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Department Of Civil Engineering Industrial Training Report
Reinforcement is used to take tensile force and as well as compressive force. Only reinforcement bars carry tensile stress where compressive stress is carried by both reinforcement bars and concrete depending on the situation carry load. Normally steel bar is used as reinforcement because it has enough strength and steel bars are available at required strength with fair cost. In early days wooden bar also was used for this purpose.
Concrete is weak in tension and strong in compression. Reinforcement is provided to enhance the moment carrying capacity of structure. Tor steel bars are used as reinforcement material at our site. T-32, T-25, T-20, T-16 and T-10 bars which are imported from Singapoor, Malasiya and local product also and length of 6m and 12m, are used in our site.
The reinforcement is laid according to the structural drawings. After that the following checks are done.
- Diameter & type of bars
- No of stirrups
- No of links
- Lap length
- Starter bars if needed
- Cover blocks & Covering
- Regular binding of bars together
- Placement of tension & compression bars
If there are any construction joints, they should be chipped and cleaned to bond properly.
7.1 INDICATION OF REINFORCEMENT IN DRAWING
Engineering drawings is a language of communication of details. Therefore there is a standard to indicate reinforcement in drawing such as
6T16-101-8 B or T
This means that
‘6’- No of bars
‘T’ - Indicate tor steel
‘16’- Indicate the diameter of the bar in mm
‘101’- Indicate the bar mark
‘8’- Indicate the distance between two bars in inches
‘T’- Indicate the position of the bar (top bar)
‘B’ - Indicate the position of the bar (bottom bar)
7.2 BAR BENDING WORK
Bar bending is carried by machinery method and manual. For minor works it was done manually with the help of cutting lever. Skilled bar benders are involved in this process. The bar bender do his job according to bar schedule which is given to him. It includes
â- Size & type of bar
â- Number of bar
â- Bending length
â- Crank length position
- Shape of bar
- Bar mark
- Name & type of structural member
- Place & Grids between those grids it is used
For transferring the stress at the end of bars have to be welded or lapped at the end. The lap should not be less than the anchorage length. The minimum length of lap in tension and compression is 45 times of diameter of the bar. The types and numbers of bars are planed earlier and cut. Sufficient numbers of stirrups are also made by bar benders. Before placing the steel bars, they are cleaned well.
Lap length – 45 times of bar diameter
Anchorage length – 40 times of bar diameter
Stirrup anchorage length – 10 times of bar diameter
Diameter of ¦
Figure 7.1: Lapping of Bars
7.3 PLACING OF REINFORCEMENT
All reinforcement should be placed in correct positions as shown in drawing. It should not be allowed to move when concreting. Cover blocks are used to place reinforcement bars with cover. Cover blocks are made up of 1:3 cement & sand mortars. Cover blocks are not removed after concreting and it is important element. Its’ strength should not be less than to strength of concrete. Sizes of cover blocks vary according to the structural members. The cover block thickness of, slab is 25mm, beam, column, lift wall& retaining wall are 30mm and shoring side & water retaining structures are 40mm.
When beams are bound it is kept out of the pit of beam for the easy of work. First top bars of beam and stirrups are bound. Then other bars are inserted and bound. While reinforcement of beam is brought down after finishing the work, very nicely it is get down without affecting the formwork.
Usually for all structural members lap is not allowed at a same level for all bars. And alternative bars having laps at a same level. But columns have laps at a same level because of difficulties of binding of long bars and reinforcement is crowded at the junction of beam and column. Lap is avoided at critical position of tension and compression.
7.4 SCHEDULE OF BARS
Schedule of bars gives the number of bars required in each type for a reinforcement network. By having bar schedule following advantages can be achieved.
- Time saving in the reinforcement work
- Amount of steel required can be exactly calculated. So purchase in excess or high transport cost can be avoided.
- Wasting of cut off pieces can be reduced give more efficiency as reinforcements are expensive.
- Laps occurring in tension portion can be avoided.
- Bar benders can easily understand.
- Easy to handle at site.
While we make a bar schedule we should know about the tension and compression how to acting on the member. And lap should not be allowed to be in the tension portion and critical compression portion also. When we are doing schedule for slab if part of slab is finished for the balance work we take site measurement for required length of reinforcement. The typical lap position for beam is shown below:
Figure: 7.2 TYPICAL LAP POSITION OF BEAM
Figure: 7.3 TYPICAL END BAR OF BEAM
Slab is a kind of plate, which can carry loads. Basically slabs are divided in to two types as one-way spanning slab and two-way spanning slab. Depending on the type of slab the amount of reinforcement, which placed to resist the moments, is different. Most probably slabs are reinforced only for tension and compression is supported by concrete. Usually for middle of slab reinforcement is placed at bottom and edge (near the beam or supports) of slab reinforcement is placed at top. But basement floor slab reinforcement is placed in opposition way. Therefore middle of slab reinforcement is at top and edge (around the beam) of slab reinforcement is at bottom. Between the top bar and bottom bar of the slab should be maintained while concreting it may be disturbed due to load which is the load of workers, instrument and concrete flow pipe. Overall reinforcement cover is checked by leveling instrument. To keep the space between those two bars space bar is used. Shape of space bar is shown in the Figure: 7.4. Reinforcement for slab is shiwn in Figure: 7.9 & Figure: 7.10.
Beam is the load-transferring member building, which transfer load from slab to column. In the beams tension is supported by reinforcement and compression is supported by concrete and reinforcement. In most occasions, middle of beam undergoes sagging moment and beam nearby support undergoes hogging moment. For supporting the higher tension, additional tension bars are inserted at bottom of middle and top of edge therefore nearby support. But basement floor reinforcement is in the opposition way. For basement floor beams additional tension bars are inserted at top of middle and bottom of nearby pile caps. While bars are bound to the beam space between two bars should not be less than 1.5 times bar diameter. This minimum space is required for, to form an enough bonding between reinforcement concrete and to minimize the damage while impact load is applied on the beam reinforcement take dynamic movement.
Column is the main member to transfer the load to foundation. Column is mainly designed for carrying the compression. But it is designed to support the moment also while considering the eccentricity and side force therefore wind load. In column compression is carried by concrete and reinforcement. Column reinforcement is mainly designed for support the compression and small amount of tension that is caused by moment. For large size bars (T 32, T25, etc.) lap is held at same level. Due to the handling problem and if the lap is in the junction of column and beam concreting is very difficult some time huney homb may occur. Column typical lap is shown in the figure 7.4. Probably most of columns reinforcements are symmetrical about two axes. First when bars are bound verticality of bar is maintained.
Figure : 7.5 COLUMN TYPICAL REINFORCEMENT
7.8 Stair case
Stair case is same as slab reinforcement. Therefore tension reinforce is provided at bottom and at the edges tension reinforcement is provided at top. Here in addition to slab some tension reinforcement is provided junctions. Typical reinforcement is shown in Figure: 7.6.
Stirrup is main element in supporting the shear force. If we consider column, stirrups are used for to hole the vertical bar together. Due to load column mar bulk in this situation cracks may appear in vertical direction. Without happening bulk stirrups bound the column. In the beam, there is more probability to happen the shear failer because it transfer load horizontally. Beam is not a continuous element that is the compound of cement, sand, aggregate and reinforcement. These elements are bound by bond, which may fail in transferring the shear stress. Of cause we use stirrups to carry out the shear stress for beams and columns. In our site stirrups are made by bar bending machine. Stirrup shape is shown in the figure 7.6
FIGURE: 7.6 Stirrup shape
7.10 TYPICAL PILE CAP REINFORCEMENT
Pile cap reinforcement is shown in Figure: 7.11