Post on 08-Apr-2018
Comparison of three techniques for tying rebar on freeway bridge decks
Jim Albers, Steve Hudock, Yong-Ku Kong, Brian Lowe, Sunil
Sudhakaran, Edward KriegNational Institute for Occupational Safety & HealthOrganizational Science & Human Factors Branch,
Division of Applied Research & TechnologyCincinnati, OH
BackgroundContractor requests NIOSH assistance
Construction reinforcing concrete contractor requests that NIOSH
n Evaluate ironworkers’ exposures to WMSD risk factors during rebar tying
n Investigate effectiveness of a portable battery powered rebar tying tool for reducing exposures to WMSD risk factors
BackgroundRebar tying on freeway bridge
Contractors’ employees install and tie rebar together to reinforce the concrete deck of new elevated freeway bridgen Estimated 2.2 million ties for length of
bridge
Rebar is tied together using an automatic tier and traditional pliersn 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
MethodsField study/Quasi-experimental design
Repeated measures-randomized ordered studyn Eight ironworkers tie rebar using 3 randomly
assigned tying techniquesw Independent variables: 3 tying techniques (pliers, power tier, and power
tier + extension) w Dependent variables: trunk position and hand-wrist motion
Data collectedn Workers’ demographic & work historyn Dynamic wrist motion (dominant hand) using
biaxial goniometers (Biometrics, Gwent, UK) n Video-tape for observational analysis of trunk
posture (MVTA, 2001)n 10 point “perceived exertion” questionnaire (Borg,
1982)
MethodsGoniometric data
Flexible goniometers used to measure wrist position and motion in the flexion-extension (F/E), ulnar-radial (U/R), and pronation-supination (P/S) planes of wrist motion
Statistics calculated for wrist position, velocity, & acceleration
MethodsLow back & economic data
Observational data
n Computer assisted timed activity analysis using the Multimedia Video Task Analysis system [UW-Madison, 2001]
Economic data
n Productivity affect
n 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 datan 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 Tying rebar: 10 yrs 4 mos (8 yrs)
n Job site: 1 yr 4 mos (1 yr)
0
20
40
60
80
100
120
Pliers PT PT+E Pliers PT PT+E Pliers PT PT+E
Flexion/Extension Ulnar/Radial Pronation/Supination
Tool Type by Wrist Plane
an
gu
lar
deg
rees/s
eco
nd
Mean
Std Dev
Mean High UE MSD Risk* Mean Low UE MSD Risk*
*Marras & Schoenmarklin, 1993P= Pliers; PT= Automatic tier; PT+E= Automatic tier + extension
ResultsWrist velocity (deg/s)
ResultsWrist acceleration (deg/s2)
0
200
400
600800
1000
12001400
1600
1800
2000
Pliers PT PT+E Pliers PT PT+E Pliers PT PT+E
Flexion/Extension Ulnar/Radial Pronation/Supination
Tool Type by Wrist Plane
angu
lar d
egre
es/s
econ
d2
Mean
Std Dev
Mean High UE MSD Risk* Mean Low UE MSD Risk*
*Marras & Schoenmarklin, 1993PT= Power tier; PT+E= Power tier + extension
ResultsTrunk posture analysis
Time in extreme (>90°) forward flexion
n Tying with pliers - ~100%
n 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
SignificancePliers vs PT NSPliers vs PT+E p<0.0001)PT vs PT+E p<0.0001)
ResultsTying productivity
Ties completed in 2.5 minutes (n=8)
n Pliers - 42 ties
n PT - 84 ties
n PT+E - 52 ties
SignificancePliers vs PT p<0.0001Pliers vs PT+E p<0.001PT vs PT+E p<0.0001
ResultsPerceived effort1 survey responses (N=5)
1.22.85.8Low back
5.02.84.8Hand-wrist
Power tier + extensionPower tierPliersLocation
Mean perceived effort (0-10) during tying
1 Borg, 1982
Significance Hand-Wrist Low-BackPliers vs PT p<0.05 p<0.0001Pliers vs PT+E NS p<0.0001PT vs PT+E p<0.05 p<0.01
Economic analysisAssumptions of payback analysis
Productive hrs/year = 1440 hrs (180 days)
Hourly wage = $32.20n VA Davis-Bacon prevailing wage - $22.45 + $9.75 (benefits)
200% productivity increase with power tier
Tool Costs (assume annual replacement)n Pliers - $20
n Power tier $3070 (RB392 w/ 2 extra batteries)
n Tier battery charger - $3,000/year (generator + gasoline)
Wire Costn Pliers - $0.005/tie
n RB392 - $0.017/tie
Economic analysisPower tier payback period
0.23 yr$171,520.00$39,080.004
0.23 yr$127,902.00$29,310.003
0.23 yr$84,284.00$19,540.002
0.25 yr$38,866.00$9,770.001
Payback PeriodcBenefitbCostaTiers
a 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.
DiscussionStudy only evaluated ironworkers exposures to WMSD risk factors during rebar tyingWorkers lacked experience using the extension for the power tiern 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 laboratoryn e.g., production constraints, scheduling, environment,
instrumentation care, etc.
Conclusions
For tying rebar at ground leveln Using a pliers exposes ironworkers to a high level of risk for
upper extremity and low back musculoskeletal disorders (UEMSD)
n Using a power tier (PT and PT+E) reduces rapid & repetitive wrist motion associated with UEMSDs
n Using a PT+E eliminates extreme forward flexion
n 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.