CONCRETE-FILLED TUBES FOR ACCELERATED BRIDGE …

Tenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

CONCRETE-FILLED TUBES FOR ACCELERATED BRIDGE CONSTRUCTION

M.T. Stephens1, L. Berg1, D. E. Lehman2, and C.W. Roeder3

ABSTRACT Circular concrete filled tubes (CFTs) are practical structural elements which offer an efficient and economical alternative to conventional reinforced concrete and steel construction. However, the use of CFTs in bridge construction is currently limited in the US primarily uncertain design expressions and unreliable connections. A research program was undertaken at the University of Washington to develop connection details to use CFT columns in bridge systems. Two types connections currently being investigated, including column-to-foundation and column-to-cap beam. Research on the column-to-foundation connection is more mature and discussed in depth; the cap beam-to-column research is more recent and only preliminary results are presents. Two variations of an embedded CFT column-to-foundation connection were experimentally and analytically evaluated in which the steel tube was embedded in surrounding concrete. Study parameters included type of connection, straight seam or spiral weld tube, diameter to thickness ratio (D/t), material strengths, embedment depth, and axial load. Results from the numerical and experimental studies were used to develop engineering expressions for design of the CFT column-to-foundation connection. The second phase of the connection study focuses on the column-to-cap beam connection which offers many unique design constraints including congested joint reinforcing and limits on cap beam geometry. Four variations of the CFT column-to-cap beam connection are introduced; one full strength connection in which the CFT column is embedded into the cap beam, and three partial strength connections in which headed reinforcing bars extend from the column into the cap beam. The full strength connection was designed using expressions from the CFT column-to-foundation connection to transfer the plastic moment capacity of the CFT, while the capacity of the partial strength connections was controlled by the longitudinal reinforcing ratio of the headed reinforcing bars in the connection region. A limited numerical parameter study conducted on the full strength connection indicates that strength and ductility objectives can be achieved using the embedded connection detail even within the unique design constraints of the cap beam.

1Graduate Student Researcher, Dept. of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195 2Associate Professor, Dept. of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195 3Professor, Dept. of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195 Stephens M.T., Berg L., Lehman, D.E., and Roeder, C.W. Seismic Design of Circular Concrete Filled Tube Bridge Pier Connections for Accelerated Bridge Construction. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

Tenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

Concrete-Filled Tubes for Accelerated Bridge Construction

M.T. Stephens1, L. Berg1, D. E. Lehman2, and C.W. Roeder3

ABSTRACT Circular concrete filled tubes (CFTs) are practical structural elements which offer an efficient and

economical alternative to conventional reinforced concrete and steel construction. However, the use of CFTs in bridge construction is currently limited in the US primarily uncertain design expressions and unreliable connections. A research program was undertaken at the University of Washington to develop connection details to use CFT columns in bridge systems. Two types connections currently being investigated, including column-to-foundation and column-to-cap beam. Research on the column-to-foundation connection is more mature and discussed in depth; the cap beam-to-column research is more recent and only preliminary results are presents. Two variations of an embedded CFT column-to-foundation connection were experimentally and analytically evaluated in which the steel tube was embedded in surrounding concrete. Study parameters included type of connection, straight seam or spiral weld tube, diameter to thickness ratio (D/t), material strengths, embedment depth, and axial load. Results from the numerical and experimental studies were used to develop engineering expressions for design of the CFT column-to-foundation connection. The second phase of the connection study focuses on the column-to-cap beam connection which offers many unique design constraints including congested joint reinforcing and limits on cap beam geometry. Four variations of the CFT column-to-cap beam connection are introduced; one full strength connection in which the CFT column is embedded into the cap beam, and three partial strength connections in which headed reinforcing bars extend from the column into the cap beam. The full strength connection was designed using expressions from the CFT column-to-foundation connection to transfer the plastic moment capacity of the CFT, while the capacity of the partial strength connections was controlled by the longitudinal reinforcing ratio of the headed reinforcing bars in the connection region. A limited numerical parameter study conducted on the full strength connection indicates that strength and ductility objectives can be achieved using the embedded connection detail even within the unique design constraints of the cap beam.

Introduction

Concrete-filled steel tube (CFT) components provide an attractive alternative to conventional methods of construction. CFTs optimize the mechanical contributions of both the concrete fill and steel tube while increasing construction efficiency. The steel tube eliminates the need for traditional transverse and longitudinal reinforcing steel as well as formwork. The steel tube is placed at the optimal location to resist moment, thereby maximizing strength and stiffness while minimizing weight and material requirements. The concrete fill, in turn, restrains local and

1Professor, Dept. of Civil Engineering, University of Somwherehill, Somewhere, XY 12345 2Graduate Student Researcher, Dept. of Civil Engineering, University of Somwherehill, Somewhere, XY 12345 Stephens M.T., Berg L., Lehman, D.E., and Roeder, C.W. Seismic Design of Circular Concrete Filled Tube Bridge Pier Connections for Accelerated Bridge Construction. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

global tube buckling, supports compressive stress demands, and offers large stiffness to meet functionality performance objectives. The resulting component has large stiffness, strength, and ductility, which are desirable properties for seismic design [1, 2, 3, 4]. Despite these advantages, CFTs are not commonly used in bridge construction in the US due to a limited availability of economical and practical connections. A research study was undertaken at the University of Washington to enhance the use of circular CFT in bridge construction through development of foundation and cap beam to CFT column connections. Two CFT column-to-foundation connections have been studied using extensive experimental and analytical research. These findings have resulted in design expressions for a seismic resisting connection that exceeds the performance of current RC column-to-foundation connections. The current phase of the research focuses on the CFT column-to-cap beam connection. Numerical models were developed to study unique aspects of CFT column-to-cap beam connection, including geometric constraints and congested joint reinforcing. Several potential CFT column-to-cap beam connections are introduced, and preliminary numerical results are presented. Future work will include an experimental evaluation of this connection.

Foundation Connection

Connections between CFTs and other structural components are often different and more complex than those used in steel or reinforced concrete construction because of the composite nature of the CFT component. A foundation connection capable of sustaining demands from the full plastic moment capacity of the CFT and provides large inelastic deformation capacity during seismic loading has been developed as shown in Figure 1 [3]. The connection is a fully restrained moment connection which employs a flange or annular ring welded to the base of the steel tube to provide anchorage and efficient shear and moment transfer to the surrounding concrete and reinforcement as is illustrated by the compression struts in Figure 1a. There are no reinforcing bars in the tube or dowels penetrating from the tube into the foundation; the force transfer is solely accomplished by the anchorage provided by the tube. The foundation or pile cap is designed to normal depth, design loads, and shear and flexural reinforcement.

Figure 1. Proposed foundation connection.

Two variations of the embedded foundation connection have been developed, as shown in Figure 1b and Figure 1c. Using a monolithic version of the connection, the steel tube and annular ring are temporarily anchored within the foundation reinforcing, and the foundation and CFT-column are cast simultaneously as shown in Figure 1b. For the second variation, known as the embedded option, the footing is cast with a recess formed by a light weight corrugated pipe with an inner diameter slightly larger than the outside diameter of the annular ring as is shown in Figure 1c. The tube and ring are placed into the void after the foundation is cast, and the recess between the tube and corrugated pipe is filled with high strength fiber reinforced grout to anchor the column into the foundation. For both options, the steel tube is filled with low shrinkage self-consolidating concrete to complete the CFT member, and no vibration is required [4]. Experimental Program The experimental program to evaluate the behavior of the CFT column-to-foundation connections consisted of a series of 19 large scale specimens which simulated approximately a half scale bridge column [4, 5, 6, 7]. The details and geometry of a typical specimen are shown in Figure 2, while specimen descriptions and nominal material strengths are summarized in Table 1. The diameter and thickness of the steel tube in a majority of the specimens were 20-in. and 0.25-in. respectively; resulting in a diameter-to-thickness ratio (D/t) of 80. This exceeds the limiting D/t ratio specified in ACI [8], AISC [9], and AASHTO [10] design specifications. The annular ring in all specimens extended 4-in. (102-mm) and 2-in. (51-mm) from the outer and inner diameter of the steel tube respectively. The dimensions of the footing as well as the primary flexure reinforcing were selected to provide adequate strength for the foundation to minimize the influence of footing size on the failure mode, resist Mp of the CFT without yielding, and to represent a scale model of a typical bridge footing. The primary flexure reinforcing consisted of #6 (19-mm diameter) bars spaced at 4-in. (102-mm) on-center at the top and bottom, while shear and transverse flexural reinforcing varied from specimen to specimen to evaluate the influence of these parameters. The specimens were tested using a self-reacting test frame with a horizontal actuator to apply a cyclic lateral load and a Universal Testing Machine (UTM) to apply a constant vertical gravity load [4]. The imposed displacement history for a majority of the specimens was based on the ATC-24 protocol [11]. A majority of the specimens were subjected to approximately 10% of the gross compressive load capacity of the CFT column.

Figure 2. Typical specimen geometry and reinforcing.

Table 1. Experimental parameters and material properties. Spec. le/D Connection Type

Study Parameter Fy, ksi (MPa)

Fu, ksi (MPa)

f’c, ksi (MPa)

Tearing Drift

Mmax/MP,PSDM Failure Mode

1 0.6 Monolithic connection No vertical reinforcing

75 (520)

88 (605)

11 (76)

3.5% 0.88 Cone pullout

2 0.6 Monolithic connection Vertical Reinforcing

75 (520)

88 (605)

11 (76)

4.2% 0.92 Cone pullout

3 0.9 Monolithic connection Embedment depth

75 (520)

88 (605)

10 (69)

8.0% 1.13 Ductile tearing

4 0.6 Recessed connection Embedment depth

75 (520)

88 (605)

10 (69)

6.5% 0.98 Partial pullout

5 0.9 Recessed connection Flexible underlay

75 (520)

88 (605)

11.3 (78)


6 0.75 Recessed connection Flexible underlay

75 (520)

88 (605)

11.9 (82)


7 0.75 Monotonic axial load only Punching with 9-in (225-mm) depth

75 (520)

88 (605)

9.3 (64)

NA NA Monotonic punching

8 0.75 Cyclic axial load only Punching with 9-in (225-mm) depth

75 (520)

88 (605)

9.4 (65)

NA NA Cyclic punching

9 0.9 Recessed connection Galvanized tube

75 (520)

88 (605)

10 (69)


10 0.9 Recessed connection – galvanized Near-fault load history

75 (520)

88 (605)

9.7 (67)


11 0.9 Recessed connection 0.15Po axial load

75 (520)

88 (605)

9.3 (64)


12 0.9 Recessed connection 0.2Po axial load

75 (520)

88 (605)

69 (10)


13 0.8 Monolithic connection Straight seam tube

49 (340)

60 (417)

8.7 (60)


14 0.775 Recessed connection Straight seam tube

49 (340)

60 (417)

9.4 (65)


15 0.775 Recessed connection Spiral weld tube/evaluate le

51 (355)

78 (540)

7.8 (54)


16 0.8 Monolithic connection Spiral weld tube/evaluate le

51 (355)

78 (540)

8.7 (60)



51 (355)

78 (540)

9.9 (68)

7.4% 1.31 Ductile tearing w/ cracking


51 (355)

78 (540)

10.2 (70)


19 0.62 Recessed connection Larger, 762-mm tube 0.05Po axial load

50 (355)

78 (540)

11.2 (77)


Experimental Results A summary of the experimental results is included in Table 1, the moment drift behaviors of several specimens are plotted in Figure 3, and typical failure modes are shown in Figure 4.The theoretical plastic moment capacity of the CFT calculated using the plastic stress distribution method (PSDM), which is shown with a dashed line in each of the subfigures of Figure 3. This method is illustrated in several references [4, 9]. Drift is defined as the lateral displacement recorded at the top of the column divided by the column height above the footing. As the testing program was so large, only the hysteretic performances of selected specimens are discussed here to demonstrate the influence of the embedment depth on connection behavior. Specimens 1 and 3 demonstrate the strength and ductility as well as typical failure modes of adequately and inadequately embedded specimens. In summary, ductility of the inadequately embedded connections (Specimen 1) was ultimately limited by foundation damage as illustrated in Figure 4a, while the failure mode of the adequately embedded connections (Specimen 3) was characterized by ductile tearing of the steel tube as shown in Figure 4c, which occurred at the location of local buckling (see Figure 4b). More details on the experimental

results are available in the reference reports [4, 5, 6, 7].

Figure 3. Moment-drift plots: a) Specimen 1 and b) Specimen 3

Figure 4. Photos of connection behavior: a) cone pullout of Specimen 1, b) typical local

buckle at 4% connection rotation for Specimen 3, c) typical tearing failure.

Design Expressions

Results from the experimental investigation were used to develop design expressions for a CFT column-to-foundation connection capable of transferring the plastic moment capacity of the CFT. Specifically, expressions were developed to detail the annular ring, calculate the required embedment depth to eliminate conical pullout failure, and to determine the required depth of concrete below the tube to prevent punching failure. The proposed design expressions are introduced briefly here with detailed explanations available in reference material [4]. Annular Ring

The annular ring is welded to the end of the tube to provide anchorage and transfer stress to the concrete and reinforcement in the footing. The ring is made of steel of the same yield stress and thickness as the steel tube. The required dimensioning of the ring to provide adequate anchorage is the subject of an ongoing numerical investigation which is discussed later in this paper. The ring is welded to the tube using complete joint penetration (CJP) welds or fillet welds on both the inside and outside of the tube designed to develop the full tensile capacity of the tube. Embedment Depth The cone pullout model shown in Figure 5a was used to calculate the required embedment depth for the tube into the foundation. This model equilibrates the ultimate tensile stress in the steel tube with the shear strength of a concrete cone over that same region. The experimental results

-1.5

-1

-0.5

0

0.5

1

1.5

-10 -7.5 -5 -2.5 0 2.5 5 7.5 10

M/M

P,C

FT

Drift (%)

b) Specimen 3

M/MP,PSDM = 1

-1.5

-1

-0.5

0

0.5

1

1.5

-10 -7.5 -5 -2.5 0 2.5 5 7.5 10

M/M

P,C

FT

Drift (%)

a) Specimen 1

M/MP,PSDM = 1

were used to determine the limiting concrete shear strength n√f’c. Specifically, the model was evaluated for each specimen, and the corresponding n value was calculated via equilibrium. Figure 5b compares the n values from each specimen to performance, deformation capacity, and ultimate failure. Based on this assessment, a limiting concrete shear strength of 6√f’c-psi (0.5√f’c-MPa) is proposed, and the required embedment depth to eliminate the potential for foundation failure, le, is given in Equation 1 as:

(1)

where Do is the outside diameter of the annular ring and corrugated pipe for the monolithic and grouted connections respectfully, D, t, and fu are the diameter, thickness, and ultimate stress of the steel tube, and f’cf is the compressive strength of the foundation concrete in psi.

Figure 5. a) cone pullout model, b) experimental stress demand.

Punching Shear Adequate concrete depth must be included below the tube to suppress punching shear failure in the foundation. Using the ACI punching shear requirements for footings in single shear [8] as a basis, Equation 2 was developed to calculate the minimum footing depth df to avoid punching shear failure. Cc and Cs in this equation are the compressive forces in the concrete and steel due to the combined axial load and bending moment as computed by the PSDM.

. (2)

Cap Beam Connection

The experimental investigation of the CFT column-to-foundation connection provided unique and valuable design expressions to support the use of CFT columns in bridge construction. However, full realization of the system requires development of a range of CFT column-to-cap beam connections. This connection offers additional challenges including constraints on the cap beam width and height and congested joint reinforcing, which are parameters that have not been thoroughly investigated. Furthermore, precast elements need to be considered in construction of

the super structure as they are necessary for the optimization of accelerated bridge construction. In an effort to meet these diverse challenges, the continuing phase of this research program includes development of the CFT column-to-cap beam connection. Several proposed CFT column-to-cap beam connections are illustrated in Figure 6. Figure 6a shows a fully restrained embedded connection, which is similar to the column-to-foundation connection described above. This connection uses the grouted connection detail, with a void cast into a precast inverted-t beam (as shown; note an RC cap beam can also be utilized). The precast cap beam is placed onto the column after the column is set, and the recess between the tube and corrugated pipe is filled with high strength fiber reinforced grout. The connections shown in Figures 6b, 6c, and 6d are partial strength connections in which reinforcing bars are developed in the CFT and cap beam. The longitudinal reinforcing ratio in these connections will be less than the effective reinforcing ratio of the CFT, and therefore the plastic moment capacity of the CFT will not be achieved. Figure 6b shows welded reinforced concrete (welded RC) connection in which a ring of headed reinforcing bars is welded into the steel tube and are developed into the cap beam. The connection shown in Figure 6c employs the same welded detail within the tube; however the reinforcing bars are de-bonded in the column-to-cap beam interface region with the intent of increasing ductility. Figure 6d shows a traditional reinforced concrete (RC) connection in which both transverse and longitudinal reinforcing extends from the CFT column into the cap beam, and cover is provided between the longitudinal reinforcing and steel tube within the column. The reinforcing bars in the three RC connections provide axial, moment, and shear transfer. These connections can be integrated into precast elements using ducts or a void similar to that described for the grouted connection.

Figure 6. Proposed CFT pier-to-cap beam connections.

Numerical Model A preliminary series of nonlinear analyses were performed on the embedded cap beam connection shown in Figure 6a using the commercially available finite elements analysis software ABAQUS. An overview of the model is shown in Figure 7. Model geometry included the embedded CFT column-to-cap beam connection, the CFT column, and the reinforced concrete cap beam. To improve computational efficiency, a half model was developed taking advantage of the symmetry plane parallel to the direction of loading and the center of the specimen. The end nodes at the top of the cap beam were fully restrained to simulate boundary conditions in future tests, and lateral loading was applied by assigning displacements Δx along the x-axis to the bottom nodes of the concrete fill and steel tube. The loading direction was

transverse to the direction of traffic for the initial parameter studies. A constant axial load of 10% the crushing load of the CFT was applied to the bottom nodes of the column. The concrete components, steel tube, and reinforcing steel were modeled using the 8-node continuum element with reduced integration (C3D8R), the 4-node shell element with reduced integration (S4R), and the 2 node truss element (T3D2) respectively. Gap elements were used at every geometrically common point between the steel tube and concrete elements to simulate bond stress between the concrete by combining the confining contact stress with a coefficient of friction to develop shear stresses at the interface. The reinforcing steel and concrete components in the cap beam were spatially assembled, and the ABAQUS Embedded constraint relationship was defined to perfectly embed the reinforcing bars into the concrete. Parametric Studies A limited series of parametric studies were conducted to evaluate the influence of differing parameters on the performance of the proposed CFT column-to-cap beam connection. Specifically, the performance of the embedded connection shown in Figure 6a was evaluated for differing cap beam widths and annular ring diameters. The parameter studies were conducted in the longitudinal direction of the cap beam using a 20-in (508-mm) diameter tube with a nominal yield strength of 49-ksi (340-MPa). Shear and flexure reinforcing was included in the cap beam according to requirements in the 2010 Caltrans Seismic Design Criteria [11]; the reinforcement was assigned a nominal yield strength of 60-ksi (414-MPa). The depth and length of the cap were held constant at 24-in. (610-mm) and 76-in. (1.93-m) respectfully. Five cap beam widths ranging from 2D to 3D (half widths of D to 1.5D in the half model) as well as two differing annular ring diameters (Do=D+32t and Do=D+16t) were evaluated. The cap beam width parameter study was conducted using an annular ring with an outer diameter Do = D+32t while the cap beam width for the annular ring diameter study was held constant at 2D or 40-in (1.02-m). An embedment depth of 0.7D was selected for all parameter studies.

Figure 7. Overview of finite element model of embedded CFT column-to-cap beam connection

Moment-drift relationships for the cap beam width and annular ring diameter parameter studies are plotted in Figures 8a and 9a respectively, while inelastic deformation at 10% drift in these parameter studies has been plotted in Figures 8b and 9b. Figure 8 illustrates that although cap beam damage increased as the width decreased, the strength of the embedded connection

exceeded the plastic moment capacity of the CFT for all widths analyzed. This suggests that the embedded connection can achieve strength and ductility performance objectives within the geometric constraints of the cap. Figure 9 shows that decreasing the outer diameter of the annular ring, Do, from D+32t to D+16t does not influence the moment-drift relationship or significantly increase cracking in the cap beam.

Figure 8. Embedded connection cap beam width parameter study: a) moment drift

relationships and b) cap beam cracking at 10% drift.

Figure 9. Embedded connection annular ring diameter parameter study: a) moment drift

relationships and b) cap beam cracking at 10% drift.

Conclusions and Future Work

Two variations of an embedded CFT column-to-foundation connection – a monolithic connection and a recessed, grouted connection, were developed and experimentally evaluated. Results from the experimental analysis indicate that the proposed connection is effective and practical. Specimens with adequate embedment depth were capable of achieving drift capacities far in excess of the maximum seismic design without degradation of the system and minimal damage to the footing and column. CFT column-to-foundation connection design requirements were developed based on these results. Several potential CFT column-to-cap beam connections were developed based on the experimental investigation of the CFT column-to-foundation connection. Several CFT cap beam connections were proposed including an embedded connection similar to the grouted CFT foundation connection. Limited parameter studies conducted on the proposed embedded cap beam connection suggest that this connection can achieve strength and ductility objectives within the constraints of the cap beam. Additional numerical and experimental analyses need to be conducted on potential CFT cap beam connections such that simplified design expressions can be developed. Ongoing

research at the University of Washington has extended the numerical analysis to include several partial strength connections as well as to evaluate the influence of different parameters on connection behavior. Results from these analyses have been used to design large scale specimens to evaluate the performance of these connections in the transverse and longitudinal directions of the cap beam. Results from these experiments will be used to validate numerical analyses to develop design expressions for a variety of CFT cap beam connections.

Acknowledgements

Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number DAAD19-03-2-0036, the California Department of Transportation (CALTRANS) and the Washington State Department of Transportation (WSDOT). The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory, the U.S. Government, CALTRANS or WSDOT. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation heron. The authors gratefully acknowledge the financial support of these organizations. In addition, the advice provided by Ron Bromenschenkel, Michael Cullen, and Peter Lee of CALTRANS, Bijan Khalighi of WSDOT and Jon Tirpak of the Advanced Technology Institute and the Vanadium Technology Partnership.

References

1. Roeder, C.W., Cameron, B., Brown, C. Composite Action in Concrete Filled Tubes. ASCE Journal of Structural Engineering 1999. 125 (5): 477-484.

2. Roeder, C.W., Lehman, D.E., and Thody, R. Composite Action in Concrete Filled Tubes. AISC Engineering Journal 2009. 46 (4): 229-242.

3. Roeder, C.W., Lehman, D.E., and Bishop, E. Strength and Stiffness of Circular Concrete Filled Tubes. ASCE Journal of Structural Engineering 2010. 135 (12): 1545-1553.

4. Lehman, D.E., Roeder, C.W. Foundation Connections for Circular Concrete-Filled Tubes. Journal of Constructional Research 2012. 78: 212-225.

5. Kingsly, A., Williams, T., Lehman, D.E., and Roeder, C.W. Experimental Investigation of Column-to-Footing Connections for High Strength Vanadium Steel concrete Filled Tube Construction. International Journal of Steel Structures 2005. 5 (4): 377-387.

6. Williams, Travis. Experimental Investigation of High Strength Concrete Filled Steel Tubes in Embedded Column Base Foundation Connections. A thesis submitted in partial fulfillment of the degree of Master of Science in Civil Engineering 2007. University of Washington, Seattle WA.

7. Lee, Jason. Experimental Investigation of embedded Connections for Concrete Filled Tube Column Connections to Combined Axial-Flexural Loading. A thesis submitted in partial fulfillment of the degree of Master of Science in Civil Engineering 2011. University of Washington, Seattle WA.

8. ACI. Building Code Requirements for Structural Concrete and Commentary. American Concrete Institute: Farmington Hills, MI, 2010.

9. AISC. Specification for Structural Steel Buildings. American Institute of Steel Construction: Chicago, Illinois, 2010.

10. ATC-24. Guidelines for Testing Steel Components. Applied Technology Council: Redwood City, CA, 1992.

11. Caltrans. Seismic Design Criteria Version 1.6. California Department of Transportation: Sacramento, CA, 2010.

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CONCRETE-FILLED TUBES FOR ACCELERATED BRIDGE …