease. Occup Environ Med 58:735-746.
UW-Madison [2001] Users Manual for Multimedia Video Task
Analysis™. Ergonomic Analysis and Design Research
Consortium, University of Wisconsin-Madison.
Vi, P [2004] Personal communication on January 10, 2005
between P. Vi, Construction Safety Association of Ontario and Jim
Albers, Division of Applied Research & Technology, National
Institute for Occupational Safety & Health.
For more information
Jim Albers
Project Officer
Control Technology for the Construction Industry
National Institute for Occupational Safety & Health
4676 Columbia Parkway, MS C-24
Cincinnati, OH 45226
Phone: 513-533-8168
E-mail: JAlbers@cdc.gov