Preparing quantity for steel bars in every construction is a laborious work. It needs a skilled and experienced quantity surveyor, estimators or engineers to create one. Every section, sub-section, and parts of the plans with reinforcing bars should be included as missing one would be costly and ruinous in the long run.
Determining the reinforcing steel bars quantity for every section of the construction project is not the only thing that construction managers have to consider but also anticipating the actual needed on site. Some projects may need more than what is on the plan. Like for example, a column needs to be lengthened further due to embankments to attain the needed elevation and the project timeline is on its dead ends. Although this should be addressed by change orders, that is where skilled and efficient construction managers come in as they should be able to anticipate it first hand.
Mat Foundation
Considering a large amount on quantities for matt foundations, it is required to a lot ample amount of wastage for preparing steel bar quantity. By definition, mat or raft foundation transmit the load from the column, walls and the weight of the structure above it through the entire ground area. Being the stress distributor to the ground, it has to be designed by engineers to act as a slab and foundation. Thus, it is essentially a thickened slab but acts as the main foundation commonly ranging from 0.5 meters to 1.00 meter used in condominiums, buildings and treatment plants.
The following should help engineers on points to consider in determining the reinforcing bar quantity for matt foundation.
• Anticipate if it is required to use reinforcing bars for formworks and scaffolds for support and stability.
• Check the plans for the bottom and top bars’ spacing, length, and width. Consider the hooks and bends on each both sides as designed and add it to the total length.
• Inspect the plan for designed splicing lengths and add it to the total length. Splicing is the linking of two different reinforcing bars to attain the required total length.
• Examine the plan for other bars needed say, chair bar. Due to the lengthy height of mat foundation, designers use chair bars as concrete spacers to separate top bars and bottom bars.
Below is a sample calculation for mat foundation rebar quantity.
t =1.00 m
W = 100 m
L = 100 m
S = 0.3 m
Commercial length per reinforcing bars = 12 l.m. , (to be deducted with splicing length @ 0.8m)
Top bars
Transverse quantity = W/s= 100/0,3=333.3,say 334 pcs100 l.m.
100 + t + t = 100 + 1 + 1 = 102 l.m., 334 pcs (Additional length for hooks)
334 X 102 = 34,068 total linear meter
(Total linear meter)/(commercial length-splicing)= 34,068/(12-0.8)=3,042 pcs-12meter length reinforcing bars
Longitudinal quantity = L/s= 100/0,3=333.3,say 334 pcs100 l.m.
100 + t + t = 100 + 1 + 1 = 102 l.m., 334 pcs (Additional length for hooks)
334 X 102 = 34,068 total linear meter
(Total linear meter)/(commercial length-splicing)= 34,068/(12-0.8)=3,042 pcs-12meter length reinforcing bars
Total Quantity for TOP BARS
Transverse QTY + Longitudinal QTY = 3,042 + 3,042 = 6,084 pcs – 12mL reinforcing bars
Calculating bottom bars is the same as the top bars. It is then the engineer’s discretion as to how much wastage factor will he add.
Columns
Columns are pillars used in buildings to transmit loads from upper ground to lower ground or to the foundation. Being the bridge from level to level structures, quantifying the reinforcing bars for column has to be prepared thoroughly.
Here is a guide on determining the quantity for reinforcing bars for column.
• Scrutinize the plan for reinforcing bars diameter, grade, and length per floor level. Do not forget to add the additional length from the foundation to the desired floor level lines should there be a need for embankments.
• Add the bend’s length per end to the initial level-to-level length. This is applicable for main bars and extra bars.
• For the ties, consider the bends, seismic hooks and total perimeter (minus the offsets) of the column. Most quantity surveyors and estimators count the ties per column as some column may be longer than the other column.
• Check the column should if there are additional bars required and add the required percentage for wastage.
Sample calculation:
W = 0.4 m
L = 1.0 m
H = 3.0 m
Main bar pcs. = 16
Total number of ties = 20 pcs.
Commercial length of reinforcing bars = 8.0 l.m.
Main bar
Length = H + hooks per ends = 3.0 m + 0.4 + 0.4 = 3.8 m (assumed length for hooks)
Total Length for main bar = L x no. of pieces = 3.8 x 16 = 60.8 l.m.
Total number of pieces = (Total Length)/(Commercial Length)= 60.8/8=7.6,say 8 pcs-8meter Length rebar
Reinforcing Ties
Length per ties is the perimeter, P = 2W + 2L = 2(0.4) + 2(1.0) = 2.8 m
Total length = Length x No. of ties = 2.8 x 20 = 56 linear meter
Total number of pieces = (Total length)/(Commercial length) 56/8=7 pieces-8 meter length rebars
Slab on Grade
Slab-on-grade is the floor that is earth exposed. Being exposed to moisture and direct seismic activity due to the ground, there should be no room for missed reinforcing bars not included in the total quantity. Quantity surveyors should be very keen on estimating reinforcing bars for slab on grade as missing even a single bar would be disastrous.
• The following will help engineers on determining the quantity of reinforcing bars in slab on grade.
Check the area of the slab on grade and scrutinize the plan carefully.
• Some area, especially near the stairs, may need dowels. Add the required dowels and its length as per designed plan.
• Add the hooks and splicing (as discussed above) to the total length. Take note that splicing lengths for matt foundation may differ to the splicing of slab on grade.
• Some slab on grade may be thickened near walls or door openings. Inspect should there be variations to seismic hooks and bends.
• Add the required percentage wastage to the total computed quantities.
Sample calculation.
Temperature bars
L = 10.0 m
W = 10.0 m
t = 0.6 m
s = 0.25 m
Commercial length of reinforcing bars = 12.0 l.m.
Transverse bars (same calculation for longitudinal bars because this is a square SOG)
No. of pieces = W/s= 10/0.25= 40 pieces-12 linear meter
Total number of pieces = Transverse + Longitudinal = 40 + 40 = 80 pcs – 12 linear meter reinforcing bars
Note: 12 linear meters commercial length is used instead of 10 to cater the seismic hooks per ends.
Read also: Structural Engineering And Its Importance In Designing A Building
Suspended Slab
Suspended slabs are technically slabs that are elevated. These may be in a form of canopy, terrace or the entire floor itself. Computing the required reinforcing bars may be tricky as there are two forms of slabs, one-way slab, and two-way slab. Further, there are areas that should be deducted like the stair openings or duct-work line openings. Thus, only experienced quantity surveyors should be the one doing these calculations.
The following guide should help you determine the required reinforcing bars for suspended slabs.
• Determine the tops bars spacing as per transverse and longitudinal bars.
• Add the hooks and splicing to the length as co mputed above.
• Any openings like stairs opening should be deducted to the calculations.
•Examine the plan if there are bends due to change in floor elevations. Suspended slabs that are associated with water are essentially lowered at least an inch from the rest of the entire floor like comfort rooms and terraces. Having being lowered, there may also be changes to the structural details of the slabs. These bends should be included in the calculations of the reinforcing bars.
• Add the needed dowels for masonry works like walls and concrete railings on the total quantity.
• Multiply the required percentage provision for the wastage factor.
Quantity calculation for suspended slab is technically the same with the slab-on-grade and mat foundation. Please see the calculations on top.
Quantifying and determining the reinforcing bars for any construction projects should be done only by competitive quantity surveyors and engineers. Missing one would mean missing the actual needed monetary needs required to finish the project on time. Business as it is, one needs to finish the project on time to get away with any liquidation damages. Further, if construction managers finish the project on time, it would mean savings which are good for the construction project. Thus, it is very essential to do it right thing the first time before any further damages happen.
