Reinforcement detail drawing & Preparation of bar bending schedule - Necessity for every builder

Transcription

1 Reinforcement detail drawing & Preparation of bar bending schedule - Necessity for every builder Reji Zachariah., B.Tech, M.B.A., M.S Chief Executive S&R Consultants, Kochi, Kerala, India, (Mobile , rejizac2004@gmail.com) INTRODUCTION Preparation of highly detailed reinforcement drawing and bar bending schedule is an essential requirement in the construction field, all over the world except in India!! With this we can achieve a high level of quality control at site and will be advantageous in various aspects of construction. Unlike the practice of doing reinforcement cutting and bending in a separate workshop, as in UK and USA, based on the bar bending schedule prepared by engineers in the design office, the practice in India has been doing reinforcement cutting and bending at the site only which is based on the bar bending schedule prepared at site by site engineers or supervisors. In the present construction industry, the preparation of the bar bending schedule for the reinforcement work is by site engineers or the supervisors which is being done in India is becoming a bad practice. Due to lack of quality supervisors and unskilled labours in the site, our quality control in bar bending and laying is not up to the standard. Also proper cutting of bars is not done which results in cutting wastage.the fact is that the site engineers or the supervisors are not fully aware of all the design considerations that was made for the project, regarding the anchoring, curtailment and lapping of reinforcement bars and their positions, particularly in respect of ductile detailing for earthquake-resistant structures. So these schedules are to be prepared by the engineers in the design office only. In the use of high grade concrete and steel, quality plays a major role. In order to prepare the bar bending schedule, the design office can have the information from site about the stock lists for bar diameters and their lengths available at the site. Fe 500 bars cannot be bend acutely as we bend the Fe 415 bars. Also there are rebending considerations to be followed. Proper scheduling, & bending with a mechanized system ensures this required quality level. Let us focus on the detailed drawing to be done for reinforcement detailing (rebar) in reinforced cement concrete construction. Here rebars are drawn to 5mm accuracy as per the detailing rules in IS or British codes. The cost of a rebar shop drawing production

2 can be in the range of Rs 3.5 to Rs 5 per square feet considering the quality standards maintained, whereas the advantages can be Rs 80- Rs 100/- as shown below. Thus 1 lakh square feet structure shop drawing preparation can save you about Rs 80 lakh. It is sad to know that every builder is loosing this amount in most of his projects. This accuracy gives us, what you see is what you get format (WYS WYG), or in other words photolike drawings of the reinforcement layout. Every bar is marked with different number, at the same time providing the same number for the same type of bars. A schedule is prepared with all types of bars counted and sorted. So details of every bar can be seen in the schedule. This is called bar bending schedule system (BBS). This schedule can be used to cut and bend the bars to the required shape in a cut and bend factory and also in a site. As the steel can be cut and can be brought to the site in a ready made form, Author would like to call it ready mix steel as like our favorite ready mix concrete. Let us go through the different aspects of the preparation of BBS. BAR BENDING AND SCHEDULING (BBS) Part drawing of slab bottom reinforcement details is shown below. As more details are put in the drawing it is ideal to show top and bottom bar separately in the drawings. Every bar is also marked with a number called bar mark (Exambles-1, 2, 8 etc). This bar mark is the identifier for the bar in the BBS. Shape code defines the shape of the bar. For example shape code 20 and shape code 37 (As per British code) or as per the equivalent IS code respectively is shown below. The shape code 20(As per British code) or 3A (IS: ) means that it is a straight bar. Bar mark -1, 2, 8

3 SLAB BOTTOM BAR Barmark-48, 50 ` SLAB TOP BAR B A SHAPE CODE 38 C

4 BEAM LAYOUT BEAM DETAILS

5 The only difference between the ordinary drawing and the detailed drawing with bar bending schedule is that in the latter case all the bars will be tagged with bar mark as per the legends. For example: In the former case we only provide 2-#16 which gives only the details of the number and diameter of the bar. In the latter case, 2-# represents all the details including the bar mark (312). A typical bar bending schedule is shown below. Bar Bar Tot Shape Type Size Mem No Len(Bar)MM Mark No Bars Code A B C 1 T T T T T T T T T T T T T T T BAR BENDING SCHEDULE Length of the bars is measured from the drawing and can be entered in a schedule. Thus the number of the bars as per length given in the schedule can be counted and sorted from the drawing. At a glance, the challenge that has been put forward is that detailers need to show the minute details of the drawing as explained below. For example: Bend length at the end of the reinforcement bar (beam) has to be calculated based on how much development length is available in the column itself. If the width of the column is high in the direction of the beam, the beam bar length may not be required to be bend at all. Let us go through the advantages with bar bending schedules.

6 ADVANTAGES WITH BAR BENDING SCHEDULES (BBS) 1. Scheduling and proper bending is strongly recommended for Fe 500. Fe 500 saves 10% compared to Fe 415 steel used presently. 2. Cutting and bending in a cut and bend factory avoids the wastage completely (5-7 %). With BBS, bars can be cut with planning to reduce the wastage in a site with even the present setup. 3. There is a general tendency to group slabs and beams in the usual design methods. In BBS, it is a must to detail every member separately to account finer geometry and different forces coming on the structure in the modern design methods. Instead of grouping members as all members are detailed separately gives reduction in steel as every member is individually reinforced to resist what it has to. Finer detailing saves about 5 to 15% steel. 4. Better quality control at site. 5. Better estimation of steel. 6. Real time estimation data, with the design. 7. Better control on stock of steel actually required. 8. Theft and pilferage of steel can be reduced. 9. Economical order quantity for better project management 10. Bench marking quantity and quality requirements. 11. Optimize your design based on the quantity of steel. 12. Steel bending and cutting can commence even before the form work is done. 13. Steel bending can be done at a separate site, marked and then can be assembled at site, if there is space limitations. 14. Project time can be reduced as the bars can be cut and bend before form work is done. 15. What you see in the drawing is what you get at the site. 16. With a quality data set, other management softwares (ERP systems) can work on it. 17. A paper less office concept in the construction industry and associated advantages. 18. Total length of bars calculated using Engineering formula, leaves nothing to approximation. 19. Mechanization of bending and cutting is possible. (Cut and bend systems) Reduces labour and time but increases the reliability. 20. As the works gets organized, smaller contractors can work on the project at lesser rate.

7 COST ANALYSIS Consider 1 lakh sqft building with 15 stories, with 5kg/sqft steel usage. Cost of steel considered at Rs 40 per kg. With taxes and bar bending added, this can be considered Rs 50 per Kg. 1. With BBS the bending quality of Fe500 can be ensured. So that one can ensure 10% saving with Fe 500 compared to Fe 415, cost saving is 0.5 kg steel results in Rs 25 per sqft. 2. Reduce cutting wastage, Considering 5% wastage reduction due to planning in cut and bend operations 5kg/sqft x 0.05 = 0.25 kg = Rs 12.5 per sqft 3. Better quality controls at site due to photolike drawings add Rs 2 per sqft. 4. Better Estimation of steel add Rs 2 per sqft, for an error margin of 2% 5. Detailing without grouping saves 5% reduction in steel. 5kg/sqft x 0.05 = 0.25 kg = Rs 12.5 per sqft 6. Project time can be reduced as the bars can be cut and bend even before the formwork is done. Time savings due to automation considered is two days for every floor which gives about one month total our 15 storied structure. Saving in interest cost for one month can be calculated as follows. Consider 1 month reduction in construction period of 12 months Cost of building 1 lakh x Rs 2500 per sqft = 25 crores. One month per annum = 25 lakhs which is Rs 25 per sqft 7. Savings in labour in steel bending to the tune of Rs 2 per kg, which is Rs 10 per sqft. 8. As the work gets organized, smaller contractors can work on the project at lesser rate which is not considered here with similar advantages in other items also.. Adding the savings side together it is Rs 89 per sqft. PROJCTS DONE WITH BBS This method was partially followed in the following builder s projects which were found to be practical in our construction methods and sites.

8 Holy Faith, Ernakulam Haritha, Thrissur Skyline, Ernakulam Spring Flower, Kakkanadu CONCLUSION As explained reinforcement detail drawing improves the total quality control which is badly required. It can help in redirecting the construction industry to an overall organized system with higher efficiency and reduction in cost per square feet. Many ERP packages are not working in the construction industry due to absence of quality data like steel quantity which is very difficult to calculate accurately in the manual methods. Preparing BBS is an answer to this issue. In other parts of world, they are in a position to locate every bar with manufacture s identity, engineer s and contractor s identity. This is achieved as BBS database is connected with quality control and suppliers details. Lot of unnecessary workforce like, different levels of supervisory mason, contractor and engineer s time can be replaced with this system, thus enabling smaller contractors to take bigger works. Engineer s time is actually getting shifted from site to the designer s table where all the decisions with respect to the reinforcement is taken. This reduces the builders cost as a high level of drawing automation is possible now a days. A good level of detailing requires that the design to be frozen after the initial planning and designing to an accuracy of 5mm. It is practically very difficult to change the drawings and bbs once it is completed, with the day to day changes occurring from our style of managing projects. As in this type of projects, it is a good practice to complete the entire project drawings in one go so that structural cost of the project can be estimated to a high degree of accuracy. This method is a green technology and adds to carbon credit saved.