BIM News

4D helps ‘see’ risks in projects


Having a comprehensive understanding of risks is critically important in construction projects.

However, traditional methods of assessing the risks do not enable them to be visualised. Project managers traditionally detail risk in reports, supported by inventories, sketches and (usually) Gantt Charts. With incomplete information, decision making is compromised.

The arrival of 4D models, to some extent addresses, the problem. However, evidence suggests that the potential of 4D is not being fully utilised.

In creating construction schedules, a planner follows a basic three-stage process:
Design interpretation: the planner uses the design information to illustrate and visualise the work. Traditionally, this is based on 2D drawings supplemented by sketches and the process produces a list of the activities required to construct the project (sometimes referred to as work breakdown structure, WBS);
Resource allocation: the next stage is to define the resources required to achieve the defined tasks in terms of manpower, machinery and materials and based on those resources, allocate durations to the key activities; and
Scheduling: the final stage is to determine the dependency between activities and arrange them in a logical way to optimise the construction sequence.

The common output of the above process is a critical path method (CPM), applied to calculate the longest path of planned activities over the project duration, which in turn determines the earliest/latest each activity can start/finish. In simple terms, this process identifies which activities make up the ‘longest path’ (also known as critical path) and those which can be delayed without extending the project duration (also known as ‘total float’).

To assist the planners, software tools are available based on two primary methodologies: Gantt Chart, where activities are illustrated by horizontal lines representing tasks over a specified duration, and Line of Balance, where specific activities are represented as lines, the slope of which represents the rate at which the task can be executed.

Gantt Charts are very useful to represent the high-level principal activities of a complex project, but can be very difficult to understand when applied to visualise large numbers of tasks or even smaller numbers with a high number of dependencies.

Line of Balance, which includes the concept of location as well as time, is an excellent method of visualising the flow of project activity. While Line of Balance has many advantages
over Gantt planning at a detailed level, it is a less understood methodology and software tools are not yet commonplace.

Common 4D applications 

4D combines a 3D model with the construction schedule, producing a dynamic representation of key stages in the construction process in both time and space. This enables the construction sequence to be more easily communicated than by the simple use of charts and sketches.

The use of 4D has grown rapidly in recent years as 3D models become more widely available. It is therefore not surprising that 4D CAD is increasingly being used to support construction planning. Adding ‘time’ facilitates the natural progression from 3D to 4D.

It is already well understood that linking elements from a 3D model to activities in planning tools can create a space and time (4D) model. This helps the planner visually link task to workplace thereby facilitating the creation of a logical construction sequence.

However, this process is only using the 3D geometric model to review the planners programme rather than assist with the process from the beginning.

Most projects involve fragmented practices. Each contractor/subcontractor develops its own discipline specific programmes, which are not easily co-ordinated hence giving rise to potential conflicts (logistics and space), during construction.

On a complex project, many planners (incorporating multiple disciplines) can be involved in the process. Being able to work in a collaborative environment from the early project phases is the only real solution.

This concept is less widely understood and yet brings with it the real power of 4D – the ability to integrate 4D into the entire planning process. A summary of the current 4D model approaches and their limitations will provide a better understanding of this concept.

Current 4D planning methods 

A 4D model is developed by linking 3D model elements (product breakdown structure – PBS) to construction activities (WBS), which are created using the manual or the semi-automated approach.

In the manual process, the 3D model and programme are developed independently and then the PBS and WBS are manually linked via a 4D tool (although some CAD systems offer this tool as an integral element of their software). This provides a ‘review only’ facility and although useful, it has limited value generally and none at all in the collaborative planning environment.

In the semi-automated process, the PBS and WBS are linked within a database (usually to some pre-defined classification system). To produce the simulation, it is still necessary to have a 3D model and a pre-defined programme, but the link creation is automated.

While this is an improvement compared to manual linking, the same limitations apply. Clearly, to move to a true 4D construction planning environment another solution is required.

Collaborative 4D planning

If the multidisciplinary planners can work in a shared environment, then it is possible to plan collaboratively – that is, work not only on their own programmes but also interactively in relative real time. In this environment, it is possible to co-ordinate programmes much as the designers would co-ordinate trades in a combined 3D model.

The critical factor is the 3D model itself, because if all planners use this as the starting point, then all planning considerations will originate from a common data environment.

As the 3D model is the source of PBS, having the correct file format and information is critical – these can be provided by a building information model (BIM).

Given a 3D model created in the BIM environment, PBS information of a building such as doors, windows, beams and columns, can be generated easily from its internal data specifications.
Furthermore, the planner can interrogate the BIM and its elements to obtain additional information such as materials, dimensions, compositions and zoned data to facilitate the creation of the WBS.

Another factor is that if BIM has been coordinated, then spatial conflict and collisions have already been identified and rectified, enabling the planners to concentrate fully on coordinating their sequence.

In this shared environment, planners can work collaboratively at three levels:
• High: social interaction is facilitated amongst the planners by an improved communication network;
• Mid: system interaction allows planners to interact with the BIM to analyse the PBS and interrogate elements to facilitate WBS definition; and
• Low: system broadcasting/access to updated simulations to planners and other stakeholders to help eliminate conflicts and communicate construction process.


The creation and application of 4D process and technology is not being fully realised in many applications. As a result, most applications simply use the process to produce visualisations
and review programmes.

However, using BIM as the starting point, 4D can be used as an integral part of the planning process. By incorporating multiple trades, benefits that traditional approaches miss, will be realised. These benefits include: project visualisation – as an aid to communication at all project phases; schedule verification – single and multiple trades; planned Vs actual simulations (requires updating of model during construction); elimination of conflicts; ‘What if’ scenario planning; temporary works – analysis of workflow; site logistics and layout; and training tool for planners.