Local bridge Hydraulics 101

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. Check the design/overtopping flood against guidelines for the ADT level. Bridge Hydraulic Checklist • 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 MnDOTstandard 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. Scour codes Bridge Hydraulic Checklist

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. 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. Advantages • Very economical short spans bridge • Ease of design and detail • Adaptable to curved alignments Concrete slab spans • 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. Continuous spans Concrete slab spans Common Designs

28. Concrete slab spans

29. “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. Concrete slab spans

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

33. Prestressed concrete beam bridges

34. Timber slab spans

35. Advantages • 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 Timber slab spans • 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. Rule of thumb • Normally 1 or 3 spans with a maximum span length of about 40 feet. Timber slab spans Simple spans Common Designs

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 culvertstandards

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. 3149.2B2 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 5-395.115 for details. • Minimum distance between barrels is 6”.

50. Precast Concrete box culverts