Question

• determine structural number
You should design a roadway that is to carry 100,000 people a day
(excluding truck occupants) 50,000 in design direction. The vehicles that
carry these people include; (1) personal vehicles (average occupancy of 1.5
people per car) 2 2 kip single axles, and (2) buses with 2 24 kip single axles
(assume they are filled with an average of 40 passengers). There are also
trucks on the roadway and all trucks have 1 40 kip triple axle, 1 34 kip
tandem axle, and 1 8 kip single axle. You know that the soil CBR is 8. the
initial PSI is 4.5 and the TSI is 2.5. Other values to be used:
•
All drainage coefficients = 1.0
•
Reliability = 90%
•
Overall standard deviation of traffic = 0.50

Answer 1

First calculate the Number of ESALS that is being loaded on the road.

1 kip = 1000 lb

50000 people travel per day (excluding truck occupants) in one direction.

cars are 1.5 per/veh and buses are 40 per/veh

So we need to find out the number or cars, buses and trucks.

So N_cars *1.5 + N_buses*40 = 50000

No other informatin is given about the proportion of cars to buses.Lets assume Number of buses = 5% of Number of cars, and Number of trucks = 5% of

Assume V is the total number of vehicles.

so we get N_buses = 0.05*V

N_trucks = 0.05V

N_cars = 0.9V

So we have N_cars *1.5 + N_buses*40 = 50000

or (1.5*0.9V) + (40*0.05V) = 50000

So we get V = 14926 vehicles

So N_cars = 0.9*V = 13434

N_buses = 746

N_trucks = 746

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Now calculate the total ESALS being loaded onto roadway in year, assume lane distribution factor = 1

Truck Factor = Total weight(lb) on the vehicle/18000

So ESAL for any category = Truck Factor * AADT * 365

Now that we have ESALS, we can calcutate the Structural Number

Given that CBR = 8

So we know that

So we get MR = 2555 * 8^0.64 = 9669 PSI

So from AASHTO flexible pavemnet design equation we know that

where

W₁₈ = predicted number of 18 kip loads i.e ESALs = 10 * 10⁶

Z_R = standard normal deviate = -1.282 for a 90% reliability level

S₀ = combined standard error of traffic prediction and performance predictions given to be 0.50

S_N = Structural number (an index that is indicative of the total pavement thickness required) = a₁D₁ + a₂D₂m₂ + a₃D₃m₃ where D is in inches

Delta PSI = difference between initial design serviceability index P₀ and terminal design serviceability index P_t

= 4.5 - 2.5 = 2.0

M_R = subgrade resilient modulus (in psi) = 9669

So we get

And if you calculate it out SN = 3.2227

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