Mitigation of Exterior Beam Rotation in Bridge Construction Through Experimental Investigation of Different Bracing Systems
Bridge contractors use different bracing systems to prevent exterior girder rotation in bridge construction. The reason behind this rotation is the unbalanced eccentric loads induced from loading the deck overhang, which can cause excessive twisting in the exterior girders and excessive overhang deflections. This situation can lead to several problems during construction and over the life of the bridge. Contractors traditionally use block-and-tie systems, which are formed by a combination of tie bars and timber blocks, to mitigate exterior girder rotation. However, block-and-tie systems do not always perform as expected and their efficiency to prevent rotation not yet been evaluated. This paper investigates current block-and-tie systems and presents an analysis of improved rotation prevention systems that can be implemented in the field. Both the block-and-tie systems and the new proposed bracing alternatives were tested in a twin steel girder system [1.8 m × 4.6 m (6 ft. × 15 ft.)] that simulated a steel girder bridge bay. The prototype was eccentrically loaded at mid span through the application of a vertical load in a bracket which was mounted in one of the girders, simulating the loading from the bridge overhang. Forty-five bracing combinations were tested by varying the number of bracing elements and the spacing between them and obtaining girder rotation corresponding to the mid span of the loaded girder. The forty-five cases were divided into nine different groups to allow a clear and easy comparison between bracing alternatives. The results show very promising potential replacements of the current temporary bracing systems which, in many cases, offer minimal protection against exterior girder rotation.
KeywordsDeck overhang Exterior girder rotation prevention Stability issues Block-and-tie systems New bracing systems
The unbalanced loads from the deck overhang can result in significant torsion applied to the exterior girders resulting in excessive overhang deflections which can create instability issues and problems with the finished grade of the concrete deck. These rotations occur in both concrete and steel girder bridges. Non-uniform deck thickness can lead to construction safety concerns (overturning failures, hazardous condition for construction personnel) and maintenance problems (premature deck cracking, long-term corrosion, deck steel reinforcement exposure) (Yang et al. 2010; Fasl 2008). In addition, local instability can be a major concern in steel plate girder bridges with large web slenderness values and initial imperfections (Shokouhian and Shi 2015; Gupta et al. 2006; Sayed-Ahmed 2005; Kala et al. 2009). Global buckling can also be a hazard if the tendency in rotation in the exterior girders is assumed to be a rigid body rotation (Yang et al. 2010; Haskett et al. 2009).
One experimental study has been conducted to evaluate bracing systems for concrete girders (Yang et al. 2010). Suppliers have funded several experimental investigations in collaboration with DOTs to evaluate their products in concrete girders (Ariyasajjkorn 2006; Lackey 2006; Clifton 2008; Clifton and Bayrak 2008). However, and up to the knowledge of the authors, no previous experimental research has been implemented addressing the effectiveness of bracing systems for use with steel girder bridges, where the risk of stability issues is great in comparison to concrete girders.
Therefore, the present paper has two main expectations: (1) evaluation of the bracing systems currently implemented by contractors (block-and-tie systems) during bridge construction, and (2) analysis of improved rotation prevention systems that can be implemented in real bridge construction. Both the block-and-tie systems and the proposed bracing alternatives are tested experimentally using a twin steel girders system [1.8 m × 4.6 m (6 ft. × 15 ft.)] that simulates a real steel girder bridge bay.
2 Test Setup
A point load of 11.12 kN (2,500 lbs.) was applied to one girder (considered the exterior girder) to simulate the overhang construction loading that occurs during the construction of an actual bridge. A vertical point load was applied from a manually operated jack attached to the bracket with a steel chain. A hydraulic jack was connected to a load cell. The load cell was bolted to a reaction frame fixed to the strong floor. The load was applied gradually in five steps of 2.22 kN (500 lbs.) each at the tip of the horizontal leg of the bracket at a distance of 62.23 cm (24.5 in.) from the centerline of the loaded girder.
The girders were instrumented with dual-axis tilt sensors at mid span (at top flange and web intersection) of loaded girder. The rotation in this location is the most relevant because it dictates the deflection of the deck overhang.
3 Description of Rotation Prevention Systems
Transverse Tie Bar (TT)
Unadjusted Biagonal Tie Bar (UDT)
Adjusted/Straight Diagonal Tie Bar (ADT)
Intermediate Cross Frames
Combined application of tie bars and compression struts is the conventional practice in bridge construction to prevent exterior girder rotation.
4 Test Matrix
Test designations for Groups 1, 2 and 3
Test designations for Groups 4, 5 and 6
Test designations for Groups 7 and 8
Test designations for Group 9
5 Experimental Results
Group 1 (No Bracing System)
The measured rotation for this case is 2.73º (Fig. 11) which is the largest rotation observed during testing. This is the worst case scenario for the exterior girder since there are no rotation prevention systems available to resist the rotation resulting from the applied load.
Group 2 and 3 (Transverse Ties Combined with Additional Bracing Elements)
As per Figs. 12 and 13, using only transverse tie bars as a girder rotation prevention system works well to prevent rotation effectively. Adding timber blocks or horizontal pipes cannot effectively reduce the girder rotation. It can be seen from Figs. 12 and 13, the combination of transverse tie bars and diagonal pipes can effectively reduce the exterior girder rotation by approximately 60% as compared with the conventional rotation prevention system (TT + TB). The lowest measured rotations were 0.87° and 0.76° for the 4TT + 4DP (Group 2) and 3TT + 3DP (Group 3) configurations, respectively.
Group 4 and 5 (Adjusted Diagonal Ties Combined with Additional Bracing Elements)
As shown in Figs. 14 and 15, adjusted diagonal tie bars show similar characteristics as the transverse tie bars without compression struts. Applying only adjusted diagonal tie bars shows the maximum girder rotation in Groups 4 and 5. Adding horizontal pipes and timber blocks with adjusted diagonal tie bars can reduce the rotation by approximately 50% when compared with conventional rotation prevention systems (TT + TB). It is suggested by the researchers that using horizontal pipes works better than timber blocks. The horizontal pipes are easier to install and do not require shimming during installation of the blocks. The lowest experimental rotations were 1.18° and 1.00° for the 4ADT + 4HP (Group 4) and 3ADT + 3HP (Group 5) configurations, respectively.
Group 6 and 7 (Unadjusted Diagonal Ties Combined with Additional Bracing Elements)
The installation of this type of unadjusted diagonal tie bars was a complex task for the researchers and time consuming as well. S-shaped bending as shown in Fig. 6 introduces initial defect in the unadjusted diagonal tie bars. Based on the results as shown in Figs. 16 and 17, the initial defect due to S-shaped bending made them ineffective in preventing rotation.
Group 8 (Two Cross Frames Combined with Additional Bracing Elements)
In this group, two intermediate cross frames located 107 cm (3.5 ft.) from mid span increase the overall stiffness of the structure. In this group, the significance of tension carrying member (transverse tie bars and adjusted diagonal tie bars) was recognized. The 2CF + 4TT configuration shows the lowest rotation (1.08°) in the group. As shown in Fig. 18, adding two intermediate cross frames reduces the exterior girder rotation by 50% as compared with the conventional rotation prevention system (TT + TB).
Group 9 (One Cross Frame Combined with Additional Bracing Elements)
In this group, one intermediate cross frame with two different configurations was used including (1) one intermediate cross frame located at mid span (CF@MS), and (2) one intermediate cross frame located at 107 cm (3.5 ft.) from mid span which was identified as cross frame not at mid span (CF N@MS). When the CF@MS are combined with 3 or 4 TT, a 50% reduction in the rotation is observed when compared with the system containing only one cross frame. These results demonstrate that the combination of 1-CF at mid span plus TT is an extremely effective method to decrease rotation. 1CF N@MS did not perform well alone. Adding transverse tie bars reduced the girder rotation effectively.
The conventional rotation prevention systems (TT + TB or UDT + TB) are not effective in preventing rotation during overhang bridge deck construction.
The TT + DP, ADT + HP, or intermediate cross frames at the exterior panels configurations are improved alternatives compared to the current practice.
The improved rotation prevention systems can reduce rotations by up to 50% compared with the conventional girder rotation prevention systems.
This project was made possible through funding from the Illinois Department of Transportation (IDOT) through the Illinois Center of Transportation (ICT); this support is very much appreciated. The authors would also like to thank Mr. Darren Green, Carlos Merino Calvo, SLU, for his help throughout this study.
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