CE 535, Spring 2002 Preliminary Design Example 1 / 6

Transcription

1 CE 535, Spring 2002 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 535, Spring 2002 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 535, Spring 2002 Preliminary Design Example 3 / 6 Lane Width = 12ft, page 2 in course notes, for two-way traffic, rural, Desirable, Major Collector Shoulder Width = 6 ft (same page in course notes) Min. Bridge Width = 2 x 12ft + 2 x 6ft + 2 x 1.3 ft (Jersey barrier rail) = 38.6 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) > 1ft above WSE_50 for stream crossings > navigation requirement for rivers with barge traffic (set by Army Corps of Engineers) Elevation of low steel = 245ft + 1 ft Elevation Low Steel = 246 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 AutoCAD drawing of stream cross-section, A_50_natural = 10,000 sf, OK

4 CE 535, Spring 2002 Preliminary Design Example 4 / 6 4. Locate Abutments (CL of bearings) Try #1: Try South Abutment bearing at Sta 500, Elev 246 (low steel elevation) And North Abutment at Sta 900, Elev 246, from AutoCAD, A_50_prov d = 7,237sf Try #2: S.Abut. at Sta 550, N. Abut. At Sta 850, Use Try #1 A_50_prov d = 5,976sf, NG A_50_prov d = 7237 sf Station Elevation N. Abutment N. 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 = 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 = 22ft (from AutoCAD drawing) Cost/sf = $65/sf for BT72 with pier ht between 40 to 60 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 Estimated Bridge Cost = $1,144,000

5 CE 535, Spring 2002 Preliminary Design Example 5 / 6 Cost of Approach Embankments: Max. Fill Ht. = 18ft 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 3 13ft h_ ft L ft S_L S_S 3 3 V_side_slope_section 43,400 36,350 cf V_side_slope 79,750 cf road_width 44 V_road_section 140,800 68,200 cf V_road 209,000 cf h_abut 18 V_front_slope 14,256 cf Total Embankment Fill 382,756 cf 14,176 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

6 CE 535, Spring 2002 Preliminary Design Example 6 / 6 Assume $5/cy for embankment fill. Cost of embankments = $5/cy x 14,200cy /embankment x 2 embankments Cost of Embankments = $142,000 Total Cost of bridge + approach embankments = $1,144,000 + $142,00 Total Estimated Cost of bridge + approach embankments = $1,286,000