Study of contribution of equipment and plant in throughput of construction processes

Equipment Capacity: Productive Scenario

The contribution of construction equipment for a given throughput has two components: the number or measurement of the equipment and the duration of mobilization of the equipment. Contribution of equipment is therefore measured in geometry-duration units. Primarily, in the simplest scenario, quantity and duration of the equipment can be scaled from the throughput of work and capacity of equipment. Hence, the primary imperative of the contribution is the equipment capacity –  based on which, construction equipment may be classified into the following types:

Performing Equipment

Performing equipment is a machine with a characteristic productivity – that is, output for unit operating time. Concrete drum mixers, excavators, tripod winch rigs are examples. More often than not, performing equipment is countable in numbers, so that contribution can be measured in hours. The operating time of the machine can be expressed as a proportion [kQ] of the geometry throughput of work [Q] and [1/k] being the productivity. The number of machines required is given by [kQ/D] where [D] is the duration of work. Work duration is governed by the equipment with the lowest productivity and by limitations of the work-front.

When two or more unit machines are combined, their respective performance are regarded as mutually independent. If two or more machines produce a combined tandem productivity, such as drilling rigs and drilling tools, the tandem operation of the machines is considered as an activity by itself of duration throughput.

During a combination of two or more machines, due to the difference in the productivities of each machine, like for example equipment A is 1.5 times faster than B,  there could be three possible strategies:

1. Scaled Mobilization Strategy: Number of units deployed is inversely proportional to the productivity; Three units of B are mobilized for every two units of A.
2. Over-mobilization Strategy: Faster machines work slower than capacity or with intermittent idling;  One unit each of A and B are mobilized, permitting A to be 33.3% idle. [This relationship is created by specifying the float strategy of workgroup itself as stand-by, instead of scalable.]
3. Split-mobilization Strategy: Faster machines are demobilized intermittently; say One unit each of A and B are mobilized in three segments, but A is demobilized after 66.7% of each segment. [This relationship is created during work front mobilization.]

Scaled mobilization is the preferential strategy, as there is no idling or remobilization cost. However, sometimes completely scaling may not be possible due to low work volume, confined workspace, longer duration assigned or high mobilization cost. Besides, it should be noted that if/as the machine cannot be mobilized in fractional units, there is a non-zero idle time.

Modular Equipment

Modular equipment, such as shuttering, barricades, traffic cones, tremie pipes or scaffolds provide characteristic coverage (geometry per unit equipment). Although the geometric measurement of equipment is given by the coverage, the duration for which the equipment is required in service is governed by factors other than itself.

When the duration is defined by a fixed specification of work (such as 24 hours stripping time for column concrete), we shall call the duration of each task or take-off as task duration. Service duration is expressed as a proportion [kQd] of geometric throughput [Q] and task duration [d] where [1/k] is the coverage. The number of modules required is given by [kQd/D] where [D] is the duration of the entire work. Thus, ten columns could be cast (at equal cost) in ten days with one module, or in one day with ten modules. Note that work duration cannot be reduced below task duration, by any incremental mobilization and also that any partial mobilization increases duration, at equal cost.  More often than not, task duration coincides with reporting interval, that is one day, so that contribution of modular equipment can be measured in geometry units of the equipment.

On the other hand, when the duration of coverage is given by a supported work (such as in case of traffic diversion), the duration is not fixed. Consequently, scaling of the coverage is dependant on the scaling of supported work. The coverage should hence be considered as an integral part of the supported work, in the same workgroup and quantified by related takeoff. This enables to scale the quantity of coverage based on the quantity of supported work, and maintain operating time as work duration. It should be noted that similar to the case of works with task duration, any partial mobilization results in an increase of total duration, at equal cost. This is because if the total duration [D] should remain the same, in say [N] equal segments of partial mobilization, then it should be possible to complete each segment in a duration of [D/N]. However, due to scalability of the supported work, it should also then, be possible to scale the entire work to be completed in a duration of [D/N]. Therefore, it is necessary to scale the coverage only when the supported activities are not optimized.

Modular equipment can involve step-wise complementary relations. For example in shuttering, length of runners depends on the type of sheets and the number of props depend on type of runners. Hence runner-sheet couple and prop-runner couple may be modelled as constituent nominal throughput procedures. The workforce required for installation and dismantling of modular equipment should be considered along with quantification of equipment.

Cases of Plant Scenario

Characteristics of builders equipment, other than capacity, that determine its contribution to an activity are as follows:

Difficult Scaling

Sometimes, the throughput of work is measured in a unit that is not scaleable to the output of an otherwise productive or modular equipment. For example, the effort of a cutting machine is not directly related to the tonnage or areas of finished shapes of steel plates or ply-boards. Similarly, the capacity of a shuttering prop is measured by the safe working load, whereas shuttering is measured in area. There are three possible approaches for assessing the contribution, in such cases.

1. Auxiliary Quantification: A rigorous approach is to quantify the work additionally in the same unit as the equipment output, that is to measure cutting sections, welding lengths and bending angles. This method is a truly scalable model but makes the quantity survey cumbersome.
2. Abstract Quantification: A contribution-factor that is suitable to the case, is chosen from a given set of abstract contribution factors, that is extensive cutting, sparse welding, dense propping etc. This method is an approximately scalable model.
3. Quantification by Judgement: The third alternative is to specify the productivity by engineering experience or judgement, case by case. This is effected by considering the equipment to be mobilized in unknown quantities during a workgroup. The quantity and operating duration is specified during take-off by experienced quantity surveyors. Any scaling is done manually, as and when required.

In many cases, it is possible to model non-homogeneous equipment as a combination of the three methods above.

Assembly Setup

In the productive model for performing equipment or modules, it was assumed that the setup time of equipment is insignificant compared to operating time. However, certain equipment such as scaffolds or tower cranes, fall protection, require a long sequence of erection and dismantling. Assemblies are considered to be on mobilization for the duration between its respective installation and dismantling. The installation and dismantling is determined by the precedence relationships of other works in the project. For example, suspended floor shuttering is an assembled plant with predefined duration, whereas scaffolding and hoardings are assembled plants without predefined duration. The period of service starts after installation is done. Assemblies are composed using geometry units – positive during assembly and negative during dismantling. The actual sequence of assembly/dismantling, if any is addressed by sub-schedule within the same workgroup [like beam/slab soffit, side shutters, soffit removal, prop removal etc] or partial assembly on different workgroups [like additional segments of scaffolds in height/width]. Most assemblies may be considered as modular equipment; however, they may house a performing equipment as in tower cranes and builders hoist.

Supporting Devices

Machines or coverage units whose output hardly affects work throughput such as mobile crane, kentledge, man-lift, derricks or access ladder, but are required to complete the task shall be called supporting devices. Supporting devices operate for the entire duration of the associated work or a part thereof, usually providing support to another workforce. The productivity or coverage of the device is composed as same as supported workforce. For instance, a man-lift provides access to the worker or mobile crane holds the load until installation. Supporting mobilization may be modelled as a duration activity where the device is on standby. The unit of supported activity should be so chosen that the substitution of the device with an equivalent does not alter its throughput or preferably still when the device does not have a productive. If a device has a substitute of different capacity and if the throughput of supported work is likely to be affected by replacement of the device with a substitute, it is advisable to consider the equipment as an integral module or a productive machinery. It may be noted that an integral module is modelled as an activity integral to a workgroup, whereas supporting device is modelled as an equipment that supports another workforce.

Infrastructure

Equipment that are mobilized during the entire duration of a procedure, one or more workgroups or whole project are classified as infrastructure. These include quality control devices, testing instruments, surveying instruments, PPE, safety devices, power supply, water supply, shelter, storage, cleaning etc. The number or coverage of infrastructure is directly quantified during takeoff using measures of mobilized time, geometric measurement of equipment and ratio of operation time to mobilized time.

Modelling Strategies

The different scenarios in which equipment contribute to construction processes may be summarized (in the order of appearance in the aforesaid discussion) as follows:

1. Tandem Model: Two or more equipment are composed using duration units in a procedure of duration unit. The productivity of the procedure is the combined productivity of all equipment. That is, one hour of procedure X using equipment A and B of unit productivity is equivalent to 0.6 hours when A improves productivity by 25% and B improves productivity by 33%
2. Homogeneous Model: Equipment is composed using duration units in a procedure of geometry unit. By default, all resources are scaleable based on throughput and duration. Alternately, resources may be slowed down (over-mobilized), idled (float) or demobilized (split) based on the relative performance of the machines.
3. Task Module Model: Equipment is composed using a unit of time in a procedure of geometry unit. A task duration requires to be specified in addition to geometry during take-off. This model is now not in use; for unit task duration, an integral module model may be used.
4. Integral Module Model (including task modules with unit task duration): Equipment is composed using a geometry-duration unit in a coverage activity of geometry-unit. The coverage activity is included in the same workgroup as supported work, so that duration of the workgroup is regarded as the duration of coverage and hence the module. The workforce for installation and dismantling the module is included in the composition of coverage.
5. Auxiliary Activity Model: The auxiliary activity (say, cutting) is composed using a secondary geometry (say, area), similar to a regular composition, with equipment (say, cutting machine) as usual in duration units. Auxiliary activity is then specified as an add-on to the main activity ^# (say steel fabrication), with a tentative contribution. During take-off, the area of cutting may be quantified or tentative contribution may be superseded by judgement.
6. Abstract Activity Model: Two or three auxiliary compositions are defined using primary geometry (say, the weight of fabrication) with an abstract contribution for activity/equipment (say, cutting or cutting machine). One of these abstract compositions, (such as extensive or sparse cutting) is selected during take-off. The composition may be adjusted fine using effort factors in take-off.
7. Arbitrary Model (Direct Mobilization) for non-analytic equipment: During take-off, the quantity and duration of auxiliary equipment (say, cutting machine) is specified by judgement, as direct mobilization (without using any composition) in the workgroup (say, structural steel fabrication) using the equipment. To remind the quantity surveyor, the auxiliary equipment may also be composed as stand-by to the main activity, in which case it may be considered as a special-purpose direct mobilization. If a directly-mobilized resource is also included as performing in one or more activity within the same workgroup, the calculated contribution is used to ascertain the operating time.
8. Assembly Model: Equipment in positive geometry units during installation and negative geometry units during dismantling. When a performing machine (such as hoist winch) is housed in an assembly, it is also regarded as mobilized since its first installation. If the machine is included in one or more activity during the installed period (such as lift of masonry), the calculated contribution is used to ascertain the operating time.
9. Abeyance Model: Equipment is composed as stand-by in procedure with duration throughput. Equipment with duration units in the same procedure operates in tandem and equipment on stand-by unit operates during the entire duration of the activity.
10. Direct mobilization of Infrastructure: This is the same as direct mobilization of non-analytic equipment. Quantity and operating duration (if applicable) is specified at take-off in all the workgroups using the equipment. To remind the quantity surveyor, different equipment may be clubbed together as a stand-by activity. If a directly-mobilized resource is also included as performing in one or more activity within the same workgroup, the calculated contribution is used to ascertain the operating time.

The scenarios may be rephrased based on the modelling criteria as follows:

1. Equipment composed as duration in a procedure will be scaled. If the procedure is geometric, scaling is mutually independent. If the procedure is duration, scaling is combined or tandem. {homogeneous or tandem; also abstract machines}
2. Equipment composed of geometry units in a procedure, the procedure requires either a task duration or a corresponding dismantling procedure. Task-duration, if applicable is specified during take-off. {Integral task or assembly; also abstract modules}
3. Equipment composed as stand-by in a procedure with geometry throughput will be directly mobilized during takeoff {Infrastructure or nonanalytic}. Equipment composed as stand-by in a procedure that is included in another procedure, will be directly mobilized
4. Equipment composed as stand-by in a procedure with duration throughput will be mobilized to the duration of procedure {Idle supports}.
5. An activity with geometric throughput composed as stand-by in a procedure, require separate secondary take-off {Auxiliary}
6. An activity of stand-by throughput comprises only of stand-by work-force or add-on material.

Behaviour during multiple definitions

When equipment is directly mobilized over one or more workgroups or as assembly between its respective installation and dismantling, it could also be redundant as a scalable contributor to one or more works in the mobilized period. Equipment contribution calculated based on the throughput and productivity is then, considered as the operating time of equipment in the plant. If the calculated requirement exceeds stationed plant capacity, an additional requirement is mandated; otherwise, the difference is written off as idle time.

Behaviour during Replacement

When one equipment can voluntarily substitute another with a different productivity or coverage, these machines/modules are said to be productivity substitutes (such as excavators in terms of bucket size). Operating time of machine or mobilized quantity of modules, change according to the productivity of the selected substitute. When one equipment must necessarily substitute another due to a specification of work, they are called characteristic substitutes (such as crane capacity or boom length).

When equipment is directly mobilized, substitution does not alter the mobilized time. However, operating time may be altered by the substitution. It does not make any change on mobilized time or operating time in case of idle supports. While making a characteristic substitution of modular units or assemblies the workforce for its erection/dismantling should be comparable to each other. Due to this reason, erection/dismantling of assemblies, or modular units shall be preferred with nominal throughput, so that replacement by a substitute allows for modification of installation workforce.

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