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Constructability Review through Bar Bending Schedule preparation

Summary

  1. Bar bending schedule preparation for cost control
  2. Constructability review through BBS
  3. BBS is not a commodity

There is no way around it – reinforcing steel forms a large part (around 10%) of the total cost of building structures. This is just the cost of material without taking into account the affiliated costs from procurement to fit (logistics, storage, cutting, bending, and installing) of rebar. In India, most activities within the procurement to fit stages are managed by labour which is considerably cheap in India.

We are therefore quick to overlook some of the practices that can help control, if not eliminate, the hidden costs associated with the affiliated activities. This lack of attention extends as far as not knowing the exact quantum of steel built into the structure which can lead to major surprises at later stages in the project. Given the dynamic nature of this industry, we need detailed assessment of steel quantum at various stages of the project. The more emphasis we give to this critical planning and monitoring activity, the better control we will have over the project budget.

We are regularly approached by real estate developers as well as infrastructure contractors regarding bar bending schedule preparation for the projects that they are executing. It is evident from our numerous interactions that the true value of rebar detailing is not completely understood by the stakeholders most impacted by it.

Quantification of rebar can be achieved in one of three ways as have been discussed below.

Thumb rule calculations

As indicated, this method of rebar quantification is quite basic and revolves around accepting the structural consultant’s ball park. This kg/sq.ft. or kg/cum of concrete measures are commonly known as the thumb rule and many in the industry swear by it because it costs absolutely nothing to produce (does it really though?).

While structural consultants guarantee optimization (for cost) of reinforcement in a structure, they have no real incentive to do so. In fact, optimization for complex structures requires additional time and efforts and is therefore quite a disincentive for them. We have come across a number of cases where developers have enlisted our services to assess detailed rebar quantities midway through the project, because actual steel consumption was not in line with the consultant’s ball park.

It is easy to see that this strategy is highly likely to result in cost overrun, irrespective of the scale of the project. Even a small villa or a single tower project is likely to see a budget burst if a thumb rule is “fixed” at the inception of the project, even if the difference for such projects will be small in absolute terms. Owners of small projects have an easy choice to make i.e. ignore a scientific, methodical assessment based on principles of quantity estimation. 

The difference becomes evident when the same thumb rule is applied to larger projects as well. In effect, the thumb rule mechanism often turns out to be the costliest approach.

Next we shall take a look at another approach which is much more detailed and offers a theoretical level of accuracy that is difficult to implement on the ground.

Rebar detailing drawings/shop drawings

Design drawings give an overall representation of the structural elements without detailing the exact number of bars and the way they are to be placed and tied. Moreover, the design drawings often do not provide for the rebar arrangements at construction joints, the location of which is decided ad-hoc during execution. Design drawings must ideally be put through the process of detailing before construction.

This not only produces a system-generated schedule for cutting and bending of bars (therefore deriving the exact quantity of diameter-wise bars) but also the drawings which the site workers can actually utilise to place and fix rebar. Rebar detailing can be performed using software platform. CADS RC is one of popular and globally accepted software’s The process ensures 100% compliance to design intent as well as codal requirements while detailing rebar. Aside from compliance, this process also leads to a number of RFIs for the architect / designer before the drawing is issued to the project site.

These RFIs may be regarding clashes observed while attempting to create the shop drawings or some missing information without which the execution guideline is incomplete. In other words, it highlights constructability issues which would otherwise invariably result in workers either idling at site or proceeding without complete information. Effectively, the time and money spent in doing a good job during the detailing stage saves time or money or both many times over at the execution stage (by either avoiding idling or rework due to errors).

That being said, rebar detailing as an engineering process costs money in the form of engineer’s time, software platform and the specific understanding of detailing principles. An experienced site engineer may be fairly conversant with producing bar bending schedules for individual slabs, beams, columns, walls as and when these elements approach the construction stage. However, rebar shop drawings and bar bending schedule preparation is a very specific skillset that they do not usually possess.

The full benefit of this process can be leveraged only if the cutting and bending of reinforcement bars is done in an off-site facility (cut and bend factory). As with any factory-manufactured-site-assembled process, this too is not the most amenable to changes in design. Given our peculiar inability to finalize designs till (and sometimes even after) construction, rebar detailing has not seen rapid adoption in India, in spite of having evident commercial benefits. Here, we haven’t even considered the possibility of optimizing the schedule by using cut pieces of rebar, which is possible if the detailing and shop drawings are prepared in advance for many pours at a time.

Another reason of non-adoption is the siloed specialization approach that we follow in the industry. Individual functions have a mandate of saving cost but only within their own domains. The design team’s “lookout” is to issue usable drawings to the project team as early as possible and at the lowest cost. Similarly the “lookout” of the execution team is to construct using minimum resources available to them (drawings and their best understanding of them). Further, the procurement teams do exceedingly well at applying commercial pressure on agencies that are affiliated with the project. When we try to showcase our rebar detailing services as a cost improvement initiative, our customers are often unclear whether this should be taken up by the design team or the execution team.

There is an element of change management here. Traditional practices are not easy to override either because they worked in the old context or because changing the process requires a complete paradigm shift for the organization. To my mind, while the industry’s ecosystem evolves to look at this as a best practice, there is a third option which can be looked upon as the middle path which is talked about below.

Value added bar bending schedule preparation

There is an optimal form of processing of drawings before moving to execution. I say processing of drawings because this is not meant to be just preparation of bar bending schedules for reinforcement which has become highly commoditized. As mentioned previously, any site engineer or quantity surveyor can be enlisted for bar bending schedule preparation. It can also be outsourced to rebar detailing companies that offer this service. However, with extensive commercial pressure put on this process, the output is bound to be an operational translation of design into bar bending schedules without any assessment of constructability.

The optimal value would be derived by performing a review that produces BBS and also helps generate RFIs which would act as an early warning in terms of constructability. Of course, without preparing shop drawings, it would not be possible to detect potential clashes within and across elements. On the other hand, the expenditure incurred in performing this review would be somewhere moderate.

In summary, we need construction managers to wear the project manager’s hat and look at a comprehensive picture of the project cost. This would enable them to identify the benefit of the value-added BBS and implement it on their projects.

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