Table of Contents
Steel Reinforcement
Steel is the alloy of iron and carbon as a main constituent and other constituent such as aluminum, chromium, nickel, etc are also used depending upon the quality needed. Steel is extensively used in different field such as construction, automobiles, military, and machinery appliances. Here our main focus is on steel reinforcement used in building.
In building construction also, steel is extensively used in constructing steel or concrete structure. I think it is not needed to talk about the use of steel in steel structure because all structural members are made up of steel. Let’s talk about concrete structure. Steel is used as reinforcement in structural member along with concrete for tensile strength. In many place, steel is used for form-work and scaffolding works. Steel is used for making entrance gate, main-door/shutter-door, railing, spiral stair and grill-items.
Types of Steel:
- Mild steel Bar:
Surface of the mild steel bar plain and round in shape and used in the special purpose such as dowels bars at the expansion joints.
| Ultimate strength | Yield strength | Elongation | |
| Up to 20 mm | 410 | 250 | 23% |
| 20-50mm | 410 | 240 | 23% |
2. Deformed bar
a. TMT ( Thermo-Mechanically Treated bars)
TMT are hot treated bars that are high in strength used in RCC works. It is the newest induction within the MS steel bars group with the superior’s properties like ductility, strength, welding ability, bending ability and top quality standard in international levels.
b. HYSD ( High Yield Strength Deformed Bars)
HYSD are cold twisted steel bar with lugs, ribs, projection or deformation on the surface. It is extensively and majorly used for reinforcement purposes in the construction.
Different Grade of deformed bar:
Fe 415 (1.25) 14.5%
Due to higher uniform elongation, it can be used in RCC works as well as in earthquake prone zone.
Fe500 (1.08) 12%
High corrosion resistance, excellent bendability and great resistance on dynamics loading
Fe 550 (1.06) 10%
Utilized in RCC construction exposed to coastal, marine or underground condition. *economically infeasible for the common man uses.
Fe600 (1.1) 10%
Provide better toughness and more strength; used for large construction. *economically infeasible for the common man uses.
Fe400D (1.12) 18%
Fe415S (1.25) 20%
Fe500D (1.1) 16%
Fe500S (1.25) 18%
The number in the bracket is the ratio of ultimate strength to the yield strength. The number after Fe indicated the yield strength of that steel bar in N/mm2. The percentage mentioned indicates the elongation percentage of the steel bar.
D and S are the label for different types and qualities of steel bar. And the basic difference of having D and S is elongation and the ratio of ultimate strength to yield strength.
Ribs and Lugs:
Have you ever noticed deformed bar? If yes, then you have probably seen two longitudinal projection and numerous inclined ring like projection at regular intervals. Lugs on TMT bar strengthen the ribbing pattern by its angular spacing on the surface of bars. It provides higher interlocking of the bond formed between materials. It provides enhance anchorage and gripping effect in concrete structure so as to hold the structure in place and avoid slippage. It helps to create better grip so it can bond with concrete without any slippage.
What does Fe 500D stands for?
The Fe indicates iron out of which deformed bar is made. The number 500 indicates the minimum yield strength of the bar in N/mm2. And the letter D stands for ductility which means the steel bar has higher percentage of elongation.
The area of steel reinforcement required depends upon the
- Type of structure and structural member
- Grade of steel
- Load and loading pattern on the structural members
- Grade of concrete
Grade selection of steel reinforcement:
After having the overview on the different grade of steel, most probable you got the idea about it. Let’s further illustrate. Due to lower strength, mild steel (fe 250) is not used in structural member of the building. Also it has low bonding strength due to its plane surface area. But the good point is, mild steel is more ductile (ie. elongation 23%). The steel reinforcement normally used in the building construction is deformed bar and the grade is Fe 415. In design and construction of RCC building in seismic Zones III and above, it gives better result if steel with elongation more than 14.5% is used. And if we look for the elongation of Fe500 and above, we found elongation less than this (less ductile). But if the construction site is in seismic Zone I and II, then it will be better to go for Fe 500 and above.
You may have question that why only Fe 415, we can go for Fe 500D, Fe 500S or Fe 550D in seismic zone III and above, can’t we? Yes you can because they have more elongation than 14.5% but….
Since the steel with D and S are prepared with superior qualities of raw material and with high technology, these steel reinforcements are more expensive, limited manufacturing industry and un-available for all location.
Steel reinforcement in different structural member of building
The minimum number of steel reinforcement required for the rectangular column is 4, each at the corner and circular column is 6. The minimum and maximum percentage of steel reinforcement used in column is 0.8% and 6% of cross-sectional area. But it is not recommended to use greater than 4% for non-lapping zone. Structural engineers always suggest to make the size of column smaller by optimum use of steel (nearly 4%) but the least lateral dimension should not be less than 300 mm. There are different plus point of this, 1. Decrease the dead load of the building, 2. Gives more space for use 3. Save cost in formwork and finishing works.
In column of residential building, the minimum diameter of steel bar used is 12 and 16mm. And for commercial building, the diameter of steel bars used are greater than 20mm.
The minimum number of steel reinforcement required for beam is 4, each at the corner. The minimum percentage of steel reinforcement used in beam is 0.85/fy for tensile strength and maximum percentage is 4% of cross-sectional area. The least lateral dimension (width) of beam should not be less than 200 mm. The diameter of longitudinal reinforcement used in beam should not be less than 12 mm.
The amount of reinforcement in slab is indicated with the diameter of steel bar and center to center spacing. The minimum percentage of steel for slab is 0.12% for HYSD steel and 0.15% for mild steel (Fe 250) and the maximum percentage is restricted to 2 % but can be used up to 4%. The diameter of steel reinforcement used may be 10 or 12 mm, no matter the types of building.
Footing:
In footing also the amount of reinforcement is indicated by diameter and spacing of bar rather than numbers of bar used. The minimum and maximum percentage of steel can be used for the footing is same as the slab.
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