www.sefindia.org

[N. Prabhakar]

Dear Sefians,

Preparation of Reinforcement Detail Drawings

The new code has dealt in detail about the structural analysis of Tall Buildings, ie. more on the theoretical aspect.  Very little is written about the preparation of reinforcement detail drawings required for construction of thesel buildings.

The end product of any structural design is the detail drawing which is sent to site for execution.  Drawing is the language of engineers.  It has to be drawn clearly showing all the necessary details so that they are all understood by the site engineer and executed accordingly.  But, unfortunately, not much is written in the new code about drawing preparation.  By this glaring omission, the new code is incomplete by all means and it is not serving its full purpose.

Regarding the reinforcement details in the drawings for buildings,  the common practice amongst many structural consultants in the country  is to prepare the details in a schedule table (Please see the attached cartoon I had drawn in the Indian Concrete Journal few years ago) giving details of top and bottom bars  and spacing of stirrups, etc. without showing reinforcement details in plan  or elevation and sections of the structural members as required. The dangers of this method are that firstly, many site engineers and steel bar benders or fixers do not know much about design aspect of the building and codal requirements for detailing.  Secondly, in the common practice of Structural Engineer visiting the site to check the reinforcement that is placed before placing concrete,   time normally devoted by the structural engineer to check it  is not sufficient  to do justice to the checking work.  It is not  done in a systematic manner, which may lead to inadequate bar anchorages at supports, under reinforcement at places, curtailment and lapping bars at wrong locations, etc.  This method of detailing falls very much short of the standard method of detailing of reinforced concrete members as required by our own Indian codes and other international codes.

From the list of important  towns and cities given in IS: 1893, it is noticed that large number of big cities like Kolkata, Mumbai, Delhi, Chennai,  Ahmedabad, Chandigarh, etc. and in north and north-east of India, fall under the seismic zones of III and above.  This indicates a broad spectrum of tall structures in the country fall under the category that need to be designed and detailed as Earthquake Resistant  requiring ductile detailing as per IS: 13920.  Because of stringent  rules specified in IS: 13920, the current practice of showing reinforcement details of structural members in a schedule table, as mentioned earlier, will not work.  The drawings have to be total with details of each and every bar shown in plan/elevation and sections with bar bending schedule.
     
The best ways to prepare reinforcement details on drawings are done by showing  reinforcement bars for floor slabs, roofs, foundations, and  beams  both in plan and sections, and for columns and walls  reinforcement bars  shown in  both elevations and sections.  Every reinforcement bar on the drawing  of one type/shape is given a bar mark with a bar bending schedule for all the bars on the drawing.  By this procedure, the drawings show all the details , without  leaving any option for the site engineer to deviate otherwise.  Lapping of bars have to be provided only at locations shown on the drawing.  The site will have to strictly adhere to the detail drawings.   The above procedure of preparing detail drawings is followed in UK  as I know.

Considering the above points, preparation of details drawings with bar bending schedule  as mentioned above, and proof-checking of them by external structural auditors  should be mandatory for all tall buildings.  These points have to be made clear in the new code.

With best wishes,

N. Prabhakar, BE CEng(I) MIStructE (UK) MIE (Ind)
Chartered Structural Engineer
Vasai (E), Pin 401 208