IBC Evaluation of Modular Press-Brake-Formed Steel Tub Girders for Short Span Bridge Applications

Transcription

1 IBC Evaluation of Modular Press-Brake-Formed Steel Tub Girders for Short Span Bridge Applications The International Bridge Conference (Engr. Society of Western PA) June 7-10, 2016 Gregory K. Michaelson, Ph.D. (Marshall University) Karl E. Barth, Ph.D. (West Virginia University) Michael G. Barker, Ph.D. (University of Wyoming) Daniel R. Snyder (Steel Market Development Institute)

2 Outline Background Proposed System Details Experimental Testing Analytical Methods Assessment of Flexural Capacity Feasibility Studies Benefits of Proposed System

3 Background Concept has been in place since 1970s Prefabricated Press- Formed Steel T-Box Girder Bridge System AISC Engineering Journal Taly & GangaRao, 1979 (Significant input into the original concept development was given by Dr. Larry Luttrell, a key authority on cold bending of steel.)

4 Background (cont d) Why didn t this take off? Concrete deck option would require refinement of expensive fabrication details Lack of proof of concept testing Lack of industry support Similar systems have been proposed that include: Prestressing components Inefficient girder cross-sections Complex fabrication details

5 Current Systems Tokai University system: Bending Behavior of Composite Girders with Cold Formed Steel U Section Journal of Structural Engineering Nakamura 2002 System contains prestressing: Requires longitudinal posttensioning in deck Increases complexity/cost Structurally inefficient: Small bottom flange

6 Current Systems (cont d) Tricon Con-Struct: Pretopped modules with galvanized steel press-brakeformed tub girders System contains prestressing: Increases complexity/cost Limited fabricators to implement Con-Struct Decreases skew options Limits deck options Plates are 3/8 thick (susceptible to corrosion)

7 Proposed System Bridge Technology Center: Modules with steel pressbrake tub girders Galvanized or weathering Modules are joined using UHPC longitudinal closure pours Modules can be shipped to site pre-topped or with a variety of deck options

8 Short-Span Steel Bridge Alliance

9 Design Methodology Goal: utilize standard plate widths 84, 96, etc. Maintain 1:4 web slope, 5t radii, and 6 b tf Consistent w/ AASHTO Spec. Optimize girder dimensions to attain maximum capacity

10 Design Methodology (cont d)

11 Design Methodology (cont d) Resulting girder depths: 60 plate: d = plate: d = plate: d = plate: d = plate: d = plate: d = 34 All composite section properties can be found in the manuscript.

12 Experimental Testing Testing was conducted on composite flexural specimens: 84 7/16 PL Dimensions shown below:

13 Experimental Testing (cont d)

14 Experimental Testing (cont d)

15 Experimental Testing (cont d)

16 Analytical Methods FEA was completed using Abaqus v.6.10-ef2 S4R shell elements were employed to simulate the girder and deck von Mises material laws governed steel behavior A smeared cracking model incorporating tension stiffening was employed for concrete behavior

17 Analytical Methods (cont d) Using strain-compatibility methods, estimates of girder capacity were obtained: Steel was assumed to behave linearly until F y Concrete in compression was assumed to have a uniform stress of 0.85 f c Neutral axis depth was iterated until equilibrium was attained. Moments were then summed to obtain capacity.

18 Analytical Methods (cont d) Example of straincompatibility approach: M n = 2646 ft-kip Depth Along Cross-Section (in) Depth Along Cross-Section (in) Strain Stress (ksi)

19 Composite Section Capacity In order to evaluate the applicability of AASHTO Specifications, a parametric matrix of composite girders was developed (resulting in 900 girders): 18 girders (previously described) 50-ksi and 70-ksi steel employed 25 deck options 5 deck thicknesses (7 to 11 in 1 increments) 5 deck widths (defined based on out-to-out width of the girder

20 Composite Section Capacity (cont d) For a composite girder to be considered compact: F y 70 ksi D/t w 150 True for all 18 girders The following limit must be satisfied: 2D t w cp 3.76 E F yc

21 Composite Section Capacity (cont d) According to AASHTO Specifications, for compact composite girders, the following model is used to compute the capacity: M n M p Dp 0.1Dt Dp M D D 0.42D Dt p t p t

22 Composite Section Capacity (cont d) In order to evaluate the applicability of AASHTO Specifications, a parametric matrix of composite girders was developed (resulting in 900 girders): 18 girders (previously described) 50-ksi and 70-ksi steel employed 25 deck options 5 deck thicknesses (7 to 11 in 1 increments) 5 deck widths (defined based on outto-out width of the girder M n M p Dp 0.1Dt Dp M p Dt Dp 0.42Dt Dt

23 Noncomposite Section Capacity In order to assess noncomposite girder capacity, the stability of the cross-section must be evaluated First, the governing buckling modes of the girders were determined. This was performed using CUFSM: Operates through use of the constrained finite strip method Available from JHU (Schaefer and Ádány 2006)

24 Noncomposite Section Capacity (cont d) Typical results for press-brake-formed tub girders: 7 PL 84" 7/16" 6 5 M cr / M y M y Half Wavelength (in)

25 Noncomposite Section Capacity (cont d) Summary of results for press-brake-formed tub girders:

26 Noncomposite Section Capacity (cont d) Therefore, global lateral-torsional buckling will govern for the proposed system: M cr EI 2 2 y x 4 GJL Cw L x EIy Iy Note that the ± results from the direction of applied moment: Section is stronger if the larger flange is in compression.

27 Noncomposite Section Capacity (cont d) Using the proposed equation, the theoretical buckling load was found to be 92.3 kip This agrees closely with experimental results (elastic LTB). At approximately 95 kip: M cr EI 2 2 y x 4 GJL Cw L x EIy Iy

28 Noncomposite Section Capacity (cont d) Torsional properties of the proposed press-brake-formed steel tub girder are derived using provisions specified by Galambos (1968) and the Guide to Stability Design Criteria for Metal Structures. C w S 3 tw 2 n 1 x I A x J 3 t ds 2 3 x y y da 2Yo

29 Benefits of Proposed System System advantages: Utilizes standard plate widths (based on availability) Optimized to achieve maximum structural capacity Majority of steel in the bottom flange Increased torsional stiffness Equations for girder design have been provided.

30 Benefits of Proposed System (cont d) Currently, the BTC is working with Buchanan Co. IA on an IBRD project on a press-brake-formed tub girder demonstration project. WVDOH is also working on two demonstration projects as well. We are also looking for opportunities with other agencies interested in deploying this technical concept!

31 Questions? Thank You!