Iowa Department of Transportation
Iowa Highway Research Board
Brent Pharesbphares@iastate.edu email >
Bridge Research Engineer, BEC
Behrouz Shafeishafei@iastate.edu email >
Structural Engineer, BEC
About the research
Although bridge designers commonly use rules-of-thumb with regard to the geometry of the bridge deck overhang, these rules-of-thumb generally consider only the deck strength and deflection limits, and the effect due to construction loads during deck placement is often overlooked.
This project investigated exterior girder rotation, girder and formwork deflection, and the effect of skew during bridge deck placement through a field review of construction practices and both a numerical, analytical study and a parametric, sensitivity study using calibrated finite element models.
Three bridge construction projects were selected for field evaluation during deck placement. The projects were selected for their representation of variables of interest that included skew, relative girder depth, and span length.
The parametric study involved six different parameters that were identified to investigate: brace strength, skew angle, diaphragm spacing, girder spacing, span ratio, and girder flange thickness. Three load cases were studied for each of the six parameters. Timber blocking and compression struts as temporary bracings were modeled using compression-only elements by providing the axial stiffness of the members.
Key findings from the field review of construction practices and from the analytical and parametric studies, as well as overall findings, are included in this report, along with recommendations for future research.
Of note, it appears that the cause of any possible deck thinning (or the greatest deflection and/or rotation of the girders) is a result of differential deflections, and not from the rotation of the exterior girders. While the bracing methods considered in this research were effective at restraining girder rotation for straight bridges or those with low skew, they were not as effective at reducing differential deflections caused by the concentrated screed load.
While differential deflections during deck placement are not surprising due to this load location, the field data showed that, while the bridge does begin to return to its original location, there is permanent differential deflection of the bridge cross-section even after the screed load is off the bridge.