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Improved Demand Predictions on Shear Studs for Composite Steel Bridge Design

Description:

To ensure full composite action in a steel-girder bridge, shear studs must be provided along the length of a bridge at the interface between the concrete deck and the steel section to resist the interface shear and prevent slip along the interface. Currently, designers determine the stud proportions and the required stud pitch to satisfy both the fatigue and strength limit states. There have been instances where some designs have appeared to have an excessive number of shear studs. Shear stud quantity and subsequent installation increases steel bridge fabrication cost. In addition to undesirable increases in costs, an excessive number of shear studs produces unsafe conditions since the studs can pose a tripping hazard for iron workers.

In 2019, the National Steel Bridge Alliance (NSBA) created a task group to review recent and historical research conducted on shear studs and to develop a ballot of recommend changes to the shear stud design provisions within the AASHTO LRFD Bridge Design Specifications (BDS). The NSBA shear stud task group identified a number of proposed changes including the following:

  • Changing the finite-life fatigue design equation from a linear-log curve to a log-log curve with a slope of 1/5
  • Modifying the limit on the maximum nominal resistance of a shear stud at the strength limit state, Qn, to 0.7 times its tensile strength, while increasing the resistance factor for shear studs, ϕsc, from 0.85 to 1.0
  • Modifying the pitch provisions to accommodate clustered shear studs in precast concrete decks as used in accelerated bridge construction applications
  • Decreasing the minimum pitch from 6 to 4 stud diameters
  • Eliminating all mentions of “channel” shear connectors

As a result of these proposed provisions, the required stud pitches at the fatigue and strength limit states are more nearly equivalent in many instances, which can lead to an overall reduction in the number of required shear studs.

Aside from the modified pitch provisions for clustered shear studs, the above changes mostly reflect proposed revisions to the resistance of a shear stud. The task group also determined that the demand on shear studs for composite steel-girder bridges warranted further investigation. The group identified the following two areas in which further research is necessary to understand the demand on shear studs for steel-girder bridges at the fatigue limit state:

  • Allowing friction and cohesion as mechanisms to assist in transferring the horizontal shear force between a concrete deck and the top flange of a steel girder
  • Further investigation of the applicability of the current AASHTO LRFD BDS horizontal shear flow equation (VQ/I) to shear studs.

Currently Section 6 of the AASHTO LRFD BDS only allows the horizontal shear force between a concrete deck and steel girder to be transferred through the shear studs. However, field studies on steel girder bridges designed as noncomposite have indicated that some level of unintended composite action is present despite having no shear studs. Further, AASHTO LRFD BDS Section 5 allows for the horizontal shear force to be transferred through the steel reinforcing bars as well as through friction and cohesion between the concrete deck and the top flange of concrete girders. Given that some shear force appears to be transferred between the concrete deck and girder through friction and cohesion, it seems logical to account for this behavior in determining the stud demand for steel-girder bridges, which may further decrease the number of shear studs that are required at the fatigue limit state.

The current specified method of determining the fatigue shear force range on shear studs is by using the classic shear flow equation, where shear force per unit length = VQ/I. In a composite girder, this shear flow equation assumes that shear is continuously transferred between the concrete deck and steel girder even though shear studs are placed at discrete locations along the girder length. A recent analytical study showed that when shear studs were placed at a large spacing, significant differences were found between the shear stud demands predicted by the AASHTO LRFD BDS and those predicted by a calibrated finite element model1. In order to evaluate this postulate further, shear studs must be instrumented on newly designed bridges, and controlled load tests must be conducted after the bridge is in service to determine the measured demands on shear studs.

By investigating the above topics, it is anticipated that the demand on shear studs at the fatigue limit state will be reduced, which will reduce the quantity of required shear studs without compromising the safety of steel girder bridges. A reduction in the number of shear studs will also increase the efficiency of fabrication and erection as well as decreasing the number of tripping hazards during construction.

Objective:

The objective of this research is to develop more accurate demand predictions on shear studs for composite steel bridge design. Results from this research are anticipated to reduce the specified demand on shear studs at the fatigue limit state, which should lead to a further decrease in the number of shear studs required on composite steel girder bridges.

COBS Prioritized Objective(s)(“2018 SCOBS Strategic Plan”) Addressed by this Problem Statement:

  1. Extend Bridge Service Life: N/A2. Assess Bridge Condition: N/A3. Maintain and Enhance a Knowledgeable Workforce: The research findings would increase the overall knowledge for the design loads applied to shear studs for composite steel girders.4. Maintain and Enhance AASHTO Specifications: The research findings would be implemented into the AASHTO LRFD Bridge Design Specifications.5. Accelerate Bridge Delivery and Construction: Fewer shear studs required would lead to more efficient fabrication and erection of steel girder bridges.6. Optimize Structural Systems: This research statement aligns with this objective because the potential benefit would be to use shear studs more efficiently.7. Model and Manage Information Intelligently: N/A8. Contribute to National Policy: N/A

Benefits:

Proposed changes from this research should further reduce the number of shear studs required on steel girder bridges, which will reduce engineering costs, improve fabrication and construction efficiency, and improve erection safety.

Related Research:

The authors are aware of one research study with limited test results that measured the effect of friction between a concrete deck and composite steel girder2. The authors are not aware of any research studies that have measured the applicability of the classic shear flow equation on shear studs of bridges in service.

Tasks:
  • Task 1 – Literature Review – The research team will perform a literature review to find and evaluate relevant past studies.
  • Task 2 – Laboratory Experimental Testing – The research team will perform experimental tests in a laboratory to evaluate the effect of friction on shear stud demands for different concrete deck and steel girder top flange combinations. The concrete contact surfaces to be tested should include cast-in-place concrete, precast concrete, and grout. The steel girder surfaces to be tested should include mill scale, blast cleaned, and coated. Zinc rich primers are believed to represent a lower bound on the friction coefficient of coated steel surfaces.* Task 3 – Field Testing – The research team will perform controlled load testing on bridges in service to determine the actual demand on shear studs when also accounting for friction and cohesion shear force transfer between the deck and steel girder.* Task 4 – Develop Specification Language – Based upon the research findings, the research team will propose revisions to the AASHTO LRFD BDS in the form of draft specifications and commentary.
  • Task 5 – Design Examples – The research team will prepare design examples using the proposed specifications to illustrate the change in the required number of shear studs to satisfy the strength and fatigue limit states.
  • Task 6 – Final Report – The research team will document and submit the findings and recommendations in a final report.
Implementation:

The findings in this study will be implemented through potential modifications to the AASHTO LFRD Bridge Design Specifications.

Relevance:

Shear studs are used on every composite steel girder bridge and can be required in large quantities by the AASHTO LRFD BDS. Results from this research are expected to show that the demands on shear studs can be reduced which will allow for more economic designs, increase fabrication and construction efficiency, and increase safety.

Sponsoring Committee:AKB20, Steel Bridges
Research Period:Longer than 36 months
Research Priority:High
RNS Developer:Farris, Provines, Ocel, Prinz
Source Info:Ovuoba, B., and Prinz, G.S. (2018). “Analysis of shear stud fatigue demands in composite bridge girders having varied stud pitch, girder depth, and span length.” Journal of Bridge Engineering, ASCE, 23(11).

Hillhouse, B., and Prinz, G.S. (2020). “Effects of clustering and flange surface friction on headed shear stud demands.” Journal of Bridge Engineering (ASCE), [Accepted for Publication].
Date Posted:04/09/2020
Date Modified:05/06/2020
Index Terms:Stud connectors, Composite structures, Steel bridges, Bridge design, Fatigue (Mechanics),
Cosponsoring Committees: 
Subjects    
Highways
Design
Bridges and other structures

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