Appc
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EVALUATION OF THE ANCHOR BOLT COMPONENTS KINZUA BRIDGE COLLAPSE By Eric Kaufmann, Ph.D. and Robert Connor, Ph.D. ATLSS Engineering Research Center, Lehigh University Material Properties and Fracture Evaluation of Anchor Bolts I.
Material Properties
Physical and mechanical properties of the original 1882 foundation anchor bolts, and the collar and bolt extension added in the 1900 re-construction were determined from a fractured assemblage recovered from the site (see Figure 1). Chemical composition and metallurgical microstructure was determined for each of the component materials. A tabulation of the compositions is provided in Table 1. TABLE 1 – Chemical Composition of Anchorage Components
1882 Anchor Bolt (1-1/4” dia.) 1900 Collar (2-1/4” O.D.) 1900 Bolt (1-1/2” dia.)
C
Mn
Wt% P
0.017
0.006
0.082
0.013
0.14
0.028
0.31
0.12
0.05
0.23
0.094
0.53
0.085
0.078
<0.001
S
Si
Microstructures of the materials are shown in Figure 2. The microstructure and chemical composition of the original 1882 1-1/4” anchor bolt is characteristic of wrought iron of this period with a very low carbon ferrite matrix and high concentration of large elongated non-metallic inclusions. The collar and 1-1/2 in. bolt added in the 1900 design was determined to be wrought iron and mild steel, respectively.
Figure 1 Typical Failed Anchorage Where Fracture Developed in the Original 1882 Anchor Bolt.
1
“Evaluation of the Anchor Bolt Components – Kinzua Viaduct Collapse” by Eric Kaufman, Ph.D. & Robert Conner, Ph.D., ATLSS/Lehigh University
(a)
(b)
(c)
Figure 2 Microstructures of the Anchorage Components. a) 1882 Wrought Iron Anchor Bolt b) 1900 Wrought Iron Collar c) 1900 1-1/2 in. Steel Bolt. [Mag. 100X ] Due to the small size of the collar and bolt extension and yielded condition of the fractured 1-1/4 in. anchor bolt mechanical property tests were limited to hardness measurements in unyielded areas to estimate the original tensile strength of the material. A summary of the hardness measurements and corresponding estimated tensile strength of each component is given in Table 2. The measurements indicate that the tensile strength of each of the component materials is similar with strengths ranging from 55 ksi to 62 ksi and typical of wrought iron and mild steel from this period. TABLE 2 Mechanical Properties of Anchorage Components Avg. Hardness (HRB) 1882 Anchor Bolt (1-1/4” dia.) 1900 Collar (2-1/4” O.D.) 1900 Bolt (1-1/2” dia.)
70.6
Approx. Tensile Strength (ksi) 62
Approx. Tensile Capacity (kips) 76
64.1
55
99
68.8
60
84
2
“Evaluation of the Anchor Bolt Components – Kinzua Viaduct Collapse” by Eric Kaufman, Ph.D. & Robert Conner, Ph.D., ATLSS/Lehigh University
II.
Tensile Load Capacity
Also shown in Table 2 is the tensile load capacity of each anchorage component based upon the original nominal dimensions. According to the original 1882 design drawings foundation anchor bolts were fabricated from 1-1/4 in. “upset” bars where the upsetted end was threaded with a 1-1/2 in. thread. Anchor bolt capacity shown in Table 2 is computed using the lesser 1-1/4 in. nominal bar diameter. The nominal diameter of the collar was taken as 2-1/4 in. based upon measurements of several collars recovered from the field. Table 2 indicates that in its original condition the weakest component of the anchorage assembly was the 1-1/4 in. anchor bolt (76 kips). Furthermore, field observations of foundation anchor bolts indicated that, in general, the greatest corrosion loss in the anchorage system appeared to occur in this bolt compared to the collar or 1-1/2 in. steel bolt which tends to widen this strength disparity. Corrosion loss appeared to vary from small as for the bolt shown in Figure 1 where the diameter in the fractured region was about 1.2 in. to approaching the full cross-section in locations below the foundation surface. Failure of the anchor bolt under these circumstances is consistent with field observations of failed anchorages where the collar was not cracked. III.
Fracture Examination
Visual and microscopic examination of 1-1/4 in. anchor bolt fractures indicated failure by a ductile fracture mechanism resulting from tensile overload. Gross plastic deformation and necking was observed in the fracture region as shown in Figure 3. Viewed at higher magnifications with a scanning electron microscope (SEM) the fracture surface showed dimple fracture characteristic of ductile fracture (see Figure 3 inset).
Figure 3 Typical 1-1/4 in. Dia. Anchor Bolt Fracture Showing Ductile Fracture.
3
“Evaluation of the Anchor Bolt Components – Kinzua Viaduct Collapse” by Eric Kaufman, Ph.D. & Robert Conner, Ph.D., ATLSS/Lehigh University
Collar fractures with multiple longitudinal “splits” were observed in a large number of anchorages (see Figure 4). Visual and microscopic examination of fractures indicated that some of these fractures existed prior to the collapse. Figure 4 shows a collar crack surface after removing corrosion from the crack surface. The flat and smooth surface with the absence of plastic deformation in the fracture region is indicative of fatigue cracking. Viewed at higher magnifications with the SEM also supported fracture by a fatigue mechanism (see Figure 4). The fatigue cracks appear to have propagated through the entire cross-section of the collar under a lateral cyclic load and effectively reduced the tensile capacity of the collar to nil prior to the collapse. An additional crack part way through the collar wall likely developed during the collapse as the cracked collar was pryed open. The crack surface is also seen in Figure 4 after fracturing the remaining crosssection at cryogenic temperatures. Examination of the crack surface with the SEM indicated fracture by a ductile fracture mechanism consistent with fracture during the collapse.
4
“Evaluation of the Anchor Bolt Components – Kinzua Viaduct Collapse” by Eric Kaufman, Ph.D. & Robert Conner, Ph.D., ATLSS/Lehigh University
Fatigue Fracture
Ductile Fracture
Cleavage Fracture
Figure 4 Fatigue and Ductile Fracture Areas in a Failed Collar. (Cleavage Fracture Introduced in Laboratory to Expose Crack Surfaces).
5
Material Properties
Physical and mechanical properties of the original 1882 foundation anchor bolts, and the collar and bolt extension added in the 1900 re-construction were determined from a fractured assemblage recovered from the site (see Figure 1). Chemical composition and metallurgical microstructure was determined for each of the component materials. A tabulation of the compositions is provided in Table 1. TABLE 1 – Chemical Composition of Anchorage Components
1882 Anchor Bolt (1-1/4” dia.) 1900 Collar (2-1/4” O.D.) 1900 Bolt (1-1/2” dia.)
C
Mn
Wt% P
0.017
0.006
0.082
0.013
0.14
0.028
0.31
0.12
0.05
0.23
0.094
0.53
0.085
0.078
<0.001
S
Si
Microstructures of the materials are shown in Figure 2. The microstructure and chemical composition of the original 1882 1-1/4” anchor bolt is characteristic of wrought iron of this period with a very low carbon ferrite matrix and high concentration of large elongated non-metallic inclusions. The collar and 1-1/2 in. bolt added in the 1900 design was determined to be wrought iron and mild steel, respectively.
Figure 1 Typical Failed Anchorage Where Fracture Developed in the Original 1882 Anchor Bolt.
1
“Evaluation of the Anchor Bolt Components – Kinzua Viaduct Collapse” by Eric Kaufman, Ph.D. & Robert Conner, Ph.D., ATLSS/Lehigh University
(a)
(b)
(c)
Figure 2 Microstructures of the Anchorage Components. a) 1882 Wrought Iron Anchor Bolt b) 1900 Wrought Iron Collar c) 1900 1-1/2 in. Steel Bolt. [Mag. 100X ] Due to the small size of the collar and bolt extension and yielded condition of the fractured 1-1/4 in. anchor bolt mechanical property tests were limited to hardness measurements in unyielded areas to estimate the original tensile strength of the material. A summary of the hardness measurements and corresponding estimated tensile strength of each component is given in Table 2. The measurements indicate that the tensile strength of each of the component materials is similar with strengths ranging from 55 ksi to 62 ksi and typical of wrought iron and mild steel from this period. TABLE 2 Mechanical Properties of Anchorage Components Avg. Hardness (HRB) 1882 Anchor Bolt (1-1/4” dia.) 1900 Collar (2-1/4” O.D.) 1900 Bolt (1-1/2” dia.)
70.6
Approx. Tensile Strength (ksi) 62
Approx. Tensile Capacity (kips) 76
64.1
55
99
68.8
60
84
2
“Evaluation of the Anchor Bolt Components – Kinzua Viaduct Collapse” by Eric Kaufman, Ph.D. & Robert Conner, Ph.D., ATLSS/Lehigh University
II.
Tensile Load Capacity
Also shown in Table 2 is the tensile load capacity of each anchorage component based upon the original nominal dimensions. According to the original 1882 design drawings foundation anchor bolts were fabricated from 1-1/4 in. “upset” bars where the upsetted end was threaded with a 1-1/2 in. thread. Anchor bolt capacity shown in Table 2 is computed using the lesser 1-1/4 in. nominal bar diameter. The nominal diameter of the collar was taken as 2-1/4 in. based upon measurements of several collars recovered from the field. Table 2 indicates that in its original condition the weakest component of the anchorage assembly was the 1-1/4 in. anchor bolt (76 kips). Furthermore, field observations of foundation anchor bolts indicated that, in general, the greatest corrosion loss in the anchorage system appeared to occur in this bolt compared to the collar or 1-1/2 in. steel bolt which tends to widen this strength disparity. Corrosion loss appeared to vary from small as for the bolt shown in Figure 1 where the diameter in the fractured region was about 1.2 in. to approaching the full cross-section in locations below the foundation surface. Failure of the anchor bolt under these circumstances is consistent with field observations of failed anchorages where the collar was not cracked. III.
Fracture Examination
Visual and microscopic examination of 1-1/4 in. anchor bolt fractures indicated failure by a ductile fracture mechanism resulting from tensile overload. Gross plastic deformation and necking was observed in the fracture region as shown in Figure 3. Viewed at higher magnifications with a scanning electron microscope (SEM) the fracture surface showed dimple fracture characteristic of ductile fracture (see Figure 3 inset).
Figure 3 Typical 1-1/4 in. Dia. Anchor Bolt Fracture Showing Ductile Fracture.
3
“Evaluation of the Anchor Bolt Components – Kinzua Viaduct Collapse” by Eric Kaufman, Ph.D. & Robert Conner, Ph.D., ATLSS/Lehigh University
Collar fractures with multiple longitudinal “splits” were observed in a large number of anchorages (see Figure 4). Visual and microscopic examination of fractures indicated that some of these fractures existed prior to the collapse. Figure 4 shows a collar crack surface after removing corrosion from the crack surface. The flat and smooth surface with the absence of plastic deformation in the fracture region is indicative of fatigue cracking. Viewed at higher magnifications with the SEM also supported fracture by a fatigue mechanism (see Figure 4). The fatigue cracks appear to have propagated through the entire cross-section of the collar under a lateral cyclic load and effectively reduced the tensile capacity of the collar to nil prior to the collapse. An additional crack part way through the collar wall likely developed during the collapse as the cracked collar was pryed open. The crack surface is also seen in Figure 4 after fracturing the remaining crosssection at cryogenic temperatures. Examination of the crack surface with the SEM indicated fracture by a ductile fracture mechanism consistent with fracture during the collapse.
4
“Evaluation of the Anchor Bolt Components – Kinzua Viaduct Collapse” by Eric Kaufman, Ph.D. & Robert Conner, Ph.D., ATLSS/Lehigh University
Fatigue Fracture
Ductile Fracture
Cleavage Fracture
Figure 4 Fatigue and Ductile Fracture Areas in a Failed Collar. (Cleavage Fracture Introduced in Laboratory to Expose Crack Surfaces).
5
