|4.11 Choice of Equipment and Standard Production Rates|
Typically, construction equipment is used to perform essentially repetitive operations, and can be broadly classified according to two basic functions: (1) operators such as cranes, graders, etc. which stay within the confines of the construction site, and (2) haulers such as dump trucks, ready mixed concrete truck, etc. which transport materials to and from the site. In both cases, the cycle of a piece of equipment is a sequence of tasks which is repeated to produce a unit of output. For example, the sequence of tasks for a crane might be to fit and install a wall panel (or a package of eight wall panels) on the side of a building; similarly, the sequence of tasks of a ready mixed concrete truck might be to load, haul and unload two cubic yards (or one truck load) of fresh concrete.
In order to increase job-site productivity, it is beneficial to select equipment with proper characteristics and a size most suitable for the work conditions at a construction site. In excavation for building construction, for examples, factors that could affect the selection of excavators include:
By comparing various types of machines for excavation, for example, power shovels are generally found to be the most suitable for excavating from a level surface and for attacking an existing digging surface or one created by the power shovel; furthermore, they have the capability of placing the excavated material directly onto the haulers. Another alternative is to use bulldozers for excavation.
The choice of the type and size of haulers is based on the consideration that the number of haulers selected must be capable of disposing of the excavated materials expeditiously. Factors which affect this selection include:
Dump trucks are usually used as haulers for excavated materials as they can move freely with relatively high speeds on city streets as well as on highways.
The cycle capacity C of a piece of equipment is defined as the number of output units per cycle of operation under standard work conditions. The capacity is a function of the output units used in the measurement as well as the size of the equipment and the material to be processed. The cycle time T refers to units of time per cycle of operation. The standard production rate R of a piece of construction equipment is defined as the number of output units per unit time. Hence:
The daily standard production rate Pe of an excavator can be obtained by multiplying its standard production rate Re by the number of operating hours He per day. Thus:
where Ce and Te are cycle capacity (in units of volume) and cycle time (in hours) of the excavator respectively.
In determining the daily standard production rate of a hauler, it is necessary to determine first the cycle time from the distance D to a dump site and the average speed S of the hauler. Let Tt be the travel time for the round trip to the dump site, To be the loading time and Td be the dumping time. Then the travel time for the round trip is given by:
The loading time is related to the cycle time of the excavator Te and the relative capacities Ch and Ce of the hauler and the excavator respectively. In the optimum or standard case:
For a given dumping time Td, the cycle time Th of the hauler is given by:
The daily standard production rate Ph of a hauler can be obtained by multiplying its standard production rate Rh by the number of operating hours Hh per day. Hence:
This expression assumes that haulers begin loading as soon as they return from the dump site.
The number of haulers required is also of interest. Let w denote the swell factor of the soil such that wPe denotes the daily volume of loose excavated materials resulting from the excavation volume Pe. Then the approximate number of haulers required to dispose of the excavated materials is given by:
While the standard production rate of a piece of equipment is based on "standard" or ideal conditions, equipment productivities at job sites are influenced by actual work conditions and a variety of inefficiencies and work stoppages. As one example, various factor adjustments can be used to account in a approximate fashion for actual site conditions. If the conditions that lower the standard production rate are denoted by n factors F1, F2, ..., Fn, each of which is smaller than 1, then the actual equipment productivity R' at the job site can be related to the standard production rate R as follows:
On the other hand, the cycle time T' at the job site will be increased by these factors, reflecting actual work conditions. If only these factors are involved, T' is related to the standard cycle time T as:
Each of these various adjustment factors must be determined from experience or observation of job sites. For example, a bulk composition factor is derived for bulk excavation in building construction because the standard production rate for general bulk excavation is reduced when an excavator is used to create a ramp to reach the bottom of the bulk and to open up a space in the bulk to accommodate the hauler.
In addition to the problem of estimating the various factors, F1, F2, ..., Fn, it may also be important to account for interactions among the factors and the exact influence of particular site characteristics.
Example 4-9: Daily standard production rate of a power shovel
A power shovel with a dipper of one cubic yard capacity has a standard operating cycle time of 30 seconds. Find the daily standard production rate of the shovel.
For Ce = 1 cu. yd., Te = 30 sec. and He = 8 hours, the daily standard production rate is found from Eq. (4.6) as follows:
In practice, of course, this standard rate would be modified to reflect various production inefficiencies, as described in Example 4-11.
Example 4-10: Daily standard production rate of a dump truck
A dump truck with a capacity of 6 cubic yards is used to dispose of excavated materials at a dump site 4 miles away. The average speed of the dump truck is 30 mph and the dumping time is 30 seconds. Find the daily standard production rate of the truck. If a fleet of dump trucks of this capacity is used to dispose of the excavated materials in Example 4-9 for 8 hours per day, determine the number of trucks needed daily, assuming a swell factor of 1.1 for the soil.
The daily standard production rate of a dump truck can be obtained by using Equations (4.7) through (4.10):
Hence, the daily hauler productivity is:
Finally, from Equation (4.12), the number of trucks required is:
implying that 8 trucks should be used.
Example 4-11: Job site productivity of a power shovel
A power shovel with a dipper of one cubic yard capacity (in Example 4-9) has a standard production rate of 960 cubic yards for an 8-hour day. Determine the job site productivity and the actual cycle time of this shovel under the work conditions at the site that affects its productivity as shown below:
Work Conditions at the Site
Factors Bulk composition 0.954 Soil properties and water content 0.983 Equipment idle time for worker breaks 0.8 Management efficiency 0.7
Using Equation (4.11), the job site productivity of the power shovel per day is given by:
The actual cycle time can be determined as follows:
Noting Equation (4.6), the actual cycle time can also be obtained from the relation T'e = (CeHe)/P'e. Thus:
Example 4-12: Job site productivity of a dump truck
A dump truck with a capacity of 6 cubic yards (in Example 4-10) is used to dispose of excavated materials. The distance from the dump site is 4 miles and the average speed of the dump truck is 30 mph. The job site productivity of the power shovel per day (in Example 4-11) is 504 cubic yards, which will be modified by a swell factor of 1.1. The only factors affecting the job site productivity of the dump truck in addition to those affecting the power shovel are 0.80 for equipment idle time and 0.70 for management efficiency. Determine the job site productivity of the dump truck. If a fleet of such trucks is used to haul the excavated material, find the number of trucks needed daily.
The actual cycle time T'h of the dump truck can be obtained by summing the actual times for traveling, loading and dumping:
Hence, the actual cycle time is:
The jobsite productivity P'h of the dump truck per day is:
The number of trucks needed daily is:
so 8 trucks are required.