For any RCC frame member it is in general to use numerous steel bars to satisfy the required steel area rather than using one high diameter steel bar. What would be the reason for that?
It supports and help to keep the shape . Keep the concrete intact during the tensile loading conditions. The fracture toughness and damage tolerance is hence improved.
We calculate the steel area required in design calculation. Now the question is what should be the combination of different standard bars.My perception is:
1. Bars of standard dimension is only manufactured and it is not available on case-to-case basis.
2. Lateral confinement through better detailing and increase of ductility would not be possible by one bar. Note that we differentiate in standard practice between confined and unconfined concrete (cover). Do you have confined concrete if we use one bar?
3. On the other hand, use of bars of lower dimension is also a problem from constructional view point. If you use very low diameter bars, casting concrete (using vibrator etc.) shall often be a problem.
So have a compromise. There may be some more reasons.
Limit the surface cracking and for water tightness - absolutely critical - have a look at the British Standard on this matter
We do this because,
- One large diameter bar calls for significant development length which possibly is not practical.
- Concrete confinement cannot be achieved through a single bar, regardless of its diameter.
- Small number of rebars is not a good idea to arrest shrinkage, thermal, and structural cracks. Sometimes a well distributed reinforcement is more critical than the overall rebar cross section.
- Most available experimental results were obtained based on the current practice (large number of small rebars).
The answers have been quite amazing. Thank you everyone for taking your time out and answering. I was facing this question while I was studying corrosion effect in structural damage.
There is a major study on this matter at the UIUC in 2000 - look for a PhD by one of Hawkins' students - sorry cannot remember the name.
JMN
I'm not a specialist of reinforced concrete, but based on the principles from fibre reinforced composites, I see a few arguments that could be invoked.:
1- The larger the steel part, the higher the likelihood of having a strength reducing defect in a given unit length. Therefore, as the reinforcement cross-section increases, there is a decrease of its ultimate tensile stress.
2- Having a multitude of small reinforcements instead of a big one increases the surface to volume ration of the reinforcements. As the load is transferred from the concrete to the steel wires by shear, an increased surface of transmitting the load reduces the shear stress at the interface.
3- Multiple distributed wires will make stress in the concrete part more uniform as each rebar will be able to react to local stresses more easily.
4- Redundancy: in the event of the failure of a single bar or of the interface, The load will be transferred to the neighboring bars which are likely to be able to deal with the overload, at least temporarily. This would increase the parts reliability.
Food for thought I guess. May be others can improve on my answer.
Excellent answer - merely add that circular wound shear reinforcement will tie the small bars together and there for improve 3.
Not sure about the bridge - I see research gate as an area that Masters students will use so this type of excellent discussion helps them understand the sorts of discussions that occur between highly competent engineers - such as you and I.
I saw in the Hunter Valley 36 mm bars at 90 mm spacing on a 45 degree incline with 1 metre lifts - bundles were so big the concrete had to be hand placed carefully - contractor paid for a better solution
I have no problem with any of the previous answers but in more basic terms, sharing a force between an equivalent number of smaller bars distributes the load into the surrounding concrete with lower stresses at the concrete/steel interface. Avoidance of high stress concentrations reduces (micro)cracking and improves the durability of the concrete component.
Reinforcement in RCC structure is used to provide (i) adequake ductility in the structure which is other missing in concrete, (ii) stability to the concrete through its unique rib design which provides resistance to slippgage of concrete during collapse, (iii) complicated shape to the structure which is otherwise may not be possible.
It is not possible to achieve all these objectives by using a single rebar of higher diameter and it necessitates use of multiple rebars as per design of the structure.
- Badges
- Science topic