Appendix B
Bridge Classification
Bridge and vehicle classification allows vehicle operators to avoid bridge failure due to overloading. Vehicle operators may drive across without restrictions if their vehicles' class numbers are less than or equal to the bridge class number. This appendix provides a quick method of estimating bridge capacity in the field.
BRIDGE SIGNS
All classified vehicles and bridges in the theater of operations require classification signs. Bridge signs are circular with a yellow background and black inscriptions. Sign diameters are a minimum of 16 inches for one-lane bridges and 20 inches for two-lane bridges. A two-lane bridge classification sign has two numbers, side by side, on its sign (see Figure B-1). The number on the left is the bridge classification when both lanes are in service simultaneously. The number on the right indicates the classification if the bridge is carrying one-way traffic and the vehicles proceed along the centerline of the bridge. For bridges with separate classifications for wheeled and tracked vehicles (dual classification), use a special circular sign that indicates both classifications (see Figure B-1, right side). Classify bridges greater than class 50 as dual-class bridges. Use a separate rectangular sign, if necessary, to show bridge width limitations.
Figure B-1. Bridge classification signs
WIDTH AND HEIGHT RESTRICTIONS
Table B-1 lists width restrictions for bridges. If a one-lane bridge does not meet width requirements, post a rectangular warning sign under the classification sign showing the actual clear width. For a two-lane bridge, downgrade the two-way classification to the highest class for which it does qualify (one-way class is not affected). If the minimum overhead clearance is less than 4.3 meters, post a sign with the limited clearance.
Table B-1. Minimum roadway widths
Roadway Width |
Bridge Classification |
One-Way |
Two-Way |
|---|---|---|
2.75 to 3.34 |
12 |
0 |
3.35 to 3.99 |
30 |
0 |
4 to 4.49 |
60 |
0 |
4.5 to 4.99 |
100 |
0 |
5 to 5.4 |
150 |
0 |
5.5 to 7.2 |
150 |
30 |
7.3 to 8.1 |
150 |
60 |
8.2 to 9.7 |
150 |
100 |
9.8+ |
150 |
150 |
| NOTE: The minimum overhead clearance for all classes is 4.5 m. | ||
CLASSIFICATION PROCEDURES
Tables B-2 through B-9 and Figures B-2 through Figure B-26 are used to determine the bridge classification of various bridges.
Table B-2. Notations
Variable |
Definition |
|---|---|
b |
stringer width, in inches |
bd |
concrete slab width, in feet |
be |
effective slab width, in feet |
be1 |
effective slab width for one-lane traffic, in feet |
be2 |
effective slab width for two-lane traffic, in feet |
bR |
curb-to-curb roadway width, in feet |
d |
stringer depth, in inches |
df |
depth of fill, in inches |
di |
interior stringer depth (d - 2tf), in inches |
fbDL |
allowable bearing stress of the stringer, in ksi |
L |
span length, in feet |
Lc |
maximum brace spacing, in feet |
Lm |
maximum span length, in feet |
m |
moment capacity per stringer, in kip-feet |
mDL |
dead-load moment per stringer, in kip-feet |
MDL |
dead-load moment for entire span, in kip-feet |
mLL |
live-load moment per stringer, in kip-feet |
MLL |
live-load moment per lane, in kip-feet |
MLL1 |
live-load moment for one-lane traffic, in kip-feet |
MLL2 |
live-load moment for two-lane traffic, in kip-feet |
Nb |
number of braces |
NL |
number of lanes |
NS |
total number of stringers in the span |
N1 |
effective number of stringers for one-lane traffic |
N2 |
effective number of stringers for two-lane traffic |
PLC |
provisional load class |
R |
rise of arch, in feet |
S |
section modulus, in cubic inches |
Sb |
actual brace spacing, in feet |
Sc |
section modulus of the composite section, in cubic inches |
Ss |
center-to-center stringer spacing, in feet |
tc |
crown thickness, in inches |
td |
average deck thickness, in inches |
teff |
effective deck thickness, in inches |
tf |
flange thickness, in inches |
tpl |
plate thickness, in inches |
tw |
web thickness, in inches |
tx |
thickness factor |
T1 |
one-lane, tracked-vehicle classification |
T2 |
two-lane, tracked-vehicle classification |
v |
shear capacity per stringer, in kips |
vDL |
dead-load shear per stringer, in kips |
VDL |
estimated dead-load shear of span, in kips |
vLL |
live-load shear per stringer, in kips |
VLL |
live-load shear per lane, in kips |
Ws |
stringer weight, in lbs/ft |
W1 |
one-lane, wheeled-vehicle classification |
W2 |
two-lane, wheeled-vehicle classification |
Xpl |
plate thickness factor |
%lam |
percent of lamination |
Figure B-2. Timber- or steel-trestle bridge with timber deck
Figure B-3. Steel-stringer bridge with concrete deck (noncomposite construction)
Figure B-4. Concrete steel-stringer bridge (composite construction)
Figure B-5. Concrete T-beam bridge with asphalt wearing surface
Figure B-6. Concrete-slab bridge with asphalt wearing surface
Figure B-7. Masonry-arch bridge
Table B-3. Properties of timber stringers
Rectangular Stringers |
Rectangular Stringers |
||||||
|---|---|---|---|---|---|---|---|
Nominal |
m |
v |
Lm(ft)4 |
Nominal |
m |
v |
Lm(ft)4 |
4 x 6 |
4.80 |
2.40 |
7.14 |
16 x 16 |
136.50 |
25.60 |
19.10 |
4 x 8 |
8.53 |
3.20 |
9.50 |
16 x 18 |
172.80 |
28.80 |
21.50 |
4 x 10 |
13.33 |
4.00 |
11.90 |
16 x 20 |
213.00 |
32.00 |
23.80 |
4 x 12 |
19.20 |
4.80 |
14.30 |
16 x 22 |
258.00 |
35.20 |
26.20 |
6 x 8 |
12.80 |
4.80 |
9.50 |
16 x 24 |
307.00 |
38.40 |
28.60 |
6 x 10 |
20.00 |
6.00 |
11.90 |
18 x 18 |
194.40 |
32.40 |
21.50 |
6 x 12 |
28.80 |
7.20 |
14.30 |
18 x 20 |
240.00 |
36.00 |
23.80 |
6 x 14 |
39.20 |
8.40 |
16.70 |
18 x 22 |
290.00 |
9.60 |
26.20 |
6 x 16 |
51.20 |
9.60 |
19.10 |
18 x 24 |
346.00 |
43.20 |
28.60 |
6 x 18 |
64.80 |
10.80 |
21.50 |
Round Stringers (Nominal Size is Diameter) | |||
8 x 8 |
17.07 |
6.40 |
9.50 |
||||
8 x 10 |
26.70 |
8.00 |
11.90 |
8 |
10.05 |
5.70 |
9.50 |
8 x 12 |
38.40 |
9.60 |
14.30 |
9 |
14.31 |
7.20 |
10.70 |
8 x 14 |
52.30 |
11.20 |
16.70 |
10 |
19.63 |
8.80 |
11.90 |
8 x 16 |
68.30 |
12.80 |
19.10 |
11 |
26.10 |
10.60 |
13.10 |
8 x 18 |
86.40 |
14.40 |
21.50 |
12 |
33.90 |
12.70 |
14.30 |
8 x 20 |
106.70 |
16.40 |
23.80 |
13 |
43.10 |
15.00 |
15.50 |
8 x 22 |
129.10 |
17.60 |
26.20 |
14 |
53.90 |
17.40 |
16.70 |
8 x 24 |
153.60 |
19.20 |
28.60 |
15 |
67.50 |
20.20 |
17.80 |
10 x 10 |
33.30 |
10.00 |
11.90 |
16 |
80.40 |
22.60 |
19.10 |
10 x 12 |
48.00 |
12.00 |
14.30 |
17 |
98.20 |
26.00 |
20.20 |
10 x 14 |
65.30 |
14.00 |
16.70 |
18 |
114.50 |
28.60 |
21.50 |
10 x 16 |
85.30 |
16.00 |
19.10 |
19 |
137.10 |
32.40 |
22.60 |
10 x 18 |
108.00 |
18.00 |
21.50 |
20 |
157.10 |
35.40 |
23.80 |
10 x 20 |
133.30 |
20.00 |
23.80 |
21 |
185.20 |
39.60 |
24.90 |
10 x 22 |
161.30 |
22.00 |
26.20 |
22 |
209.00 |
42.70 |
26.20 |
10 x 24 |
192.00 |
24.00 |
28.60 |
23 |
243.00 |
47.60 |
27.30 |
12 x 12 |
57.60 |
14.40 |
14.30 |
24 |
271.00 |
50.80 |
28.60 |
12 x 14 |
78.40 |
16.80 |
16.70 |
25 |
312.00 |
56.20 |
29.70 |
12 x 16 |
102.40 |
19.20 |
19.10 |
26 |
351.00 |
60.80 |
30.90 |
12 x 18 |
129.60 |
21.60 |
21.50 |
27 |
393.00 |
65.60 |
32.10 |
12 x 20 |
160.00 |
24.00 |
23.80 |
28 |
439.00 |
70.50 |
33.30 |
12 x 22 |
193.60 |
26.40 |
26.20 |
29 |
487.00 |
75.60 |
34.50 |
12 x 24 |
230.00 |
28.80 |
28.60 |
30 |
540.00 |
81.00 |
35.70 |
14 x 14 |
91.50 |
19.60 |
16.70 |
31 |
595.00 |
86.40 |
36.80 |
14 x 16 |
119.50 |
22.40 |
19.10 |
32 |
655.00 |
92.10 |
38.00 |
14 x 18 |
151.20 |
25.20 |
21.50 |
33 |
718.00 |
98.00 |
39.20 |
14 x 20 |
186.70 |
28.00 |
23.80 |
34 |
786.00 |
104.00 |
40.40 |
14 x 22 |
226.00 |
30.80 |
26.20 |
35 |
857.00 |
110.20 |
41.60 |
14 x 24 |
269.00 |
33.60 |
28.60 |
36 |
933.00 |
116.60 |
42.80 |
1 If d > 2b, bracing is required at the midspan and at both ends. |
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