# Box Culvert Design Using Visual Basic-6.0 (Part-5)

Rajib Barua | 6:42 AM |

In case of concealed slab

Effective width, E = 1.75 * 8

= 14 ft.

Therefore, let us assume, the dispersion width is 14 ft considering following provisions of PCA manual, AASHTO-92, section 3.8.2.3,

Impact factor for wheel load, I = 0.0%

Weight of each vertical wall = 4 * 0.15 * 1 [consider 1 ft strip]

= 0.6 k/ft

The transmission and distribution of wheel loads through soil generally assumed to follow the principles of elasticity.

When the depth of fill is less than 2 ft, AASHTO specifies that, the wheel load be distributed as an exposed slab, when the depth of fill is more than 2 ft, concentrated loads are to be uniformly distributed over a square, the side of which are equal to 1.75 times the depth of fill.

Therefore, concentrated load ( conceal slab ) :

Live load from rear wheel, per ft of slab

= {16(1+Impact factor/100)}/14

= 1.1428 k/ft [Impact factor = 0%]

Live load from front wheel, per ft width of slab,

= {4(1+Impact factor/100)}/14

= 0.2857 k/ft

Concentrated load ( exposed slab ) with no impact

= 16 ( 1+0/100 ) / 4.57

= 3.5 k

Coefficient of active earth pressure is calculated by using the Rankine’s formula,

ka = (1-sin 300) / (1+sin300 )

= 0.33

But in our case, we consider co-efficient of active earth pressure at rest condition,

ka = (1-sin 300 )

= 0.5

The effect of live load surcharge when acting along ( with approach slab), will be uniformly lateral load.

Uniformly lateral load due to surcharge equivalent to 2.6ft earth fill

h= 8+9/12+4+9/12

= 13.5

h1 = 8+9/12

= 8.75

Lateral pressure at the bottom of vertical wall

= 0.5*0.12*13.5

= 0.81 k/ft

Lateral pressure at centerline of top slab