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

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

Bridge Hydraulic Checklist What do we look for?

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.

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 400-1499 25 year ≥ 1500 50 year Generally find that design is governed by factors other than ADT.

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.

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”

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.

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.

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

Bridge Hydraulic Checklist Scour codes

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

Bridge Hydraulic Checklist

Hydraulic engineer’s recommendation table

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.

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.

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.

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

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

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.

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

Concrete slab spans

Concrete slab spans

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.

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

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)

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. 22-28-22 0° to 30° 18 to 20 in. 27-34-27 23 in. 37-46-37 24 in. 40-50-40 Variable (23”-37”) 46-58-46 Variable (24”- 38”) 48-60-48

Concrete slab spans

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.

Prestressed concrete beam bridges

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

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.

Prestressed concrete beam bridges

Timber slab spans

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

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°

Timber slab spans Timber Slab Timber Slab

Steel Truss Pedestrian bridge spans

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.

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.

Steel Truss Pedestrian bridge spans

Precast Concrete box culverts

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.

Precast Concrete box culverts

Precast Concrete box culvert standards

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.

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.

Precast Concrete box culverts

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”.

Precast Concrete box culverts

Precast Concrete 3-sided buried structures

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

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;  

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;

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;

Precast Concrete arch structures

Precast Concrete arch structures

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

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

Concrete slab spans

Concrete slab spans Bridge No. 50592

Bridge No. 50592

Bridge No. 50592 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.

Bridge No. 50592 Projected ADT = 813 Projected ADT Minimum OT Flood 0-10 2 year 11-49 5 year 50-399 10 year 400-1499 25 year ≥ 1500 50 year

Bridge No. 50592 Road sag pt. elev. = 1236.9

Bridge No. 50592 The overtopping flood is greater than Q100, therefore the design flood is Q100.

Bridge No. 50592 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. = 1235.3 Q100 TW is Elev. 1235.3 (HW-0.2 stage inc.)

Bridge No. 50592 Structure depth check

Bridge No. 50592 S = 47 feet Min Slab depth = 47 + 10 = 1.9 feet or 1’-11” 30

Bridge No. 50592 Followed up by a slab design check

Bridge No. 50592 Check proposed channel bottom 55 - 60 feet up and down stream Proposed = 57 feet

Hydraulic report Hydraulic Engineer’s Rec. on plan

Bridge No. 50592 Verify Waterway Area, stream velocity and Channel Bottom width using our internal Mathcad spreadsheet.

Bridge 50592

Bridge 50592

Bridge 50592

Plan values 1362 sq ft Bridge 50592 5.4 fps 57 ft 1235.3 ft

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

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

Prestressed concrete beam bridges

Bridge No. 32570

Bridge No. 32570

Bridge No. 32570

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

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)

Bridge No. 32570 Projected ADT < 50 Projected ADT Minimum OT Flood 0-10 2 year 11-49 5 year 50-399 10 year 400-1499 25 year ≥ 1500 50 year

Bridge No. 32570 Road sag pt. elev. = 1447.8

Bridge No. 32570 The overtopping flood is greater than Q100, therefore the design flood is Q100.

Bridge No. 32570 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. = 1447.4 Q100 TW is Elev. 1447.5 (say ok)

Bridge No. 32570

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

Hydraulic report Hydraulic Engineer’s Rec. on plan

Bridge No. 32570 Verify Waterway Area, stream velocity and Channel Bottom width using our internal Mathcad spreadsheet.

Bridge 32570

Plan values Bridge 32570 520 sq ft 6.2 fps 36 ft 1447.4 ft

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

State aid bridge hydraulics QuestionS ?