Request for bridge scour analysis for complex pier foundations

Size: px

Start display at page:

Download "Request for bridge scour analysis for complex pier foundations"

Tracy Goodwin
5 years ago
Views:

1 Technical Memorandum To: Theresa Maahs-Henderson, Stantec Dale Grove, Stantec Keith Farquhar, HNTB Corporation From: Hugh Zeng, P.E. & Mark Abrahams, HZ United, LLC Date: 03/22/2016 RE: Request for bridge scour analysis for complex pier foundations HZU has completed the requested bridge scour analysis for complex pier foundation. Our findings are based on the updated HEC-RAS model (as discussed below). The proposed bridge geometry was provided for two design alternatives, 4-span and 5-spans bridge. Scour analysis was conducted using the procedure outlined in FHWA HEC-18 for scour at complex pier foundations. The revised HEC-RAS model included the confluence of the Baudette River and the Rainy River. A newly created junction in the HEC-RAS model represents the river confluence. Discharges of the Baudette River was obtained from the USGS Streamstat website, which incorporates USGS Regression Equation using GIS drainage boundaries and the stream gauge data. Because the drainage tributary area of the Baudette River is much smaller than that of the Rainy River, coincidental factor was considered to correlate the peak discharges from each river. The results of the HEC-RAS model with the confluence showed minor water surface increases. The Baudette River connects to the Rainy River near a right angle. The velocity is slow compared to the Rainy River. Sand delta bars formed at the mouth of the Baudette River indicate a low stream energy from the tributary. As a result, the effects of the Baudette River on the mainstream is minimum. The stream attack-angle of 18.7 was unchanged in the scour calculation. The scour calculation took into account of both contraction scour and pier scour. The contraction scour was calculated from the HEC-RAS model. HEC-18 equations were used to predict the pier scour instead of using HEC-RAS model due to the complex pier footing configurations, which consist of pile caps over pile groups. Results for the two proposed pier configuration bridges are provided below: 1

2 Table 1: Complex Pier Foundation Scour Analysis (100-yr Event) Alternative 1: Continuous Steel I- Girder, 4-Span Alternative 1: Continuous Steel I- Girder, 5-Span Contraction Scour (ft) Pier Stem Scour [ft] Pile Cap (Footing) Scour [ft] Pile Group Scour [ft] Total Scour [ft] Table 2: Complex Pier Foundation Scour Analysis (500-yr Event) Alternative 1: Continuous Steel I- Girder, 4-Span Alternative 1: Continuous Steel I- Girder, 5-Span Contraction Scour (ft) Pier Stem Scour [ft] Pile Cap (Footing) Scour [ft] Pile Group Scour [ft] Total Scour [ft] The results shown above are preliminary and subject to revision. The predicted scour depths are comparable to the 30ft scour depth, which was documented in a June 19, 2009 MnDOT memorandum. Our analysis shows only a slight difference between the two proposed pier configurations. Preliminary bridge design provides identical pier geometry for the two proposed alternatives, so the only source of variance is due to any impact in water surface and velocity in the channel at the Bridge. It should also be noted that this analysis was conducted using a pier skew at 18.7 relative to the channel thaweg. Aligning piers normal to the channel flow will effectively reduce the overall scour estimates. Detailed calculations for the complex scour analysis are attached. 2

3 Attachment A: Complex Scour Calculations for 4-span Alternative

4 S.P Scour for Complex Pier Foundations Baudette Bridge PE 3 Pier Alternative Rev Date By Ck 0 03/22/2016 MBA HZ Live-bed or clear water scour Ref: FHWA HEC-18, sect. 6.3 Live-Bed Contraction Scour V * = shear velocity in the upstream section g = Acceleration of gravity (32.2 ft/s 2 ) y 1 = average depth in the upstream main channel = ft (7.83 m) S 1 = slope of energy grade line of main channel (ft/ft) = ft/s (0.084 m/s) T = fall velocity of bed material based on the D 50, (use D 50 = 0.50 mm) for fall velocity in English units multiply T in m/s by 3.28 Use T = 10 C, ω 0.06 m/s Use live-bed scour. 1

5 100-yr Event 1. Contraction Scour Depth Contration Scour computed from HEC-RAS river analysis. Refer to HEC-RAS model for contraction scour computation and inputs < R:\1502-Baudette Br PE\Design\Calculations\HEC-RAS>. Channel Contraction Scour: Ref: HEC-18, sect. 7.5 Scour for Complex Pier y s = Total Scour from superposition of components 100-yr event 2. Pier Stem Scour Depth (7.23) Where: f = Distance between front edge of pile cap or footing and pier 9 ft. (2.74 m) a pier = pier width = 7 ft (2.13 m) h 1 = h 0 + T = height of the pier stem above the bed before scour = 0 K h pier = Coefficient to account for the height of pier stem above bed and shielding effect by pile cap overhang distance f in front of pier stem. 2

6 100-yr Event K h pier 0.32 (from figure 7.6) For 100-year event (from HEC-RAS output) WSE = ft ( m) Ground = ft ( m) y 1 = 25.7 ft (7.83 m) V 1 = 3.74 fps (1.14 m/s) K 1 = correction factor for pier nose shape; round nose = 1.0 K 2 = correction factor for angle of attack of flow; K 3 = correction factor for bed condition; (From Table 7.3, use plane bed) = 1.1 g = acceleration of gravity = 32.2 ft/s 2 3

7 3. Pile Cap (Footing) Scour Depth Use Case 2; bottom of pile cap is on or below bed 3025 Harbor Lane, Suite 121, Plymouth, MN yr Event y f = distance from the bed (after degredation, contraction scour, and pier stem scour) to the top of footing. k s = Grain roughness, use estimate D 84 for sand, 2.5 mm = ft (0.0025m) Average velocity of flow at the exposed footing (V f ) is determined using the following: (7.25) The scour component equation for the footing can be written as: (7.24) K W = wide pier factor for shallow flow, not applicable. a pc = pile cap width = 20.0 ft (6.10 m) 4

8 4. Determination of Pile Group Scour Depth Component 3025 Harbor Lane, Suite 121, Plymouth, MN yr Event Where: a proj = sum of non-overlapping projected widths of piles = 6.67 ft K sp = coefficient for pile spacing a = 1.33 ft (16 in) s = 4 ft (7.28) K sp 0.49 (from figure 7.11) 5

9 100-yr Event K m = coefficient for number of aligned rows 1.4 (from figure 7.12) (7.29) (7.30) K h pg 0.45 (from figure 7.13) (7.31) Total Pier Scour (100yr): Total Scour (100yr): 6

10 Scour Design Check Flood Frequency (500-yr) 1. Contraction Scour Depth 3025 Harbor Lane, Suite 121, Plymouth, MN yr Event Contration Scour computed from HEC-RAS river analysis. Refer to HEC-RAS model for contraction scour computation and inputs < R:\1502-Baudette Br PE\Design\Calculations\HEC-RAS>. Channel Contraction Scour: 2. Pier Stem Scour Depth Where: f = Distance between front edge of pile cap or footing and pier 9 ft. a pier = pier width = 7 ft h 1 = h 0 + T = height of the pier stem above the bed before scour = 0. K h pier = Coefficient to account for the height of pier stem above bed and shielding effect by pile cap overhang distance f in front of pier stem. 7

11 K h pier 0.32 (from figure 7.6) For 500-year event (from HEC-RAS output) WSE = ft ( m) Ground = ft ( m) y 1 = ft (8.36 m) V 1 = 4.10 fps (1.25 m/s) 3025 Harbor Lane, Suite 121, Plymouth, MN yr Event K 1 = correction factor for pier nose shape; round nose = 1.0 K 2 = correction factor for angle of attack of flow; K 3 = correction factor for bed condition; (From Table 7.3, use plane bed) = 1.1 g = acceleration of gravity = 32.2 ft/s 2 3. Pile Cap (Footing) Scour Depth Use Case 2; bottom of pile cap is on or below bed y f = distance from the bed (after degredation, contraction scour, and pier stem scour) to the top of footing. k s = Grain roughness, use estimate D 84 for sand, 2.5 mm = ft Average velocity of flow at the exposed footing (V f ) is determined using the following: 8

12 500-yr Event ft/s The scour component equation for the footing can be written as: K W = wide pier factor, not applicable. a pc = pile cap width = 20.0 ft 4. Determination of the Pile Group Scour Depth Component Where: a proj = sum of non-overlapping projected widths of piles = 6.67 ft K sp = coefficient for pile spacing a = 1.33 ft (16 in) s = 4 ft, (7.28) K sp 0.49 (from figure 7.11) 9

13 500-yr Event K m = coefficient for number of aligned rows 1.4 (from figure 7.12) (7.29) (7.30) K h pg 0.45 (from figure 7.13) 10

14 500-yr Event (7.31) Total Pier Scour (500yr): Total Scour (500yr): 11

15 Attachment B: Complex Scour Calculations for 5-span Alternative

S.P. 3905-09 Scour for Complex Pier Foundations Baudette Bridge PE 4 Pier Alternative Rev Date By Ck 0 03/22/2016 MBA HZ Live-bed or clear water scour

2 ft/s 2 ) y 1 = average depth in the upstream main channel = 25.69 ft (7.83 m) S 1 = slope of energy grade line of main channel (ft/ft) = 0.

16 S.P Scour for Complex Pier Foundations Baudette Bridge PE 4 Pier Alternative Rev Date By Ck 0 03/22/2016 MBA HZ Live-bed or clear water scour Ref: FHWA HEC-18, sect. 6.3 Live-Bed Contraction Scour V * = shear velocity in the upstream section g = Acceleration of gravity (32.2 ft/s 2 ) y 1 = average depth in the upstream main channel = ft (7.83 m) S 1 = slope of energy grade line of main channel (ft/ft) = ft/s T = fall velocity of bed material based on the D 50, (use D 50 = 0.50 mm) for fall velocity in English units multiply T in m/s by 3.28 Use T = 10 C, ω 0.06 m/s Use live-bed scour. 1

17 100-yr Event 1. Contraction Scour Depth Contration Scour computed from HEC-RAS river analysis. Refer to HEC-RAS model for contraction scour computation and inputs < R:\1502-Baudette Br PE\Design\Calculations\HEC-RAS>. Channel Contraction Scour: Ref: HEC-18, sect. 7.5 Scour for Complex Pier y s = Total Scour from superposition of components 100-yr event 2. Pier Stem Scour Depth (7.23) Where: f = Distance between front edge of pile cap or footing and pier 9 ft. (2.74 m) a pier = pier width = 7 ft (2.13 m) h 1 = h 0 + T = height of the pier stem above the bed before scour = 0 K h pier = Coefficient to account for the height of pier stem above bed and shielding effect by pile cap overhang distance f in front of pier stem. 2

18 100-yr Event K h pier 0.32 (from figure 7.6) For 100-year event (from HEC-RAS output) WSE = ft ( m) Ground = ft ( m) y 1 = 25.7 ft (7.83 m) V 1 = 3.81 fps (1.16 m/s) K 1 = correction factor for pier nose shape; round nose = 1.0 K 2 = correction factor for angle of attack of flow; K 3 = correction factor for bed condition; (From Table 7.3, use plane bed) = 1.1 g = acceleration of gravity = 32.2 ft/s 2 3

19 3. Pile Cap (Footing) Scour Depth Use Case 2; bottom of pile cap is on or below bed 3025 Harbor Lane, Suite 121, Plymouth, MN yr Event y f = distance from the bed (after degredation, contraction scour, and pier stem scour) to the top of footing. k s = Grain roughness, use estimate D 84 for sand, 2.5 mm = ft (0.0025m) Average velocity of flow at the exposed footing (V f ) is determined using the following: (7.25) The scour component equation for the footing can be written as: (7.24) K W = wide pier factor for shallow flow, not applicable. a pc = pile cap width = 20.0 ft (6.10 m) 4

20 4. Determination of Pile Group Scour Depth Component 3025 Harbor Lane, Suite 121, Plymouth, MN yr Event Where: a proj = sum of non-overlapping projected widths of piles = 6.67 ft K sp = coefficient for pile spacing a = 1.33 ft (16 in) s = 4 ft (7.28) K sp 0.49 (from figure 7.11) 5

21 100-yr Event K m = coefficient for number of aligned rows 1.4 (from figure 7.12) (7.29) (7.30) K h pg 0.45 (from figure 7.13) (7.31) Total Pier Scour (100yr): Total Scour (100yr): 6

22 Scour Design Check Flood Frequency (500-yr) 1. Contraction Scour Depth 3025 Harbor Lane, Suite 121, Plymouth, MN yr Event Contration Scour computed from HEC-RAS river analysis. Refer to HEC-RAS model for contraction scour computation and inputs < R:\1502-Baudette Br PE\Design\Calculations\HEC-RAS>. Channel Contraction Scour: 2. Pier Stem Scour Depth Where: f = Distance between front edge of pile cap or footing and pier 9 ft. a pier = pier width = 7 ft h 1 = h 0 + T = height of the pier stem above the bed before scour = 0. K h pier = Coefficient to account for the height of pier stem above bed and shielding effect by pile cap overhang distance f in front of pier stem. 7

23 K h pier 0.32 (from figure 7.6) For 500-year event (from HEC-RAS output) WSE = ft ( m) Ground = ft ( m) y 1 = ft V 1 = 4.17 fps (1.27 m/s) 3025 Harbor Lane, Suite 121, Plymouth, MN yr Event K 1 = correction factor for pier nose shape; round nose = 1.0 K 2 = correction factor for angle of attack of flow; K 3 = correction factor for bed condition; (From Table 7.3, use plane bed) = 1.1 g = acceleration of gravity = 32.2 ft/s 2 3. Pile Cap (Footing) Scour Depth Use Case 2; bottom of pile cap is on or below bed y f = distance from the bed (after degredation, contraction scour, and pier stem scour) to the top of footing. k s = Grain roughness, use estimate D 84 for sand, 2.5 mm = ft Average velocity of flow at the exposed footing (V f ) is determined using the following: 8

24 500-yr Event ft/s The scour component equation for the footing can be written as: K W = wide pier factor, not applicable. a pc = pile cap width = 20.0 ft 4. Determination of the Pile Group Scour Depth Component Where: a proj = sum of non-overlapping projected widths of piles = 6.67 ft K sp = coefficient for pile spacing a = 1.33 ft (16 in) s = 4 ft, (7.28) K sp 0.49 (from figure 7.11) 9

25 500-yr Event K m = coefficient for number of aligned rows 1.4 (from figure 7.12) (7.29) (7.30) K h pg 0.45 (from figure 7.13) 10

26 500-yr Event (7.31) Total Pier Scour (500yr): Total Scour (500yr): 11

A STUDY OF LOCAL SCOUR AT BRIDGE PIERS OF EL-MINIA

A STUDY OF LOCAL SCOUR AT BRIDGE PIERS OF EL-MINIA Dr. Gamal A. Sallam 1 and Dr. Medhat Aziz 2 ABSTRACT Bridges are critical structures that require a substantial investment to construct and serve an important