1. Local bridge Hydraulics 101
State aid bridge
Dave Conkel
State Aid Bridge Engineer
Brian Homan
State Aid Bridge Plans Engineer

2. State aid bridge hydraulics
Bridge Hydraulic Checklist
(What do we look for)
Common Bridge Types
Typical Spans and Structure Depths
Low Member Criteria
Examples
Slab Span
Prestressed Concrete Beam Bridge

3. Bridge Hydraulic Checklist
What do we look for?

4. Bridge Hydraulic Checklist
Check Q100, V100 and drainage area versus up and down stream bridges.
Check proposed channel bottom width and flow line elevation vs. channel profile and typical up and down stream channel sections on the plan.
Channel width at bridge usually should be within 10% of the natural channel bottom width.

5. Bridge Hydraulic Checklist
Check the design/overtopping flood against guidelines for the ADT level.
Projected ADT
Minimum OT Flood
0-10
2 year
11-49
5 year
50-399
10 year
25 year
≥ 1500
50 year
Generally find that design is governed by factors other than ADT.

6. Bridge Hydraulic Checklist
A bridge opening is typically sized to match the natural channel bottom width with 1v:2h side slopes to match approximate in-place roadway profiles
Essentially the design/overtopping flood is derived from an interrelationship between structure type, roadway profile, and hydrology.

7. Bridge Hydraulic Checklist
Note: the waterway opening or replacement structure recommendation must reflect “the lowest acceptable grade line and the smallest waterway opening consistent with the constraints imposed on the projects”

8. Bridge Hydraulic Checklist
Check that design flood elevation is below the roadway overtopping flood elevation.
The design flood is equal to the overtopping flood when the overtopping flood is less than the Q100.
When the overtopping flood is greater than Q100, the design flood is always Q100.

9. Bridge Hydraulic Checklist
Verify that the Flood of Record or Apparent High Water Elevation seem to make sense with the Q100 or Design Flood elevation.
Verify that the Q100 event information shows up on the plan, along with the Design Flood event (if different from Q100). The DNR requires Q100 information to be shown on all plans.

10. Bridge Hydraulic Checklist
Check that the information in the Hydraulic Engineer’s Recommendation Table on the bridge survey sheet matches the hydraulics letter.
Table needs to match the MnDOT standard format
Check water velocity through the structure
Should be no more than 8 to 9 fps
Check that stage is not increased significantly from existing condition
Should not be more than a 0.5 foot increase
Bridge Survey sheet needs to contain a Scour Code

11. Bridge Hydraulic Checklist
Scour codes

12. Bridge Hydraulic Checklist
Check that the Low Member elevation meets the appropriate elevation for the structure type, flood event frequency, velocity, and any debris issues
Note that Design Headwater “HW” is required to be shown on General Plan and Elevation sheet, while Tailwater “TW” is used to determine Low Member Elevation
In most cases:
TW = HW – stage increase
TW is given on the Risk Assessment

13. Bridge Hydraulic Checklist

14. Hydraulic engineer’s recommendation table

15. Bridge Hydraulic Checklist
Check that the Skew Angle of the bridge matches the Hydraulic Analysis from the consultant.
Verify Waterway Area and Channel Bottom width using our internal Mathcad spreadsheet.

16. Bridge Hydraulic Checklist
For other than single span bridges, check pier scour elevation.
This may be used as a check, but should not be used for design stability of new structures.

17. Bridge Hydraulic Checklist
Verify Riprap is properly sized for the design flood velocity.
In cases where overtopping is greater than the 100 year event, verify if Riprap is properly sized for 500-year or overtopping flood, whichever comes first.

18. Bridge Hydraulic Checklist
Verify that the correct riprap details are shown.
RIPRAP SIZING DETAILS
SIZE OF AVG.
GRANULAR FILTER
WATER VELOCITY
AGGREGATE
REQUIREMENTS
(UP TO) F.P.S.
CLASS I
3"
3.86
CLASS II
6"
5.46
CLASS III
9"
6.69
CLASS IV
12"
7.73
CLASS V
15"
8.64
SPEC BOOK ALSO DENOTES THAT THE GRANULAR FILTER SHALL BE 6" UNLESS
OTHER DIMENSIONS ARE SPECIFIED (PROBABLY IN THE HYDRAULICS REC.).
OCCASIONALLY YOU WILL SEE A PAY ITEM FOR "QUARRY RUN RIPRAP", WHICH IS
BASICALLY RIPRAP THAT HAS A MORE ANGULAR SHAPE TO IT. THIS IS HELPFUL
FOR THE HIGHER VELOCITIES, WHERE IT INTERLOCKS WITH ITSELF BETTER.
Old Rip Rap detail

19. Bridge Hydraulic Checklist
Verify that the correct riprap details are shown.
Bridges

20. Bridge Hydraulic Checklist
Standard detail is not for use with Class V riprap.
Risk is that larger riprap may tear the filter fabric.
Can use Class V riprap with granular filter material.

21. State aid bridge hydraulics
Bridge Hydraulic Checklist
(What do we look for)
Common Bridge Types
Typical Spans and Structure Depths
Low Member Criteria
Examples
Slab Span
Prestressed Concrete Beam Bridge

22. Concrete slab spans

23. Concrete slab spans

24. Concrete slab spans Advantages Disadvantages
Very economical short spans bridge
Ease of design and detail
Adaptable to curved alignments
Disadvantages
Requires heavy false work over stream
Typical multi-span arrangements require additional substructure units
Maximum skew at 45 degrees
Span length limited to 65 feet.

25. Concrete slab spans Rules of thumb
Skew effects can be ignored for skew angles ≤ 20°
End spans should be approximately 80% of the center span length to balance moments and prevent uplift.
Slab type bridges should not be used for bridges with skew angles greater than 45°
Span range of 15 to 40 feet for simple span cast-in-place concrete slab bridges

26. Concrete slab spans
When haunches are required, use linear haunches in accordance with the following:
Minimum slab depth at the pier = 1.33*[(S+10)/30] (Includes wear course if present)
Minimum slab depth in non-haunched area = 0.8*[(S+10)/30] (Includes wear course if present)

27. Concrete slab spans Common Designs Continuous spans Slab Thickness
Number of Spans
Span Limit
Constant 3
50 Ft. middle span
Variable
65 Ft. middle span
Common Designs
Slab Thickness
Span Arrangement
Skew
16 to 18 in.
0° to 30°
18 to 20 in.
23 in.
24 in.
Variable (23”-37”)
Variable (24”- 38”)

28. Concrete slab spans

29. Concrete slab spans “TW” denotes tailwater or stage.
Any low member elevations less than the above criteria will require a complete structural design for buoyant and lateral forces due to stream flow, ice and debris. Consideration must be given to the possibility that the debris will increase the upstream water surface elevation.
A higher low member elevation can be used when the roadway design dictates or there are hydraulic considerations such as increased flood damage potential to upstream properties.

30. Prestressed concrete beam bridges

31. Prestressed concrete beam bridges
Advantages
Very economical for spans ≥40 ft.
Beam details are standardized
Adaptable to most geometric conditions
Durable with low maintenance
Disadvantages
Not a shallow depth structure
Shipping limitations may limit the use of longer spans
Cannot be curved to fit difficult geometrics

32. Prestressed concrete beam bridges
Simple spans
Common Designs
PCB Size
Span Limits
14RB
40 Ft.
18RB
50 Ft.
22RB
60 Ft.
27M
80 Ft.
36M
100 Ft.
MN45
130 Ft.
MN54
145 Ft.
MN63
160Ft.
82MW
190Ft.
96MW
205Ft.
PCB Size
Span
Skew, 45°max
36 in.
≤ 85 Ft.
0° to 20º
45 in.
≤ 115 Ft.
54 in.
≤ 130 Ft.

33. Prestressed concrete beam bridges

34. Timber slab spans

35. Timber slab spans Advantages Disadvantages
Timber can be constructed in virtually any weather conditions
Do not require special equipment for installation, and can normally be constructed without highly skilled labor
Presents a natural and aesthetically pleasing appearance in the natural surroundings
Disadvantages
The use of timber bridges are limited to low-volume roads with ADT under 750
Asphalt wearing surface tends to crack from differential deck deflections
Not the most economical solution

36. Timber slab spans Rule of thumb
Normally 1 or 3 spans with a maximum span length of about 40 feet.
Slab Thickness
Span Limit
10”
17 Ft.
12”
25 Ft.
14”
30 Ft.
16”
36 Ft.
18”
40 Ft.
Simple spans
Common Designs
Slab Thickness
Span Arrangement
Skew
14 in.
3 spans, 18’ to 30’
0° -30º
16 in.
3 spans, 28’ to 34’
18 in.
3 spans, 34’ to 40’ 0°

37. Timber slab spans
Timber Slab
Timber Slab

38. Steel Truss Pedestrian bridge spans

39. Steel Truss Pedestrian bridge spans
Prefabricated Steel Bridges are ideal for recreation and low volume vehicular applications
The bridges are shop manufactured with primarily welded connections then shipped to the site ready for installation. Limited field assembly is required for most projects.

40. Steel Truss Pedestrian bridge spans
Typical designs allow for clear spans from 20 to 200 feet. Under certain circumstances, special designs can extend spans to 250 feet. Bridges can be in single or multiple span configurations.
Clear spans up to 100 feet can be fabricated and shipped as one piece if contractor capabilities and site considerations allow. Longer spans are built with bolted field splices and shipped as multiple sections.

41. Steel Truss Pedestrian bridge spans

42. Precast Concrete box culverts

43. Precast Concrete box culverts
Advantages include quick installation, low cost bridge, and typically low maintenance. Presents a natural and aesthetically pleasing appearance in the natural surroundings
Disadvantages include span limitations of 20’ for single barrel, possible debris build-up with multiple barrel arrangements, and a lack of a natural stream bottom for fish unless the invert is lowered and riprapped.

44. Precast Concrete box culverts

45. Precast Concrete box culvert standards

46. Precast Concrete box culverts
Fill heights of less than 2’-0” require a distribution slab over the fill area of roadway and shoulders.
Class 1 culverts with 2’ to 3’ of fill and all class 2, 3, and 4 culverts do not require a distribution slab.
Cast-in-place distribution slabs to be 6” thick with No. 16 bars at 1’-0” transversely and No. 16 bars at 1’-0” longitudinally.

47. Precast Concrete box culverts
Distribution slab joints must be centered over barrel segments.
Provide 3” minimum granular material per MnDOT spec B2 between barrel and distribution slab.
If the fill height range extends into more than one class, use the class with the largest steel areas. Check maximum and minimum fill heights over the full area of roadway and shoulders.

48. Precast Concrete box culverts

49. Precast Concrete box culverts
If the distance between double barrels is less than 2’-0” use either pea rock or lean mix backfill (MnDOT spec. 2520) between the culverts as approved by the engineer.
If pea rock is used, provide approved grout seepage cutoff core, minimum 12” thick, between the culvert’s two ends. See standard figure for details.
Minimum distance between barrels is 6”.

50. Precast Concrete box culverts

51. Precast Concrete 3-sided buried structures

52. Precast Concrete 3-sided buried structures
Offer an alternative for short span bridges up to 42 feet. Larger spans may be considered on a case-by-case basis
Advantages include quick installation and natural stream bottom
Disadvantages include a higher cost than cast-in-place bridges
Typical Spans on local system 24, 28, 32, & 42

53. Precast Concrete 3-sided buried structures
In General, precast 3-sided structures may be used where:
Design span is less than or equal to 42 feet (12.8 m). Larger spans may be considered on a case-by-case basis, but only with prior approval of the Bridge Design Engineer. Span is measured from inside face of sidewalls along the longitudinal axis of the unit;
Rise is less than or equal to 13 feet (4m). rise is measured from top of footing/pedestal wall to bottom of top slab;

54. Precast Concrete 3-sided buried structures
Fill height is less than or equal to 10 feet (3m) but is greater than or equal to 3 feet (1m). fill heights larger than 10 feet (3m) may be considered on a case-by-case basis, but only with prior approval of the Bridge Design Engineer;
Skew is less than 30O;
No foundation limitations exist such as unusually weak soil;

55. Precast Concrete 3-sided buried structures
No site access limitations exist for transporting and erecting the three-sided structures;
Clogging from debris or sediment precludes the use of multiple barrel structures;

56. Precast Concrete arch structures

57. Precast Concrete arch structures

58. Bridge Hydraulic Checklist
Verify that the correct riprap details are shown.
Concrete Arches
Design 2A
Design 3
Design 2B
Design 4
Design 1

59. State aid bridge hydraulics
Bridge Hydraulic Checklist
(What do we look for)
Common Bridge Types
Typical Spans and Structure Depths
Low Member Criteria
Examples
Slab Span
Prestressed Concrete Beam Bridge

60. Concrete slab spans

61. Concrete slab spans
Bridge No

62. Bridge No

63. Bridge No
3 Span C-SLAB Bridge – 124’-8” long (out to out of deck)
Velocity = 5.4 FPS
Class III riprap
Skew angle = 0 degrees
Existing bridge is 112.6’ long (11% increase in length)
Held to 0.5% minimum profile grade requirement (for drainage), which resulted in a minimum grade increase.

64. Bridge No. 50592 Projected ADT = 813 Projected ADT Minimum OT Flood
0-10
2 year
11-49
5 year
50-399
10 year
25 year
≥ 1500
50 year

65. Bridge No
Road sag pt. elev. =

66. Bridge No
The overtopping flood is greater than Q100, therefore the design flood is Q100.

67. Bridge No
Design Velocity is 5.4 FPS for a Q100 event, but ADT guidelines would suggest a minimum Q25 event for 813 ADT.
Q25 event would mean lowering profile significantly, which would create extensive grading cost.
Therefore Q100 design was deemed appropriate for this site.
Low Member is Elev. =
Q100 TW is Elev (HW-0.2 stage inc.)

68. Bridge No
Structure depth check

69. Bridge No
S = 47 feet
Min Slab depth = = 1.9 feet or 1’-11” 30

70. Bridge No
Followed up by a slab design check

71. Bridge No. 50592 Check proposed channel bottom
feet up and down stream
Proposed = 57 feet

72. Hydraulic report
Hydraulic Engineer’s
Rec. on plan

73. Bridge No
Verify Waterway Area, stream velocity and Channel Bottom width using our internal Mathcad spreadsheet.

74. Bridge 50592

75. Bridge 50592

76. Bridge 50592

77. Plan values
1362 sq ft
Bridge 50592
5.4 fps
57 ft ft

78. Bridge No. 50592 Check riprap size RIPRAP SIZING DETAILS
SIZE OF AVG.
GRANULAR FILTER
WATER VELOCITY
AGGREGATE
REQUIREMENTS
(UP TO) F.P.S.
CLASS I
3"
3.86
CLASS II
6"
5.46
CLASS III
9"
6.69
CLASS IV
12"
7.73
CLASS V
15"
8.64
SPEC BOOK ALSO DENOTES THAT THE GRANULAR FILTER SHALL BE 6" UNLESS
OTHER DIMENSIONS ARE SPECIFIED (PROBABLY IN THE HYDRAULICS REC.).
OCCASIONALLY YOU WILL SEE A PAY ITEM FOR "QUARRY RUN RIPRAP", WHICH IS
BASICALLY RIPRAP THAT HAS A MORE ANGULAR SHAPE TO IT. THIS IS HELPFUL
FOR THE HIGHER VELOCITIES, WHERE IT INTERLOCKS WITH ITSELF BETTER.
Velocity = 5.4 FPS
Minimum Class III riprap required

79. State aid bridge hydraulics
Bridge Hydraulic Checklist
(What do we look for)
Common Bridge Types
Typical Spans and Structure Depths
Low Member Criteria
Examples
Slab Span
Prestressed Concrete Beam Bridge

80. Prestressed concrete beam bridges

81. Bridge No

82. Bridge No

83. Bridge No

84. Bridge No. 32570 Prestressed concrete beam bridges
Single Span PCB Bridge – 80’-11” long (out to out of deck)
36M beams spaced at 9’-0”
Velocity is 6.2 FPS
Class IV riprap
Skew angle = 0 degrees

85. Bridge No. 32570 Low Member is Elev. 1447.4 – Q100 is Elev. 1447.5
Existing bridge is 64’ long timber beam span (26% increase in length)

86. Bridge No. 32570 Projected ADT < 50 Projected ADT Minimum OT Flood
0-10
2 year
11-49
5 year
50-399
10 year
25 year
≥ 1500
50 year

87. Bridge No
Road sag pt. elev. =

88. Bridge No
The overtopping flood is greater than Q100, therefore the design flood is Q100.

89. Bridge No
Velocity is 6.2 FPS and is designed for Q100 event, but ADT guidelines would suggest a minimum Q5 event for <50 ADT.
Q5 event would mean lowering profile significantly, which would create extensive grading cost. Plus the inplace east approach is already steep.
Therefore Q100 design is appropriate for this site.
Low Member is Elev. =
Q100 TW is Elev (say ok)

90. Bridge No

91. Bridge No. 32570 Proposed = 36 feet Check proposed channel bottom
approx. 43 feet
Proposed = 36 feet
approx. 35 feet

92. Hydraulic report
Hydraulic Engineer’s
Rec. on plan

93. Bridge No
Verify Waterway Area, stream velocity and Channel Bottom width using our internal Mathcad spreadsheet.

94. Bridge 32570

95. Plan values
Bridge 32570
520 sq ft
6.2 fps
36 ft ft

96. Bridge No. 32570 Check riprap size RIPRAP SIZING DETAILS
SIZE OF AVG.
GRANULAR FILTER
WATER VELOCITY
AGGREGATE
REQUIREMENTS
(UP TO) F.P.S.
CLASS I
3"
3.86
CLASS II
6"
5.46
CLASS III
9"
6.69
CLASS IV
12"
7.73
CLASS V
15"
8.64
SPEC BOOK ALSO DENOTES THAT THE GRANULAR FILTER SHALL BE 6" UNLESS
OTHER DIMENSIONS ARE SPECIFIED (PROBABLY IN THE HYDRAULICS REC.).
OCCASIONALLY YOU WILL SEE A PAY ITEM FOR "QUARRY RUN RIPRAP", WHICH IS
BASICALLY RIPRAP THAT HAS A MORE ANGULAR SHAPE TO IT. THIS IS HELPFUL
FOR THE HIGHER VELOCITIES, WHERE IT INTERLOCKS WITH ITSELF BETTER.
Velocity = 6.2 FPS
Class IV riprap called for in plans, conservatively

97. State aid bridge hydraulics
QuestionS ?