IA-57 in Butler County - Bridge Engineering Center at Iowa State

IA-57 in Butler County - Bridge Engineering Center at Iowa State

Test and Evaluation Report
Testing and Evaluation of the IA-57 Bridge in Butler County, IA
October 30, 2009
Prepared For
Iowa Department of Transportation
Office of Bridges and Structures
800 Lincoln Way
Ames, IA 50010
Prepared By
Bridge Engineering Center
2901 South Loop Dr.
Suite 3100
Ames, IA 50010
(515) 294-8103
Introduction
On August 25, 2009, the IA-57 Bridge (maintenance #1224.9S057, FHWA #16420), in Butler
County, IA was load tested using an Iowa Department of Transportation tandem axle snooper
truck. The load truck had a front axle weight of 17,650 lb, and a rear axle weight of 18,575 lb ea.
(total weight 54,800 lb). The bridge is a single span bridge, consisting of two main steel girders,
G1 and G2, seven steel stringers, transverse floor beams, and a concrete deck which is composite
only with girders G1 and G2. Strengthening angles were added to both faces of each of the
exterior stringers near the bottom flange in 1987; refer to Fig. 1 for a typical cross-section. Fig. 2
shows a plan view of the instrumentation layout. For reference, the data will be referred to with
respect to the designations shown in Fig. 3. Strain data were collected at two locations: 1)
midspan of the bridge at the top and bottom flanges of each girder (see Fig. 3a); 2) midspan of the
interior panel (east of bridge midspan) at the top and bottom flanges of each girder and stringer as
well as on the interior strengthening angle on stringers S1 and S7 (see Fig. 3b). The load truck was
driven from east to west at a crawl speed (2-5 mph) along multiple paths summarized in Fig. 3.
Figure 1. Typical Cross-Section of IA – 57 Bridge, Butler County, IA.
21'-9" CL to CL
Girders
3'-8 78"
Strain Gage
Location (Typ.)
30'-0" Roadway
4 Spaces @ 3'-6 13
16"
CL
Roadway
3'-8 78"
N
8'-4"
16'-8"
50'-0" CL to CL Bearings
25'-0"
Figure 2. Plan View of Instrumentation Layout of IA – 57 Bridge, Butler County, IA
a. Midspan of Bridge.
Figure 3. Instrumentation Plan and Load Cases for IA-57 Bridge Test.
b. Midspan of Interior Panel (East of Bridge Midspan)
Figure 3. Instrumentation Plan and Load Cases for IA-57 Bridge Test.
Test Results
Data will be presented from the five individual load cases, LC1 through LC5, as well as from
several two lanes loaded cases. The two lanes loaded cases were comprised of LC2+LC4,
LC1+LC3, and LC3+LC5.
Figure 4 illustrates the bottom flange strain data versus truck position for the two main girders and
the seven stringers for Load Case 1 and 3 for the previously mentioned strain gages at midspan of
the interior panel (east of bridge midspan.) These two figures provide a general understanding of
the pattern and magnitude of strains collected for this testing and are typical of what would be
expected for the given load cases. As can be seen, the strain magnitudes are highest when the
truck is closest to any single gage. Listed in Table 1 are the peak strains for each girder for each
load case, including the two lanes loaded cases for the midspan of the interior panel.
A closer look at the individual girder strain data (top and bottom flange) indicates some magnitude
of composite action exists at both the girders and the stringers for all load cases, even though plans
for this structure indicate shear lugs are only present on girders G1 and G2. Figure 5 through 8
illustrate the strain profile for the top and bottom flanges of girder G1, stringer S3, stringer S5, and
girder G2, respectively, in the midspan of the interior panel from Load Case 1. These graphs are
representative of the girder strain behavior for all load cases in the midspan of the interior span.
For all load cases, composite action appeared to be most prevalent in the stringers directly under
the load truck and the exterior girders.
a. Load Case 1
Figure 4. Bottom Flange Girder Strains.
b. Load Case 3
Figure 4. Bottom Flange Girder Strains.
Table 1. Measured Peak Strains for Single and Two-Lanes Loaded, Midspan of Interior Panel.
Midspan of Interior Panel Peak Strains (Microstrain)
S1
G1
S2
S3
S4
S5
S6
G2
S7
LC1
0.7
6.8
7.0
15.4
21.9
33.0
39.5
59.3
32.3
LC2
4.6
17.8
20.3
46.3
61.0
69.2
55.5
46.9
12.3
LC3
8.2
30.7
39.9
85.1
80.2
72.5
45.7
31.4
8.3
LC4
13.1
44.9
47.3
79.2
58.7
38.5
22.3
17.7
4.5
LC5
31.9
56.4
33.5
38.6
21.9
13.5
8.7
7.0
0.6
LC2 + LC4
17.4
62.7
66.9
126.1
119.3
107.4
77.8
65.2
17.1
LC1 + LC3
8.6
36.1
42.4
90.2
84.4
79.5
55.4
66.5
27.6
LC3 + LC5
32.5
75.8
55.6
96.9
92.4
78.4
50.3
37.5
9.2
Figure 5. Top and Bottom Flange Strains, Girder 1, Load Case 1.
Figure 6. Top and Bottom Flange Strains, Stringer 3, Load Case 1.
Figure 7. Top and Bottom Flange Strains, Stringer 5, Load Case 1.
Figure 8. Top and Bottom Flange Strains, Girder 2, Load Case 1.
Presented in Figs. 9 through 16 are the peak bottom flange strains for each girder and stringer at
midspan of the East panel (Fig. 3b) for load cases 1 through 5 and the two lanes loaded cases
(LC2+ LC4, LC1+LC3, and LC3+LC5), respectively. These figures give a basic understanding of
the distribution of strain relative to load placement.
Figure 9. Peak Bottom Flange Strains, Load Case 1.
Figure 10. Peak Bottom Flange Strains, Load Case 2.
Figure 11. Peak Bottom Flange Strains, Load Case 3.
Figure 12. Peak Bottom Flange Strains, Load Case 4.
Figure 13. Peak Bottom Flange Strains, Load Case 5.
Figure 14. Peak Bottom Flange Strains, Two Lanes Loaded (Load Case 2 + 4.)
Figure 15. Peak Bottom Flange Strains, Two Lanes Loaded (Load Case 1 + 3.)
Figure 16. Peak Bottom Flange Strains, Two Lanes Loaded (Load Case 3 + 5.)
Being a two girder bridge with stringers and floor beams, Fig. 9 through 16 only give a general
perspective on the load distribution characteristics of the bridge. Therefore, evaluation of the load
distribution characteristics via calculation of load fractions for the girders and stringers
independent of one another provides a better approximation. Equation 1, along with the measured
bottom flange strains, was used to evaluate the load distribution characteristics (Load Fraction) of
the bridge for single and two lanes loaded cases.
Equation 1.
LF =
εi
∑ε
i =1
where,
, decimal percentage of a single truck
n
i
ε i = maximum measured bottom flange strain from ith girder/stringer
Listed in Table 2 are the calculated Load Fractions for the seven stringers for individual Load
Cases 1 through 5 as well as Load Fractions for the investigated two-lane loaded case at the
midspan of the interior panel. For a single lane loaded the maximum load fraction on an interior
and exterior stringer are 0.32 and 0.31, respectively; for two lanes loaded the maximum load
fraction for an interior and exterior stringer are 0.24 and 0.04, respectively. Illustrated in Fig. 17
are the load fractions for the single lane loaded cases and the two lanes loaded case.
Table 2. Calculated Stringer Load Fractions for Single and Two-Lanes Loaded, Midspan of Interior Panel.
Load Fractions
LC1
LC2
LC3
LC4
LC5
LC2 + LC4
S1
S2
S3
S4
S5
S6
S7
0.01
0.02
0.02
0.05
0.31
0.04
0.06
0.08
0.13
0.16
0.20
0.13
0.10
0.19
0.22
0.32
0.17
0.20
0.13
0.20
0.26
0.23
0.14
0.24
0.19
0.27
0.22
0.15
0.11
0.20
0.21
0.17
0.13
0.08
0.08
0.14
0.31
0.07
0.02
0.01
0.00
0.04
Figure 17. Stringer Load Fractions for All Load Cases at Midspan of Interior Panel.
Listed in Table 3 are the calculated Load Fractions for the two girders for individual Load Cases 1
through 5 as well as Load Fractions for the investigated two-lane loaded case at the midspan of the
interior panel as well as the midspan of the bridge. For a single lane loaded the maximum load
fraction on either girder at the midspan of bridge and midspan of the interior panel are 0.86 and
0.87, respectively; for two lanes loaded the maximum load fraction on either girder at the midspan
of bridge and midspan of the interior panel are 0.52 and 0.50, respectively. Illustrated in Figs. 18
and 19 are the load fractions for the single lane loaded cases and the two lanes loaded case for the
midspan of the interior and panel and midspan of the bridge, respectively.
Table 3. Calculated Girder Load Fractions for Single and Two-Lanes Loaded, Midspan of Bridge &
Midspan of Interior Panel.
Load Fractions
Midspan Bridge
LC1
LC2
LC3
LC4
LC5
LC2 + LC4
Midspan Int. Panel
G1
G2
G1
G2
0.14
0.29
0.48
0.65
0.83
0.48
0.86
0.71
0.52
0.35
0.17
0.52
0.13
0.28
0.51
0.71
0.86
0.50
0.87
0.72
0.49
0.29
0.14
0.50
Figure 18. Girder Load Fractions for All Load Cases at Midspan of Interior Panel.
Figure 19. Girder Load Fractions for All Load Cases at Midspan of Bridge.
Individuals from the BEC involved in the load testing and data reduction for this testing include:
Dr. Terry Wipf, Doug Wood, Travis Hosteng, and Peter Moreau (undergraduate student).