DESIGN OF BOX CULVERT
3.1 DESIGN OF ONE UNIT BOX CULVERT
When the depth of fill is 2’ or more, concentrates load is to be uniformly distributed over a square, the side of which are equal to 1.75 times the depth of fill, shear is calculated as for dead load when the depth of fill is less than 2’, AASHTO specifies that the wheel load be distributed as an exposed slab.
When areas from several concentrations overlap, total load is considered uniformly distributed over the area, defined by the outside limits of the individual area. The total width of distribution should not exceed the total width of supporting slab. For single span the effect of live load may be neglected when the fill is more than 8’ and exceeds the span length. For multiple spans, the live load may be neglected when the depth of fill exceeds the distance between end supports or abutment.
If the calculated live load and impact moment (based on a distribution loading) exceeds the live load and impact moment due to a concentrated load, the lower values should be used.
3.1.1 SPECIFICATION DATA :
- Water way opening = 1 cell ( 4 ft * 4 ft )
= 4 * 4 ft2
= 16 ft2
2. Foundation type-unyielding foundation
3. Live load—HS-20-44
4. Height of fill above slab = 8 ft.
3.1.2 DESIGN CRITERIA :
1. Unit weight of concrete = 150lb/ft
2. Unit weight of soil = 120lb/ft
1. Compressive stress of concrete at 28 days, f/c = 3000psi
2. Allowable compressive stress, fc = 0.45 f/c
= 1350 psi
3. Yield point stress of reinforcement, fy = 60,000psi
4. Allowable stress of reinforcement, fs = 0.4fy
3.1.3 DESIGN CONSTANT :
n = Es /Ec
= 29 * 10 /( 57000*3000 )
r = fs / fc
= 24,000 / 1350
k = n / (n+r)
j = 1- k/3
R = 0.5 X fc X k X j
= 0.5 X 1350 X 0.336 X 0.888
. .. n = 9
k = 0.336
j = 0.888
R = 201