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Structural capacity and fatigue performance of ASTM A709 Grade 50CR steel
Construction and Building Materials xxx (xxxx) xxx
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Structural capacity and fatigue performance of ASTM A709 Grade 50CR steel Weizhuo Shi a, Behrouz Shafei a,b,c,⇑, Brent Phares a,b a
Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA, United States Institute for Transportation (InTrans), Iowa State University, Ames, IA, United States c Department of Materials Science and Engineering, Iowa State University, Ames, IA, United States b
h i g h l i g h t s A holistic investigation was carried out on the use of corrosion-resistant steel. Various performance measures were extracted from full-scale load and fatigue tests. The structural capacity and fatigue performance were systematically assessed. Predictions were made on the long-term performance and remaining capacity. Benefits of corrosion-resistant steel were quantified comparing to other alternatives.
a r t i c l e
i n f o
Article history: Received 14 February 2020 Received in revised form 12 September 2020 Accepted 17 October 2020 Available online xxxx Keywords: ASTM A709 Grade 50CR steel Corrosion resistance Full-scale structural tests Composite section Fatigue characteristics
a b s t r a c t ASTM A709 Grade 50CR steel includes a range of 10.5% to 12.5% chromium, which greatly enhances the corrosion resistance of this type of steel in comparison to conventional steels widely used for structural applications. Despite the wealth of information on the durability of ASTM A709 Grade 50CR steel in corrosive environments, there was a gap in the literature concerning how this type of steel responds to the loads that the structures experience during their service life. To investigate this critical aspect and facilitate the future use of corrosion-resistant steel, the current study devised a holistic structural testing program with a focus on bridge applications. For this purpose, a full-scale girder was designed, fabricated, and tested under a four-point bending setup. The girder’s ability to meet the design expectations was assessed by comparing the results to the requirements of AASHTO Bridge Design Specifications. In addition, several tensile and fatigue tests were carried out to obtain an in-depth understanding of the performance of ASTM A709 Grade 50CR steel under both monotonic and cyclic loading scenarios. This led to the development of load–displacement, stress–strain, and S-N curves. The investigations were then expanded to quantify the remaining capacity of three different types of steel, i.e., corrosion-resistant steel, weathering steel, and carbon steel. For this purpose, a set of simulations were performed to estimate the corrosion rate and section loss of the girders under consideration over time. The outcome of this holistic study contributed to providing the insight necessary for the safe and efficient design of structural members made with corrosion-resistant steel. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction A growing investment is made every year on the repair and replacement of deteriorating structures. For steel bridges, in particular, corrosion is the main cause of deterioration, which adversely affects the long-term integrity and performance of this category of
⇑ Corresponding author at: Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA, United States. E-mail address: [email protected] (B. Shafei).
bridges. There is a wide range of environmental stressors that influence the rate of corrosion, including temperature, humidity, and exposure to corrosive agents. While exposure to such stressors is largely unavoidable, selection of alternative steels, including those resistant to corrosion, can minimize the vulnerability of steel bridges to harsh environmental conditions and increase their expected service life. Among available alternatives, ASTM A709 Grade 50W steel, which is characterized as weathering steel, has been commonly used to avoid corrosion-induced damage to bridge structures. The bridges made with this type of steel, however, are
https://doi.org/10.1016/j.conbuildmat.2020.121379 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: W. Shi, B. Shafei and B. Phares, Structural capacity and fatigue performance of ASTM A709 Grade 50CR steel, Construction and Building Materials, https://doi.org/10.1016/j.conbuildmat.2020.121379
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tions, strains, and stresses at various locations of the bridge. The obtained results indicated a satisfactory performance. In 2016, a four-span bridge, comprising of a combination of four A709 Grade 50W and two A709 Grade 50CR steel girders, was built in Iowa. This bridge, which is under a long-term performance monitoring program, has experienced no structural issues to date. The main material properties and fabrication process of ASTM A709 Grade 50CR steel have been documented in Sharp et al. [9]. In particular, this type of steel meets the minimum yield strength requirement, i.e., 50 ksi (345 MPa), and the fracture toughness requirement of 15 lb.ft (20 J) for non-fracture critical members and 25 lb.ft (34 J) for fracture critical members. While the initial cost of structural members built with ASTM A709 Grade 50CR steel can be higher than that of structural members built with conventional steels, they benefit from a lower maintenance need and longer service life. With the justification of using corrosion-resistant steel from a life-cycle perspective, a few research studies have been conducted on the accelerated and in-situ corrosion of ASTM A709 Grade 50CR steel coupons [3,10,11]. Among them, Fletcher [3] examined the corrosion characteristics of stainless steel with different chromium contents, including 9%-chromium steel (9Cr), 7%-chromium and 2%-silicon steel (7Cr2Si), and 7%-chromium and 2%-aluminum steel (7Cr2Al). The results were then compared to other structural steels, such as A36 carbon steel and A588 weathering steel. All the three chromium steels showed a lower corrosion rate than the two structural steels. This was further confirmed by the X-ray diffraction analyses of corrosion products. Seradj [10] also performed a study to examine the corrosion of ASTM A709 Grade 50CR steel with simulating coastal exposure condition. A588 weathering steel and highperformance steel were considered in the study as reference points. After two years of exposure, ASTM A709 Grade 50CR steel showed a superior corrosion resistance in comparison to the other two steels. To investigate the corrosion of stainless steel for bridge applications, an accelerated corrosion investigation was conducted by Groshek [11]. After 80 cycles, it was reported that ASTM A709 Grade 50CR steel can offer a considerably higher corrosion resistance than weathering steel, even in highly-corrosive atmospheric environments. This was attributed to the fact that chromium oxide molecules formed on the surface of ASTM A709 Grade 50CR steel are smaller (and more packed) than the iron oxide molecules produced on conventional weathering steel, thus, the steel substrate is better isolated from aggressive agents. The promising results obtained from corrosion tests motivated further consideration of ASTM A709 Grade 50CR steel for various structural applications, especially where multiple environmental and mechanical stressors are present. This was explored in the current study through a holistic set of coupon and full-scale structural tests.
still found to be in need of frequent maintenance and repair, as the protective patina may not be sufficient, especially in extreme exposure conditions [1,2,3,4]. In a study completed by Cook [5], it was found that the presence of excessive chloride ions can be detrimental to the formation of patina, questioning the long-term performance of weathering steel used in the coastal bridges exposed to sea salts and the bridges located in cold regions, where deicing salts are commonly spread on roadways [6]. McDad et al. [2] reported a similar issue in the survey conducted for the Texas Department of Transportation. The protective film was found not well developed in a number of weathering steel bridges due to the cycles of wetting and drying. In a separate study, Crampton et al. [4] inspected more than 30 bridges in Iowa to document the quality of patina in weathering steel bridges. The bridges were categorized based on their local environments. A series of tests were then carried out to rate the condition state of patina on a scale of 1 to 5, where 1 indicated a substantial failure of patina, while 5 indicated a well-developed patina with satisfactory protection. With the survey outcome skewed to 1, the delamination of patina from the surface of weathering steel was evident in several cases. Such delamination was observed to permit the penetration of moisture and salt into rust interlayers, requiring early repair and maintenance to prevent corrosion-induced structural damage. To address the long-standing issues associated with the corrosion of conventional structural steels, ASTM A709 Grade 50CR steel has gained growing attention, especially for bridge applications [7,8]. Despite the fact that a number of studies have been conducted on the corrosion resistance of stainless steel, there was a gap in the literature on the structural performance and fatigue characteristics of this type of steel. Thus, it was important to ensure that, while ASTM A709 Grade 50CR steel offers superior corrosion resistance, it can also meet the design requirements from the perspective of Strength and Serviceability Limit States. This motivated the current study to evaluate the response of ASTM A709 Grade 50CR steel in both coupon and full-scale setups. In the absence of similar studies, the composite behavior of the girders made with this type of steel was investigated through experimental tests. The results were then compared to the design expectations outlined in AASHTO Bridge Design Specifications. The testing program was then supplemented with a set of tensile and fatigue tests to obtain a full understanding of the performance of ASTM A709 Grade 50CR steel under both monotonic and cyclic loading scenarios. The original data obtained from the experimental tests paved the way to expand the investigations to understand and quantify the long-term benefits of corrosion-resistant steel. This was achieved with a set of simulations that captured the corrosion rate and section loss of three different types of steel over time. This provided a detailed insight on how the structural performance can be maintained if the corrosion-resistant steel is used instead of conventional weathering and carbon steels.
3. Full-scale experiments 2. Material properties and field applications
3.1. Main mechanical properties
ASTM A709 Grade 50CR steel is a new addition to the group of structural stainless steel materials. This type of steel has a nominal 12% chromium (a minimum of 10.5% and a maximum of 12.5%), which significantly enhances its resistance to corrosion in comparison to other structural steels. While ASTM A709 Grade 50CR steel has been utilized in aggressive environmental conditions, such as those experienced in coal preparation and processing facilities, the civil infrastructure applications of this type of steel have been limited to date. In 2004, a short-span, box-girder bridge was built using ASTM A709 Grade 50CR steel over the Glenn-Colusa Main Canal in Colusa County, California. Shortly after the bridge was opened to traffic, load tests were performed to measure the deflec-
The composition of ASTM A709 Grade 50CR steel used in this study is provided in Table 1, along with the composition specified for ASTM A709 Grade 50W and A36. Based on the compositions provided in this table, there is a high percentage of chromium in ASTM A709 Grade 50CR steel. This leads to the formation of a chromium oxide layer (instead of iron oxide rust) on the surface, which protects the steel substrate from further corrosion. To determine the main mechanical properties, such as yield strength, ultimate tensile strength, modulus of elasticity, and elongation prior to fracture, of ASTM A709 Grade 50CR steel, a set of tensile tests were conducted under a monotonic loading scenario by using a universal testing machine. The geometry of coupons was decided follow2
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W. Shi, B. Shafei and B. Phares Table 1 Composition of different types of steel investigated in the current study. Steel
C
Mn
P
S
Si
Cr
Ni
N
Mo
ASTM A709 50CR ASTM A709 50W ASTM A36
0.03 0.11 0.26
1.50 1.10–1.35 –
0.040 0.020 0.040
0.010 0.006 0.050
1.00 0.40 0.40
10.5–12.5 0.45–0.70 –
1.50 0.30 –
0.030 0.015 –
– 0.02–0.08 –
ing ASTM E8 [12]. As documented in Table 2, the average yield strength and modulus of elasticity are determined to be 68.7 ksi (473.7 MPa) and 31,366 ksi (216.3 GPa), respectively. For comparison purposes, this table also provides the mechanical properties specified for the conventional weathering steel. As indicated in the referenced table, ASTM A709 Grade 50CR steel fulfills the requirements of mechanical strength and ductility similar to ASTM A709 Grade 50W steel.
span to capture the vertical displacement of the concrete deck during the test. Two LVDTs were placed horizontally at the interface between the steel girder and the concrete deck to record any longitudinal slip. They were placed at the mid-span and close to one of the end supports. The details of the instrumentation layout have been depicted in Fig. 5.
3.2. Specimen design and fabrication
Before the ultimate load test on the full-scale test setup, preliminary tests were conducted on the ASTM A709 Grade 50CR steel girder (with no concrete deck) and the steel–concrete composite girder, both at the elastic range. While the load level was kept very low to avoid introducing any residual strains/stresses, these two preliminary tests helped verify the test setup and calibrate the calculations. After completing the preliminary tests, the specimen was loaded to failure under a four-point bending setup. The details of the specimen failure at the ultimate load is shown in Fig. 6. The load–deflection relationship measured at the mid-span of the specimen is shown in Fig. 7. Based on the recorded deflections, the flexural ductility factor, l, was found to be approximately 7.5, based on l ¼ du =dy , where du and dy are the deflections at the ultimate and yield points, respectively. As shown in Fig. 7, in the initial stage of loading, the load–deflection curves were linear and no cracks were found on the deck. As the load was increased, the first crack initiated at the bottom surface of the deck at the mid-span, propagating toward the neutral axis, which was in the concrete deck. It can be observed that the stiffness of the beam setup started to degrade when the applied load reached close to 150 kip (670 kN), indicating that the girder has yielded. When the applied load was further increased and the specimen reached its ultimate capacity, spalling of the concrete deck was observed due to stress concentrations under the point load areas, where the beam failed because of concrete crushing. With plastic deformations at the ultimate load, a satisfactory flexural behavior was recorded for the ASTM A709 Grade 50CR steel–concrete composite specimen. No difference was observed in the load–deflection measured at the bottom of the steel girder and at the left and right of the concrete deck. This indicated that the lateral-torsional buckling was prevented, primarily because the lateral stiffness of the deck was adequate. Fig. 8 presents the longitudinal strain distributions in the steel and concrete subjected to bending at the mid-span cross-section with respect to each level of loading. This figure highlights that the magnitude of the longitudinal strain in both steel and concrete increased consistent with increasing the applied load. The strain at the bottom of the girder was close to 2000 me under the loading
3.3. Results and findings
To evaluate the structural performance of ASTM A709 Grade 50CR steel in a full-scale setting, a 52 ft.-9 in. (16.1 m) long girder was designed, replicating the same details used in the stainless steel bridge that is under an ongoing long-term monitoring program in Iowa. Fig. 1 shows the plate girder profile and the section geometry for the full-scale test setup. The width and thickness of the top and bottom flange were 12 in. (30.5 cm) and 1 in. (2.5 cm), respectively. A total of 111 shear studs were welded on the top flange with the spacing of 1 ft.-5 in. (43.2 cm). The depth and thickness of the girder web were 36 in. (91.5 cm) and 3/4 in. (1.9 cm), respectively. A concrete deck was cast on the steel plate girder to provide a steel–concrete composite section. The geometry of the steel–concrete composite section is shown in Fig. 2. The width and thickness of the deck were 7.5 ft. (228.6 cm) and 8 in. (20.3 cm), respectively. The average compressive strength of concrete determined from multiple cylinder tests was 6143 psi (42 MPa). Fig. 3 presents the key steps for the fabrication of the full-scale specimen. After curing for 28 days, the specimen was tested under a four-point bending scenario to evaluate the composite flexural behavior of the ASTM A709 Grade 50CR steel–concrete section. The specimen was simply supported and subjected to a load setup shown in Fig. 4. The two loading points were spaced at 15 ft. (4.6 m) away from each other. The applied loads were recorded by two load cells attached to the loading actuators. Three sets of instrumentation devices were installed to collect the structural response data during the test: electrical resistance strain gages (ERSGs), direct current displacement transducers (DCDTs), and linear variable differential transformers (LVDTs). A total of 15 ERSGs were attached to the top surface of the concrete deck and the longitudinal rebars embedded in the deck to measure the longitudinal strain. In addition, 12 ERSGs were placed on the top and bottom flange of the girder at the quarter and mid-span. A total of 5 DCDTs were installed vertically underneath the girder to record deflections. The DCDTs were placed at the quarter and mid-span, as well as close to the two end supports. A set of two DCDTs were also placed at the bottom of the deck at the mid-
Table 2 Summary of the tensile test results and mechanical properties of ASTM A709 Grade 50CR and 50W steel (1 ksi = 6.9 MPa). Mechanical Property
Yield Strength, ksi Tensile Strength, ksi Modulus of Elasticity, ksi Elongation in 2 in. (50 mm), %
Experimental Results
ASTM Standard
C-1
C-2
C-3
Average
A709 50CR
A709 50W
67.8 87.0 31,155 17.9
67.8 86.9 31,399 18.9
70.7 88.8 31,144 17.1
68.7 87.6 31,366 18.2
50.0 70.0 29,000 18.0
50.0 70.0 29,000 18.0
3
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Fig. 1. Details of the plate girder specimen, including dimensions, shear studs, and stiffeners (1 ft. = 30.5 cm, 1 in. = 2.54 cm).
7'-6''
8''
#6 BAR
36''
12'' Fig. 2. Geometry of the composite beam section (1 in. = 2.54 cm).
In addition to the full-scale test, analytical calculations were performed to estimate the moment capacity of the ASTM A709 Grade 50CR steel–concrete composite section, following AASHTO Bridge Design Specifications. The calculations were based on the classic beam theory, in which the plane section is assumed to remain plane after bending. Fig. 8 confirmed the validity of the assumption for the beam under consideration. Global and local buckling was checked and ensured to be prevented so that a plastic behavior can be achieved. To further refine the calculations, two approaches were considered in this study, as illustrated in Fig. 10. Approach 1 included the material properties based on the design values. Noting that the yield strength of the ASTM A709 Grade 50CR steel was 50 ksi (345 MPa) and the compressive strength of the concrete deck was
level equal to 60% of the ultimate load, at which the ASTM A709 Grade 50CR steel girder started to yield. It was observed that, in general, a linear strain distribution can be obtained until close to 90% of the ultimate load. This indicated a satisfactory composite behavior between the steel girder and the concrete deck. The change of longitudinal slip with respect to the applied load was measured with the LVDTs (as shown in Fig. 9). It can be observed that the relationship between the applied load and the recorded slip was linear when the load was below the yield point. With further increasing the load, the longitudinal slip at the end support increased significantly from the yield point, i.e., 0.01 in. (0.25 mm) to 0.14 in. (3.50 mm). The slip was found to have minimum effect on the composite behavior and load bearing capacity. 4
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W. Shi, B. Shafei and B. Phares
(a) Formwork and reinforcement
(b) Concrete placement
(c) Concrete deck curing
(d) Actuator setup
Fig. 3. Main steps to fabricate and setup the specimen for the full-scale four-point loading test.
ACTUATOR
SPREADER BEAM
Midspan 25'-10''21 6''
51'-9''
6''
Fig. 4. Loading configuration for the ASTM A709 Grade 50CR steel–concrete composite deck (1 ft. = 30.5 cm).
applications. The fatigue tests were performed with a variety of axial load amplitudes using an MTS hydraulic testing machine. The loading protocol and other supporting details for the fatigue tests can be found in Table 4. As reflected in this table, the loading protocol was in a sinusoidal shape with the stress ratio, R, (i.e., min stress/max stress) of 1 to fully replicate a tension–compression loading scenario, capturing the most extreme demand. In the fatigue testing program designed for the current study, the maximum stress was set at 75 percent of the yield strength, and the stress amplitudes were then reduced with an interval of 5 percent of the yield strength, until the fatigue limit was reached. Two specimens were included for each stress amplitude. The hourglass shape specimens for fatigue tests were cut from the ASTM A709 Grade 50CR steel plates according to ASTM E466 [13] (Fig. 11). Before conducting the cyclic loading tests for fatigue assessment, a preliminary compressive test was conducted to ensure the stability of the coupons. In this preliminary test, lateral displacement was measured as the load was monotonically increased. When the buckling initiated, the applied stress had reached 68 ksi (468 MPa), i.e., equivalent to 100% yield strength. This preliminary test confirmed that the geometry design used for fatigue coupons does not cause any stability issues. The tests were continued until
4 ksi (28 MPa), the entire steel section was utilized up to the yield point. The compressive stress in the concrete was assumed 0:85f c and the tensile strength of the concrete was neglected. The plastic neutral axis was then determined based on the force equilibrium. Approach 2 included the calculations based on the experimental strain and stress distributions across the section. The plastic moment was then found as the summation of the moments caused by the forces about the plastic neutral axis. Table 3 provides a comparison between the experimental test results and analytical calculations for the yield and ultimate bending moments. As summarized in this table, the predictions obtained from the design equations are in a very good agreement with the results obtained from the tests, indicating that the ASTM A709 Grade 50CR steel girder meets the design requirements. 4. Fatigue experiments 4.1. Test details Further to full-scale beam tests, fatigue tests were performed on ASTM A709 Grade 50CR steel coupons to predict their service life under the cyclic loads that they commonly experience in structural 5
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Fig. 5. Details of the instrumentation plan used to record various structural response measures during the full-scale tests (1 ft. = 30.5 cm).
made in Boyer [19] for hot-rolled steel coupons. The modified fatigue limit was calculated to be between 26.2 ksi (180.5 MPa) and 28.6 ksi (197 MPa). Considering that the corresponding value specified in AASHTO Bridge Design Specifications is 24 ksi (170 MPa), it was found that ASTM A709 Grade 50CR steel can provide adequate fatigue resistance similar to conventional structural steels. In the absence of similar studies in the literature on the high-cycle fatigue of this type of steel, the fatigue behavior of ASTM A709 Grade 50CR steel obtained from the experimental tests was compared to that of other conventional structural steels, including ASTM A709 Grade 70W and A36 steels [20]. According to Fig. 13, the highcycle fatigue performance of ASTM A709 Grade 50CR steel falls well within the range of performance predicted for other structural steels. This confirms the satisfactory fatigue life of ASTM A709 Grade 50CR steel, further to providing superior corrosion resistance.
either the coupons were fractured or the number of fatigue cycles reached 10,000,000 cycles. 4.2. Prediction of fatigue limits The stress-life method was employed in the current study to predict the fatigue life (a.k.a., S-N curve). This method was deemed appropriate, as the bulk material’s response was elastic, emphasizing on overall stresses, rather than local ones [14–17]. To determine the S-N curve for ASTM A709 Grade 50CR steel, the stresses recorded in each fatigue test were plotted versus the number of cycles to failure in a log–log format. The relation between the alternating stress amplitude, rA , and the corresponding number of cycles, N f , can be mathematically expressed as rA ¼ aðN f ÞB , where a represents the fatigue strength coefficient and B is the fatigue strength exponent. The constants a and B were obtained from a regression analysis of the stress and fatigue life data obtained from the experiments conducted in the current study. The values of a ¼ 258:61 and B ¼ 0:138 were found to generate the S-N curve
5. Prediction of remaining structural capacity over time Due to the cross-sectional loss resulted from the formation and accumulation of corrosion products, the structural capacity is found to drop over time, diminishing the safety margins anticipated for structural members. To obtain a holistic assessment of the remaining capacity of structural steel members and quantify how this capacity is affected in long term by the choice of steel, three different types of steel, i.e., corrosion-resistant steel, weathering steel, and carbon steel, are compared in the current study. To predict the time-dependent loss of thickness, the power function provided in Equation (1) can be employed:
with R2 ¼ 0:874. The fatigue limit for ASTM A709 Grade 50CR steel subjected to high-cycle fatigue loads has been reported in Fig. 12. The two red arrows in this figure indicate the runouts, at which the fatigue life was above the specified 10,000,000 cycles. The fatigue limit was defined as the stress amplitude between the smallest stress with failure and the largest stress without failure [18]. According to Fig. 12, this limit was found to be between 37.4 ksi (257.9 MPa) and 40.8 ksi (281.3 MPa) for ASTM A709 Grade 50CR steel under consideration. Since the surface was gently polished in the coupons used for the fatigue tests, a surface roughness correction factor of 0.7 was considered in the current study, following the recommendations
C ¼ Atn 6
ð1Þ
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Fig. 6. Damage to the full-scale specimen under the ultimate load.
300 Measurement stopped
250 Concrete crushing
Load (kip)
200
150 Steel yielding
100
1/2 Span
50
1/4 Span
0 0
3
6
9
12
15
Deflection (in) Fig. 7. Load-displacement at the quarter and mid-span (1 in. = 25.4 mm, 1 kip = 4.5 kN).
the loss of metal thickness can exceed 2150 mm for carbon steel, 950 mm for weathering steel, and 95 mm for corrosion-resistant steel after 75 years. The loss of the section thicknesses over time was further employed in the current study to estimate the drop in the moment capacity of the steel girder sections. For this purpose, the section loss was assumed uniform along the entire perimeter of the girder section, except for the top surface, which was considered to be attached to the deck. Based on a detailed analytical study that was initiated from the composite section tested in the current study (Fig. 2), the time-dependent moment capacity was predicted for the three different types of steel. According to Fig. 15, while the drop in the moment capacity remained negligible in the sections made of corrosion-resistant steel, the sections made of carbon steel
where C is the average corrosion penetration depth; t is the exposure time; and A and n are the two constants, which vary based on the type of steel. For carbon steel, A and n can be considered equal to 70.60 mm and 0.79, respectively, under severe corrosive conditions [1]. For weathering steel, A and n change to 119.26 mm and 0.48, respectively. The assumed values have been reported by Groshek [11] based on the thickness loss data obtained for ASTM A709 Grade 50W steel subjected to extreme environmental conditions. According to the thickness loss data obtained from Fletcher [3] and Groshek [11], ASTM A709 Grade 50CR steel experiences a section loss approximately one-tenth of that measured for ASTM A709 Grade 50W steel. For further demonstration, Fig. 14 provides the estimated thickness loss of structural steel members over a service life of 75 years. As reflected in this figure, 7
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48 42
Girder height (in)
P/Pu=0.0 36
P/Pu=0.3 P/Pu=0.5
30
P/Pu=0.6 24
P/Pu=0.7 P/Pu=0.8
18
P/Pu=0.9 12 6 0 -2000
0
2000
4000
6000
8000
Longitudinal strain (10-6) Fig. 8. Strain distribution at the mid-span’s cross section (1 in. = 2.54 cm).
300
250
Load (kip)
200
150
100 Mid-span
50
End-span 0 0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Longitudinal slip at steel-concrete interface (in) Fig. 9. Load-longitudinal slip relationship (1 in. = 25.4 mm, 1 kip = 4.5 kN). Note that the slip remained almost zero at the mid-span for the entire test.
Ps Pc
Ps Pt2
Pw Pt1 Pt Fig. 10. Calculations to obtain the ultimate flexural capacity: (a) based on AASHTO Bridge Design Specifications, and (b) based on the experimental stress distribution across the section.
ments from the individual section capacity to the overall structural response of the girders. As a result of section loss due to corrosion, it was found that the moment of inertia and consequently stiffness of the girder are adversely impacted, leading to excessive deflec-
and weathering steel experienced a drop of 22% and 10% after 75 year, respectively. Since the serviceability of the steel structures directly depends on the magnitude of deflections, a separate analytical investigation was conducted to expand the scope of assess8
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W. Shi, B. Shafei and B. Phares Table 3 Comparison of the ultimate flexural strengths (1 kipft = 1.4 kNm).
Experimental Test Results Calculations based on AASHTO Bridge Design Specifications
Approach 1 Approach 2
Yield Moment (kipft)
Ultimate Moment (kipft)
Ultimate Load (kip)
3100.5 2651.3 3457.5
5167.5 4380.9 5497.4
260.0 220.0 276.6
Table 4 Overview of the fatigue testing parameters (1 ksi = 6.9 MPa; 1 in = 25 mm; 1 kip = 4.5 kN). ID.
Frequency (Hz)
Stress ratio
Max. stress (ksi)
Min. stress (ksi)
Thickness (in.)
Width (in.)
Area (in2)
Max. load (kip)
Min. load (kip)
F1 F2 F3 F4 F5 F6
8 8 8 8 8 8
1 1 1 1 1 1
51.00 47.60 44.20 40.80 37.40 34.00
51.00 47.60 44.20 40.80 37.40 34.00
0.523 0.520 0.519 0.520 0.519 0.519
1.000 0.990 1.001 1.001 0.999 0.999
0.523 0.515 0.520 0.521 0.518 0.518
26.67 24.50 22.96 21.30 19.39 17.63
26.67 24.50 –22.96 21.30 19.39 17.63
Hydraulic Grip
Hydraulic Actuator
Fig. 11. Fatigue test setup and the dimensions of the specimens (1 in. = 25.4 mm).
80 Fitted line Fatigue test data
Alternating stress (ksi)
70 60
50
40
30 10,000
100,000
1,000,000
10,000,000
Number of cycles Fig. 12. S-N curve developed for ASTM A709 Grade 50CR steel based on the fatigue tests performed in the current study. 9
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90 Current experiments
80
Alternating stress (ksi)
ASTM A709 Grade 50CR steel 70
ASTM A709 Grade 70W steel
60
ASTM A36 steel
50 40 30 20 10 0 0
500,000
1,000,000
1,500,000
2,000,000
Number of cycles Fig. 13. Fatigue behavior of ASTM A709 Grade 50CR steel in comparison to the other conventional structural steels (1 ksi = 6.9 MPa).
2500
3.00
Carbon steel 2000
Weathering steel
2.75
Corrosion-resistant steel
Corrosion-resistant steel
Deflection (in)
Section loss (µm)
Carbon steel
Weathering steel
1500
1000
500
Experiment
2.50
2.25
2.00
1.75
0
1.50
0
15
30
45
60
75
0
Time (year)
15
30
45
60
75
Time (year) Fig. 16. Maximum girder deflection predicted as a function of age for three different types of steel (1 in. = 25.4 mm).
Remaining moment capacity (%)
100 Carbon steel
Weathering steel
Corrosion-resistant steel
deflection changes under the experimental yield load (i.e., 150 kip (670 kN)), depending on the choice of steel. As reflected in this figure, the percentage of increase in the maximum girder deflection is 24.3% and 9.7% for carbon steel and weathering steel after 75 years, respectively. However, there was no major change in the maximum deflection of the girder made of corrosionresistant steel. This highlights the promise of ASTM A709 Grade 50CR steel in meeting both strength and serviceability requirements.
95
90
85
80
6. Conclusions 75 0
15
30
45
60
75
This study provided a holistic investigation of the structural performance of ASTM A709 Grade 50CR steel, in terms of bending moment capacity and fatigue resistance, through a set of laboratory tests and analytical calculations. For this purpose, a fullscale specimen consisting of an ASTM A709 Grade 50CR steel plate girder attached to a concrete deck was built and subjected to a four-point bending scenario to evaluate various structural response measures, such as strains, stresses, and deflections. Comparisons were then made with the design requirements provided by AASHTO Bridge Design Specifications to ensure that this type of steel meets the expectations for Strength and Serviceability
Time (year) Fig. 15. Percentage of drop in the moment capacity as a function of age.
tions. To quantify this important aspect, the deflection of the bridge girder subjected to the full-scale test was estimated assuming three different types of steel. After the validation of the analytical calculations with the experimental test results, the maximum girder deflections were determined as a function of timedependent section loss. Fig. 16 depicts how the maximum girder 10
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W. Shi, B. Shafei and B. Phares
tion, Writing - review & editing, Funding acquisition, Supervision. Brent Phares: Methodology, Investigation, Writing - review & editing, Funding acquisition.
Limit States. To investigate how ASTM A709 Grade 50CR steel responds to cyclic loads, a set of fatigue tests were also performed up to 10,000,000 cycles. The high-cycle fatigue test data was then converted to an S-N curve. This curve predicted how the magnitude of alternating stresses changes in ASTM A709 Grade 50CR steel as a function of number of cycles. The main conclusions made from this study are as follows:
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
From the tensile tests, the main mechanical properties, such as yield strength, ultimate tensile strength, modulus of elasticity, and elongation prior to fracture, of ASTM A709 Grade 50CR steel were determined. With an average yield strength and modulus of elasticity of 68.7 ksi (473.7 MPa) and 31,366 ksi (216.3 GPa), respectively, ASTM A709 Grade 50CR steel was found to fulfill the requirements of mechanical properties similar to ASTM A709 Grade 50W steel. This was in addition to an average ductility of 7.5 recorded in the experiments. From the full-scale bending tests, the ASTM A709 Grade 50CR steel girder was found to yield at the loading level equal to 60% of the ultimate load. A linear strain distribution was maintained until close to 90% of the ultimate load, indicating a satisfactory composite behavior between the ASTM A709 Grade 50CR steel girder and the concrete deck. This was further supported by monitoring the load-slip relationship at both middle and end of the specimen. Analytical calculations were performed with two different approaches to evaluate the structural capacity of the steel girder-concrete deck system, following AASHTO Bridge Design Specifications. The predictions provided upper and lower bounds for the values reported from the experimental tests. This confirmed the ability of ASTM A709 Grade 50CR steel to meet the structural performance requirements. From high-cycle fatigue tests carried out on ASTM A709 Grade 50CR steel coupons, the fatigue limit was reported to be between 37.4 ksi (257.9 MPa) and 40.8 ksi (281.3 MPa). Even after a correction factor of 0.7 was applied to capture the surface roughness, the fatigue limit remained above 24 ksi (170 MPa), which has been recommended by AASHTO Bridge Design Specifications for structural steels. Additional tests are recommended to evaluate the fatigue strength of welded and bolted connection made with this type of steel. The drop of structural capacity over time was predicted in the girder setup under consideration using three different types of steel, i.e., corrosion-resistant steel, weathering steel, and carbon steel. Upon comparing the results, it was determined how the long-term benefits of ASTM A709 Grade 50CR steel can be realized by maintaining the expected performance and safety margins in both short and long term. In the long-term study, the extent of capacity loss due to corrosion did not reach a level that would change the mode of failure of the composite beams. This observation remains valid for most of corroding steel–concrete beams, mainly because the composite action is maintained by the welded shear studs embedded in concrete. Change of failure mode, however, is possible if an advanced stage of corrosion is reached.
Acknowledgement The authors would like to acknowledge the Iowa Department of Transportation for sponsoring this project. The contents of the paper reflect the conclusions and opinions of the authors and do not necessarily express the views of the funding agency. References [1] P. Albrecht, A. Naeemi, Performance of Weathering Steel in Bridges, NCHRP Report 272, National Cooperative Highway Research Program, Washington, DC, 1984. [2] B. McDad, D.C. Laffrey, M. Dammann, R.D. Medlock, Performance of Weathering Steel in TxDOT Bridges, Project 0-1818 (2000). [3] F.B. Fletcher, Improved Corrosion-Resistant Steel for Highway Bridge Construction, Report No, Federal Highway Administration, McLean, VA, 2011. [4] Crampton, D. D., Holloway, K. P., and Fraczek, J. (2013), Assessment of Weathering Steel Bridge Performance in Iowa and Development of Inspection and Maintenance Techniques, Report No. RB17-012, Iowa Department of Transportation, Ames, IA. [5] D.C. Cook, Spectroscopic identification of protective and non-protective coatings on steel structures in marine environments, Corros. Sci. 47 (10) (2005) 2550–2570. [6] Khatami, D. and Shafei, B. (2021), Impact of climate conditions on deteriorating reinforced concrete bridges in the U.S. Midwest region, ASCE Journal of Performance of Constructed Facilities, 35(1), 04020129 1-11. [7] H. Seradj, Oregon’s ASTM A1010 Bridges, Iowa Department of Transportation A1010 Steel Workshop, Ames, IA, 2015. [8] W.F. Via, Harrop, Virginia’s Initial Experience with an ASTM A1010 Plate Girder Bridge, VTCA Spring Conference, Virginia Department of Transportation, Richmond, VA, 2017. [9] Sharp, S. R., Provines, J. T., Moruza, A. K., Via, W. F., and Harrop, K. N., Virginia’s First Corrosion-Resistant ASTM A1010 Steel Plate Girder Bridge, Report No. FHWA/VTRC 20-R10, Charlottesville, VA, 2019. [10] H. Seradj, Oregon’s First ASTM A1010 Bridge and Risk Management, World Steel Bridge Symposium, Grapevine, TX, 2012. [11] I.G. Groshek, Corrosion Behavior of ASTM A1010 Stainless Steel for Applications in Bridge Components, Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA, 2017. [12] ASTM E8-16, Standard Test Methods for Tension Testing of Metallic Materials, ASTM International, West Conshohocken, PA, 2016. [13] ASTM E466-15, Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials, ASTM International, West Conshohocken, PA, 2015. [14] Y.-L. Lee, Fatigue Testing and Analysis: Theory and Practice, Elsevier Butterworth-Heinemann, Burlington, MA, 2005. [15] S. Arabi, B. Shafei, B. Phares, Fatigue analysis of sign-support structures during transportation under road-induced excitations, Journal of Engineering Structures 164 (2018) 305–315. [16] S. Arabi, B. Shafei, B. Phares, Investigation of fatigue in steel sign-support structures under diurnal temperature changes, J. Constr. Steel Res. 153 (2019) 286–297. [17] S. Arabi, B. Shafei, Multi-stressor fatigue assessment of steel sign-support structures: A case study in Iowa, Journal of Engineering Structures 200 (2019) 1–11. [18] Dowling, N. (1999), Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture and Fatigue, 2nd Edition, NJ [19] H.E. Boyer, Atlas of Fatigue Curves, ASM International, Metals Park, OH, 1986. [20] H. Chen, G.Y. Grondin, R.G. Driver, Characterization of fatigue properties of ASTM A709 high performance steel, J. Constr. Steel Res. 63 (6) (2007) 838–848.
CRediT authorship contribution statement Weizhuo Shi: Methodology, Investigation, Writing - original draft. Behrouz Shafei: Conceptualization, Methodology, Investiga-
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Structural capacity and fatigue performance of ASTM A709 Grade 50CR steel Weizhuo Shi a, Behrouz Shafei a,b,c,⇑, Brent Phares a,b a
Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA, United States Institute for Transportation (InTrans), Iowa State University, Ames, IA, United States c Department of Materials Science and Engineering, Iowa State University, Ames, IA, United States b
h i g h l i g h t s A holistic investigation was carried out on the use of corrosion-resistant steel. Various performance measures were extracted from full-scale load and fatigue tests. The structural capacity and fatigue performance were systematically assessed. Predictions were made on the long-term performance and remaining capacity. Benefits of corrosion-resistant steel were quantified comparing to other alternatives.
a r t i c l e
i n f o
Article history: Received 14 February 2020 Received in revised form 12 September 2020 Accepted 17 October 2020 Available online xxxx Keywords: ASTM A709 Grade 50CR steel Corrosion resistance Full-scale structural tests Composite section Fatigue characteristics
a b s t r a c t ASTM A709 Grade 50CR steel includes a range of 10.5% to 12.5% chromium, which greatly enhances the corrosion resistance of this type of steel in comparison to conventional steels widely used for structural applications. Despite the wealth of information on the durability of ASTM A709 Grade 50CR steel in corrosive environments, there was a gap in the literature concerning how this type of steel responds to the loads that the structures experience during their service life. To investigate this critical aspect and facilitate the future use of corrosion-resistant steel, the current study devised a holistic structural testing program with a focus on bridge applications. For this purpose, a full-scale girder was designed, fabricated, and tested under a four-point bending setup. The girder’s ability to meet the design expectations was assessed by comparing the results to the requirements of AASHTO Bridge Design Specifications. In addition, several tensile and fatigue tests were carried out to obtain an in-depth understanding of the performance of ASTM A709 Grade 50CR steel under both monotonic and cyclic loading scenarios. This led to the development of load–displacement, stress–strain, and S-N curves. The investigations were then expanded to quantify the remaining capacity of three different types of steel, i.e., corrosion-resistant steel, weathering steel, and carbon steel. For this purpose, a set of simulations were performed to estimate the corrosion rate and section loss of the girders under consideration over time. The outcome of this holistic study contributed to providing the insight necessary for the safe and efficient design of structural members made with corrosion-resistant steel. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction A growing investment is made every year on the repair and replacement of deteriorating structures. For steel bridges, in particular, corrosion is the main cause of deterioration, which adversely affects the long-term integrity and performance of this category of
⇑ Corresponding author at: Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA, United States. E-mail address: [email protected] (B. Shafei).
bridges. There is a wide range of environmental stressors that influence the rate of corrosion, including temperature, humidity, and exposure to corrosive agents. While exposure to such stressors is largely unavoidable, selection of alternative steels, including those resistant to corrosion, can minimize the vulnerability of steel bridges to harsh environmental conditions and increase their expected service life. Among available alternatives, ASTM A709 Grade 50W steel, which is characterized as weathering steel, has been commonly used to avoid corrosion-induced damage to bridge structures. The bridges made with this type of steel, however, are
https://doi.org/10.1016/j.conbuildmat.2020.121379 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: W. Shi, B. Shafei and B. Phares, Structural capacity and fatigue performance of ASTM A709 Grade 50CR steel, Construction and Building Materials, https://doi.org/10.1016/j.conbuildmat.2020.121379
W. Shi, B. Shafei and B. Phares
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tions, strains, and stresses at various locations of the bridge. The obtained results indicated a satisfactory performance. In 2016, a four-span bridge, comprising of a combination of four A709 Grade 50W and two A709 Grade 50CR steel girders, was built in Iowa. This bridge, which is under a long-term performance monitoring program, has experienced no structural issues to date. The main material properties and fabrication process of ASTM A709 Grade 50CR steel have been documented in Sharp et al. [9]. In particular, this type of steel meets the minimum yield strength requirement, i.e., 50 ksi (345 MPa), and the fracture toughness requirement of 15 lb.ft (20 J) for non-fracture critical members and 25 lb.ft (34 J) for fracture critical members. While the initial cost of structural members built with ASTM A709 Grade 50CR steel can be higher than that of structural members built with conventional steels, they benefit from a lower maintenance need and longer service life. With the justification of using corrosion-resistant steel from a life-cycle perspective, a few research studies have been conducted on the accelerated and in-situ corrosion of ASTM A709 Grade 50CR steel coupons [3,10,11]. Among them, Fletcher [3] examined the corrosion characteristics of stainless steel with different chromium contents, including 9%-chromium steel (9Cr), 7%-chromium and 2%-silicon steel (7Cr2Si), and 7%-chromium and 2%-aluminum steel (7Cr2Al). The results were then compared to other structural steels, such as A36 carbon steel and A588 weathering steel. All the three chromium steels showed a lower corrosion rate than the two structural steels. This was further confirmed by the X-ray diffraction analyses of corrosion products. Seradj [10] also performed a study to examine the corrosion of ASTM A709 Grade 50CR steel with simulating coastal exposure condition. A588 weathering steel and highperformance steel were considered in the study as reference points. After two years of exposure, ASTM A709 Grade 50CR steel showed a superior corrosion resistance in comparison to the other two steels. To investigate the corrosion of stainless steel for bridge applications, an accelerated corrosion investigation was conducted by Groshek [11]. After 80 cycles, it was reported that ASTM A709 Grade 50CR steel can offer a considerably higher corrosion resistance than weathering steel, even in highly-corrosive atmospheric environments. This was attributed to the fact that chromium oxide molecules formed on the surface of ASTM A709 Grade 50CR steel are smaller (and more packed) than the iron oxide molecules produced on conventional weathering steel, thus, the steel substrate is better isolated from aggressive agents. The promising results obtained from corrosion tests motivated further consideration of ASTM A709 Grade 50CR steel for various structural applications, especially where multiple environmental and mechanical stressors are present. This was explored in the current study through a holistic set of coupon and full-scale structural tests.
still found to be in need of frequent maintenance and repair, as the protective patina may not be sufficient, especially in extreme exposure conditions [1,2,3,4]. In a study completed by Cook [5], it was found that the presence of excessive chloride ions can be detrimental to the formation of patina, questioning the long-term performance of weathering steel used in the coastal bridges exposed to sea salts and the bridges located in cold regions, where deicing salts are commonly spread on roadways [6]. McDad et al. [2] reported a similar issue in the survey conducted for the Texas Department of Transportation. The protective film was found not well developed in a number of weathering steel bridges due to the cycles of wetting and drying. In a separate study, Crampton et al. [4] inspected more than 30 bridges in Iowa to document the quality of patina in weathering steel bridges. The bridges were categorized based on their local environments. A series of tests were then carried out to rate the condition state of patina on a scale of 1 to 5, where 1 indicated a substantial failure of patina, while 5 indicated a well-developed patina with satisfactory protection. With the survey outcome skewed to 1, the delamination of patina from the surface of weathering steel was evident in several cases. Such delamination was observed to permit the penetration of moisture and salt into rust interlayers, requiring early repair and maintenance to prevent corrosion-induced structural damage. To address the long-standing issues associated with the corrosion of conventional structural steels, ASTM A709 Grade 50CR steel has gained growing attention, especially for bridge applications [7,8]. Despite the fact that a number of studies have been conducted on the corrosion resistance of stainless steel, there was a gap in the literature on the structural performance and fatigue characteristics of this type of steel. Thus, it was important to ensure that, while ASTM A709 Grade 50CR steel offers superior corrosion resistance, it can also meet the design requirements from the perspective of Strength and Serviceability Limit States. This motivated the current study to evaluate the response of ASTM A709 Grade 50CR steel in both coupon and full-scale setups. In the absence of similar studies, the composite behavior of the girders made with this type of steel was investigated through experimental tests. The results were then compared to the design expectations outlined in AASHTO Bridge Design Specifications. The testing program was then supplemented with a set of tensile and fatigue tests to obtain a full understanding of the performance of ASTM A709 Grade 50CR steel under both monotonic and cyclic loading scenarios. The original data obtained from the experimental tests paved the way to expand the investigations to understand and quantify the long-term benefits of corrosion-resistant steel. This was achieved with a set of simulations that captured the corrosion rate and section loss of three different types of steel over time. This provided a detailed insight on how the structural performance can be maintained if the corrosion-resistant steel is used instead of conventional weathering and carbon steels.
3. Full-scale experiments 2. Material properties and field applications
3.1. Main mechanical properties
ASTM A709 Grade 50CR steel is a new addition to the group of structural stainless steel materials. This type of steel has a nominal 12% chromium (a minimum of 10.5% and a maximum of 12.5%), which significantly enhances its resistance to corrosion in comparison to other structural steels. While ASTM A709 Grade 50CR steel has been utilized in aggressive environmental conditions, such as those experienced in coal preparation and processing facilities, the civil infrastructure applications of this type of steel have been limited to date. In 2004, a short-span, box-girder bridge was built using ASTM A709 Grade 50CR steel over the Glenn-Colusa Main Canal in Colusa County, California. Shortly after the bridge was opened to traffic, load tests were performed to measure the deflec-
The composition of ASTM A709 Grade 50CR steel used in this study is provided in Table 1, along with the composition specified for ASTM A709 Grade 50W and A36. Based on the compositions provided in this table, there is a high percentage of chromium in ASTM A709 Grade 50CR steel. This leads to the formation of a chromium oxide layer (instead of iron oxide rust) on the surface, which protects the steel substrate from further corrosion. To determine the main mechanical properties, such as yield strength, ultimate tensile strength, modulus of elasticity, and elongation prior to fracture, of ASTM A709 Grade 50CR steel, a set of tensile tests were conducted under a monotonic loading scenario by using a universal testing machine. The geometry of coupons was decided follow2
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W. Shi, B. Shafei and B. Phares Table 1 Composition of different types of steel investigated in the current study. Steel
C
Mn
P
S
Si
Cr
Ni
N
Mo
ASTM A709 50CR ASTM A709 50W ASTM A36
0.03 0.11 0.26
1.50 1.10–1.35 –
0.040 0.020 0.040
0.010 0.006 0.050
1.00 0.40 0.40
10.5–12.5 0.45–0.70 –
1.50 0.30 –
0.030 0.015 –
– 0.02–0.08 –
ing ASTM E8 [12]. As documented in Table 2, the average yield strength and modulus of elasticity are determined to be 68.7 ksi (473.7 MPa) and 31,366 ksi (216.3 GPa), respectively. For comparison purposes, this table also provides the mechanical properties specified for the conventional weathering steel. As indicated in the referenced table, ASTM A709 Grade 50CR steel fulfills the requirements of mechanical strength and ductility similar to ASTM A709 Grade 50W steel.
span to capture the vertical displacement of the concrete deck during the test. Two LVDTs were placed horizontally at the interface between the steel girder and the concrete deck to record any longitudinal slip. They were placed at the mid-span and close to one of the end supports. The details of the instrumentation layout have been depicted in Fig. 5.
3.2. Specimen design and fabrication
Before the ultimate load test on the full-scale test setup, preliminary tests were conducted on the ASTM A709 Grade 50CR steel girder (with no concrete deck) and the steel–concrete composite girder, both at the elastic range. While the load level was kept very low to avoid introducing any residual strains/stresses, these two preliminary tests helped verify the test setup and calibrate the calculations. After completing the preliminary tests, the specimen was loaded to failure under a four-point bending setup. The details of the specimen failure at the ultimate load is shown in Fig. 6. The load–deflection relationship measured at the mid-span of the specimen is shown in Fig. 7. Based on the recorded deflections, the flexural ductility factor, l, was found to be approximately 7.5, based on l ¼ du =dy , where du and dy are the deflections at the ultimate and yield points, respectively. As shown in Fig. 7, in the initial stage of loading, the load–deflection curves were linear and no cracks were found on the deck. As the load was increased, the first crack initiated at the bottom surface of the deck at the mid-span, propagating toward the neutral axis, which was in the concrete deck. It can be observed that the stiffness of the beam setup started to degrade when the applied load reached close to 150 kip (670 kN), indicating that the girder has yielded. When the applied load was further increased and the specimen reached its ultimate capacity, spalling of the concrete deck was observed due to stress concentrations under the point load areas, where the beam failed because of concrete crushing. With plastic deformations at the ultimate load, a satisfactory flexural behavior was recorded for the ASTM A709 Grade 50CR steel–concrete composite specimen. No difference was observed in the load–deflection measured at the bottom of the steel girder and at the left and right of the concrete deck. This indicated that the lateral-torsional buckling was prevented, primarily because the lateral stiffness of the deck was adequate. Fig. 8 presents the longitudinal strain distributions in the steel and concrete subjected to bending at the mid-span cross-section with respect to each level of loading. This figure highlights that the magnitude of the longitudinal strain in both steel and concrete increased consistent with increasing the applied load. The strain at the bottom of the girder was close to 2000 me under the loading
3.3. Results and findings
To evaluate the structural performance of ASTM A709 Grade 50CR steel in a full-scale setting, a 52 ft.-9 in. (16.1 m) long girder was designed, replicating the same details used in the stainless steel bridge that is under an ongoing long-term monitoring program in Iowa. Fig. 1 shows the plate girder profile and the section geometry for the full-scale test setup. The width and thickness of the top and bottom flange were 12 in. (30.5 cm) and 1 in. (2.5 cm), respectively. A total of 111 shear studs were welded on the top flange with the spacing of 1 ft.-5 in. (43.2 cm). The depth and thickness of the girder web were 36 in. (91.5 cm) and 3/4 in. (1.9 cm), respectively. A concrete deck was cast on the steel plate girder to provide a steel–concrete composite section. The geometry of the steel–concrete composite section is shown in Fig. 2. The width and thickness of the deck were 7.5 ft. (228.6 cm) and 8 in. (20.3 cm), respectively. The average compressive strength of concrete determined from multiple cylinder tests was 6143 psi (42 MPa). Fig. 3 presents the key steps for the fabrication of the full-scale specimen. After curing for 28 days, the specimen was tested under a four-point bending scenario to evaluate the composite flexural behavior of the ASTM A709 Grade 50CR steel–concrete section. The specimen was simply supported and subjected to a load setup shown in Fig. 4. The two loading points were spaced at 15 ft. (4.6 m) away from each other. The applied loads were recorded by two load cells attached to the loading actuators. Three sets of instrumentation devices were installed to collect the structural response data during the test: electrical resistance strain gages (ERSGs), direct current displacement transducers (DCDTs), and linear variable differential transformers (LVDTs). A total of 15 ERSGs were attached to the top surface of the concrete deck and the longitudinal rebars embedded in the deck to measure the longitudinal strain. In addition, 12 ERSGs were placed on the top and bottom flange of the girder at the quarter and mid-span. A total of 5 DCDTs were installed vertically underneath the girder to record deflections. The DCDTs were placed at the quarter and mid-span, as well as close to the two end supports. A set of two DCDTs were also placed at the bottom of the deck at the mid-
Table 2 Summary of the tensile test results and mechanical properties of ASTM A709 Grade 50CR and 50W steel (1 ksi = 6.9 MPa). Mechanical Property
Yield Strength, ksi Tensile Strength, ksi Modulus of Elasticity, ksi Elongation in 2 in. (50 mm), %
Experimental Results
ASTM Standard
C-1
C-2
C-3
Average
A709 50CR
A709 50W
67.8 87.0 31,155 17.9
67.8 86.9 31,399 18.9
70.7 88.8 31,144 17.1
68.7 87.6 31,366 18.2
50.0 70.0 29,000 18.0
50.0 70.0 29,000 18.0
3
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Fig. 1. Details of the plate girder specimen, including dimensions, shear studs, and stiffeners (1 ft. = 30.5 cm, 1 in. = 2.54 cm).
7'-6''
8''
#6 BAR
36''
12'' Fig. 2. Geometry of the composite beam section (1 in. = 2.54 cm).
In addition to the full-scale test, analytical calculations were performed to estimate the moment capacity of the ASTM A709 Grade 50CR steel–concrete composite section, following AASHTO Bridge Design Specifications. The calculations were based on the classic beam theory, in which the plane section is assumed to remain plane after bending. Fig. 8 confirmed the validity of the assumption for the beam under consideration. Global and local buckling was checked and ensured to be prevented so that a plastic behavior can be achieved. To further refine the calculations, two approaches were considered in this study, as illustrated in Fig. 10. Approach 1 included the material properties based on the design values. Noting that the yield strength of the ASTM A709 Grade 50CR steel was 50 ksi (345 MPa) and the compressive strength of the concrete deck was
level equal to 60% of the ultimate load, at which the ASTM A709 Grade 50CR steel girder started to yield. It was observed that, in general, a linear strain distribution can be obtained until close to 90% of the ultimate load. This indicated a satisfactory composite behavior between the steel girder and the concrete deck. The change of longitudinal slip with respect to the applied load was measured with the LVDTs (as shown in Fig. 9). It can be observed that the relationship between the applied load and the recorded slip was linear when the load was below the yield point. With further increasing the load, the longitudinal slip at the end support increased significantly from the yield point, i.e., 0.01 in. (0.25 mm) to 0.14 in. (3.50 mm). The slip was found to have minimum effect on the composite behavior and load bearing capacity. 4
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(a) Formwork and reinforcement
(b) Concrete placement
(c) Concrete deck curing
(d) Actuator setup
Fig. 3. Main steps to fabricate and setup the specimen for the full-scale four-point loading test.
ACTUATOR
SPREADER BEAM
Midspan 25'-10''21 6''
51'-9''
6''
Fig. 4. Loading configuration for the ASTM A709 Grade 50CR steel–concrete composite deck (1 ft. = 30.5 cm).
applications. The fatigue tests were performed with a variety of axial load amplitudes using an MTS hydraulic testing machine. The loading protocol and other supporting details for the fatigue tests can be found in Table 4. As reflected in this table, the loading protocol was in a sinusoidal shape with the stress ratio, R, (i.e., min stress/max stress) of 1 to fully replicate a tension–compression loading scenario, capturing the most extreme demand. In the fatigue testing program designed for the current study, the maximum stress was set at 75 percent of the yield strength, and the stress amplitudes were then reduced with an interval of 5 percent of the yield strength, until the fatigue limit was reached. Two specimens were included for each stress amplitude. The hourglass shape specimens for fatigue tests were cut from the ASTM A709 Grade 50CR steel plates according to ASTM E466 [13] (Fig. 11). Before conducting the cyclic loading tests for fatigue assessment, a preliminary compressive test was conducted to ensure the stability of the coupons. In this preliminary test, lateral displacement was measured as the load was monotonically increased. When the buckling initiated, the applied stress had reached 68 ksi (468 MPa), i.e., equivalent to 100% yield strength. This preliminary test confirmed that the geometry design used for fatigue coupons does not cause any stability issues. The tests were continued until
4 ksi (28 MPa), the entire steel section was utilized up to the yield point. The compressive stress in the concrete was assumed 0:85f c and the tensile strength of the concrete was neglected. The plastic neutral axis was then determined based on the force equilibrium. Approach 2 included the calculations based on the experimental strain and stress distributions across the section. The plastic moment was then found as the summation of the moments caused by the forces about the plastic neutral axis. Table 3 provides a comparison between the experimental test results and analytical calculations for the yield and ultimate bending moments. As summarized in this table, the predictions obtained from the design equations are in a very good agreement with the results obtained from the tests, indicating that the ASTM A709 Grade 50CR steel girder meets the design requirements. 4. Fatigue experiments 4.1. Test details Further to full-scale beam tests, fatigue tests were performed on ASTM A709 Grade 50CR steel coupons to predict their service life under the cyclic loads that they commonly experience in structural 5
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Fig. 5. Details of the instrumentation plan used to record various structural response measures during the full-scale tests (1 ft. = 30.5 cm).
made in Boyer [19] for hot-rolled steel coupons. The modified fatigue limit was calculated to be between 26.2 ksi (180.5 MPa) and 28.6 ksi (197 MPa). Considering that the corresponding value specified in AASHTO Bridge Design Specifications is 24 ksi (170 MPa), it was found that ASTM A709 Grade 50CR steel can provide adequate fatigue resistance similar to conventional structural steels. In the absence of similar studies in the literature on the high-cycle fatigue of this type of steel, the fatigue behavior of ASTM A709 Grade 50CR steel obtained from the experimental tests was compared to that of other conventional structural steels, including ASTM A709 Grade 70W and A36 steels [20]. According to Fig. 13, the highcycle fatigue performance of ASTM A709 Grade 50CR steel falls well within the range of performance predicted for other structural steels. This confirms the satisfactory fatigue life of ASTM A709 Grade 50CR steel, further to providing superior corrosion resistance.
either the coupons were fractured or the number of fatigue cycles reached 10,000,000 cycles. 4.2. Prediction of fatigue limits The stress-life method was employed in the current study to predict the fatigue life (a.k.a., S-N curve). This method was deemed appropriate, as the bulk material’s response was elastic, emphasizing on overall stresses, rather than local ones [14–17]. To determine the S-N curve for ASTM A709 Grade 50CR steel, the stresses recorded in each fatigue test were plotted versus the number of cycles to failure in a log–log format. The relation between the alternating stress amplitude, rA , and the corresponding number of cycles, N f , can be mathematically expressed as rA ¼ aðN f ÞB , where a represents the fatigue strength coefficient and B is the fatigue strength exponent. The constants a and B were obtained from a regression analysis of the stress and fatigue life data obtained from the experiments conducted in the current study. The values of a ¼ 258:61 and B ¼ 0:138 were found to generate the S-N curve
5. Prediction of remaining structural capacity over time Due to the cross-sectional loss resulted from the formation and accumulation of corrosion products, the structural capacity is found to drop over time, diminishing the safety margins anticipated for structural members. To obtain a holistic assessment of the remaining capacity of structural steel members and quantify how this capacity is affected in long term by the choice of steel, three different types of steel, i.e., corrosion-resistant steel, weathering steel, and carbon steel, are compared in the current study. To predict the time-dependent loss of thickness, the power function provided in Equation (1) can be employed:
with R2 ¼ 0:874. The fatigue limit for ASTM A709 Grade 50CR steel subjected to high-cycle fatigue loads has been reported in Fig. 12. The two red arrows in this figure indicate the runouts, at which the fatigue life was above the specified 10,000,000 cycles. The fatigue limit was defined as the stress amplitude between the smallest stress with failure and the largest stress without failure [18]. According to Fig. 12, this limit was found to be between 37.4 ksi (257.9 MPa) and 40.8 ksi (281.3 MPa) for ASTM A709 Grade 50CR steel under consideration. Since the surface was gently polished in the coupons used for the fatigue tests, a surface roughness correction factor of 0.7 was considered in the current study, following the recommendations
C ¼ Atn 6
ð1Þ
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W. Shi, B. Shafei and B. Phares
Fig. 6. Damage to the full-scale specimen under the ultimate load.
300 Measurement stopped
250 Concrete crushing
Load (kip)
200
150 Steel yielding
100
1/2 Span
50
1/4 Span
0 0
3
6
9
12
15
Deflection (in) Fig. 7. Load-displacement at the quarter and mid-span (1 in. = 25.4 mm, 1 kip = 4.5 kN).
the loss of metal thickness can exceed 2150 mm for carbon steel, 950 mm for weathering steel, and 95 mm for corrosion-resistant steel after 75 years. The loss of the section thicknesses over time was further employed in the current study to estimate the drop in the moment capacity of the steel girder sections. For this purpose, the section loss was assumed uniform along the entire perimeter of the girder section, except for the top surface, which was considered to be attached to the deck. Based on a detailed analytical study that was initiated from the composite section tested in the current study (Fig. 2), the time-dependent moment capacity was predicted for the three different types of steel. According to Fig. 15, while the drop in the moment capacity remained negligible in the sections made of corrosion-resistant steel, the sections made of carbon steel
where C is the average corrosion penetration depth; t is the exposure time; and A and n are the two constants, which vary based on the type of steel. For carbon steel, A and n can be considered equal to 70.60 mm and 0.79, respectively, under severe corrosive conditions [1]. For weathering steel, A and n change to 119.26 mm and 0.48, respectively. The assumed values have been reported by Groshek [11] based on the thickness loss data obtained for ASTM A709 Grade 50W steel subjected to extreme environmental conditions. According to the thickness loss data obtained from Fletcher [3] and Groshek [11], ASTM A709 Grade 50CR steel experiences a section loss approximately one-tenth of that measured for ASTM A709 Grade 50W steel. For further demonstration, Fig. 14 provides the estimated thickness loss of structural steel members over a service life of 75 years. As reflected in this figure, 7
W. Shi, B. Shafei and B. Phares
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48 42
Girder height (in)
P/Pu=0.0 36
P/Pu=0.3 P/Pu=0.5
30
P/Pu=0.6 24
P/Pu=0.7 P/Pu=0.8
18
P/Pu=0.9 12 6 0 -2000
0
2000
4000
6000
8000
Longitudinal strain (10-6) Fig. 8. Strain distribution at the mid-span’s cross section (1 in. = 2.54 cm).
300
250
Load (kip)
200
150
100 Mid-span
50
End-span 0 0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Longitudinal slip at steel-concrete interface (in) Fig. 9. Load-longitudinal slip relationship (1 in. = 25.4 mm, 1 kip = 4.5 kN). Note that the slip remained almost zero at the mid-span for the entire test.
Ps Pc
Ps Pt2
Pw Pt1 Pt Fig. 10. Calculations to obtain the ultimate flexural capacity: (a) based on AASHTO Bridge Design Specifications, and (b) based on the experimental stress distribution across the section.
ments from the individual section capacity to the overall structural response of the girders. As a result of section loss due to corrosion, it was found that the moment of inertia and consequently stiffness of the girder are adversely impacted, leading to excessive deflec-
and weathering steel experienced a drop of 22% and 10% after 75 year, respectively. Since the serviceability of the steel structures directly depends on the magnitude of deflections, a separate analytical investigation was conducted to expand the scope of assess8
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W. Shi, B. Shafei and B. Phares Table 3 Comparison of the ultimate flexural strengths (1 kipft = 1.4 kNm).
Experimental Test Results Calculations based on AASHTO Bridge Design Specifications
Approach 1 Approach 2
Yield Moment (kipft)
Ultimate Moment (kipft)
Ultimate Load (kip)
3100.5 2651.3 3457.5
5167.5 4380.9 5497.4
260.0 220.0 276.6
Table 4 Overview of the fatigue testing parameters (1 ksi = 6.9 MPa; 1 in = 25 mm; 1 kip = 4.5 kN). ID.
Frequency (Hz)
Stress ratio
Max. stress (ksi)
Min. stress (ksi)
Thickness (in.)
Width (in.)
Area (in2)
Max. load (kip)
Min. load (kip)
F1 F2 F3 F4 F5 F6
8 8 8 8 8 8
1 1 1 1 1 1
51.00 47.60 44.20 40.80 37.40 34.00
51.00 47.60 44.20 40.80 37.40 34.00
0.523 0.520 0.519 0.520 0.519 0.519
1.000 0.990 1.001 1.001 0.999 0.999
0.523 0.515 0.520 0.521 0.518 0.518
26.67 24.50 22.96 21.30 19.39 17.63
26.67 24.50 –22.96 21.30 19.39 17.63
Hydraulic Grip
Hydraulic Actuator
Fig. 11. Fatigue test setup and the dimensions of the specimens (1 in. = 25.4 mm).
80 Fitted line Fatigue test data
Alternating stress (ksi)
70 60
50
40
30 10,000
100,000
1,000,000
10,000,000
Number of cycles Fig. 12. S-N curve developed for ASTM A709 Grade 50CR steel based on the fatigue tests performed in the current study. 9
W. Shi, B. Shafei and B. Phares
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90 Current experiments
80
Alternating stress (ksi)
ASTM A709 Grade 50CR steel 70
ASTM A709 Grade 70W steel
60
ASTM A36 steel
50 40 30 20 10 0 0
500,000
1,000,000
1,500,000
2,000,000
Number of cycles Fig. 13. Fatigue behavior of ASTM A709 Grade 50CR steel in comparison to the other conventional structural steels (1 ksi = 6.9 MPa).
2500
3.00
Carbon steel 2000
Weathering steel
2.75
Corrosion-resistant steel
Corrosion-resistant steel
Deflection (in)
Section loss (µm)
Carbon steel
Weathering steel
1500
1000
500
Experiment
2.50
2.25
2.00
1.75
0
1.50
0
15
30
45
60
75
0
Time (year)
15
30
45
60
75
Time (year) Fig. 16. Maximum girder deflection predicted as a function of age for three different types of steel (1 in. = 25.4 mm).
Remaining moment capacity (%)
100 Carbon steel
Weathering steel
Corrosion-resistant steel
deflection changes under the experimental yield load (i.e., 150 kip (670 kN)), depending on the choice of steel. As reflected in this figure, the percentage of increase in the maximum girder deflection is 24.3% and 9.7% for carbon steel and weathering steel after 75 years, respectively. However, there was no major change in the maximum deflection of the girder made of corrosionresistant steel. This highlights the promise of ASTM A709 Grade 50CR steel in meeting both strength and serviceability requirements.
95
90
85
80
6. Conclusions 75 0
15
30
45
60
75
This study provided a holistic investigation of the structural performance of ASTM A709 Grade 50CR steel, in terms of bending moment capacity and fatigue resistance, through a set of laboratory tests and analytical calculations. For this purpose, a fullscale specimen consisting of an ASTM A709 Grade 50CR steel plate girder attached to a concrete deck was built and subjected to a four-point bending scenario to evaluate various structural response measures, such as strains, stresses, and deflections. Comparisons were then made with the design requirements provided by AASHTO Bridge Design Specifications to ensure that this type of steel meets the expectations for Strength and Serviceability
Time (year) Fig. 15. Percentage of drop in the moment capacity as a function of age.
tions. To quantify this important aspect, the deflection of the bridge girder subjected to the full-scale test was estimated assuming three different types of steel. After the validation of the analytical calculations with the experimental test results, the maximum girder deflections were determined as a function of timedependent section loss. Fig. 16 depicts how the maximum girder 10
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W. Shi, B. Shafei and B. Phares
tion, Writing - review & editing, Funding acquisition, Supervision. Brent Phares: Methodology, Investigation, Writing - review & editing, Funding acquisition.
Limit States. To investigate how ASTM A709 Grade 50CR steel responds to cyclic loads, a set of fatigue tests were also performed up to 10,000,000 cycles. The high-cycle fatigue test data was then converted to an S-N curve. This curve predicted how the magnitude of alternating stresses changes in ASTM A709 Grade 50CR steel as a function of number of cycles. The main conclusions made from this study are as follows:
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
From the tensile tests, the main mechanical properties, such as yield strength, ultimate tensile strength, modulus of elasticity, and elongation prior to fracture, of ASTM A709 Grade 50CR steel were determined. With an average yield strength and modulus of elasticity of 68.7 ksi (473.7 MPa) and 31,366 ksi (216.3 GPa), respectively, ASTM A709 Grade 50CR steel was found to fulfill the requirements of mechanical properties similar to ASTM A709 Grade 50W steel. This was in addition to an average ductility of 7.5 recorded in the experiments. From the full-scale bending tests, the ASTM A709 Grade 50CR steel girder was found to yield at the loading level equal to 60% of the ultimate load. A linear strain distribution was maintained until close to 90% of the ultimate load, indicating a satisfactory composite behavior between the ASTM A709 Grade 50CR steel girder and the concrete deck. This was further supported by monitoring the load-slip relationship at both middle and end of the specimen. Analytical calculations were performed with two different approaches to evaluate the structural capacity of the steel girder-concrete deck system, following AASHTO Bridge Design Specifications. The predictions provided upper and lower bounds for the values reported from the experimental tests. This confirmed the ability of ASTM A709 Grade 50CR steel to meet the structural performance requirements. From high-cycle fatigue tests carried out on ASTM A709 Grade 50CR steel coupons, the fatigue limit was reported to be between 37.4 ksi (257.9 MPa) and 40.8 ksi (281.3 MPa). Even after a correction factor of 0.7 was applied to capture the surface roughness, the fatigue limit remained above 24 ksi (170 MPa), which has been recommended by AASHTO Bridge Design Specifications for structural steels. Additional tests are recommended to evaluate the fatigue strength of welded and bolted connection made with this type of steel. The drop of structural capacity over time was predicted in the girder setup under consideration using three different types of steel, i.e., corrosion-resistant steel, weathering steel, and carbon steel. Upon comparing the results, it was determined how the long-term benefits of ASTM A709 Grade 50CR steel can be realized by maintaining the expected performance and safety margins in both short and long term. In the long-term study, the extent of capacity loss due to corrosion did not reach a level that would change the mode of failure of the composite beams. This observation remains valid for most of corroding steel–concrete beams, mainly because the composite action is maintained by the welded shear studs embedded in concrete. Change of failure mode, however, is possible if an advanced stage of corrosion is reached.
Acknowledgement The authors would like to acknowledge the Iowa Department of Transportation for sponsoring this project. The contents of the paper reflect the conclusions and opinions of the authors and do not necessarily express the views of the funding agency. References [1] P. Albrecht, A. Naeemi, Performance of Weathering Steel in Bridges, NCHRP Report 272, National Cooperative Highway Research Program, Washington, DC, 1984. [2] B. McDad, D.C. Laffrey, M. Dammann, R.D. Medlock, Performance of Weathering Steel in TxDOT Bridges, Project 0-1818 (2000). [3] F.B. Fletcher, Improved Corrosion-Resistant Steel for Highway Bridge Construction, Report No, Federal Highway Administration, McLean, VA, 2011. [4] Crampton, D. D., Holloway, K. P., and Fraczek, J. (2013), Assessment of Weathering Steel Bridge Performance in Iowa and Development of Inspection and Maintenance Techniques, Report No. RB17-012, Iowa Department of Transportation, Ames, IA. [5] D.C. Cook, Spectroscopic identification of protective and non-protective coatings on steel structures in marine environments, Corros. Sci. 47 (10) (2005) 2550–2570. [6] Khatami, D. and Shafei, B. (2021), Impact of climate conditions on deteriorating reinforced concrete bridges in the U.S. Midwest region, ASCE Journal of Performance of Constructed Facilities, 35(1), 04020129 1-11. [7] H. Seradj, Oregon’s ASTM A1010 Bridges, Iowa Department of Transportation A1010 Steel Workshop, Ames, IA, 2015. [8] W.F. Via, Harrop, Virginia’s Initial Experience with an ASTM A1010 Plate Girder Bridge, VTCA Spring Conference, Virginia Department of Transportation, Richmond, VA, 2017. [9] Sharp, S. R., Provines, J. T., Moruza, A. K., Via, W. F., and Harrop, K. N., Virginia’s First Corrosion-Resistant ASTM A1010 Steel Plate Girder Bridge, Report No. FHWA/VTRC 20-R10, Charlottesville, VA, 2019. [10] H. Seradj, Oregon’s First ASTM A1010 Bridge and Risk Management, World Steel Bridge Symposium, Grapevine, TX, 2012. [11] I.G. Groshek, Corrosion Behavior of ASTM A1010 Stainless Steel for Applications in Bridge Components, Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA, 2017. [12] ASTM E8-16, Standard Test Methods for Tension Testing of Metallic Materials, ASTM International, West Conshohocken, PA, 2016. [13] ASTM E466-15, Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials, ASTM International, West Conshohocken, PA, 2015. [14] Y.-L. Lee, Fatigue Testing and Analysis: Theory and Practice, Elsevier Butterworth-Heinemann, Burlington, MA, 2005. [15] S. Arabi, B. Shafei, B. Phares, Fatigue analysis of sign-support structures during transportation under road-induced excitations, Journal of Engineering Structures 164 (2018) 305–315. [16] S. Arabi, B. Shafei, B. Phares, Investigation of fatigue in steel sign-support structures under diurnal temperature changes, J. Constr. Steel Res. 153 (2019) 286–297. [17] S. Arabi, B. Shafei, Multi-stressor fatigue assessment of steel sign-support structures: A case study in Iowa, Journal of Engineering Structures 200 (2019) 1–11. [18] Dowling, N. (1999), Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture and Fatigue, 2nd Edition, NJ [19] H.E. Boyer, Atlas of Fatigue Curves, ASM International, Metals Park, OH, 1986. [20] H. Chen, G.Y. Grondin, R.G. Driver, Characterization of fatigue properties of ASTM A709 high performance steel, J. Constr. Steel Res. 63 (6) (2007) 838–848.
CRediT authorship contribution statement Weizhuo Shi: Methodology, Investigation, Writing - original draft. Behrouz Shafei: Conceptualization, Methodology, Investiga-
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