Principle of choosing the size of reinforced concrete cross section
Before starting to calculate the reinforced concrete structure, the first thing we need to do is choose the preliminary cross-sectional dimensions for the structural objects in the calculated work. So what is the principle of choosing this section size, here are 3 key factors you need to follow when choosing a preliminary section
Evaluate the reasonableness of the selected section size based on the reinforcement ratio.
- The ratio of reinforcement that I want to mention here is μ=As/A
- With As: is the area of reinforcement, A: is the cross-sectional area
- When the preliminary cross-section is selected for the structure, and the reinforcement for the structural member is calculated. The next thing is that we need to calculate the reinforcement ratio μ of that structural member and then compare it with a reasonable interval. That is, μ must satisfy the condition
- Because if μ is too small, it proves that the cross-sectional size is too large compared to the bearing capacity requirements, and vice versa, if μ is too large, it proves that the cross-sectional size is too small compared to the capacity requirement. bearing capacity. Therefore μ must be within a reasonable range
- For each type of structural member, there is a reasonable μ interval, to know exactly this parameter, please find articles on structural calculation such as columns, beams, floors, ..
Aesthetic factors of reinforced concrete cross-section dimensions
You need to combine structural drawings with architectural drawings to come up with the most optimal cross-sectional size plan that is both capable of bearing and aesthetically consistent with the given architectural plan.
Factors on construction electrical control when choosing the size of reinforced concrete cross section
- You need to choose the size of the section so that the fabrication and unification of the formwork is convenient for placing reinforcement and concrete.
- Usually choose wall and slab thickness as multiples of 1 or 2cm
- Choose beam and column cross section dimensions in multiples of 2, 5 or 10cm
Principles of the structure of the frame and reinforcement mesh
Reinforcing steel placed into the structure must not leave each bar but must be connected to form a frame or mesh
The frame consists of longitudinal and transverse reinforcement, used in beams and columns
The grid consists of reinforcements placed in two perpendicular directions, used in floor slabs and walls.
The intersection between the reinforcement in the frame and the mesh can be connected by tying and welding.
The frame and net are made of loose rods, and the buttons are fastened with soft steel wire (Ø0.8-Ø1).
The advantage of the fastening method is that it is possible to arrange the reinforcement flexibly, in accordance with the bearing capacity of the structure. Therefore, reinforcement is used reasonably and economically. But the downside is slow construction.
To overcome the above disadvantage of the fastening method, a welding method has been made by having welded frames and meshes fabricated in specialized facilities by welding the contact point where the reinforcement intersects.
Bearing reinforcement and structural reinforcement
Rebar in the frame and mesh, depending on its role and task, will be divided into 2 types: Load-bearing reinforcement and structural reinforcement.
Bearing reinforcement, also known as calculated reinforcement, is used to bear the stresses arising from the action of loads, they are determined or checked by calculation.
Structural reinforcement is placed into the structure with many different effects:
- To link the load-bearing reinforcement into a frame or mesh
- To keep the position of load-bearing reinforcement during construction
- To reduce the uneven shrinkage of concrete
- To withstand stress due to temperature change
- To prevent the expansion of cracks
- To distribute the effect of concentrated loads,…
In fact, structural reinforcement is also subjected to forces, but they are not calculated but placed according to regulations and experience.
Reinforcement protection layer
The layer of concrete protecting the reinforcement is calculated from the outer edge of the concrete to the nearest outer edge of the reinforcement.
The protective layer works to ensure the simultaneous working of the reinforcement and the concrete in all stages, as well as to protect the reinforcement from the effects of air, temperature and the like.
Two types of reinforcement protection layer you need to clearly distinguish
- Protection layer of C2 . bearing longitudinal reinforcement
- Protection layer of structural reinforcement, C1 . belt reinforcement
Regulations for protective class of reinforcement
- In all cases, the thickness of the protective layer must not be less than the specified reinforcement diameter ( C >= D )
- In addition, the thickness of the reinforcement protection layer must not be less than the Co value with the following provisions:
- With bearing reinforcement
- In slab and wall thickness:
- From 100mm or less: Co=10mm (15mm)
- Over 100mm or more: Co=15mm (20mm)
- In beams and ribs with height:
- Less than 250mm: Co=15mm ( 20mm )
- Greater than or equal to 250mm: Co=20mm (25mm)
- In column: Co=20mm (25mm)
- In foundation beam: Co= 30mm
- In the nail:
- Mounting: Co=30mm
- Whole block with concrete lining: Co=35mm
- Whole block without concrete lining: Co=70mm
- With structural reinforcement, belt reinforcement
- When the cross-sectional height h<20mm, then Co=10mm (15mm)
- When the cross-sectional height h>=250mm, then Co=15mm (20mm)
- Attention
- Values in brackets (…) apply to outdoor structures or wet places
- For structures affected by the marine environment (salt water), the thickness should be increased according to TCVN 327-2004
- For structures in strongly aggressive environments, additional cladding or special protective measures are required
Spacing of reinforcement
The purpose of the reinforcement is placed with a wide enough gap t so that the concrete mortar can easily pass through and so that around each reinforcement there is a concrete layer sufficient to ensure the adhesion force.
According to current reinforced concrete standards, clearance t >=(Ømax;to)
When the reinforcement has a horizontal or oblique position when pouring concrete
- With reinforcement placed below: to=25mm
- With reinforcement placed on: to=30mm
- When reinforcement is placed more than two layers, with the upper layers to=50mm (except the bottom two layers)
- If a rammer is used to compact the concrete, the gap t in the upper layer should be ensured for the beam to pass through
When the reinforcement is placed vertically at the time of pouring concrete
- In this case to=50mm
- If there is systematic control of the aggregate size, it can be reduced to 35mm but not less than 1.5 times the maximum size of the coarse aggregate.
Special case
- The special case is that it is allowed to arrange the reinforcement bars in pairs, with no gap between them
- The method of pairing must follow the direction of movement of the concrete mortar and the required clearance t>=1.5Ø
Reinforced anchor
The purpose of reinforcement anchors is to promote the bearing capacity because the reinforcement is firmly anchored to the concrete in the connection or support area.
Methods of anchoring reinforcement:
- Straight anchor
- Anchor with angle of α=45-90 degrees
- Standard hook type anchor (U-hook)
The section of reinforcement anchorage from the end of the bar to the section perpendicular to the longitudinal axis of the member where it is used with its entire bearing capacity (calculated with the total calculated strength).
- At the same time, the anchor should not be more than the value
- At the same time the anchor paragraph >= lmin
- The above values are given in the following table:
- In case the area for anchoring the reinforcement is not enough to place the anchorage according to the above requirements, additional anchoring measures such as welding to the end of the anchor bars can be used. At this time, the anchor plate needs to be calculated according to the local bearing capacity and the anchor length should not be less than 10.Ø
- Example of regulations for longitudinal reinforcement anchoring for beams
Rebar connection
Rebar connection occurs in the case when the length of the steel bar is not enough or if the steel wall is too long, it will be an obstacle for construction (when the steel bar has to be erected).
Methods of joining reinforcement
- Solder joint
- Binding
- Connecting with a telescopic tube
Regulations when connecting reinforcement by welding method
The two most common welding methods currently used to connect steel cups are:
- Contact welding: Welded by using specialized welding machines, used to extend bars with a diameter of more than 10mm and the ratio of the diameters of the two bars is not less than 0.85mm
- Arc Welding: Welded by using a large electric current to melt the metal of the welding rod and the steel to be welded so that they bond together. The clamp bar can be used or not during welding with this method
- When arc welding using clamps, it can be done with four welds on either side or two on one side
- When arc welding without clamping rods, it is necessary to bend the ends of the reinforcement and then overlap each other so that the shafts of the two rods are aligned
- Confrontation arc welding is often used for large diameter steel bars and requires a trough for lining
The size of the weld seam is specified as follows:
- Thickness (also known as height Hh): Taken as 1/4 of the diameter of reinforcement but not less than 4mm.
- Width: Taken 1/2 of the diameter of reinforcement but not less than 10mm.
- Welding seam length Lh: Determined by experiment to ensure the bearing capacity (according to the method of welding connection calculation) but not less than
- Lmin=4.Ø when using welding clamps on both sides
- Lmin=5.Ø when not using clamps, welding on both sides
- Lmin=8.Ø or 10.Ø when welding only one side
Regulations when connecting reinforcement by the method of overlapping (tie connection)
Overlapping is a way of placing two ends of the reinforcement connected to each other for a length of Lan and tying it with soft steel wire
The effect of this connection is that the force from bar 1 is transferred to the concrete by the adhesion force and then from the concrete to the steel bar 2.
Thus, in the range of concrete stacking, work must be more and more complicated than in other locations, for that reason in that area must strengthen the belt reinforcement and when construction must pay attention to ensure the quality of the concrete. concrete
Do not use the method of overlapping with steel bars with diameter Ø> 30mm
It is not recommended to overlap in the tension zone of the member subjected to bending and compression, and to eccentric tension at the places where the reinforcement is used to its full capacity.
It is not allowed to use the superimposed connection method in straight members where the entire section is in tension as well as in all cases where reinforcement of group CIV and above is used.
The length of the overlapping joint of the superimposed joint is Lan taken according to the formula
The end of the round, smooth, tensile reinforcement in the frame and the tie net should be bent
Regulations when connecting reinforcement by connecting by telescopic pipe.
The connection method of this method is to insert two steel rods to be connected into a steel pipe. The connection between the rebar and the cage can be by using a press to squeeze the tube into the reinforcement to create friction, using threaded connections or glue.
The force from this rebar is transmitted to the cage and then from the cage to the other reinforcing bar.
See also TCXD 234-1999 to understand more about this method
