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.

For more information

Jim AlbersProject OfficerControl Technology for the Construction IndustryNational Institute for Occupational Safety & Health4676 Columbia Parkway, MS C-24Cincinnati, OH 45226Phone: 513-533-8168E-mail: JAlbers@cdc.gov