CE 435/535, Fall 2003 Preliminary Design Example 1 / 6

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1 CE 435/535, Fall 2003 Preliminary Design Example 1 / 6 Perform a preliminary design for a bridge on a state highway over a large creek in rural Alabama. Produce a construction layout drawing showing profile and plan views of the bridge and approach embankments. Traffic Information. This information is used to calculate the number of lanes and the bridge width. These calculations are frequently performed by a transportation engineer. Rural location, flat terrain ADT = 5000 veh/day (2-way) % trucks = 5% (reasonable for rural area; I20/59 has 30% trucks, one of the highest in the state, for comparison) Functional Class of highway = major collector growth rate = 3% Hydraulic Information. This information is used to calculate the minimum required waterway opening (in square feet). design flood = 50-year flood (this is a state route; 25-yr flood may be used for small county roads, 100-yr flood is used for interstates) Q_50 (flow rate for 50-yr flood) = 25,000 cfs WSE_50 (water surface elevation for 50-yr flood) = 245 ft Streambed is hard clay, Vmax = max. mean velocity to prevent scour = 4 fps Stream Profile This information is from a survey along the road/bridge centerline. Station, ft Elevation, ft Design Requirements: Design for Level of Service (LOS) C No barge traffic on waterway

2 CE 435/535, Fall 2003 Preliminary Design Example 2 / 6 1. Determine Bridge Width ADT 5,000 veh/day average daily traffic % trucks 5% E T 1.5 equivalent cars per truck use 1.5 for level terrain 3.0 for rolling terrain 6.0 for Mountainous terrain ADT adjusted for truck traffic 5,125 = ADT x (1 - %_trucks + %_trucks x E T) growth rate 3% typically varies between 1% and 5%, depending on local economy, etc. design year 20 yrs into the future ADT in 20 years 9,256 vh/day, 2 way = ADT x (1 + growth_rate) 20 peak volume factor 0.12 ranges from 0.12 to Use: 0.12 for fairly constant traffic from hour to hour 0.18 for traffic with large variations, for example: rush hours DHV 1,111 vh/hr, 2 way Design Hourly Volume = ADT x peak_volume_factor Desired_LOS C Desired Level of Service ("C" is typical) vh/hr, 2 Allowable_DHV 1200 way OK for Two Lanes, LOS = "C" see Table II-5 (handout), from: Geometric Design of Highways and Streets, AASHTO Thefore, Use 2 traffic lanes

3 CE 435/535, Fall 2003 Preliminary Design Example 3 / 6 Lane Width = 12ft, page 2 in course notes, for two-way traffic, rural, Desirable, Major Collector Shoulder Width = 8 ft (same page in course notes) Min. Bridge Width = 2 x 12ft + 2 x 8ft + 2 x 1.3 ft (Jersey barrier rail) = 42.6 ft ALDOT has 44ft wide standard design (shoulder width = 44ft 24ft 2.6ft) / 2 = 8.7 ft 2. Calculate Bridge Low-Steel Elevation Use 44 ft wide bridge, out-to-out low steel = lowest part of superstructure elevation of low steel must be > 16ft above under -road for grade separations (17ft is better) > 2ft above WSE_50 for stream crossings > navigation requirement for rivers with barge traffic (set by Army Corps of Engineers) Elevation of low steel = 245ft + 2 ft Elevation Low Steel = 247 ft 3. Calculate Min. Waterway Opening (A_50_min) A_50_min = Q_50 / Vmax = 25,000 cfs / 4 fps A_50_min = 6,250 sf Vmax is usually specified by someone knowledgeable in bridge hydraulics and scour. In Alabama, Tom Flournoy with the Bridge Bureau usually visits the site and supplies Vmax. Typical values from the Army Corps of Engineers for various soil types are shown below. Table 4-1. Maximum Permissible Mean Velocities to Prevent Scour, U..S. Army Corps of Engineers Material Maximum Permissible Mean Velocity, fps Uniform graded sand and cohesionless silts 1.5 Well-graded sand 2.5 Silty sand 3.0 Gravel 6.0 From hand-drawn drawing of stream cross-section, A_50_natural 9,900 sf, OK (from AutoCAD drawing, A_50_nat = 10,000 sf)

4 CE 435/535, Fall 2003 Preliminary Design Example 4 / 6 4. Locate Abutments (CL of bearings) Try #1: Try Abutment 1 bearing at Sta 500, Elev 247 (low steel elevation) And Abutment 2 at Sta 900, Elev 247, bridge length = = 400 assume 3 spans (so that can use prestressed concrete girders, max span = 140 ) avg span = 400 / 3 spans = 133 / span approx bridge depth = 6 (AlDOT Estimated Costs Sheet) + 1 deck = 7 top of bridge = Elev = 254 from hand-drawn drawing, A_50_prov d 6800 sf > 6,250 sf = A_50_min, OK (A_50_prov d from AutoCAD = 7,237 sf) Try #2: try moving abutments closer to minimize cost of bridge Abut. 1 at Sta 550, Abut. 2 at Sta 850, A_50_prov d from AutoCAD = 5,976sf, NG Use Try #1 A_50_prov d = 7237 sf Station C L Bearing Elevation Top of Bearing Abutment Abutment Select Superstructure Type & Pier Locations 400-ft-long bridge, try 3 spans Max. prestressed girder length = 140 for a Bulb Tee 72 (BT 72) Try spans of 130, 140, 130 Station Elevation Pier Pier Superstructure Depth = 6 Girder depth (ALDOT cost sheet) + 1 (assumed deck + buildup thickness) road elev Road Surface Elev = 254

5 CE 435/535, Fall 2003 Preliminary Design Example 5 / 6 6. Estimate Construction Cost of Bridge & Embankments Bridge Cost: Deck Area = 400ft x 44ft = 17,600 sf Superstructure Type = Precast Concrete Girders, BT72 Max. Pier Ht = 23ft (from AutoCAD drawing) Cost/sf = $65/sf for BT72 with pier ht between 40 to 60 (AlDOT Estimated Bridge Cost Sheet) Note: Our max. pier height = 22 which is less than typically used for BT72 girders. Should consider using more spans with shorter girders (e.g AASHTO Type III, max span = 80 ) Bridge Cost = 17,600sf x $65/sf x (1.03) 3 Estimated Bridge Cost = $1,250,000 (Assumed 3% inflation rate to adjust cost/sf given in Year 2000 dollars) Cost of Approach Embankments: Max. Fill Ht. = 19ft at Abutments ADT_20 = 9,300 veh/day Terrain = flat From Typical Cross Section handout: use 3:1 fill slope (note: handout recommends 2:1 slope for fill heights more than 15ft assume most of embankment less than 15 high). Calculate side slope volume for each section with uniform slope. There are two sections for the left embankment: Section 1 between Sta 0+00 and Sta 4+00 Section 2 between Sta 4+00 and Sta 5+00 (centerline south abutment) Section 1 Section 2 h_1 4 14ft h_ ft L ft S_L S_S 3 3 V_side_slope_section 53,600 38,600 cf V_side_slope 92,200 cf

6 CE 435/535, Fall 2003 Preliminary Design Example 6 / 6 road_width 44 V_road_section 158,400 70,400 cf V_road 228,800 cf h_abut 19 V_front_slope 15,884 cf Total Embankment Fill 429,084 cf 15,892 cy Where: V_side_slope_section V_road_section V_front_slope =0.5*S_S*(h_1^2*L+S_L*h_1*L^2+S_L^2/3*L^3) =0.5*(h_1+h_2)*L*road_width =h_abut^2*road_width Assume $5/cy for embankment fill. Cost of embankments = $5/cy x 15,900 cy/embankment x 2 embankments Cost of Embankments = $159,000 Total Cost of bridge + approach embankments = $1,250,000 + $159,000 Total Estimated Cost of bridge + approach embankments = $1,410,000